GB2621621A

GB2621621A - Decontamination and regeneration of irradiated graphite

Info

Publication number: GB2621621A
Application number: GB2212053.9A
Authority: GB
Inventors: Cottam Martin
Original assignee: JACOBS U K Ltd; Jacobs UK Ltd
Current assignee: JACOBS U K Ltd; Jacobs UK Ltd
Priority date: 2022-08-18
Filing date: 2022-08-18
Publication date: 2024-02-21
Also published as: GB202212053D0

Abstract

A method for sonoelectrochemically decontaminating irradiated nuclear graphite. The method may comprise immersing the irradiated nuclear graphite in an electrolyte 14 subjecting it to one or more electrochemical and ultrasonic cycles. The contaminants may be collected on one or more cathodes 12. The irradiated nuclear graphite may be exposed to a positive electrical potential. The irradiated nuclear graphite may be immersed in one or more acid oxidising agents, or a molten salt electrolyte. A sonoelectrochemical device 10 for removing selected contaminants from irradiated nuclear graphite comprises an anode 22 in contact with the irradiated nuclear graphite, one or more cathodes configured to receive the selected contaminants, with the electrodes immersed in an electrolyte alongside an one or more ultrasonic transducers, and a controller 16 configured to apply ultrasound via the transducers to the electrolyte and an electric potential between the anode and one or more cathodes, separating at least one contaminant from the irradiated nuclear graphite into the electrolyte.

Description

Decontamination and Regeneration of Irradiated Graphite

FIELD OF THE INVENTION

The present invention relates to a method and device for sonoelectrochemically decontaminating irradiated nuclear graphite and, furthermore to the regeneration and reuse of graphite for energy storage devices and systems.

BACKGROUND TO THE INVENTION

There are large volumes of activated graphite associated with decommissioning Magnox and Advanced Gas Cooled reactors in the UK, and other reactor designs internationally. The large volumes of irradiated graphite are contaminated with a variety of radionuclide contamination including activated metals, actinides, lanthanoids, fission products, tritium and carbon 14. Carbon 14 has a very long half-life of 5,730 years which is a major concern for long term storage or disposal.

Owing to the high levels of radionuclide contamination, irradiated nuclear graphite is classified as Intermediate Level Waste (ILW) and must be disposed of at a Geological Disposal Facility (GDF), whose location has not been established at this time. Treating irradiated nuclear graphite waste such that the level of radionuclide contamination is reduced to Low Level Waste (LLW) classification, can offer considerable savings and risk mitigation.

Decontamination of irradiated nuclear graphite has been investigated using electrochemistry in molten salts and other oxidising agents. Electrochemical methods are capable of reducing contamination of irradiated nuclear graphite to low level waste classification levels depending on the initial contamination levels.

Studies have reported a total activity reduction of up to 80% for 60Co contaminant from irradiated nuclear graphite in a molten salt medium. However, the activity levels for carbon 14 were only reduced by up to 15%. Therefore, there is a requirement to enhance the electrochemical removal of contaminants from irradiated nuclear graphite such that the levels of carbon 14 and total activity can be reduced further.

It is against this background that the present invention has arisen.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method for sonoelectrochemically decontaminating irradiated nuclear graphite.

The present invention combines electrochemistry with sonochemistry to implement a novel sonoelectrochemical method for decontaminating irradiated nuclear graphite.

Within the context of the present invention, the term 'electrochemistry' should be understood to refer to the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outcome of a particular chemical change, or vice versa. Electrochemical processes utilize electrochemistry and include electrorefining and electrowinning processes.

Within the context of the present invention, the term 'electrorefining' should be understood to refer to processes which remove contamination from the irradiated nuclear graphite. In particular, the term electrorefining, as used herein, refers to the electrodeposition of contaminants which have been put into solution from the irradiated nuclear graphite. This process uses a similar process to that of the electrowinning process.

Within the context of the present invention, the term 'electrowinning' should be understood to refer to processes that remove contamination from solution and subsequently deposit the contamination on cathodes. The term electrowinning, as used herein, should be understood to refer to the electrodeposition of contaminants which have been previously leached into solution from the irradiated nuclear graphite. In this process, an electrical current passes from an inert anode (graphite where the oxidation occurs) through a leach solution consisting of dissolved metal ions, contamination and graphene derivatives. These constituents are recovered from the solution and deposited in an electroplating process onto the cathode where the reduction occurs.

Within the context of the present invention, the term 'contamination' or 'contaminants' should be understood to include, but not be limited to, activated metals, actinides, lanthanoids, fission products, tritium, carbon 14, graphene and graphene derivatives.

Within the context of the present invention, the term 'sonochemistry' should be understood to refer to the use of ultrasound to form acoustic cavitation in liquids, resulting in the initiation or enhancement of the chemical activity in the solution. There are three classes of sonochemical reactions and the term sonochemistry as used herein should be understood to refer to any one of these classes of reactions. The three classes of sonochemical reactions are: homogeneous sonochemistry of liquids, heterogeneous sonochemistry of liquid-liquid or solid-liquid systems, and, overlapping with the aforementioned, sonocatalysis (the catalysis or increasing, the rate of a chemical reaction with ultrasound): Within the context of the present invention, the term 'acoustic cavitation' should be understood to refer to the formation, growth and then collapse of microbubbles in solution. The collapsing bubble event is a microscopic implosion that generates high local turbulence and thermal energy. Moreover, the collapsing bubble can generate a high temperature up to 5000°C and high pressure up to 2000 atm. The implosion of the cavitation bubble also results in liquid jets of up to 280m/s velocity.

Ultrasound can greatly enhance chemical reactivity in a number of systems by as much as a million-fold; effectively acting to activate heterogeneous catalysts. In addition, in reactions at liquid-solid interfaces, ultrasound breaks up the solid pieces and exposes active clean surfaces through microjet pitting from cavitation near the surfaces and from fragmentation of solids by cavitation collapse nearby. This gives the solid reactant a larger surface area of active surfaces for the reaction to proceed over, increasing the observed rate of reaction.

Ultrasound has been applied to many electrochemical systems, and areas of electrochemistry as this technology offers several benefits over traditional technologies. The advantages are as follows: increase in electrodeposit/electroplating thickness; increase in electrochemical rates, yields and efficiencies; increase in electrodeposit porosity and hardness; increase in gas removal from electrochemical solutions; increase in electrode cleanliness and hence electrode surface activation; lowering in electrode overpotentials (due to metal depassivation and gas bubble removal generated at the electrode surface induced by cavitation and acoustic streaming); and suppression in electrode fouling.

The sonoelectrochemical method of the present invention applies ultrasound to the electrochemical removal of activated contaminants from irradiated nuclear graphite. In some embodiments, sonochemistry enhances the efficiency of the electrochemical process by the use of ultrasonic cavitation and chemical separation of the bonds between contaminants in sub surface layers and graphite pores, thereby improving decontamination factors. In some embodiments, irradiated graphite has a higher porosity than virgin graphite and ultrasonic cavitation impinges the surface of the pores to break bonds and separate the contaminants more effectively. In some embodiments, the creation of free radicals by ultrasonic cavitation also leads to chemical separation and degradation of target contamination.

In some embodiments, the sonoelectrochemical method of the present invention can be used to electrochemically and ultrasonically remove activated contaminants from irradiated nuclear graphite. In some embodiments, the sonoelectrochemical method of the present invention may minimise the contamination level of irradiated nuclear graphite to low level and very low level or out of scope waste classification.

In some embodiments, the method may comprise: immersing the irradiated nuclear graphite in an electrolyte;, and subjecting the irradiated nuclear graphite to a sonoelectrochemical treatment comprising one or more electrochemical and ultrasonic cycles.

In some embodiments, the method for sonoelectrochemically decontaminating irradiated nuclear graphite may comprise cathodes in contact with the electrolyte. In some embodiments, the method may comprise the use of one or more electrowinning cathodes to attract and collect contaminants from the electrolyte. In some embodiments, the method may comprise the use of one or more ultrasonic cathodes which act as ultrasonic probe-type emitters in direct contact with the electrolyte. In some embodiments, one or more ultrasonic cathodes may be electrowinning cathodes and collect contaminants when not applying ultrasound to the electrolyte.

