FR3150630A1 - NUCLEAR REACTOR WITH CONVECTIVE EXCHANGER - Google Patents

Abstract

Nuclear reactor provided with a nuclear core (6), arranged on a support (6a) in a bottom of a cylindrical vessel (9) oriented along a vertical axis, filled with a heat transfer fluid, in particular water, under an upper dome of the vessel, comprising an annular primary exchanger (91) surrounding a chimney (9a) for installing and extracting said core, said exchanger starting above said core, extending along the cylindrical wall of the vessel under said dome, and comprising a primary circuit constituted by the heat transfer fluid circulating in the core and in said primary exchanger by convection, for which a first part of the cylindrical wall (94) of said vessel (9) is surrounded by a first annular casing (1a) forming a cold plenum (83) placed in communication with an inlet (91a) of a secondary circuit of said exchanger (91) in the lower part of said exchanger and extending from the bottom of the vessel to under an outlet (91b) of the primary exchanger, in that a second part of said cylindrical wall is surrounded by a second annular casing (1b), above said first annular casing (1a), forming a hot plenum placed in communication with the outlet (91b) of said secondary circuit of said exchanger (91) in the upper part of said exchanger, an intermediate heat extraction circuit comprising a first pipe (81), for the arrival of cold heat transfer liquid, opening into the upper part of said first casing (1a) while a second pipe for the departure of hot heat transfer liquid exits in the upper part of said second casing (1b). Figure 2

Description

NUCLEAR REACTOR WITH CONVECTOR HEAT EXCHANGER

This disclosure relates to the field of nuclear reactors and in particular to low-power nuclear reactors intended for use as heat generators for heating networks.

The fight against climate change is leading to a reduction in the carbon footprint of all energy sectors. Housing is one of these sectors and remains heavily dependent on fossil fuels. Strong public policies are being implemented across Europe to encourage the transition to low-carbon solutions.

District heating, highly developed in northern and eastern Europe, is experiencing significant growth but remains heavily reliant on fossil fuels, particularly gas and coal. Coal, in particular, is a major contributor to pollution in densely populated urban areas. Solutions such as geothermal energy exist, but large-scale deployment is difficult. The primary approach being considered is the use of biomass, but this limited resource is coveted by several energy sectors, such as aerospace, and will inevitably lead to significant price pressures on the raw material.

Nuclear energy, which a number of countries, including France, have adopted as a means of controlling their carbon emissions in electricity production, can be considered as a potential solution in the field of district heating.

According to the International Atomic Energy Agency, around forty nuclear reactors worldwide currently co-generate electricity and heat for residential use. Numerous projects are currently under development to utilize the waste heat from nuclear reactors for district heating, particularly for the new generation of small modular reactors (SMRs) currently being developed.

However, the coupling of constraints related to electricity production and heat production does not facilitate the implementation of optimal dual solutions.

Nuclear reactors purely dedicated to district heating have been developed in the past, but none have seen concrete realization to date.

One example is the Thermos reactor, from the French Atomic Energy Commission (CEA), designed during the 1970s. However, in addition to the problems of acceptance of this technology, the power of the reactor, 100 MW, proved to be oversized compared to the capacities of local heat networks and the economic relevance of such a project appeared unconvincing compared to the construction of a coal-fired power plant.

A small-sized reactor is also described in documents WO2022/106756 A1.

The issue of the safety of a district heating reactor requires managing emergency cooling situations in the event of a fault in the installation and in particular in the case of a shutdown of the cooling fluid circulation pumps of a secondary circuit.

Document FR2 314 560 A1 describes a nuclear reactor for a district heating network comprising so-called obturator tubes replacing valves, the primary cooling circuit being constantly in open communication with a cooling basin by an exhaust port and an inlet port in the boundary walls of the primary cooling circuit, the exhaust port at least being fitted with a connection device provided for this purpose, in the form of a gas obturator tube.

US document 4,363,780 relates to a steam-generating reactor which has valves that allow steam to be released and water from a pool to be introduced into the reactor core container in the event of overheating.

Technical problem

It is desirable to build low-power reactors and therefore improve their efficiency while implementing a passive safety device adapted to cool the reactor core in the event of a failure of an active heat recovery circuit of said core.

In view of this situation, the present disclosure proposes several improvements for a nuclear reactor installation adapted to district heating.

This disclosure relates in particular to a reactor incorporating a convective heat exchanger in the reactor vessel, which transfers heat to an advanced intermediate circuit. This circuit is equipped to allow the coolant, such as water, contained in the pool to naturally cool the reactor without any operator intervention. This cooling solution covers all operating states, both normal and accidental. Therefore, this reactor will require no human intervention to ensure safety for an extended period, typically at least one week, and does not rely on any electrical source for core cooling.

The present invention relates in particular to a nuclear reactor comprising a vessel equipped with a nuclear core and a primary convective heat transfer exchanger to an intermediate circuit comprising at least one annular jacket around a portion of the cylindrical wall of the vessel.

