Safety system for nuclear facility, and method for making such a facility safe

Abstract

The invention relates to a safety system for a nuclear facility, comprising a reactor hall (4) provided with a nuclear reactor (1) in a cooling pool (3) of said reactor, characterised in that it comprises a ventilation system, for adjusting the ambient conditions of technical rooms and of the reactor hall and for filtration to protect the environment from gaseous radioactive waste. The system is provided with: a. at least one air inlet (201a) having a variable cross-section in a wall of a building (10), the air inlet opening into an intake treatment room (201) provided with a first air outlet (201b) into the reactor hall; b. a water / air exchanger (5) in the pool, comprising an inlet duct (5a) open in the reactor hall (4); c. an air outlet duct (5b) of the exchanger (5), the duct being suitable for supplying the air leaving the water / air exchanger to an outlet filtration device (203) in communication with a flue (210); d. at least one extractor (202) arranged at the outlet of the air outlet duct (5b) in a room comprising the outlet filtration device (203), and for which, when the extractor is at a standstill, the water / air exchanger and the cross-section of the at least one air inlet (201a) are configured so that, when the reactor is at a standstill, the rise in the temperature of the water in the pool heats the air in the exchanger relative to the outside temperature at the flue outlet and creates an air flow that sucks the outside air at the air inlet in order to eject it at the outlet of the flue through the filtration device (203).

Description

Description Title: SECURITY SYSTEM FOR A NUCLEAR FACILITY AND METHOD FOR MAKING SUCH AN FACILITY SAFE Technical field

[0001] This disclosure relates to the field of nuclear reactors and in particular low-power nuclear reactors intended for use as heat generators for heating networks. Prior art

[0002] The fight against global warming requires reducing the carbon footprint of all energy sectors. Housing is one of these, and remains heavily dependent on fossil fuels. Public policies that provide strong incentives to move toward low-carbon solutions are being implemented across Europe.

[0003] District heating, which is highly developed in northern and eastern Europe, is experiencing significant growth but remains highly dependent on fossil fuels, particularly gas and coal. The latter is a significant contributor to pollution in highly urbanized areas. Solutions, such as geothermal energy, exist, but are difficult to deploy on a large scale. The main avenue being considered is the use of biomass, but this limited resource is coveted by several energy sectors, such as aeronautics, and will inevitably be subject to significant pressure on the price of the fuel raw material.

[0004] Nuclear energy, which a number of countries, including France, have chosen as a means of controlling their carbon emissions in electricity production, can be considered as a potential solution in the field of urban heating.

[0005] According to the International Atomic Energy Agency, around forty nuclear reactors worldwide currently cogenerate electricity and heat for housing. Many projects are currently under development to use the "fatal" heat from nuclear reactors for district heating, particularly for the new generation of small reactors (SMR) currently in development.

[0006] However, the coupling of constraints linked to electricity production and heat production does not facilitate the implementation of optimal dual solutions.

[0007] Nuclear reactors purely dedicated to district heating have been developed in the past, but none have seen concrete implementation to date.

[0008] An 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 an achievement appeared unconvincing compared to the construction of a coal-fired power station.

[0009] A small-sized reactor is also described in documents WO2022 / 106756 A1.

[0010] The issue of the safety of a district heating reactor requires the management of emergency cooling situations in the event of a fault in the installation and in particular in the event of the shutdown of the cooling liquid circulation pumps in a secondary circuit.

[0011] Document FR2 314 560 A1 describes a nuclear reactor for a district heating network comprising so-called shutter tubes replacing valves, the primary cooling circuit being constantly in open communication with a cooling basin by an exhaust orifice and an inlet orifice in the boundary walls of the primary cooling circuit, the exhaust orifice at least being provided with a connection member provided for this purpose, in the form of a gas shutter tube.