In some embodiments, the method for sonoelectrochemically decontaminating irradiated nuclear graphite may comprise positioning one or more ultrasonic cathodes around an anode basket containing irradiated graphite blocks. In some embodiments, the irradiated nuclear graphite within the anode basket may be immersed in the electrolyte. In some embodiments, a power supply and a controller may be connected to the ultrasonic cathodes and the anode (basket containing graphite), to regulate the field potential and current density along with ultrasonic strength and frequency. In some embodiments, the ultrasonic cathodes are positioned to optimize the stand-off distance and promote ultrasonic streaming towards one or more electrowinning cathodes. In some embodiments, immersing the irradiated nuclear graphite in an electrolyte and then subjecting the graphite to a sonoelectrochemical treatment results in the mass transport of electrochemical species from the irradiated nuclear graphite into the electrolyte.

In some embodiments, the application of one or more ultrasonic cycles to the irradiated nuclear graphite significantly enhances the mass transport of electrochemical species compared to the application of electrochemical cycles alone. In some embodiments, the ultrasonic cycles enhance the conventional electrochemical processes by introducing the beneficial effects of ultrasonics and acoustic cavitation to the electrochemical molten salt and other oxidising processes. During the acoustic cavitation process, high temperatures, pressures, and cooling rates can be achieved upon the collapse of the bubble, which permit access to a range of chemical reaction space normally not accessible.

In some embodiments, the method may further comprise the step of collecting the contaminants removed from the irradiated nuclear graphite on one or more cathodes.

In some embodiments, after the irradiated nuclear graphite has been subjected to sonoelectrochemical decontamination such that the contaminants are transported into the electrolyte, they may subsequently undergo mass transport from the bulk solution to the surface of the electrowinning cathodes where they are collected. In some embodiments, the ultrasonic cathodes may apply a low level of ultrasound to the electrolyte or be placed in an 'off' state in which no ultrasound is applied to the electrolyte. In some embodiments, the ultrasonic cathodes may then be use to collect contaminants from the electrolyte.

Therefore, in some embodiments, the ultrasonic cathodes may be electrowinning cathodes.

In some embodiments, the ultrasonic cathodes and one or more cathodes collecting contaminants may be separate. In some embodiments, removing the contaminants from the electrolyte onto electrowinning cathodes minimises the generation of intermediate level waste.

In some embodiments, only the desired contaminants are electrodeposited onto the electrowinning cathodes. In some embodiments, the electrowinning cathodes may be plates, 3D open cell metal foam, or ceramic 3D porous structures. In some embodiments, a 3D open cell structure is preferable owing to its high surface area which maximises the retention of contaminants.

In some embodiments, once the electrowinning cathodes have collected contaminants, they may be removed, rinsed, dried and loaded into intermediate level waste containers for processing and disposal at a licensed burial facility.

In some embodiments, the method comprises exposing the irradiated nuclear graphite to a positive electrical potential.

In some embodiments, various electrochemical and ultrasonic methodologies may be applied to sonoelectrochemically decontaminate irradiated nuclear graphite. In some embodiments, in order to maximise the efficiency of the sonoelectrochemical processes, various experimental parameters can be optimised including the process equipment design, ultrasonic frequency, acoustic power, irradiation time, ultrasonic transducer-electrode distance, electrode materials, electrode potentials, temperature, pH, conductivity, and electrolyte compositions. In some embodiments, the parameters are optimised to maximise contaminant removal from the irradiated nuclear graphite whilst minimising weight loss from the irradiated nuclear graphite. In some embodiments, the irradiated nuclear graphite can be disposed of in low level waste facilities after treatment whereas the contamination in the electrolyte may be disposed of in intermediate level waste facilities. Therefore minimising weight loss from the graphite prevents excess graphite from entering the electrolyte and increasing the volume of intermediate level waste. In some embodiments, the parameters are dependent on the amount of irradiated nuclear graphite to be decontaminated.

In some embodiments, adjusting the frequency and power of the ultrasonic cycle facilitates the acoustic cavitation process. In some embodiments, the bursting of cavitation bubbles on the surface of the irradiated nuclear graphite substrate and inside pores dislodges contaminants embedded in them and enhances mass transfer rates by microstreaming. In some embodiments, the acoustic cavitation process facilitated by the application of ultrasound to the irradiated nuclear graphite leads to the production of highly reactive radicals such as OH, 0 and H202. In some embodiments, these highly reactive radicals act as strong oxidants to oxidize the graphite and release contaminants such as NC. In some embodiments, the creation of free radicals by ultrasonic cavitation may be the main mechanism that leads to chemical degradation of target contamination.

In some embodiments, by varying the operating parameters which include the ultrasonic power, current density, deposition potential, and ultrasonic versus electrochemical pulse times as well as the pH, temperature, and composition of the electrolyte in the sonoelectrochemical cell, the amount of the contaminant to be removed from the irradiated nuclear graphite can be optimised. In some embodiments, this may enhance the removal, transportation and loading of the contaminant onto the porous electrowinning cathodes, whilst minimising weight loss from the graphite.

Removing contaminants with acid oxidising agents In some embodiments, the method may comprise immersing the irradiated nuclear graphite in one or more acid oxidising agents. In some embodiments, the method may comprise immersing the irradiated nuclear graphite in an oxidising agent electrolyte. In some embodiments, the oxidising agents may include, but are not limited to, nitric acid, sulphuric acid, water, hydrogen peroxide, nitric acid and/or potassium permanganate. In some embodiments, the oxidising agent may be a combination of nitric acid and potassium permanganate, sulphuric acid and potassium permanganate, and/or nitric acid and sulphuric acid and water and hydrogen peroxide.

Removing contaminants with molten salt electrolyte In some embodiments, the method may comprise immersing the irradiated nuclear graphite in a molten salt electrolyte. In some embodiments, the molten salt electrolyte may comprise one or more alkali metal halide salts, alkaline earth metal halide salts, strong bases, or combinations thereof.

In some embodiments, the method may be carried out in an inert atmosphere. In some embodiments, the method may be carried out in air. In some embodiments, the inert atmosphere may be argon. In some embodiments, an inert atmosphere prevents oxidization of the molten salt electrolyte.

In some embodiments, the molten salt electrolyte may be an ionic molten salt electrolyte. In some embodiments, the ionic molten salt electrolyte may comprise one or more ionic electrolytes including, but not limited to, Lid, NaCI, KCI or LiBr amongst others. In some embodiments, a LidI-CaCl2 eutectic melt electrolyte may be used as it operates at a low temperature compared with simple salts, has a high energy efficiency and high reducing properties along with a relatively low energy cost. In some embodiments, KCI may be added to further lower the operating temperature.

In some embodiments, after removal of contaminants from the irradiated nuclear graphite into the molten salt electrolyte, the contaminants can be removed from the molten salt electrolyte and collected onto electrowinning cathodes. In some embodiments, the contaminant coated electrowinning cathodes may be removed from the sonoelectrochemical cell and loaded into ILW 3m3 and 4M boxes for storage at the Geological Disposal Facility.

In some embodiments, the anode basket containing the irradiated nuclear graphite may be removed from a sonoelectrochemical cell after undergoing a sonoelectrochemical decontamination cycle. In some embodiments, the anode basket containing the irradiated nuclear graphite may subsequently be immersed in the electrolyte of another sonoelectrochemical cell and undergo an additional sonoelectrochemical cycle. In some embodiments, the method may comprise immersing the irradiated nuclear graphite in an acid oxidising agent electrolyte and subsequently a molten salt electrolyte.

Forming and removing graphene and graphene derivatives In some embodiments, the sonoelectrochemical method for decontaminating irradiated nuclear graphite may comprise the step of forming graphene and/or graphene derivatives.

Within the context of the present invention, the term 'graphene derivatives' should be understood to include, but not be limited to, graphene oxides, reduced graphene oxide, reduced graphene oxide decorated with metallic nanoparticles or nano diamonds, functionalised graphene based nano-materials and/or graphene oxide quantum dots.

In some embodiments, the method may comprise the step of creating graphene oxide, reduced graphene oxide, graphene layers, nano-scrolls and/or nano-diamonds.

In some embodiments, the sonoelectrochemical method for decontaminating irradiated nuclear graphite may comprise the step of functionalising irradiated nuclear graphite to form graphene or graphene derivatives. In some embodiments, the irradiated nuclear graphite may be functionalised to produce graphene oxide, reduced graphene oxide and/or graphene depending on the reuse requirements and product application.