More specifically, this disclosure proposes a nuclear reactor equipped with a nuclear core, disposed on a support in the bottom of a cylindrical vessel oriented along a vertical axis, filled with a heat transfer fluid, in particular water, under an upper dome of the vessel, comprising an annular primary heat exchanger surrounding an installation and extraction stack for said core, said heat exchanger originating above said core, extending along the cylindrical wall of the vessel under said dome, and comprising a primary circuit consisting of the heat transfer fluid circulating in the core and in said primary heat exchanger by convection, for which a first part of the cylindrical wall of said vessel is surrounded by a first annular shell forming a cold plenum connected to an inlet of a secondary circuit of said heat exchanger in the lower part of said heat exchanger and extending from the bottom of the vessel to below an outlet of the primary heat exchanger, in that a second part of said cylindrical wall is surrounded by a second annular shell, above said first annular shell, forming a hot plenum communication with the outlet of said secondary circuit of said exchanger in the upper part of said exchanger, an intermediate heat extraction circuit comprising a first tube, for the inlet of cold heat transfer fluid, opening into the upper part of said first jacket while a second tube for the outlet of hot heat transfer fluid exits into the upper part of said second jacket.

The use of jackets forming hot and cold plenums increases the efficiency of heat transfer from the primary exchanger to the intermediate circuit by recovering some of the heat from the tank wall.

Advantageously, the upper dome of the tank is filled with a chemically neutral gas such as nitrogen to regulate pressure in the tank.

The tank is preferably placed in a well made in the bottom of a pool filled with the same heat transfer fluid as the intermediate circuit, said first jacket and said second jacket being surrounded by the heat transfer fluid contained in the pool.

According to an advantageous embodiment, the first envelope includes a first opening for communication of said cold plenum in the first envelope with the pool, said first opening being kept closed by a first gravity valve pushed back under the action of a first flow of said heat transfer fluid between the first pipe, supplying cold heat transfer fluid from an intermediate circuit and the second pipe supplying hot heat transfer fluid from said intermediate circuit, said first opening opening under the action of the first gravity valve in the absence of said first flow so as to allow heat transfer fluid to pass between the first envelope and the pool.

The first gravity valve may include a mass calibrated according to the first flow to close the first opening once a given first flow value is reached. This makes the valve's operation entirely passive and dependent on the presence or absence of the expected flow.

The second jacket may advantageously include a second opening for connecting the hot plenum to the pool. This second opening is held closed by a second gravity valve when the first flow exits the reactor through a second pipe. In the absence of the first flow, this second opening opens under the action of the second gravity valve, allowing heat transfer fluid to pass between the second jacket and the pool. Like the first valve, the second valve is passive, and its position is changed only by the presence or absence of the first flow.

Said second gravity valve may include a mass calibrated according to said first flux to close the second opening from a given first flux value.

The pool volume is preferably sized to ensure passive cooling of the core's residual power in the absence of any other cooling method, and to maintain the primary circuit's heat transfer fluid below a set temperature for a predetermined period. This temperature can be defined as the boiling point of the pool water or a lower temperature, and the duration can be a standard period in the industry to allow for troubleshooting, namely approximately one week.

More specifically, the first and second pipes originate from the intermediate circuit, which includes an intermediate heat exchanger and at least one pump. The intermediate heat exchanger can distribute heat from the intermediate circuit to a primary district heating distribution network supplying first buildings and substations that supply heat to second buildings.

Other features, details and advantages of the invention will become apparent from the following detailed description of non-limiting embodiments, and from the analysis of the accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a nuclear facility of the present disclosure;

FIG. 2 is a cross-sectional view of a well and an example of a reactor in its vessel;

FIG. 3 is a variant of the reactor of the FIG. 2 ;

FIG. 4 , FIG. 4 show a gravity inlet valve in two positions;

FIG. 5 , FIG. 5 show a gravity outlet valve in two positions;

FIG. 6 is a top view of a reactor core in a basket;

FIG. 7 is a side view in cross-section of a lower part of a reactor;

FIG. 8 is a top view of an intermediate plate of a nuclear core support basket;

FIG. 8 is a top view of the upper plate of a nuclear core support basket;

FIG. 8 is a top view of a lower plate of a nuclear core support basket;

FIG. 9 shows a nuclear core in a basket in side view;

FIG. 10 shows a top view of parts of a nuclear facility building of this disclosure;

FIG. 11 shows a diagram of a district heating network including a nuclear facility of this disclosure.

Description of implementation methods

The drawings and description below contain elements that can not only help to better understand the present invention, but also contribute to its definition, if necessary.

There FIG. 1 This is a schematic view of a nuclear installation comprising a nuclear reactor vessel 1 containing a nuclear reactor core with a removable lid 11. The vessel is located in a well 2 of a pool 3 filled with a cooling and radiation-blocking heat transfer fluid such as water, and more specifically demineralized water. The terms water and heat transfer fluid will be used interchangeably in the following description, given that water is the simplest heat transfer fluid to use. The pool contains a space 33 for removing the removable lid 11, one or more fuel storage racks 31, a refueling bench 32, and a water/air heat exchanger 5. The pool 3 is located in a building 4 which includes a crane 41 and its sling 42, and a refueling machine 43.

The nuclear installation described in this disclosure is particularly suitable for use as a heat generator for a heating network such as a district heating network and the FIG. 11 proposes a schematic diagram for the layout of the nuclear installation of the FIG. 1 in a district heating network.

The reactor core in vessel 9 produces heat, which is transferred to a primary heat exchanger 91 via a primary circuit 92. The heat is then transferred to an intermediate circuit 8, equipped with one or more circulation pumps 101, and finally to a secondary heat exchanger 100. This secondary heat exchanger supplies heat to a main distribution network 105, which may also include other production facilities 106, employing various technologies, both conventional and nuclear. This arrangement ensures a continuous supply of heat to tertiary heat exchangers that feed heating circuits 115, 125 for buildings 107, 108, and 109. The presence of multiple production facilities allows for heating to be provided even during maintenance periods or in the event of an incident affecting one of the production components.

