GB2330685A

GB2330685A - Production of plutonium containing products

Info

Publication number: GB2330685A
Application number: GB9722498A
Authority: GB
Inventors: Jeffrey William Hobbs; Peter Parkes
Original assignee: British Nuclear Fuels PLC
Current assignee: Sellafield Ltd
Priority date: 1997-10-25
Filing date: 1997-10-25
Publication date: 1999-04-28
Also published as: GB9722498D0

Abstract

A method of forming a plutonium-containing product comprising combining the plutonium with an oxide or oxide precursor of a metal which does not breed plutonium. For instance, a mixture containing plutonium and zirconium species may be provided. Oxides of plutonium and zirconium are co-precipitated from a mixture and the co-precipitate is formed into a dry material suitable for disposal or use as a fuel.

Description

PRODUCON OF PLUTONIUM-CONTAINING PRODUCTS This invention is concerned with the production of plutonium-containing products. More particularly, it relates to methods of producing nuclear fuel pellets and in one embodiment is concerned with the use of gel precipitation in such methods.

A reprocessing plant deals with an irradiated fuel which has a cladding comprising zirconia.

Traditionally, the cladding is removed before the irradiated fuel is dissolved in a suitable liquid medium. The dissolved U and Pu species are separated from the medium and typically reformulated into a fuel which breeds plutonium when irradiated. There is however, an increasing interest in producing a non-breeding fuel in order to avoid increases in plutonium inventories.

Gel precipitation was introduced in the early 1970's primarily as a method of producing fast reactor fuel that could be loaded directly into the fuel pin. There are two principle methods.

Internal gelation involves the formation of droplets in an immiscible liquid (silicone oil or a chlorinated hydrocarbon) or in air'. Homogeneous precipitation occurs because hexamethylene tetramine in the feed solution undergoes thermal decomposition and releases ammonia internally as the sphere drops in the hot gelation media.

External gelation, or polymer supported precipitation, involves the addition of a water soluble organic polymer to a nitrate feed solution (eg a solution of Pu(NO3)4 and Uo2 (No3) 2) 2-5. This additive maintains the structure of the droplet as it precipitates when contacted with ammonia gas or an ammonium solution.

More recently a process called total gelation has been developed, which in effect combines the internal and external routes6,7.

The gel spheres are typically washed in hot water to remove reaction products from the gelation stage and then dried, for example by extracting the water into hexanol or another solvent. The spheres are calcined in, usually, a carbon dioxide atmosphere to debond them, i.e. to remove polymer and convert the spheres to microspheres of porous metal oxide.

Gel precipitation offers a number of advantages over traditional powder fabrication methods.

The absence of dust, with its associated high plant maintenance requirements and operator dose levels, is often quoted as the most significant improvement. The benefits, however, of co-processing to give a homogeneous oxide product, the continuous operation of a process capable of remote operation and the ability to pour the inherently free flowing fully-sintered spheres directly into fuel pins are all recognised as important further advantages.

Gel sphere routes were initially developed to provide spheres for vibro packing into pins for use in fast reactors but this type of fuel could not withstand the very high ratings used. The spheres were later used as press feed for pelleting but failed to produce pellets of adequate quality. More recently gel sphere routes have been re-investigated for thermal MOX.

As mentioned above, gel precipitation processes are dust-free, enabling high levels of remote operation and shielding, and are therefore suitable for handling reprocessed streams with a low decontamination factor.

According to one aspect of the present invention there is provided a method of forming a plutonium-containing product, comprising combining the plutonium with an oxide or oxide precursor of a metal which does not breed plutonium Also provided is a non-breeding nuclear fuel containing a combination of plutonium and a metal oxide which does not breed plutonium The metal which does not breed plutonium is preferably zirconium but in some embodiments is another non-breeding metal, such as Mg, Al, Ce or Ti, for example. The metal may even be one which is fertile, for example Th, which breeds 233U. The non-breeding material may contain a mixture of metals. . Non-breeding Pu fuels will contain a much higher proportion of Pu than do thermal MOX fuels, as there is no fertile content to breed and sustain fission; for example, suitable Pu contents could be around 40 weight %.