[0012] Document US 4,363,780 relates to a steam generating reactor which includes valves allowing steam to be evacuated in the event of overheating and water from a pool to enter the reactor core container. Technical problem

[0013] It is desirable to produce low-power reactors and therefore improve their efficiency while producing a passive safety device suitable for cooling the reactor core in the event of failure of an active heat recovery circuit of said core. Statement of the invention

[0014] In view of this situation, this disclosure proposes several improvements for a nuclear reactor installation suitable for district heating.

[0015] This disclosure relates in particular to equipment designed so that the heat transfer fluid, for example consisting of water, contained in the pool can naturally cool the reactor without any intervention from an operator, a cooling solution making it possible to cover all its operating states, normal or accidental. This reactor will therefore not require any human intervention to guarantee safety for an extended period, typically at least one week, and does not use any electrical source to ensure cooling of the core.

[0016] The present disclosure relates in particular to a security system for a nuclear installation and a method for securing such an installation.

[0017] More particularly, the present disclosure proposes a safety system for a nuclear installation, comprising a reactor hall provided with a nuclear reactor in a cooling pool of said reactor which comprises a ventilation system, for adjusting the ambient conditions of technical rooms and the reactor hall and for filtration for the protection of the environment from gaseous radioactive discharges provided with: a. at least one variable-section air inlet in a wall of a building, said air inlet opening into an inlet treatment room provided with a first air outlet in the reactor hall, b. a water / air exchanger in the pool comprising an inlet duct open in the reactor hall, c. an air outlet duct from said exchanger, said duct being adapted to bring the air leaving said water / air exchanger to an outlet filtration device in communication with a chimney, d.of at least one extractor arranged at the outlet of the air outlet duct in a room comprising said outlet filtration device, and for which, with the extractor stopped, said water / air exchanger and the section of said at least one air inlet are configured so that, with the reactor stopped, the rise in the temperature of the swimming pool water heats the air in said exchanger relative to the outside temperature at the chimney outlet and creates an air flow sucking in the outside air at the level of said air inlet to spit it out at the chimney outlet through said filtration device.

[0018] The inlet treatment room is a room for treating the ambient air drawn into the installation which will notably filter the incoming air to eliminate dust and control the humidity of this drawn air.

[0019] The safety system is designed to maintain ventilation of the reactor hall directed towards the chimney even if the ventilation system fan(s) stops following a power outage, for example. In this way, any radioactive pollutants will not be able to escape freely and will remain trapped in the filtration devices.

[0020] Said at least one air inlet may comprise a diaphragm or a flap comprising a fully open position of said air inlet and a partially closed position of said air inlet.

[0021] The fully open position of said air inlet is advantageously maintained by an electromagnetic force and the partially closed position of said air inlet is obtained in the absence of said electromagnetic force.

[0022] Said electromagnetic force can be generated by an electrical device powered in parallel with the electrical supply of said extractor.

[0023] Preferably, the airflow maintains the reactor hall under negative pressure, with the extractor off. This prevents discharges through the building's air intake.

[0024] The inlet treatment room can be provided with a second air outlet to an auxiliary systems room and / or a heat transfer room and / or a loading / unloading hall forming part of the technical rooms and each provided with a third air outlet to the reactor hall. In this way a converging air circuit to the reactor hall is created.

[0025] At least one of the auxiliary systems rooms, heat transfer room and loading / unloading hall is advantageously maintained under negative pressure.

[0026] The system advantageously comprises one or more filters upstream of said chimney and downstream of said extractor. This ensures that the air leaving the chimney is free of its pollutants even when the extractor is stopped because the air circuit is maintained from the air inlet to the reactor hall and then to the extractor chimney when stopped.

[0027] Said at least one air inlet advantageously comprises a diaphragm or a flap comprising a fully open position of said air inlet under the action of an electromagnetic force and a partially closed position of said air inlet in the absence of said electromagnetic force.

[0028] Thus, a depression remains present at the air inlet of the building.

[0029] Said electromagnetic force can be generated by an electrical device powered in parallel with said extractor.

[0030] According to the present disclosure, the reactor hall is maintained in extractor depression when stopped.

[0031] In this way a circuit returning to the reactor hall with a common output is created.