In some embodiments, the functionalisation of irradiated nuclear graphite may occur through exfoliation of the graphite layers. In some embodiments, the functionalisation of graphite may use Graphite Intercalation Compounds (GICs). GICs may be formed by the insertion of atomic or molecular layers of a guest chemical between the layers in the irradiated nuclear graphite material. In some embodiments, the guest chemical may be a transition metal or a metal oxide. In some embodiments, sulphuric acid may be used to add functional groups between the layers of graphite in the presence of an oxidising reagent such as potassium permanganate. In some embodiments, the GIC may be a complex material having a formula CXm, where the ion Xn+ or Xn-is inserted (intercalated) between the oppositely charged carbon layers. In some embodiments, the intercalation between the graphite layers expands the interlayer distance and weakens the bonds which exfoliates layers and exposes the porous structure.

In some embodiments, graphene and/or graphene derivatives may be exfoliated from the irradiated nuclear graphite into the electrolyte during the sonoelectrochemical decontamination process. In some embodiments, the exfoliation of irradiated nuclear graphite aids in the decontamination process by exposing the porous structure and enabling contaminants to be removed from within.

In some embodiments, the step of forming graphene and/or graphene derivatives may take place using an acid oxidising agent electrolyte or may utilise a molten salt electrolyte. In some embodiments, graphene oxide may be formed using an acid oxidising agent electrolyte. In some embodiments, graphene may be formed using a molten salt electrolyte.

In some embodiments, the formed graphene and/or graphene derivatives may be functionalised. In some embodiments, graphene and/or graphene derivatives may be modified by tuning the composition, framework, dimension, and/or morphology. It some embodiments, graphene can be functionalised by chemical oxidation, doping, covalent and non-covalent modification, and hybridization with other materials. In some embodiments, the functionalisation of graphene may yield various products including, but not limited to, graphene oxide, nano graphene, graphene nanoribbons (GNRs), graphene nanomeshes, and graphene-polymer hybrids. In some embodiments, the properties of graphene such as its composition, size, shape, and structure, may be modified after functionalisation, leading to changes in its electronic structure, surface chemistry, solubility, and mechanical and chemical properties.

In some embodiments, functionalized graphene composites may be formed. Within the context of the present invention 'functionalised graphene composites' should be understood to refer to hybrids of graphene (or functionalized graphene) with other materials, including but not limited to small molecules, polymers, metals, inorganic compounds, and carbon nanotubes (CNTs).

Collecting graphene/graphene derivatives onto electrowinning cathodes In some embodiments, the contaminant removed from the irradiated nuclear graphite and collected on one or more cathodes may comprise graphene, graphene derivatives, 14C, activated metals, lanthanides and/or actinides.

In some embodiments, the graphene and/or graphene derivatives may be removed from the electrolyte and collected onto the electrowinning cathodes. In some embodiments, the graphene and/or graphene derivatives can interact with other contaminants in the electrolyte such as Am(iii), Th(iv), Pu(iv), Np(v), U(vi), Cs(i), Sr(ii), Eu(iii) and Tc(vii). The adsorption of contaminants onto the graphene and/or graphene derivatives may facilitate the collection and removal of contaminants from the electrolyte onto electrowinning cathodes.

In some embodiments, the irradiated nuclear graphite may first be exfoliated in an acid oxidising agent electrolyte such that graphene oxide is leached into the electrolyte and collected onto the electrowinning cathodes. In some embodiments, the irradiated nuclear graphite may subsequently be exfoliated in a molten salt electrolyte such that graphene leaches into the electrolyte and is subsequently collected and layered on top of the graphene oxide layer on the electrowinning cathodes. In some embodiments, an electrowinning cathode comprising graphene overlayed on top of graphene oxide can have an enhanced performance when subsequently used to form a battery and/or supercapacitor.

Using the graphene and/or graphene derivative coated electrowinning cathodes in energy storage devices In some embodiments, the sonoelectrochemical method of the present invention may be used to form contaminant and/or graphene and/or graphene derivative coated electrowinning cathodes. In some embodiments, the electrowinning cathodes which have collected the contaminants and/or graphene and/or graphene derivatives may be subsequently removed from the sonoelectrochemical cell and utilised in nuclear and non-nuclear applications. In some embodiments, the contaminant and/or graphene and/or graphene derivative coated electrowinning cathodes may be used in battery energy storage systems and other energy storage devices. In some embodiments, the contaminant and/or graphene and/or graphene derivative coated electrowinning cathodes may be used to convert waste storage containers into battery and/or supercapacitor energy storage systems for Intermediate Level Waste and Low Level Waste storage and disposal facilities.

In some embodiments, the sonoelectrochemical method of the present invention may be used to form nano-diamonds from the irradiated nuclear graphite. In some embodiments, collecting nano-diamonds on the electrowinning cathodes can improve the electrical conductivity of the cathode for use in energy storage systems and other devices.

Contaminant removal In some embodiments, the contaminant removed from the irradiated nuclear graphite and collected on one or more cathodes may comprise one or more of graphene, graphene derivatives, 'AC, activated metals, lanthanides and actinides. In some embodiments, the contaminant removed from the irradiated nuclear graphite and collected on one or more cathodes may be one or more of Am(iii), Th(iv), Pu(iv), Np(v), U(vi), Cs(i), Sr(ii), Eu(iii) and Tc(vii).

In some embodiments, an acid oxidising agent electrolyte or molten salt electrolyte may be used to remove 14C contamination from irradiated nuclear graphite. In some embodiments, 14C may be removed from the irradiated nuclear graphite into the electrolyte creating calcium carbonate salt (CaCO3) or calcium oxalate (CaO) + carbon dioxide (CO2) in the electrolyte. In some embodiments, the radioactive 14CO2 may be immobilised to form a stable Ca14CO3 product. In some embodiments, Cal4CO3 is regarded as a suitable waste form of 14032 because of its excellent thermal and chemical stability. In some embodiments, Ca14CO3 may be collected and removed from the electrolyte onto electrowinning cathodes.

In some embodiments, the Ca14CO3 coated electrowinning cathodes may be suitable for cementation and long-term storage or disposal.

Continuous vs. pulsed cycles In some embodiments, the method may comprise subjecting the irradiated nuclear graphite to continuous ultrasonic treatment. In some embodiments, the method may comprise applying ultrasound to the irradiated nuclear graphite in pulses. In some embodiments, ultrasonic pulses include a period in which the ultrasound is 'on', followed by a period in which the ultrasound is 'off.

In some embodiments, the method may comprise subjecting the irradiated nuclear graphite to continuous electrochemical treatment. In some embodiments, the method may comprise applying an electrochemical treatment to the irradiated nuclear graphite in pulses. In some embodiments, electrochemical pulses may include a period in which the current applied to the cell is 'on' followed by a period in which the current applied to the cell is 'off.

In some embodiments, the method may comprise subjecting the irradiated nuclear graphite to continuous electrochemical and ultrasonic cycles.

In some embodiments, the method may comprise subjecting the irradiated nuclear graphite to continuous electrochemical treatment and pulsed ultrasonic treatment.

In some embodiments, the method may comprise subjecting the irradiated nuclear graphite to a plurality of electrochemical and ultrasonic pulses. In some embodiments, the electrochemical and ultrasonic pulses may comprise application of an electric current pulse to the irradiated nuclear graphite, followed by an ultrasonic pulse. In some embodiments, electrochemical and ultrasonic pulses can be used to remove contamination from the sub-surface of the graphite and from the pores within the graphite. In some embodiments, ultrasonic pulses can be used to improve the sonochemistry of the electrolyte and the mass transport of contaminants. In some embodiments, pulsed ultrasound may be less energy demanding than continuous ultrasound cycles.

In some embodiments, the current density of the electrochemical pulses depends on the amount of irradiated nuclear graphite being decontaminated. In some embodiments, the current density of the electrochemical pulses depends on the size of the electrochemical cell and the size of the anode basket holding the irradiated nuclear graphite.

In some embodiments, current density can affect the migration rate of electrons in the solution, and therefore optimizing the current density can maximize the efficiency of the sonoelectrochemical decontamination process. In some embodiments, an increased electrochemical pulse current density may result in an increased decontamination efficiency.

In some embodiments, the method may further comprise the step of removing the contaminant coated cathodes from the device and using the cathodes in further applications. In some embodiments, the contaminant loaded cathodes may be subsequently used in nuclear and non-nuclear industries including, but not limited to, batteries, supercapacitors and fuel cells. In some embodiments, the method may further comprise the step of removing the contaminant coated cathodes from the device and using them to form energy storage devices and systems.