As will be seen later, the reactor consists of two circuits. The primary cooling circuit 92 extracts heat from the core where the chain reaction takes place, and this heat is transferred to the intermediate cooling circuit 8, which has a pressure equal to or greater than that of the primary circuit, in order to ensure that any radioactive contamination cannot spread. This intermediate circuit is connected, via a heat exchanger, to the main distribution network.

This disclosure concerns a cooling solution for a pool-immersed reactor with a core outlet temperature in the range of 70°-110°C. This reactor requires no human intervention to ensure its safety for an extended period of at least one week and does not rely on any electrical source for core cooling, thus providing inherent safety and the possibility of installing the reactor near urban areas without risk.

Firstly, this disclosure proposes to optimize heat recovery from the core. Furthermore, unlike conventional boilers using fossil fuels, nuclear reactors continue to produce heat, a few percent of the nominal power, after the chain reaction has stopped; this residual heat must therefore be able to be extracted, whether the reactor is in a normal shutdown state or in an unexpected shutdown, following an incident or accident of internal or external origin.

As seen above, the nuclear part of the reactor, which is the subject of the invention, is immersed in a pool, a solution usually used for research reactors, as shown on the FIG. 1 This system provides a water reserve ensuring a long period of autonomy without requiring any human intervention. The pool is located in a reactor hall with a controlled atmosphere that meets nuclear standards and is protected against all external hazards typically associated with nuclear facilities. The reactor hall is housed in a building whose other rooms contain all the equipment necessary for the plant's operation.

There FIG. 2 describes the operation of the primary circuit 9 and its interface with the intermediate circuit 8 when the reactor is in production mode.

The primary circuit is encapsulated in a cylindrical vessel 9 with a peripheral wall 94, which contains in its lower part the nuclear core 6, composed of fuel assemblies containing the uranium necessary for the chain reaction. These fuel assemblies can take the form of rods, as in water-cooled power reactors, or plates, as in research reactors.

To clarify, this disclosure concerns a reactor with a thermal power output of approximately 15 MW to 80 MW, comprising a vessel with a height of approximately 15 m to 25 m and a diameter of approximately 3.50 m to 4 m.

The reactor vessel 9 also contains the control rods 601, arranged at the ends of shafts 7, which can be pushed into or pulled out of the core to regulate the chain reaction and stop it, thus bringing the reactor to a safe state regardless of the initial conditions, whether normal or accidental. The shafts carrying the control rods are, as is known in the field, moved by electric motors, for example, electric motors integrated into cylindrical sleeves through which the rods pass and which are welded to a cover of the reactor vessel 9, and held in position by a magnetic field in a control device 74. In the event of a power failure to the motors, the shafts 7 are released and the rods 601 fall into the core, stopping the chain reaction.

The tank also includes an internal chimney 9a which guides a first heat transfer fluid, for example water, heated by the core, to the top of the tank by convection. An annular space allows the heat transfer fluid to flow back down to the bottom of the tank beneath the core, thus creating a closed circuit for the circulation of the heat transfer fluid through the core 6. The annular space incorporates a counter-current heat exchanger, or primary exchanger 91, which includes a primary circuit through which the first heat transfer fluid circulates. The exchanger 91 allows the heat generated by the core 6 to be transferred to the intermediate circuit 8.

The temperature of the heat transfer fluid in the tank, at the outlet of the core, is in the present application in the order of 70°C to 110°C and more precisely between 75°C and 90°C which does not require sizing the tank for high pressures and high temperatures.

The intermediate circuit 8 is a loop composed mainly of the downstream part or secondary circuit of the exchanger 91, a pumping system consisting of one or more pumps 101 electrically powered by an external network, ensuring forced circulation in the intermediate circuit and allowing to accommodate the different operating states of the reactor when it is in operation, piping 81, 82 connecting the different components and the upstream part of a secondary exchanger 100 between the intermediate circuit 8 and the main distribution network 105.

Still according to the FIG. 2 , a lower part of the cylindrical wall 94 of the tank 9 is surrounded by a first annular envelope 1a forming a cold plenum 83 of the intermediate circuit 8 which is put in communication with an inlet 91a of the secondary circuit of the primary exchanger 91 in the lower part of said exchanger.

The first annular envelope extends from the bottom of the tank to below an outlet 91b of the primary exchanger 91 in the upper part of this exchanger.

An upper part of the cylindrical wall 94 of the tank is surrounded by a second annular envelope 1b, above said first annular envelope 1a. This second annular envelope 1b encompasses the outlet 91b of the secondary circuit of the primary exchanger 91.

The radial distance between the outer wall of the first and second annular envelopes and the wall of the tank 9 is given to achieve an exchange volume adapted to the heat of the tank and the expected flow rate and is for example on the order of 20 cm to 30 cm for a reactor as defined above.

This second annular envelope forms a hot plenum of the intermediate circuit 8 in communication with the outlet 91b of said secondary circuit of the primary exchanger 91 in the upper part of said exchanger.

The intermediate circuit also includes a first tube 81, for the inlet of a cold heat transfer fluid, for example cold water, opening into the first envelope 1a in the upper part of said first envelope 1a while a second tube for the outlet of hot heat transfer fluid exits in the upper part of said second envelope 1b.