[0032] In this context, at least one of the auxiliary systems rooms, heat transfer room and loading / unloading hall is maintained under negative pressure.

[0033] The present disclosure further proposes a method for securing a nuclear installation provided with a safety system according to the present disclosure for which: a. in normal operation, the variable section of said air inlet in the wall of the building is configured in the maximum open position and said extractor draws air from the reactor hall putting the reactor hall under negative pressure and, b. during a loss of electrical power to the installation, a reactor shutdown sequence is initiated, the variable section of said air inlet in the wall of the building is configured in the reduced open position and the water / air exchanger produces a convection current drawing air from the reactor hall to evacuate it into the chimney.

[0034] According to a particular embodiment, a loss of electrical power supply to the systems of the nuclear installation also activates a passive fluid circuit for cooling the reactor by the pool so as to put the reactor into self-sufficient operation for a period compatible with the restart of said electrical power supply. Brief description of the drawings

[0035] Other characteristics, details and advantages of the invention will appear on reading the detailed description below of non-limiting exemplary embodiments, and on analyzing the appended drawings, in which:

[0036] [Fig. 1] is a schematic cross-sectional view of a nuclear facility of the present disclosure;

[0037] [Fig. 2] is a sectional view of a well and an example of a reactor in its vessel;

[0038] [Fig. 3] is a variant of the reactor of Figure 2;

[0039] [Fig. 4A], [Fig. 4B] show a gravity inlet valve in two positions;

[0040] [Fig. 5A], [Fig. 5B] show a gravity outlet valve in two positions;

[0041] [Fig. 6] is a top view of a reactor core in a basket;

[0042] [Fig. 7] is a sectional side view of a lower part of the reactor;

[0043] [Fig. 8A] is a top view of an intermediate plate of a nuclear core support basket;

[0044] [Fig. 8B] is a top view of a top plate of a nuclear core support basket;

[0045] [Fig. 8C] is a top view of a bottom plate of a nuclear core support basket;

[0046] [Fig. 9] shows a nuclear core in a basket in side view;

[0047] [Fig. 10] shows a top view of parts of a nuclear facility building of the present disclosure;

[0048] [Fig. 11] shows a diagram of a district heating network including a nuclear installation of the present disclosure. Description of embodiments

[0049] The following drawings and description contain elements which may not only serve to better understand the present invention, but also contribute to its definition, where appropriate.

[0050] Figure 1 is a schematic view of a nuclear installation comprising a nuclear reactor vessel 1 in which there is a nuclear reactor core with a removable cover 11. The vessel is arranged in a shaft 2 of a pool 3 filled with a heat transfer fluid for cooling and blocking radiation such as water and more particularly demineralized water. Water or heat transfer fluid will be used indifferently in the description below given that water is the simplest heat transfer fluid to use. In the pool there is a space 33 for removing the removable cover 11, one or more fuel storage racks 31 in the pool, a reloading bench 32 and a water / air exchanger 5. The pool 3 is located in a hall reactor 4 of a building 10 which comprises a bridge 41 and its sling 42 and a reloading machine 43.

[0051] The nuclear installation of the present disclosure is in particular suitable for serving as a heat generator for a heating network such as a district heating network and Figure 11 provides a schematic diagram of the installation of the nuclear installation of Figure 1 in a heating network.

[0052] The reactor core in the vessel 9 produces heat, transferred to a primary exchanger 91 by means of a primary circuit 92, the heat is transferred to an intermediate circuit 8 provided with one or more circulation pumps 101 to a secondary exchanger 100 which supplies heat to a main distribution network 105 on which there are possibly other production installations 106 which may be of various technologies, conventional or nuclear. This organization makes it possible to guarantee a continuous supply of heat to tertiary exchangers supplying heating circuits 115, 125 for buildings 107, 108, 109. The presence of several production installations makes it possible to provide heating even during maintenance periods or in the event of an incident on one of the production components.