In some embodiments, there is provided a sonoelectrochemical device for removing selected contaminants from irradiated nuclear graphite comprising: an anode immersed in an electrolyte and configured to be in contact with the irradiated nuclear graphite; one or more cathodes immersed in the electrolyte and configured to receive the selected contaminants from the electrolyte; one or more ultrasonic transducers immersed in the electrolyte; and a controller configured to apply ultrasound to the electrolyte through the one or more ultrasonic transducers and an electrical potential between the anode and one or more cathodes to cause the separation of at least one contaminant from the irradiated nuclear graphite into the electrolyte.

In some embodiments, the sonoelectrochemical device enables selected contaminants in the irradiated nuclear graphite to undergo anodic dissolution into the electrolyte by the application of an electrical potential and ultrasound to the irradiated nuclear graphite by the controller. The device also facilitates an electrowinning process which removes selected radioactive contaminants from the electrolyte, and then electrodeposits the selected contaminants onto the one or more cathodes.

In some embodiments, the contaminants removed from the irradiated nuclear graphite and collected on the one or more cathodes may comprise one or more of graphene, graphene derivatives, IAC, activated metals, lanthanides and actinides.

In some embodiments, the one or more ultrasonic transducers may be an ultrasonic transducer cathode. In some embodiments, immersing the ultrasonic transducers directly into the electrolyte enables ultrasound to be applied directly to the electrolyte. In some embodiments, the level of ultrasound applied to the irradiated nuclear graphite using ultrasonic transducers is above and beyond that which can be achieved by sonicating the exterior of the electrochemical device. In some embodiments, the one or more ultrasonic transducers may be the one or more cathodes configured to receive the selected contaminants from the electrolyte. In some embodiments, in which the ultrasonic cathodes act as electrowinning cathodes, low levels of ultrasound are applied to the electrolyte or the ultrasonic electrodes are switched to an 'off' state. In some embodiments, the ultrasonic transducer cathodes may be separate from the one or more cathodes configured to receive the selected contaminants from the electrolyte.

In some embodiments, multiple ultrasonic transducer cathodes may be inserted around the anode, thereby providing effective standoff distance and scalability. In some embodiments, surrounding the anode basket with multiple ultrasonic transducer cathodes provides an even distribution of the ultrasonic field on all sides of the anode basket and provides a uniform exposure of the irradiated nuclear graphite to the ultrasound. In some embodiments, the larger the anode basket and volume of electrolyte, the greater the ultrasonic power required to sonoelectrochemically treat the irradiated nuclear graphite and the more ultrasonic transducer cathodes inserted around the anode.

In some embodiments, the use of ultrasonic transducer cathodes enables the device to apply both a sonochemical and electrochemical effect to the irradiated nuclear graphite. In some embodiments, the addition of ultrasound speeds up chemical reactions and optimises the removal of contaminants from the irradiated nuclear graphite. In some embodiments, the application of ultrasound to the electrolyte through the ultrasonic transducers can optimise the size of the contaminants which are subsequently collected on the electrowinning cathodes.

In some embodiments, the use of ultrasonic transduces may prevent the build-up of solids on the electrodes and improve their performance and longevity.

In some embodiments, the ultrasonic transducers may produce high-intensity bubbles and free radicals through acoustic cavitation. In some embodiments, the ultrasonic frequencies emitted by the ultrasonic transducers may be mostly in the range of 15-25 kHz. In some embodiments, the ultrasonic intensity of the ultrasonic transducers may be up to 2000W.

In some embodiments, the distance between the ultrasonic transducers and the electrodes depends on the amount of irradiated nuclear graphite to be decontaminated. In some embodiments, the distance between the ultrasonic transducers and the electrodes is optimised for maximum effect and decontamination factors.

In some embodiments, the anode may be an anode basket. In some embodiments, the anode may be configured to be in contact with the irradiated nuclear graphite by placing irradiated nuclear graphite blocks within the anode basket prior to immersion in the electrolyte. In some embodiments the graphite blocks may be positioned length ways in the basket to promote ultrasonic streamlining towards the electrowinning cathodes.

In some embodiments, the irradiated nuclear graphite will be size reduced to expose more of the internal surfaces to facilitate the release of the contamination more effectively.

In some embodiments, the one or more cathodes may be configured to receive the selected contaminants from the electrolyte. In some embodiments, the one or more cathodes may comprise metal, metal oxides, carbon or ceramic material. In some embodiments, the electrowinning cathodes may have various profiles including but not limited to plates, foam, and other 3D porous structures.

In some embodiments, the one or more cathodes may comprise metal, metal oxide, carbon or ceramic. In some embodiments, the one or more cathodes may be a metal or ceramic foam. In some embodiments, porous metal foams such as nickel, copper and titanium may be directly utilised as an efficient cathode with excellent electrocatalytic activity and durability. In some embodiments, the high surface area of the porous metal foam enables the cathode to retain large amounts of contamination, therefore maximising the efficiency of the device.

In some embodiments, the one or more cathodes may be an open cell matrix. In some embodiments, the one or more cathodes may be a 3D printed open cell matrix. In some embodiments, the one or more cathodes may be a 3D printed open cell matrix of metals or ceramics. In some embodiments, the open cell matrix forms an engineered self-supporting structure for use as an electrowinning cathode.

In some embodiments, the high specific surface area of the metal foam and/or open cell matrix structures enables a large capacity for collecting contaminants. In some embodiments, one or more cathodes configured to receive selected contaminants with a high surface area enables shorter deposition times and greater deposition rates compared to non-porous plate electrodes for example. In some embodiments, the electrochemical device has an improved efficiency for purifying an electrolyte, and the longevity of the electrolyte can be maximised whilst minimising the generation of secondary intermediate level waste.

In some embodiments, the one or more cathodes may be hollow. In some embodiments, the one or more cathodes may be hollow such that when submerged in the electrolyte, the electrolyte contacts both the inside and outside of the cathode. In some embodiments, this maximizes the surface area of the cathode in contact with the electrolyte and maximizes the capacity of the cathode for collecting contaminants. Therefore in some embodiments, the hollow shape of the cathode may increase the efficiency with which the electrochemical device can remove contaminants and purify the electrolyte.

In some embodiments, the one or more cathodes may be coated. In some embodiments, the one or more cathodes may be configured to receive the selected contaminants from the electrolyte by pre-coating the cathodes prior to assembly of the electrochemical device. In some embodiments, the coating may be an anti-fouling coating to minimise blockages of the cathode which would otherwise prevent the cathode from removing contaminants from the electrolyte. In some embodiments, the one or more cathodes may be coated or supplemented with selective materials to enhance the collection and retention of contaminants. In some embodiments, the cathodes may be coated with an ion exchange polymer resin. In some embodiments, the ion exchange polymer resin may attract and remove contamination from the electrolyte.

In some embodiments, the electrolyte may be an acid oxidising agent. In some embodiments, the oxidising agent may include, but not be limited to, nitric acid, sulphuric acid, water, hydrogen peroxide, nitric acid and/or potassium permanganate. In some embodiments, the oxidising agent may be a combination of nitric acid and potassium permanganate, sulphuric acid and potassium permanganate, and/or nitric acid and sulphuric acid and water and hydrogen peroxide.

In some embodiments, the electrolyte may be a molten salt electrolyte. In some embodiments, the molten salt electrolyte may comprise one or more alkali metal halide salts, alkaline earth metal halide salts, strong bases, or combinations thereof.

In some embodiments, the selected contaminant removed from the irradiated nuclear graphite may be graphene and/or a graphene derivative. In some embodiments, the one or more cathodes may be configured to receive graphene and/or graphene derivatives from the electrolyte. In some embodiments, the cathodes may be configured to receive graphene, graphene oxide, reduced graphene oxide, nano-scrolls and/or nano-diamonds from the electrolyte. In some embodiments, the one or more cathodes may be coated in graphene and/or graphene derivatives after operation of the device In some embodiments, the selected contaminant removed from the irradiate nuclear graphite may belt. In some embodiments, the one or more cathodes may be configured to receive 14C or stable products thereof such as Ca14CO3.

In some embodiments, the one or more cathodes may be replaceable. In some embodiments, after selected contaminants have been electrodeposited onto the one or more cathodes, the cathodes may be removed and loaded directly into intermediate level waste containers and encapsulated with cement grout for disposal. In some embodiments, after selected contaminants have been electrodeposited onto the one or more cathodes, they may be subsequently used within nuclear and non-nuclear industries.