In operation, the intermediate circuit is designed to operate with a temperature difference of approximately 20°C to 30°C between the inlet pipe of the cold heat transfer fluid and the outlet pipe of the hot heat transfer fluid.

The first and second annular envelopes form a cylindrical interface housing, which encloses a large part of the primary tank 9, at the level of the main exchanger and is organized as follows:

1. - Le plenum d’entrée ou plenum froid 83 guide l’eau du circuit intermédiaire vers des entrées en partie basse de l’échangeur 91 intégré dans la cuve primaire 9,
2. - Dans l’échangeur 91 des plaques régulièrement réparties assurent la répartition du débit, l’efficacité de l’échange de chaleur et le maintien vibratoire de tubes d’échangeur,
3. - En partie haute, l’eau chaude sort de l’échangeur 91 pour être injectée dans le plenum supérieur d’où elle rejoint les tuyauteries chaudes du circuit intermédiaire.

- The inlet pipes 81 and outlet pipes 82 of the intermediate circuit are connected to this box.

1. - The inlet plenum or cold plenum 83 guides the water from the intermediate circuit to inlets in the lower part of the heat exchanger 91 integrated into the primary tank 9,
2. - In heat exchanger 91, regularly spaced plates ensure flow distribution, heat exchange efficiency, and vibration damping of the heat exchanger tubes.
3. - In the upper part, the hot water exits the exchanger 91 to be injected into the upper plenum from where it joins the hot pipes of the intermediate circuit.

This unit also recovers heat leaks from the primary tank and injects them into the useful heat production system.

The intermediate circuit and the pool use the same heat transfer fluid, for example water which is used to cool the reactor when it is in operation and when it is shut down.

There FIG. 3 describes one aspect of this disclosure whereby valves 85, 86 are incorporated into the envelopes 1a and 1b between said envelopes and pool 3.

These valves are configured to isolate the intermediate circuit of the pool when the pump(s) 101 are in operation but configured to put the hot and cold plenums in communication with the heat transfer fluid contained in the pool when the pump(s) 101 of the intermediate circuit are stopped which allows on the one hand the entry of the pool water into the cold plenum of the interface box and on the other hand the exit of the heated water from the hot plenum to the pool.

Indeed, when the pump is stopped—for example, in the event of a power outage affecting the pump(s) or at the operator's discretion, such as when there is no demand for heat or during maintenance—the system is designed so that, on the cold plenum side, the cold pool water descends to the inlet vent 91a in the primary heat exchanger 91, where it rises while being heated. The difference in density between the descending and ascending water provides sufficient driving force to establish natural circulation.

It should be noted that when circulation in the primary circuit stops, the reactor is designed so that the control rods descend into the core and the residual core power is only a few percent of its rated power. In the shutdown configuration of pump(s) 101, the flow rate of natural circulation between the pool and the heat exchanger will be only a small fraction of the rated flow rate, with pumping through the intermediate circuit, leading to significantly lower water velocities.

There FIG. 4 represents the inlet valve 85 in a position where the water inlet from the pool is closed and the water inlet from the intermediate circuit is open while the FIG. 4 Figure 85 represents the inlet valve in the position where the water inlet from the pool is open while the water inlet from the intermediate circuit is closed. In this example, the valve is a gravity valve comprising a flap 85 for closing an opening 83a that connects the cold plenum 83 to the pool, located above the inlet of the pipe 81 into the cold plenum. This valve includes a counterweighted paddle 86 at the point where the pipe 81 enters the casing 1a forming the cold plenum 83. FIG. 4 The flow of the heat transfer fluid F1 circulating in the intermediate circuit 8 and arriving through the inlet pipe 81 pushes the counterweighted vane which pushes the flap 85 and closes the opening 83a. FIG. 4 , the heat transfer fluid no longer circulates in the intermediate circuit 8, the counterweight descends under the action of gravity which moves the flap 85 away from the opening 83a and allows a flow F3 of water from the pool into the cold plenum while the outlet of the tubing 81 in the cold plenum is closed.

There FIG. 5 represents the outlet valve 87 in the closed position while the FIG. 5 represents the outlet valve 87 in the open position.

To the FIG. 5 The flow F1 exiting the primary heat exchanger is drawn through conduit 82 by the pump of the intermediate circuit. This flow of liquid forces valve 87 into the closed position, preventing the hot plenum water from exiting into pool 3.

In FIG. 5 The F1 flow is stopped when pump 101 of the FIG. 2 stops, the valve opens under the action of its counterweight 88 and the heated water in the exchanger 91 escapes into the pool according to the flow F3.

The outlet valve does not need to close the outlet to the intermediate circuit, which is also preferred to avoid problems

As previously mentioned, when the pump(s) are stopped, the natural circulation that occurs within the interface box is only a very small fraction of the nominal circulation with the pumps running. In this configuration, the water velocities are low and insufficient to prevent the valves from opening by gravity, as they are weighted enough to be held open by the dynamic pressure and closed when this pressure is reduced. When the pumps restart, the force of the jet is sufficient to push the valve back into the closed position and hold it there.

It should be noted that because the heat transfer fluid and the pool fluid are identical, valves 85, 87 do not necessarily need a qualified seal in the closed position, a slight tolerance on leaks being acceptable.

Due to the heat released by the core in tank 9, the heat transfer fluid circulating in this tank expands and creates a slight overpressure which is stabilized by the presence of a gas such as nitrogen in a dome 93 of the tank in the upper part of the tank.