In one class of methods, therefore, the Pu is combined with Zr. Typically, the Pu and Zr are co-precipitated. The co-precipitate may then be treated to form a dry material suitable for disposal or for use as a fuel. Co-precipitated fuels, whether with Zr or another non-breeding metal, are included in the invention, which therefore provides fuels in which the plutonium and the other metal are combined generally homogeneously.

One particularly beneficial class of processes involves dissolving the zirconium-containing cladding of fuel as well as the uranium and plutonium, to form U, Pu or U/Pu streams as well as a Zr waste stream. The Pu or U/Pu stream is blended with the Zr stream.

Gel precipitation or microsphere gelation can be carried out in various ways. In addition to internal and external gelation, referred to above, it is also possible to proceed by a preconditioning step followed by external gelation. Both internal and external gelation have the disadvantage that organic additives are added initially to increase the viscosity of the feed.

Although there may be subsequent washing stages, a significant quantity of this organic material can only be removed by debonding in carbon dioxide or air atmosphere.

Pre-conditioning followed by external gelation involves pre-conditioning the feed to a more viscous state by pre-neutralisation. The Zr(tV) feed is encouraged to form a sol (partially or totally) and form a cross-linked hydro-sol which has a much higher viscosity than the initial feed solution. If necessary, one or more viscosity modifiers, for example an organic polymer, may be added to increase the viscosity of the feed. It is desirable that any treatment to increase viscosity be performed prior to blending of the Zr (or other non-breeding material) with the Pu, in order to minimise chemical conditioning of the Pu stream.

In one preferred method of the present invention, making use of pre-conditioning followed by external gelation, the pre-conditioning is carried out by a dispersion method. A base such as ammonium hydroxide is used to precipitate the heavy metals (typically from 0.7 to 1.5 moll). Subsequent washing with water and treatment with concentrated ammonia initiates peptisation (dispersion of the precipitate) which is completed by heating and stirring. The process can be accelerated by the addition of concentrated nitric acid or an electrolyte.

Sufficient conditioning can also be provided by bubbling ammonia gas into the solution until the precipitation reaction is almost complete (80-90% in the case of thorium nitrate).

Another method of pre-conditioning makes use of a condensation procedure. This takes place when nitrate ions in solution are extracted into an organic solvent such as an aliphatic amine or hexanol. Hydrolysis of the heavy metal occurs simultaneously. Such a single extraction can provide a feed of sufficient viscosity for forrning spheres. The solution, however, can be aged by boiling prior to a second extraction.

The increased viscosity of the feed is sufficient to support the spherical shape once the drops have been formed. The higher the viscosity, the larger the drop size achievable. By way of examples, feed viscosities of approximately 13, 30 and 60 centipoise can result in sintered sphere sizes of approximately 80, 400 and 800 um respectively. The spheres may be gelled by external precipitation in an ammonia atmosphere and subsequently by immersion in an ammonium hydroxide solution.

After the gelation step and sufficient ageing time to ensure complete reaction, the spheres are washed in water and dried by, for instance, solvent extraction, humidified air or hot air, in order to reduce the water content. The spheres may then be calcined in carbon dioxide or air and sintered to the required density for use as a vibro fuel. Alternatively, the calcined spheres, by treatment to increase compressibility, can be used as a press feed.

The sintered spheres produced by the process will have an excellent microhomogeneity due to the use of the co-precipitation method of preparation. Furthermore the PuO2 could act as a phase stabiliser in the zirconia, although additional phase stabilisers may be necessary, e.g. yttria.