In some embodiments, the electrochemical device may further comprise a heater. In some embodiments, the heater may be provided in the walls and/or floor of the device to maintain the electrolyte temperature. In some embodiments in which the electrolyte is a molten salt electrolyte, the heater can be used to maintain the electrolyte in a liquid state.

In some embodiments, the controller may further comprise a power supply and an ultrasonic generator. In some embodiments, the controller may be connected to the ultrasonic cathodes and anode (basket containing graphite), to regulate the field potential and current density along with ultrasonic strength and frequency.

FIGURES

The present invention will now be described, by way of example only, with reference to the accompanying figures in which: Figure 1A shows a sonoelectrochemical decontamination cell; Figure 1B shows the sonoelectrochemical decontamination cell of Figure 1A with electrowinning cathodes; Figures 2A to 2C illustrate the decontamination process using an acid oxidising agent electrolyte Figures 3A to 3F illustrate the collection of contaminants onto electrowinning cathodes using an acid oxidising agent; Figures 4 illustrates a metal foam electrowinning cathode; Figures 5A to 5E illustrate the decontamination process using an acid oxidising agent and a molten salt electrolyte; Figures 6A to 6G illustrate the collection of contaminants onto electrowinning cathodes using an acid oxidising agent and a molten salt electrolyte; Figure 7A shows a 3m3 intermediate level waste box; Figure 7B shows electrowinning cathodes loaded into a 3m3 intermediate level waste box; and Figure 7C shows a 3m3 intermediate level waste electrowinning cathode battery.

DETAILED DESCRIPTION

According to the present invention there is provided a method and device for sonoelectrochemically decontaminating irradiated nuclear graphite. The method combines electrochemistry with sonochemistry to remove activated contaminants electrochemically and ultrasonically from the irradiated nuclear graphite and reduce contamination levels to low level waste classification.

Sonoelectrochemical cell Irradiated nuclear graphite can be decontaminated using an electrochemical device, which may be a sonoelectrochemical cell 10 as shown in Figure 1.

Ultrasonic electrodes 12 combine electrodes (cathodes) with ultrasonic probe type emitters. As shown in Figure 1, one or more ultrasonic electrodes 12 are immersed directly into the electrolyte 14 and positioned around an anode basket 22. The anode basket 22 is configured to hold the irradiated graphite blocks (not shown) such that the graphite is immersed in the electrolyte 14. The ultrasonic electrodes 12 deliver ultrasound to the irradiated nuclear graphite blocks within the anode basket 22 through the electrolyte 14. Ultrasonic frequencies are mostly in the range of 15-25 kHz, whilst ultrasonic power can reach up to 2000 W. The one or more ultrasonic electrodes 12 are attached to an ultrasonic controller 20 which comprises a power supply and an ultrasonic generator. An additional controller 16 is attached to the anode basket 22. The controller may regulate the field potential and current density along with ultrasonic strength and frequency.

The electrolyte 14 may be a molten salt electrolyte or may be an acid oxidising agent electrolyte. A molten salt electrolyte may comprise one or more alkali metal halide salts, alkaline earth metal halide salts, strong bases, or combinations thereof. For example, the molten salt electrolyte may be a LiCl/CaC12 electrolyte. The electrolyte may also comprise KC!. Additives such as CaCO3 and CO2 or CaCO3 may also be added to the electrolyte 14.

The sonoelectrochemical cell 10 is provided within a containment 18. The containment 18 may comprise air or may be sealed and maintain an inert atmosphere such as argon. When the electrolyte 14 is a molten salt electrolyte, an inert argon atmosphere prevents oxidisation of the molten salt. When the electrolyte 14 is a molten salt electrolyte, the sonoelectrochemical cell 10 may incorporate heaters (not shown) in the walls and floor to maintain the molten salt electrolyte in a liquid state at the required temperature.

As shown in Figure 1B, after removal of contaminants from the irradiated nuclear graphite s into the electrolyte 14, the anode basket 18 may be emptied and placed back into the electrolyte 14. The electrowinning cathodes 24 may then be placed in the sonoelectrochemical cell 10 and immersed in the electrolyte 14. The positioning of the ultrasonic electrodes 12 around the anode basket 22 is such that ultrasonic streaming of contaminants towards the electrowinning cathodes 24 is promoted. During the electrowinning cycle, the ultrasonic transducers may be operating at low power, or may be in an 'off' state in which no ultrasound is applied to the electrolyte. This ensures contaminants are streamed towards the electrowinning cathode 24 but prevents contaminants from being removed from the electrowinning cathodes 24 or hindering the attachment of contaminants to the electrodes.

The electrowinning cathodes 24 are connected to a cathode controller 26. The cathode controller 26 applies a current to the electrowinning cathodes 24 such that an electrolytic cell is formed with the electrolyte 14 and the anode basket 22. Upon application of current, positive ion contaminants in the electrolyte 14 are collected on the electrowinning cathodes 24. Contaminants may also be removed from the anode basket 22 and deposited on the electrowinning cathodes 24. Subsequently, the contaminant coated electrowinning cathodes 24 can be loaded into intermediate level waste boxes for storage at the geological disposal facility.

Graphene oxide synthesis in acid oxidising agent electrolyte Figure 2 shows a sonoelectrochemical cell 10 with an electrolyte 14 which is an acid oxidising agent electrolyte. An acid oxidising agent electrolyte may comprise oxidising agents such as sulphuric acid (H2SO4), potassium permanganate (KMn04) and hydrogen peroxide (H202) as an optional additive.

During the decontamination process, the anode basket 22 may be transferred between multiple tanks 19 as indicated by the arrows 28 in Figure 2. A first tank 19 illustrated by Figure 2A may contain an electrolyte 14 comprising 98% sulphuric acid. After a sonoelectrochemical decontamination cycle, the anode basket 22 containing the irradiated nuclear graphite may be removed from the tank 19 of Figure 2A and transferred to the tank 19 of Figure 2B. The tank 19 of Figure 2B may contain a 50% sulphuric acid/potassium permanganate electrolyte.

Sonoelectrochemical treatment of the irradiated nuclear graphite in sulphuric acid and potassium permanganate may result in the synthesis of graphene oxide through Graphite Intercalation Compounds (GICs). GICs are formed by the insertion of atomic or molecular layers of a guest chemical between layers in graphite material. Graphite intercalation compounds (GICs) are complex materials having a formula CXm where the ion Xn+ or Xn-is inserted (intercalated) between the oppositely charged carbon layers. Pristine graphite oxide (P60) is formed.

After the sonoelectrochemical decontamination cycle, the anode basket 22 containing the irradiated nuclear graphite may be removed from the tank 19 of Figure 2B and transferred to the tank 19 of Figure 2C, which comprises a water electrolyte. The formation of GICs between the graphite layers expands the interlayer distance and weakens the bonds which exfoliates layers and exposes the porous structure of the graphite. Sonoelectrochemical treatment in water converts the P60 into graphene oxide (GO) which involves hydrolysis of covalent sulphates and loss of all interlayer registry and associated contamination. This step further enhances the decontamination factor. The graphene oxide in water can be collected onto electrowinning cathodes 24. Alternatively, a further sonoelectrochemical decontamination process converts the graphene oxide in suspension to graphene forming a water/graphene suspension electrolyte. The graphene can then be collected onto electrowinning cathodes 24.

The sonoelectrochemical process enhances the GIC process by removing the layers of PG0 from the irradiated nuclear graphite into the electrolyte, and by exposing graphite pores to a greater degree to enhance penetration and removal of contamination within the pores. The exposed pores of the oxide graphene layer increase the release of contaminants such as 14C.

Alternatively, graphene oxide may be synthesised by water electrolytic oxidation. Concentrated sulphuric acid (98wt.%) is used to functionalize the irradiated nuclear graphite and add functional groups between the layers of graphite. Water electrolytic oxidation of the graphite intercalation concentrate (GIC) follows in 50 wt.% sulphuric acid and potassium permanganate to produce manganese heptoxide Mn207 and highly reactive radicals which facilitate removal of contamination. The sonoelectrochemical process further enhances the oxidation process and subsequent removal of contamination. Subsequently, the graphite is sonicated in water to produce an aqueous solution of electrochemical graphene oxide (EGO).

Aqueous GO can interact with contaminants in the acid oxidising agent electrolyte to facilitate their removal. Cation/GO coagulation occurs with the formation of nanoparticle aggregates of GO sheets. Aqueous GO interacts with actinides including Am(iii), Th(iv), Pu(iv), Np(v), U(vi) and typical fission products Cs(i), Sr(ii), Eu(iii) and Tc(vii). GO along with the various activated contaminants may be collected and retained from the aqueous oxidising agent electrolyte on electrowinning cathodes 24 or removed by filtration and other means.