The volume of pool 3 is specifically sized to ensure passive cooling of the residual power of core 6 in the event of a power supply problem to pump 101 and to maintain the heat transfer fluid of the primary circuit below a fixed temperature for a determined period.

This fixed temperature may in particular be a boiling point of the heat transfer fluid possibly less a safety margin of 10°C to 20°C and the determined duration may for example be 5 to 10 days and in particular one week.

The containment of radioactive materials, as is standard practice in nuclear reactors, is ensured both statically and dynamically. Static containment relies on the physical barriers placed between the nuclear materials and the environment: the fuel cladding, the primary circuit, and the building in which the reactor is located.

These three barriers are airtight and designed to maintain this airtightness even in the event of potential internal and external attacks on the facility. In particular, the third barrier, the building itself, must maintain sufficient integrity in all circumstances to contain all the nuclear materials it contains.

Back to the FIG. 1 The radioactive materials present in the reactor installation described herein will be located in pool 3, which contains the primary reactor vessel 1, the reactor refueling station 32, the storage racks 31 for new or spent fuel assemblies, and the core refueling area 33. The pool is as shown in FIG. 10 is located in reactor hall 4 of building 10.

The nuclear part of the reactor is thus immersed in the pool, a solution usually used for research reactors. This provides a water reserve guaranteeing extended autonomy without requiring any human intervention. This pool is located in the reactor hall, which has a controlled atmosphere according to nuclear standards and is protected against all external hazards, a standard consideration for nuclear facilities. Reactor Hall 4 is located in Building 10, the other rooms of which contain all the equipment necessary for the operation of the installation.

Building 10 features airtight structures—walls, roof, and access airlock—with reinforced concrete walls to ensure their integrity in the event of an internal or external attack. According to this disclosure, dynamic containment is provided by a nuclear ventilation system whereby parts of the facility containing nuclear materials, coolant, or other potentially contaminated fluids are subject to specific, nuclear-grade ventilation. A nuclear ventilation system typically fulfills two functions: first, to ensure ambient conditions that permit human activity, where necessary, and the proper functioning of equipment; and second, to filter the facility's air before its release when it may be contaminated by radioactive materials. To this end, and with reference to the FIG. 10 , outside air is drawn in through calibrated external vents 201a, dedicated and protected at the level of a suction module 201. Extraction fans 202, powered by an external electrical network, maintain in normal operation a slight negative pressure in the room or rooms likely to contain fission products in gaseous or aerosol form then the air is discharged through an atmospheric chimney 210 after passing through adsorption or filtration devices 203 which retain these possible fission products in gaseous or aerosol form. To ensure that any radioactive materials present in the facility's air can only escape through stack 210, after inlet filtration in the inlet filtration room 201a, a slight negative pressure is maintained in the rooms containing nuclear materials by means of one or more extractors 202 that draw air from the reactor hall 4. This prevents the release of radioactivity in gaseous form through small leaks, such as at door seals, because the negative pressure ensures an inflow rather than an outflow. Any remaining residue that escapes filtration is released upwards through stack 210 and is therefore extremely diluted, thus avoiding any impact on the surrounding population.

As is standard practice in nuclear reactors, in the event of an accident, rooms likely to contain radioactive materials are isolated and maintain negative pressure for a certain period, depending on the effectiveness of the isolation measures. In this case, an important aspect of this disclosure is to ensure that negative pressure is maintained over time in rooms likely to contain radioactive isotopes and in the reactor hall without requiring any operator intervention or the use of active means, such as electric fans. The aim is to guarantee the continued filtration of gaseous emissions for a period consistent with the 7 days typically considered for passive reactor cooling after the intermediate circuit coolant circulation is shut down.

The safety measures for the facility described in this disclosure include a nuclear ventilation system that ensures the containment of radioactive gases and particles that may be released within the facility. To allow personnel to work, it purifies the air and regulates temperature and humidity within specified ranges. It therefore protects the public, the environment, and personnel.

The ventilation system includes a water/air exchanger 5, located in the reactor hall pool 4 on the ventilation circuit, upstream of the extractor(s) 202.

1. de l’air frais est introduit en continu dans l’installation par une ou plusieurs entrées d’air 201a comportant chacune une ouverture calibrée ;
2. cet air est épuré et traité (chauffé ou refroidi) dans un local de filtration d’entrée 201 avant d’être distribué dans les différentes salles nécessitant une ventilation nucléaire.

In FIG. 10 The arrows indicate the air movements between the different rooms:

1. fresh air is continuously introduced into the installation through one or more air inlets 201a, each having a calibrated opening;
2. This air is purified and treated (heated or cooled) in an inlet filtration room 201 before being distributed to the various rooms requiring nuclear ventilation.

A first circuit includes an air outlet 201b in the reactor hall 4 containing reactor 1 in a cooling pool 3 of said reactor.

In reactor hall 4, air passes through an air-to-water heat exchanger 5 located in the pool. It is heated there because the pool water is, under normal operating conditions, maintained at 50°C by a dedicated pool water management circuit 220 located in the auxiliary systems room. This circuit, which also performs filtration, is configured to recover heat from the pool water using heat pumps and reinject it into the production system. An electrically powered air extractor 202, downstream of the heat exchanger, maintains negative pressure in the reactor hall.

The other rooms in building 10, which may contain radioactive elements, such as the auxiliary systems room 12, a heat transfer room 13 and the loading/unloading hall 14, are supplied with air from the inlet filtration room 201 via an air duct 201c and each is provided with a third air outlet 203a, 203b, 203c to the reactor hall 4 so that the air extracted from these rooms follows the same outlet path as the air from the reactor hall.