A fuel fabrication route based on the co-precipitation method is capable of a high level of automation and shielding and can therefore cope with reprocessed feeds with lower decontamination levels as well as reducing operator dose. Dust formation is eliminated and the process requires low levels of maintenance. Masterblend Pu-Zr reprocessed streams can be handled and the resulting fuel is, as mentioned above, a non-breeding fuel. The method may allow reduction of process steps fuel stages compared with the existing processes by, for instance, elimination of certain washing steps. The process have potential for the use of alternative firing techniques such as vacuum and oxidative sintering, for example, instead of conventional reduction in Ar/H2. It is also possible to vary the compressibility for pellet formation by the use of, for instance, carbon black dispersed in the feed to induce porosity in the debonded sphere.

The feed involved in a co-precipitation process in accordance with the present invention is stable, there being no degradation of organic material with time due to radiation.

If required, further reduction of the fertile content of the fuel can be achieved by blending the spheres with powder prior to pressing.

A further class of methods provided by the invention comprises forming gel spheres containing plutonium, debonding the spheres if they contain organic material, and then blending the spheres with a ceramic oxide powder. The spheres optionally may have been made by a co-precipitation method of the invention. Fuels obtainable using a gel sphere process form one class of products of the invention; the invention thus provides fuels in which the plutonium is in spheres or pressed spheres and the other metal (especially Zr) is in the form of a ceramic oxide powder with which the spheres are blended.

The ceramic oxide powder is a non-breeding powder, examples being ZrO2, MgO, A1203, CeO2 and TiO2. ZrO2 is a preferred oxide powder. The ceramic oxide powder may be wholly or partly formed from ThO2 which is a fertile material breeding 233U. In one aspect of the invention, however, the ceramic oxide powder may be a breeding powder.

In the preferred case where the spheres are blended with an inert ceramic oxide powder, such as ZrO2, the inert ceramic matrix provides the ideal structure in which to burn the microspheres containing materials such as PuO2, (U/Pu)O2 and (Pu/Th)O2. The irradiated fuel encased in a ceramic matrix is in a form suitable for disposal without much subsequent treatment. A fuel containing a high proportion of plutonium may be fabricated by pressing pure PuO2 spheres with such a ceramic oxide powder.

The blending process normally involves the tumbling together of the spheres and powder.

The properties of the powder, such as particle size and density, are tailored to reduce the extent of segregation during processing and to ensure good die fill.

A lubricant or binder may be added during the blending process to aid pressing and pellet formation.

Pellets can be pressed from the blend and sintered to the required density.

A fuel pellet prepared using the blending process of the present invention may be treated by such alternative firing techniques as vacuum and oxidative sintering, for example. Additives may be added to increase compressibility of the spheres for pellet formation, an example of such an additive being carbon black dispersed in the feed to induce porosity in the debonded sphere.

An advantage of the blending process is that powder handling (e.g. milling)involves only material with no radiological hazard. Radioactive material is handled as spheres with low dust levels.

All methods falling within the invention may include the routing with the plutonium of other "burnable" transuranic elements, for example neptunium or americium. The reader is referred to, for example, the first embodiment of WO 97/30456.

The invention includes a nuclear fuel reprocessing method comprising providing an aqueous spent fuel mixture including Pu and Zr, co-precipitating from said mixture oxides of Pu and Zr, and treating the co-precipitate to form a dry material suitable for disposal or use as a fuel.

Also included is a method for the preparation of a nuclear fuel pellet comprising forming gel spheres containing plutonium and organic binder debonding said spheres and blending the debonded spheres with a ceramic oxide powder. This invention also contemplates modification in which no organic binder is used and, thus, debonding is not performed. The fuels of the invention may be in the form of pellets, the pellets optionally being contained in a fuel pin which is optionally in a fuel assembly.

REFERENCES 1. J.B.W. Kanij, A.J. Noothout and 0. Votocek. The KEMA (U(VI)-process for the production of microspheres. IAEA-161 (1974) p185-195.