Graphene oxide can be converted to reduced graphene oxide (rGO) by chemical reduction using ascorbic acid as the reducing agent. Reduced graphene oxide can be synthesized from graphene oxide utilizing the sonoelectrochemical decontamination process in molten salts or acid oxidising agents using various acid and water ultrasonic processes.

Reduced graphene oxide has a higher electrical conductivity, electrochemical capacitance and cycling capability than graphene oxide, therefore enhancing the performance of batteries, supercapacitors, and superconductors. Reduced graphene oxide also has unique properties for components within fuel cells and other applications.

As illustrated by Figure 3, the electrowinning cathodes 24 can be inserted into each of the tanks 19 shown in Figure 2 after the anode basket 22 containing the irradiated nuclear graphite has been removed. This is indicated by the arrows 30 in Figure 3. Figure 3A shows electrowinning cathodes 24 immersed in a 98% sulphuric acid electrolyte. Figure 3B shows electrowinning cathodes 24 immersed in a SO% sulphuric acid/potassium permanganate electrolyte. Figure 3C shows electrowinning cathodes immersed in a water electrolyte comprising a graphene oxide suspension. Figure 3D shows the electrowinning cathodes immersed in a water electrolyte after further sonoelectrochemical treatment has converted the graphene oxide to graphene. At each stage, the electrowinning cathodes remove graphene, graphene derivatives, 'AC, activated metals, lanthanides and actinides from the electrolyte.

The electrowinning cathodes 24 may be an open cell matrix of metals or ceramics. The electrowinning cathodes 24 may be 3D printed. The electrowinning cathodes 24 may be metal foams with a 3D interconnected porous structure as shown in Figure 4. The surface of the foam is very uneven, making the true surface area of the already porous foam even larger. Porosity of the foam is typically around 90.5% and the pore size may vary from 1 to 0.1 mm depending on the metal or ceramic used. The metal foam electrowinning cathodes may be, but are not limited to, nickel, copper or titanium.

Coatings may be applied to the ceramic or metal foams to enhance the energy density and ion transfer properties for battery applications. These coatings may comprise nickel niobates, titanium niobium oxides and nickel manganese cobalt amongst others.

Ultrasound and acoustic cavitation Ultrasonic electrodes 12 enhance the electrochemical processes within the sonoelectrochemical cell 10 by introducing the process of acoustic cavitation.

As the ultrasonic electrodes 12 first apply ultrasound to the electrolyte 14, bubbles with a relatively small radius form and then grow with successive ultrasound cycles. After reaching a certain size, a bubble will become unstable, and subsequently violently collapse. The collapsing bubble event is a microscopic implosion that generates high local turbulence and thermal energy. Upon the collapse of the bubble, high temperatures, pressures, and cooling rates can be achieved, which permit access to a range of chemical reaction space normally not accessible. The collapsing bubble can generate a high temperature up to 5000°C and high pressure up to 2000 atm. The implosion of the cavitation bubble also results in liquid jets of up to 280 m/s velocity.

Ultrasound enhances the electrochemical decontamination process in several ways. Firstly, the application of ultrasound to the electrolyte 14 results in acoustic streaming. Acoustic streaming is directly proportional to the intensity of the ultrasound, the surface area of the ultrasonic emitting device and the attenuation coefficient of the medium. It is inversely proportional to the bulk solution viscosity and the speed of sound. The effect of acoustic streaming caused by ultrasound is an enhanced movement of the electrolyte 14, reducing the diffusion boundary layer and promotion of the mass transfer of electroactive species to the electrode surface.

Secondly, the application of ultrasound to the electrolyte 14 causes the movement of the acoustic cavitation bubble and results in turbulent flow. The turbulent flow increases the mass transport processes within the electrolyte 14 and at the electrode surface.

Thirdly, the application of ultrasound to the electrolyte 14 causes microjets and microstreaming. The collapsing of the acoustic bubbles on a solid surface leads to the formation of microjets being directed towards the surface of the solid material at speeds of up to 280 m/s. When a cavitation bubble bursts on the surface and inside the pores of the irradiated nuclear graphite substrate, contaminants are dislodged and mass transfer rates are enhanced. Microjets and microstreaming also have a surface cleaning effect which prevents fouling of the electrode surface (and accumulation of gas bubbles at the electrode surface) and ultimately enhances the electrodeposition process.

Additionally, the application of ultrasound to the electrolyte 14 causes shock waves which are produced at the end of the strong collapse of bubbles. The shock waves cause the erosion of the electrode surface leading to an increase in the current.

Lastly, the application of ultrasound to the electrolyte 14 enhances the chemistry of the electrolyte. Acoustic cavitation leads to the production of highly reactive radicals such as OH, H202 and 0. The radicals act as strong oxidants to oxidise contaminants such as "C. The creation of free radicals by ultrasonic cavitation may be the main mechanism that leads to chemical degradation of target contamination.

Ultrasound may be applied to the electrolyte 14 via ultrasonic electrodes 12 continuously or may be applied in pulses. Ultrasonic and current pulses may be alternated to optimise the removal of contaminants from the sub-surface and pores of the irradiated nuclear graphite and to minimise graphite weight loss. Various distributions of ultrasound and current pulses may be applied. Compared to the application of direct current alone, sonoelectrochemistry is more effective at removing contaminants, whilst pulsed current and ultrasound can be more effective at removing contaminants than continuous application. A greater current density can result in an increased mass transport of species, and therefore a more efficient decontamination process.

Effects of pulsed ultrasonic cavitation in the development of nano-diamonds When the sonoelectrochemical cell 10 comprises an acid oxidising agent electrolyte and ultrasound is applied in pulses, the process of forming graphene oxide described in Figure 2 above may further lead to the formation of nano-scrolls and nano-diamonds whilst decontaminating irradiated nuclear graphite.

Nano-diamonds can be efficiently prepared from graphite without the use of high temperatures or pressures by a multi-step procedure based on ultrasonication-induced cavitation. First, graphene oxide is formed as outlined in Figure 2. Ultrasonication of GO induces cavitation whilst a specific metal oxide such as Manganese Heptoxide Mn207 is inserted between the graphite layers as a spacer material which significantly affects the nucleation and growth of nano-diamond particles. Carbon nanomaterials, metal oxides, noble metal nanocrystals, polymers, metal-organic frameworks, and even water molecules can also be used as spacer materials. Specifically, cavitation is caused by alternating high-and low-pressure cycles initiated by the propagation of ultrasound waves. During the low-pressure cycle, these waves induced the formation of microscopic voids or bubbles in the medium that reached a certain volume and collapsed during the high-pressure cycle, which created large local increases of temperature (-5000 K) and pressure (-2000 atm) under the action of shear forces. All of these factors resulted in the modification and cleavage of GO sheets and impacted their chemistry and morphology to afford reduced GO sheets.

The shear forces involved in the acoustic cavitation can induce scrolling of the graphene sheets to produce graphene nanoscrolls. The Mn207 spacer is unstable and has a tendency to decompose with the release of oxygen. This primarily occurs at the tips and on the side walls of tubular graphene nanoscrolls and results in nano-diamond nucleation through the formation of diamond nuclei at subsurface damage sites as Mn207 decomposes.

Subsequently, damaged graphene nanoscrolls having vacancy defects can experience self-healing, which results in the formation of basic nano-diamond seeds in which carbon atoms attach to dangling bonds.

Finally, the gradual growth and warping up of these nuclei to cluster-shapes regarded as nano-diamond seeds affords crystalline diamond particles. The introduction of metal oxides to irradiated nuclear graphite during the sonoelectrochemical decontamination process enhances the properties of the graphene oxide in battery applications amongst others.

Graphene synthesis in molten salt electrolyte The sonoelectrochemical cell 10 shown in Figure 1 may be used to form graphene when comprising a molten salt electrolyte during the process of sonoelectrochemically decontaminating irradiated nuclear graphite as described herein. Compared to traditional chemical methods of synthesising graphene, using a sonoelectrochemical cell 10 and a molten salt electrolyte can reduce the number of defects in the graphene structure. By using LiCI, NaCI, KCI and LiBr electrolytes, nanotubes can be produced.