The air extracted by the extractor 202 then passes through filters of varying mesh sizes and activated carbon or equivalent traps in a filtration device 203 before being discharged through a stack 210. The heat exchanger 5, when the reactor is operating normally, can contribute to cooling the pool water, which is heated by thermal leaks from the primary reactor vessel and by any spent fuel assemblies present in the storage racks 31. It thus relieves the load on any auxiliary water temperature control system that maintains the water at 50°C. However, its primary function, according to this disclosure, is to contribute to maintaining dynamic containment in an accident situation where the reactor's power supply from the grid is lost, resulting in the loss of the exhaust fan 202 and the auxiliary pool temperature control circuit. In such an accident sequence, the reactor is immediately shut down by the dropping of control rods into the core. In addition, the lack of power causes a flap to fall or a diaphragm to close at the level of the air inlet(s) 201a.

Indeed, in normal operation, the damper or diaphragm is held open by an electromagnetic field and said air inlet 201a in the wall of building 10 is configured in the maximum open position and, when said extractor is stopped, the released damper or diaphragm reduces the surface area of the nuclear ventilation air inlet in order to adapt it to a configuration where the electric extractors are no longer in operation.

In the reactor core, fission reactions cease, but the residues from previous fissions continue to emit heat at a low level: a few percent of nominal power in the first few hours, then a few parts per thousand in the first few days. This residual heat from the fuel is released into the spent fuel pool, which gradually heats up to over 50°C, as its cooling system is not operational. The pool's water volume is sized for this temperature and remains below 80°C after one week of reactor isolation. At heat exchanger 5, the air heated by the pool water is filtered and exits only through the stack, primarily because the air inlets have been reduced to increase negative pressure in the reactor hall. The heat exchanger then ensures the stack's draft, as the outside air is at a lower temperature than the air heated by the pool water. The radioactive elements present in the air of the reactor hall are therefore, to almost 100%, trapped in the outlet filtration device 203 and the releases therefore no longer present any risk to the environment.

In the same vein, this disclosure concerns means intended to prevent the control of core reactivity from being guaranteed, which contributes to the inherent safety of the installation and strengthens the possibility of installing the reactor in urbanized areas by eliminating a whole family of risks.

In the reactor, all the requirements for varying the reactivity of the core are met by the control rods 601 which, as seen above, under the impulse of the control rods 7 operated by traditional type electromagnetic mechanisms, move vertically in the core.

One of the most severe accidents that can occur in a nuclear reactor is an uncontrolled runaway chain reaction, which would cause an exponential power surge. To prevent such a situation, prevention and mitigation measures are routinely implemented in reactors to avoid what is known as prompt criticality. These measures reduce the consequences of such an accident to an acceptable level.

In this low-power installation, for example, 15 to 80 MWth, the very small pressure difference between the reactor vessel and the surrounding pool eliminates control rod ejection accidents, which are detrimental to most power reactors. Furthermore, the maximum speed at which control rods can be extracted by the control mechanisms is very slow, as rapid power variations are unnecessary in district heating systems due to the high thermal inertia of the networks. A power surge resulting from an unexpected control rod withdrawal will allow the protection system ample time to react and lower all the rods.

The core of a nuclear reactor is the site of a chain reaction that must be sustained over time. It therefore exhibits excess reactivity, usually represented by the neutron multiplication factor between two successive generations (keff). This excess is compensated, at the beginning of the core's life, by neutron absorbers that maintain this factor at 1. These absorbers are progressively removed to compensate for the increasing neutron capture of the particles created and the residues of nuclear fission, until the reactivity reserve is no longer sufficient to maintain the core's criticality. At this point, spent fuel assemblies are removed from the core and replaced with new ones.

Nuclear regulations impose sub-reactivity criteria when the reactor is shut down, whether the fuel assemblies are still in the core, being moved for loading and unloading operations, or stored on the reactor site. The usual criterion is to guarantee, in all cold shutdown configurations, a keff < 0.95.

In the proposed solution, the configurations for stopping, reloading-unloading or storage are defined in such a way that a return to criticality is physically impossible and that regulatory criteria are naturally respected, making it possible to exclude any uncontrolled reactivity excursion.

When the reactivity reserve becomes insufficient at the end of the fuel cycle, all the control rods are inserted into the core, stopping the chain reaction and providing the regulatory anti-reactivity margin.

There FIG. 6 The example given is that of a core 6, viewed from above, equipped with plate fuel 600, but the invention is equally relevant when dealing with rod fuel, similar to that of pressurized water reactors. In the case presented, the core comprises 45 assemblies and 32 cross-shaped control rods 601 regularly distributed throughout the core. The neutron-absorbing material of the rods can be boron carbide, hafnium, or a specific metal alloy containing silver, indium, and cadmium, according to the state of the art. These 32 rods 601 are organized into several groups, each managed autonomously by means of control rods 7, schematically represented in particular in FIG. 2 .

1. - Un groupe dédié à la régulation de la puissance du réacteur.
2. - Un groupe dédié à la compensation de l’usure des assemblages combustibles.
3. - Deux groupes d’arrêt, chacun ayant la capacité d’arrêter le réacteur et d’apporter la marge d’antiréactivité règlementaire pour mener les opérations de déchargement - rechargement.