2. GB-A-2023110.

3. US 4284593.

4. B. Stringer, P.J. Russell, B.W. Davies and K.A. Danso. Basic aspects of the gelprecipitation route to nuclear fuel. Radiochimica Acta 36(1984)31.

5. GB-A-1575300.

6. Zhichang et al, The First Pacific Rim International Conference on Advanced Materials and Processing, Hanzhou, China, June 23-27, 1992.

7. Zhichang et al, the preparation of UO2 ceramic microspheres with an advanced process (TGU), China Nuclear Science and Technology Report, CHIC-0073, 1994.

Claims

CLAIMS 1. A method of forming a plutonium-containing product, comprising combining the plutonium with an oxide or oxide precursor of a metal which does not breed plutonium.
2. A method of Claim 1 which comprises providing a mixture containing Pu and Zr species, co-precipitating from said mixture oxides of Pu and Zr, and treating the coprecipitate to form a dry material suitable for disposal or use as a fuel.
3. A method of Claim 2 in which the mixture is an aqueous spent fuel mixture.
4. A method of Claim 2 in which the co-precipitation is carried out by ameans of gel precipitation method.
5. A method of Claim 4 in which the gel precipitation is an external gelation method preceded by a pre-conditioning step.
6. A method of Claim 5 in which the pre-conditioning is carried out by a dispersion method.
7. A method of Claim 5 in which the pre-conditioning is carried out by a condensation method.
8. A method of Claim 1 which comprises forming gel spheres containing plutonium, debonding the spheres if they contain organic material, and then blending the spheres with a ceramic oxide powder.
9. A method of Claim 8 in which the gel spheres are formed by precipitation within drops in ammonia to form soft sphere.
10. A method of Claim 9 in which the soft spheres are passed directly into a nonfluidised bed dryer and calcined.
11. A method of any of Claims 8 to 10 in which the ceramic oxide powder is one or more of ZrO2, MgO, Awl203, CeO2 and TiO2.
12. A method of any of Claims 8 to 11 in which a lubricant or binder is added during the blending process.
13. A method for the preparation of a nuclear fuel product which comprises forming gel spheres using a method of any of Claims 4 to 7, and subjecting the resultant spheres to a method of any of Claims 8 to 12.
14. A method of any of Claims 1 to 12 which comprises subjecting the product of the method to one or more further steps to form a nuclear fuel product.
15. A method of Claim 13 or Claim 14, wherein the nuclear fuel product is a pellet, optionally contained in a fuel assembly.
16. A nuclear fuel reprocessing method comprising providing an aqueous spent fuel mixture including Pu and Zr, co-precipitating from said mixture oxides of Pu and Zr, and treating the co-precipitate to form a dry material suitable for disposal or use as a fuel.
17. A method according to Claim 16 and substantially as herein described.
18. A method for the preparation of a nuclear fuel pellet comprising forming gel spheres containing plutonium and organic binder, debonding said spheres and blending the debonded spheres with a ceramic oxide powder.
19. A method of claim 18 wherein the ceramic oxide powder is non-breeding.
20. A non-breeding nuclear fuel containing a combination of plutonium and a metal oxide which does not breed plutonium.
21. A fuel of Claim 20 or Claim 21 in which the plutonium and the other metal are combined generally homogeneously.
22. A fuel of Claim 20 in which the plutonium is in spheres or pressed spheres and the other metal is in the form of a ceramic oxide powder with which the spheres are blended.
23. A fuel of any of Claims 20 to 22 which comprises a combination of PuO2 and ZrO2.
24. A fuel of any of Claims 20 to 23 which contains at least 30 weight % Pu.
25. A fuel of any of Claims 20 to 24 which is in the form of pellets, the pellets optionally being contained in a fuel pin which is optionally in a fuel assembly.
26. A method of Claim 1 and substantially as herein described.
27. A method of Claim 18 and substantially as herein described.
28. A non-breeding fuel substantially as hereinbefore described.