Graphene can be synthesised from irradiated nuclear graphite via electrolytic exfoliation through lithium ions intercalation, expansion, and ultrasonic cavitation. At the initial stage of electrolysis, the lithium ions from the molten LiOH electrolyte heated to 600°C, are intercalated into the spaces between the graphene layers where they are in situ reduced to form intercalation compounds (LixCy) and exert mechanical stress on the host lattice of graphite. As the abundant LixCy are introduced, the interlayer spacing of the graphene layers expands gradually. The intercalated alkali metal destabilizes the layered graphite structure and instigates the formation of carbon-based nanostructured species.

This leads to separation of the graphite structure such that individual graphene layers or stacks become detached. The graphite and electrolyte is subjected to ultrasonic cavitation wherein micro explosions and reactions occur in the interlayer space of graphene layers or stacks leading to further exfoliation.

An alternative means of exfoliation is when the graphite is immersed in water and subjected to ultrasound cavitation wherein micro explosion reactions between lithium (and Li"Cy) and water occurs in the interlayer space of graphene layers or stacks and instantly releases hydrogen. These gaseous products lead to the graphene stacks being further exfoliated.

Compared to commercial graphite, graphene formed from graphite in molten salt media has a higher electrical conductivity. The increase in the conductivity is due to the advantages of the molten salt method, which enables the production of graphene with high crystallinity and purity. The electrical conductivity of graphene synthesised from graphite via molten salt method at 600°C is 10 times higher than commercial grade graphene and 6 times greater than reduced graphene oxide. This has significant implications for enhancing the performance of batteries and/or other applications.

Graphene is also effective at capturing contaminants and therefore decontaminating the electrolyte 14 after being exfoliated from the irradiated nuclear graphite. The contamination along with graphene layers may be removed from the molten salt electrolyte and collected on cathodes using the electrowinning process illustrated by Figure 1B.

Graphene overlaved onto graphene oxide electrowinning cathode formation using acid oxidising agents and molten salts The sonoelectrochemical decontamination process may also be used to overlay graphene onto graphene oxide on electrowinning cathodes 24. Compared to commercial graphene and graphene oxide, graphene overlayed onto graphene oxide can have enhanced electrical properties. This is beneficial when using electrowinning cathodes 24 in energy storage devices in nuclear and non-nuclear applications.

The process of overlaying graphene onto graphene oxide on electrowinning cathodes 24 is shown in Figures 5 and 6. Figures SA to SC show the anode basket 22 containing the irradiated nuclear graphite being transferred between multiple tanks 19 comprising various oxidising agent electrolytes. This is indicated by the arrows 32 in Figure 5. A first tank 19 illustrated by Figure SA contains an electrolyte 14 comprising 98% sulphuric acid. After a sonoelectrochemical decontamination cycle, the anode basket 22 containing the irradiated nuclear graphite is removed from the tank 19 of Figure 5A and transferred to the tank 19 of Figure SB. The tank 19 of Figure SB contains a 50% sulphuric acid/potassium permanganate electrolyte.

Sonoelectrochemical treatment of the irradiated nuclear graphite in sulphuric acid and potassium permanganate results in the synthesis of pristine graphene oxide (P60) through Graphite Intercalation Compounds (GICs).

After the sonoelectrochemical decontamination cycle, the anode basket 22 containing the irradiated nuclear graphite may be removed from the tank 19 of Figure 5B and transferred to the tank 19 of Figure SC which comprises a water electrolyte. The formation of GICs between the graphite layers expands the interlayer distance and weakens the bonds which exfoliates layers and exposes the porous structure of the graphite. Sonoelectrochemical treatment in water converts the PG0 into graphene oxide (GO) which involves hydrolysis of covalent sulphates and loss of all interlayer registry and associated contamination. This step further enhances the decontamination factor.

Subsequently the anode basket 22 containing the irradiated nuclear graphite is immersed in a tank 19 as shown in Figure 5D which comprises a molten salt LiOH electrolyte. After sonoelectrochemical treatment in the molten salt electrolyte, the anode basket 22 is removed from the cell. Finally the anode basket 22 containing the irradiated nuclear graphite is immersed in a tank 19 comprising a water electrolyte, forming a graphene/water suspension.

As illustrated by Figure 6, electrowinning cathodes 24 can be inserted into each of the tanks 19 shown in Figure 5 after the anode basket 22 containing the irradiated nuclear graphite has been removed. This is indicated by the arrows 32 in Figure 6. Figure 6A shows electrowinning cathodes 24 immersed in a 98% sulphuric acid electrolyte. Figure 6B shows electrowinning cathodes 24 immersed in a 50% sulphuric acid/potassium permanganate electrolyte. Contaminants are removed from the electrolyte 14 onto the electrowinning cathodes 24. Figure 6C shows electrowinning cathodes 24 immersed in the water electrolyte and removal of graphene oxide onto the electrowinning cathodes 24. Figure 6D illustrates the electrowinning cathodes immersed in a molten salt electrolyte and the removal of graphene onto the electrowinning cathodes 24. Similarly, Figure 6E illustrates the electrowinning cathodes 24 immersed in the water electrolyte of Figure SE and the collection of graphene onto the electrowinning cathodes 24. By inserting the electrowinning cathodes 24 into multiple tanks 19 comprising acid oxidising electrolytes and subsequently a molten salt electrolyte, electrowinning cathodes 24 coated in graphene oxide and overlayed with graphene can be formed. Graphene overlayed onto graphene oxide (or reduced graphene) provides up to 10 times the electrical conductivity of commercial grade graphene, and by extension superior performance of energy storage systems.

Development of Graphene Metal Oxide Solid State Batteries and Anode Free Solid State Batteries and Hybrid Supercapacitor-Batteries After collection of contaminants, electrowinning cathodes 24 may be removed from the sonoelectrochemical cell 10, rinsed, dried, and loaded into intermediate level waste (ILW) containers 34 as shown in Figures 3F, 6G, 7A and 7B. Electrowinning cathodes 24 from graphite that has been decontaminated to low level waste classification may be loaded into LLW half height ISO (HHISO) containers. The electrowinning cathode loaded containers can be processed and disposed of at a licensed low level waste burial facility. Graphite classified as very low level waste (VLLW) or out of scope waste may be disposed of at permitted landfill sites or potentially reused in various applications.

Alternatively, the high electrical conductivity, large specific surface area, high-speed electron/heat mobility, and electrocatalytic properties of electrowinning cathodes 24 which have been coated with contaminants and/or graphene and/or graphene derivatives means they can be utilised to form solid state batteries and anode free solid state and hybrid supercapacitor-batteries 36 as shown in Figures 3E, 6F and 7C. Electrowinning cathodes 24 for use in solid state batteries can be produced using the sonoelectrochemical decontamination process described herein in either an acid oxidising agent or molten salt electrolyte. Alternatively or additionally, the solid state energy storage device may utilise graphene overlayed onto graphene oxide electrowinning cathodes 24. This may be achieved by acid oxidising agents or molten salts.

As shown in Figure 7C, the electrodeposited electrowinning cathodes 24 can be used to convert ILW 3m3 boxes 34 and LLW containers into solid state batteries (SSB) and anode free solid state batteries (AFSSB) and hybrid supercapacitor-battery 36 and battery energy storage systems (BESS) for ILW and LLW storage facilities. These can be used to provide power for the national grid and local residents. The electrodeposited electrowinning cathodes 24 may be incorporated within other battery types and architectures. Unlike conventional Li-ion batteries, solid state batteries are non-flammable and are considered to be intrinsically safe.

Metal foam and ceramic electrowinning cathodes 24 may be used as anodes and cathodes, can be combined with anode/cathode post insulators 38, geopolymer encapsulant 40, and solid state conducting polymer 42, and a battery casing / box liner 44 to form solid state and hybrid supercapacitor-batteries, as shown in Figure 7C. The resultant battery comprises state of the art solid state materials, which has a much greater current density and faster charging capability than lithium-ion batteries.

There are various materials and hybrids available for current collectors in solid state and anode free solid state batteries. Anode-free lithium cell architecture presents remarkable advantages in terms of both energy density and safety compared with conventional lithium-ion cells. Nanostructured graphene and graphene oxide based hybrids may be used as anodes for solid state batteries and hybrid supercapacitor-batteries.