The control bars can be organized into groups:

1. - A group dedicated to regulating the reactor's power.
2. - A group dedicated to compensating for wear on fuel assemblies.
3. - Two shutdown groups, each with the capacity to shut down the reactor and provide the regulatory anti-reactivity margin to carry out unloading and reloading operations.

The movement of each of the bars is ensured by traditional electromagnetic mechanisms acting on the control bars, and, in the event of a lack of electric current, the bars fall into the core, according to the usual principles in nuclear reactors: a deviation from a parameter in the normal operating range induces the interruption of the power supply to the mechanisms, and the bars stop the chain reaction and ensure the regulatory subcriticality.

The heart is schematically represented as FIG. 7 side view of the cut.

To allow for the extraction of the core, it is installed in a chimney surrounded by the annular heat exchanger 91. The control rods are composed of cross-shaped plates 601, also visible from above in FIG. 8 and are inserted between the 600 fuel assemblies. The control rods are attached to rods 7 as seen previously.

The core 6 is particularly small compared to that of a power reactor, leading to significant peripheral and axial neutron leakage. Typically, in research reactors, neutron economy is maintained by surrounding the core with lightweight materials that poorly capture neutrons, such as heavy water, beryllium, or graphite. These materials reflect neutrons back towards the fissile material, thus increasing performance, particularly the neutron flux, a crucial parameter for this type of reactor. In this disclosure, the core is surrounded, as shown in Figures 6 and 7, by four corner reflectors 62 and four peripheral reflectors 61 beneath the heat exchanger 91.

According to an important aspect, the heart is arranged in a basket 60. The basket is represented more precisely in FIG. 9 According to the example, it comprises a perforated base supporting the fuel assemblies and two removable perforated plates: an intermediate plate 70 covering the fuel assemblies 600, having cross-shaped slots 71 for the passage of control rods and water passage holes 72 shown in the FIG. 8 , an upper plate 73 provided with holes 74 for the passage of the control rods 7 and water passage holes 72 shown in the FIG. 8 and a lower plate 75 having water passage openings 76 which also allow the fuel assemblies to be located. The control rods are mechanically disconnectable from the control bars to allow their removal while keeping the bars in the core and therefore in the basket.

This basket has side uprights fitted with angle brackets 60a for passage of corner reflectors 62 and is a metal structure which can be made of steel and/or a neutron-transparent material such as zircaloy or aluminium.

The basket 60 allows the core to be completely removed from its tank and placed on a reloading bench 32 in the pool using an overhead crane 41 and, for example, a sling 42 or a spreader beam connected to the overhead crane and itself equipped with slings. Once in the reloading bench, whose walls are, for example, made of neutron-absorbing materials (boron steel, hafnium, cadmium, etc.), the upper internals (plates 70, 73 and bars 601) of the basket are removed to allow the unloading and reloading operations. A reloading machine positioned above the pool can remove worn assemblies and insert new assemblies into the core, and store the assemblies in the adjacent racks 31.

Once the core has been reconfigured for a new cycle, with the control rods inserted, the reverse operations of those of unloading are carried out and the basket containing the core is repositioned in the reactor by the overhead crane, the internals put in place and vessel 11 closed.

The unloaded fuel assemblies are temporarily stored in racks before being transferred by the reloading machine into transport casks which will be removed from the reactor hall by a truck.

The core exhibits its highest intrinsic reactivity when new fuel assemblies have just been introduced, at the beginning of the cycle. Minimizing this reactivity directly impacts the available antireactivity margin when the core is in its various operational configurations, both inside and outside the reactor, and thus facilitates achieving regulatory antireactivity margins.

The first way to reduce this intrinsic reactivity, while maintaining the energy produced during the cycle, is through the use of consumable poisons. This is achieved by introducing into the fuel substances whose nuclei capture neutrons, creating new isotopes that are themselves non-absorbing. These are commonly used in the nuclear industry; among the solutions considered for this disclosure are cadmium wires or gadolinium oxide, integrated into the fuel portion of the assemblies.

The use of reflectors 61 and 62 also significantly reduces the number of new fuel assemblies required for each refueling, while maintaining the same energy output during the fuel cycle. This, in turn, lowers the intrinsic initial reactivity of the core when it is no longer enveloped by its reflector blankets.

This minimization of initial reactivity, with the core exposed and without control bars, allows for a greater margin of antireactivity when the bars are inserted, thus facilitating compliance with regulatory criteria. During unloading and reloading operations, the core, in its entirety, is extracted from its reflective environment with the bars inserted, thereby increasing its margin of antireactivity until it is placed in the reloading bench.

The reactor can be refueled annually during the summer months, when heat demand is lowest, for a period of a few days during which additional production facilities on the district heating network are activated. This is also the time when maintenance is carried out and systems are tested.

1. - Arrêt du réacteur et insertion de l’ensemble des barres de contrôle 601 dans le cœur ;
2. - Egalisation des pressions du réacteur et de la piscine ;
3. - Ouverture de la cuve primaire dans la piscine, retrait et dépose du couvercle 11 dans une zone dédiée 33;
4. - Démontage et dépose des internes supérieurs de la cuve, laissant les barres de contrôle insérées dans le cœur ;
5. - déplacement, par un pont roulant 41 du hall réacteur 4 du panier comportant le cœur avec les barres de contrôle insérées entre le fond de la cuve 9 et un banc de rechargement 32 dans la piscine 3 contenant la cuve 9.

The stopping and unloading operations can be carried out according to the following sequence:

1. - Shutdown of the reactor and insertion of all 601 control rods into the core;
2. - Equalization of reactor and pool pressures;
3. - Opening of the primary tank in the pool, removal and placement of the cover 11 in a dedicated area 33;
4. - Dismantling and removal of the upper internals of the tank, leaving the control rods inserted in the core;
5. - movement, by an overhead crane 41 from reactor hall 4 of the basket containing the core with the control rods inserted between the bottom of the tank 9 and a reloading bench 32 in the pool 3 containing the tank 9.

The core, in the reloading bench, can be surrounded by neutron-absorbing materials (boron steel for example, or other neutrophilizing materials), which provide sufficient anti-reactivity to manipulate control bars and insert new assemblies.

Key characteristics include:

- A set of measures are taken on the reactor, when it is in operation, to prevent a rapid excursion of reactivity.

- The core, in operation, is surrounded by neutron-reflecting materials, allowing to limit their leakage, very significant given its small size and to reduce as much as possible its intrinsic reactivity.

Consumable poisons, which gradually disappear during the fuel cycle, also contribute to reducing the intrinsic reactivity of the core.

- The core is always installed in a basket which allows its extraction and insertion into the reactor while maintaining the presence of all the control and stop rods.

- For the operations of extracting the spent fuel and inserting the new fuel, the basket containing the core is placed in a reloading bench composed of neutrophic materials, thus allowing the manipulation of the control and stop bars.

- The core thus respects the regulatory anti-reactivity margins even in the event of mechanical failure or operator error.

The invention is not limited to the examples described above, which are given only as examples, but encompasses all the variations that a person skilled in the art could envision within the framework of the desired protection. The described configuration allows for the installation of the nuclear facility presented in urban areas.

Claims (10)

A nuclear reactor equipped with a nuclear core (6), disposed on a support (6a) in the bottom of a cylindrical vessel (9) oriented about a vertical axis, filled with a heat transfer fluid, in particular water, under an upper dome of the vessel, comprising an annular primary heat exchanger (91) surrounding an installation and extraction stack (9a) for said core, said heat exchanger originating above said core, extending along the cylindrical wall of the vessel under said dome, and comprising a primary circuit consisting of the heat transfer fluid circulating in the core and in said primary heat exchanger by convection, characterized in that a first part of the cylindrical wall (94) of said vessel (9) is surrounded by a first annular shell (1a) forming a cold plenum (83) connected to an inlet (91a) of a secondary circuit of said heat exchanger (91) in the lower part of said heat exchanger and extending from the bottom of the vessel to under an outlet (91b) of the primary exchanger, in that a second part of said cylindrical wall is surrounded by a second annular envelope (1b), above said first annular envelope (1a), forming a hot plenum connected with the outlet (91b) of said secondary circuit of said exchanger (91) in the upper part of said exchanger, an intermediate heat extraction circuit comprising a first tube (81), for the inlet of cold heat transfer fluid, opening into the upper part of said first envelope (1a) while a second tube for the outlet of hot heat transfer fluid exits into the upper part of said second envelope (1b). Nuclear reactor according to claim 1 wherein the upper dome of the vessel is filled with a chemically neutral gas (93) for pressure regulation in the vessel. Nuclear installation comprising a reactor according to claim 1 or 2 wherein the vessel is disposed in a well (2) made in the bottom of a pool (3) filled with the same heat transfer fluid as the intermediate circuit, said first shell and said second shell being surrounded by the heat transfer fluid contained in the pool. Nuclear installation according to claim 3, wherein the first shell (1a) has a first opening (83a) for communicating said cold plenum (83) in the first shell (1a) with the pool (3), said first opening (83a) being kept closed by a first gravity valve (85) pushed back under the action of a first flow of said heat transfer fluid (F1) between the first pipe (81), inlet of cold heat transfer fluid of an intermediate circuit (8) and the second pipe (82) outlet of hot heat transfer fluid of said intermediate circuit, said first opening opening under the action of the first gravity valve in the absence of said first flow (F1) so as to allow heat transfer fluid to pass between the first shell and the pool (3). Nuclear installation according to claim 4 wherein said first gravity valve comprises a mass (86) calibrated as a function of said first flux (F1) to close the first opening from a given first flux (F1) value. Nuclear installation according to claim 4 or 5 wherein the second shell (1b) has a second opening (84a) for communicating said hot plenum with the pool, said second opening being kept closed by a second gravity valve (87) in the presence of said first flow (F1) exiting the reactor through a second pipe (82), said second opening opening under the action of the second gravity valve in the absence of said first flow (F1) so as to allow heat transfer fluid to pass between the second shell (1b) and the pool (3). Nuclear installation according to claim 6 wherein said second gravity valve comprises a mass (88) calibrated as a function of said first flux (F1) to close the second opening from a given first flux value (F1). Nuclear installation according to any one of claims 3 to 7 wherein the volume of the pool (3) is sized so as to guarantee passive cooling of a residual power of the core (6) in the absence of other means of cooling and to maintain the heat transfer fluid of the primary circuit below a fixed temperature for a determined time. Nuclear installation according to any one of the preceding claims wherein the first pipe (81) and the second pipe (82) are parts of the intermediate circuit (8) comprising an intermediate exchanger (100) and at least one pump (101). Nuclear installation according to claim 9 wherein said intermediate exchanger distributes heat from the intermediate circuit to a primary district heating distribution network (105) supplying first buildings (106) and substations (110, 120) supplying heat to second buildings (107, 108, 109).