Anode materials for rechargeable batteries can be classified in three main groups depending on the mechanism of lithiation: ) Alloying materials such as Si, Ge, Sn, Al, Bi, etc., (ii) Conversion materials like transition-metal oxides (Mnx0y, NiO, Fex0y, CuO, Cu20, Mo02, etc.), metal sulfides, metal phosphides, and metal nitrides (iii) Insertion materials, such as carbonaceous compounds (graphite, porous carbon, carbon nanotubes, graphene derivatives), Ti02, Li4Ti5012, etc. Nickel niobate (NiNb206) has been examined in an "open and regular crystalline form for use as the anode in a lithium ion battery. It forms a porous, nano-scale structure that eliminates the dendrite formation that can cause short circuits and other problems. The material offers energy density of 244 mAh g-1 and retains 80%+ of its capacity across 20,000 cycles. This demonstrates promising energy storage performance of nickel niobate anodes in practical battery devices.

The high surface area of metal foam electrowinning cathodes 24 coated with nickel niobate and reduced graphene oxide, provides the basis for an efficient and long lasting anode for battery energy storage systems.

Likewise the high surface area of ceramic foam electrowinning cathodes 24 coated with nickel manganese cobalt NMC 811 and graphene, provides the basis for an efficient and long lasting cathode for battery energy storage systems.

During decontamination of the graphite, graphene oxide and graphene is deposited on the electrowinning cathodes. Reduced graphene oxide (rGO) may be synthesized from graphene oxide using ultrasound as described previously. RGO anodes and graphene cathodes combined with the porous morphology, high electrical conductivity and fast surface reactions, delivers high energy and power density needed for advanced battery and supercapacitor applications.

In order to stabilize and encapsulate the battery within the ILW 3m3 box liner and battery casing 44, a conducting polymer 42 may be used as shown in Figure 7C. Conducting polymers 42 are macromolecular, polymeric substances showing electronic conductivity only when conducting materials like reduced graphene oxide, acetylene black or metal powders have been mixed into the polymers. Intrinsic conducting polymers may also be used. Various conducting polymer composites are available with constituents designed to optimize ionic conductivity and mechanical strength.

Polyaniline (PANI) has received attention due to its unique properties. Reduced graphene oxide/polyaniline (rGO/PANI) composite was prepared by a simple and cost-effective fabrication method of formation by mixing and sonication in various conditions.

Hierarchical nanostructured conducting polymer hydrogel with high electrochemical activity. Hydrogels are polymeric networks that have a high level of hydration and three-dimensional (3D) microstructures bearing similarities to natural tissues. Hydrogels based on conducting polymers [e.g., polythiophene, polyaniline (PAni), and polypyrrole] combine the unique properties of hydrogels with the electrical and optical properties of metals or semiconductors thus offering an array of features such as intrinsic 3D microstructured conducting frameworks that promote the transport of charges, ions, and molecules.

Conducting polymer hydrogels provide an excellent interface between the electronic-transporting phase (electrode) and the ionic-transporting phase (electrolyte), between biological and synthetic systems, as well as between soft and hard materials. As a result, conducting polymer hydrogels have demonstrated great potential for a broad range of applications from energy storage devices such as biofuel cells and supercapacitors, to molecular and bioelectronics and medical electrodes. Multifunctional polyaniline (PAni) hydrogel has excellent electronic conductivity and electrochemical properties. With high surface area and three-dimensional porous nanostructures, the PAni hydrogels demonstrated potential as high-performance supercapacitor electrodes with high specific capacitance (-480 F.g(-1)), unprecedented rate capability, and cycling stability showed capacitance retention as high as -91% over 5,000 cycles and (-83% capacitance retention after 10,000 cycles).

Doped polyesteramide hydrogel, displays micropores, as well as nanometric pores, homogeneously distributed throughout the whole surface, presents better properties as a solid-like electrolyte than doped biohydrogels with pseudo-honeycomb and compact heterogeneous structures. Indeed, the polyesteramide hydrogel shows a low electrical resistance, a high capacitance and good interfacial contact with the electrode, thus meeting the electrical requirements of solid-like electrolytes for supercapacitors. The full characterization of the hydrogel solid-like electrolytes by electrochemical impedance spectroscopy (EIS) provides additional valuable data (i.e., ionic and capacitive properties) to select the most adequate system. In addition, the hydrogel morphology, which has a significant effect on the device performance, is required to be highly porous and open. The correct optimization of these parameters would improve the application of biopolymerderived hydrogels in light and wearable flexible devices.

There are numerous other electrolytes and cathode/electrolyte combinations which may be used. The electric car industry is projecting that anode free solid-state batteries (AFSSB) and hybrid supercapacitor-batteries are the way forward. Eliminating anodes simplifies the battery design, enhances performance, and reduces costs.

Porous open cell 3D electrowinning cathodes coated with RGO and graphene may be used to form high-performance hybrid supercapacitor-batteries with ultrahigh energy density for energy storage devices. Metal oxide and carbon /3D graphene and RGO nanocomposites may be used as the negative and positive electrodes with high specific capacity, ultrahigh energy density and high power density.

Another important application is the production of fuel cells. As mentioned previously graphene derived materials provide superior properties and high-performance criteria necessary for components used within fuel cells. Other applications include but not limited to, conventional and electrolytic filters, membranes, supercapacitors, solar cells, beta-voltaic radioscope power sources, and electronics.

Alpha and or Beta-voltaic radioscope power sources are devices that directly convert alpha and beta ionised radiation particles from alpha and beta emitting sources (americium 241, tritium, nickel-63 and carbon-14) from irradiated graphite into electrical energy. These contaminants retained by the electrowinning cathodes provides the source for this power which when configured with a semiconducting converter such as 4H silicon carbide, provides a long duration, compact power source for applications in remote or extreme environments like nuclear waste disposal sites and space exploration.

Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure.

"and/or" where used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example "A and/or B" is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein.

Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments which are described.

It will further be appreciated by those skilled in the art that although the invention has been described by way of example with reference to several embodiments. It is not limited to the disclosed embodiments and that alternative embodiments could be constructed without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS1. A method for sonoelectrochemically decontaminating irradiated nuclear graphite.
2. The method according to claim 1, comprising: immersing the irradiated nuclear graphite in an electrolyte; and subjecting the irradiated nuclear graphite to a sonoelectrochemical treatment comprising one or more electrochemical and ultrasonic cycles.
3. The method according to any of the preceding claims, further comprising the step of collecting the contaminants removed from the irradiated nuclear graphite on one or more contaminant coated cathodes.
4. The method according to any of the preceding claims, comprising exposing the irradiated nuclear graphite to a positive electrical potential.
S. The method according to claim 2, comprising immersing the irradiated nuclear graphite in one or more acid oxidising agents.
6. The method according to any of the preceding claims, comprising immersing the irradiated nuclear graphite in a molten salt electrolyte.
7. The method according to claim 6, wherein the method is carried out in an inert atmosphere.
8. The method according to any of the preceding claims, further comprising the step of forming graphene and/or graphene derivatives.
9. The method according to claim 3 wherein the contaminant removed from the irradiated nuclear graphite and collected on one or more cathodes comprises one or more of graphene, graphene derivatives, 14C, activated metals, lanthanides and actinides
10. The method according to any of the preceding claims, comprising subjecting the irradiated nuclear graphite to continuous electrochemical and ultrasonic cycles.
11. The method according to any of the preceding claims, comprising subjecting the irradiated nuclear graphite to a plurality of electrochemical and ultrasonic pulses.
12. The method according to claim 3 or claim 9, further comprising the step of removing the contaminant coated cathodes from the device and using them to form energy storage devices and systems.
13. A sonoelectrochemica I device for removing selected contaminants from irradiated nuclear graphite comprising: an anode immersed in an electrolyte and configured to be in contact with the irradiated nuclear graphite; one or more cathodes immersed in the electrolyte and configured to receive the selected contaminants from the electrolyte; one or more ultrasonic transducers immersed in the electrolyte; and a controller configured to apply ultrasound to the electrolyte through the one or more ultrasonic transducers and an electrical potential between the anode and one or more cathodes to cause the separation of at least one contaminant from the irradiated nuclear graphite into the electrolyte.
14. The device according to claim 13, wherein the anode is an anode basket.
15. The device according to either of claims 13 or 14, wherein the one or more cathodes comprise metal, metal oxide, carbon, or ceramic.
16. The device according to any of claims 13 to 15, wherein the one or more cathodes is a metal or ceramic foam.
17. The device according to any of claims 13 to 16, wherein the one or more cathodes is an open cell matrix.
18. The device according to any of claim 13 to 17, wherein the one or more cathodes are hollow.
19. The device according to any of claims 13 to 18, wherein the one or more cathodes are coated.
20. The device according to any of claims 13 to 19, wherein the one or more cathodes are replaceable.