[0053] As will be seen later, the reactor is composed of two circuits. The primary cooling circuit 92 extracts heat from the core where the chain reaction occurs and this heat is transferred to the intermediate cooling circuit 8, of pressure equal to or higher than the primary circuit, in order to ensure that any possible radioactive pollution cannot spread. It is this intermediate circuit which is, through an exchanger, in interface with the main distribution network.

[0054] In this context, the present disclosure relates to a cooling solution for a reactor immersed in a pool and having a core outlet temperature in a range of 70°-110°C. This reactor does not require any human intervention to guarantee its safety for an extended period of at least one week, and does not use any electrical source to ensure the cooling of the core, thus providing inherent safety and the possibility of installing the reactor near urbanized areas without risk.

[0055] Firstly, this disclosure proposes to optimise heat recovery from the core. Furthermore, as 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.

[0056] As seen above, the nuclear part of the reactor, the subject of the invention, is immersed in a pool, a solution usually used for research reactors, as shown in Figure 1. This provides a water reserve guaranteeing a large autonomy without requiring any human intervention. This pool is located in a reactor hall which has a controlled atmosphere according to nuclear criteria and protected against all external attacks always considered for nuclear installations. The reactor hall is located in a building whose other rooms contain all the equipment necessary for the operation of the installation.

[0057] Figure 2 describes the operation of the primary circuit 9 and its interface with the intermediate circuit 8 when the reactor is in production mode.

[0058] The primary circuit is encapsulated in a cylindrical vessel 9 having a peripheral wall 94 and 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 power water reactors, or plates, as in research reactors.

[0059] To clarify the ideas, this disclosure concerns a reactor with a power of approximately 15 MW to 80 MW thermal comprising a tank with a height of approximately 15 m to 25 m and a diameter of approximately 3.50 m to 4 m.

[0060] In the vessel 9 there are also the control rods 601 arranged at the end of rods 7 and which can be pushed in or out of the core to regulate the chain reaction and stop it to bring the reactor into a safe state whatever the initial conditions, normal or accidental. The rods carrying the control rods control are, as known in the field, moved by electric motors, for example electric motors integrated in cylindrical sleeves crossed by the rods and welded to a cover of the tank 9, and held in position by a magnetic field in a control device 74. In the event of an absence of supply current to the motors, the rods 7 are released and the bars 601 fall into the core, stopping the chain reaction.

[0061] The tank also comprises an internal chimney 9a for guiding by convection towards the top of the tank a first heat transfer liquid, for example water, heated by the core and an annular space for the descent of the heat transfer liquid towards the bottom of the tank under the core, which creates a closed circuit for the circulation of the heat transfer fluid passing through the core 6. The annular descent space incorporates a counter-current heat exchanger or primary exchanger 91 comprising a primary circuit in which the first heat transfer liquid circulates. The exchanger 91 allows the heat generated by the core 6 to be evacuated towards the intermediate circuit 8.

[0062] The temperature of the heat transfer fluid in the tank, at the outlet of the core, is in the present application of the order of 70°C to 110°C and more precisely between 75°C and 90°C which does not require the tank to be sized for high pressures and high temperatures.

[0063] The intermediate circuit 8 is a loop composed mainly of the downstream part or secondary circuit of the exchanger 91, of a pumping system composed of one or more pumps 101 electrically powered by an external network, ensuring forced circulation in the intermediate circuit and making it possible to accommodate the different operating states of the reactor when it is in operation, of pipes 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.

[0064] Still according to Figure 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 placed in communication with an inlet 91 a of the secondary circuit of the primary exchanger 91 in the lower part of said exchanger.

[0065] The first annular envelope extends from the bottom of the tank to under an outlet 91 b of the primary exchanger 91 in the upper part of this exchanger.

[0066] An upper part of the cylindrical wall 94 of the tank is surrounded by a second annular envelope 1 b, above said first annular envelope 1 a. This second annular envelope 1 b encompasses the outlet 91 b of the secondary circuit of the primary exchanger 91.

[0067] The radial distance between the external 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 of the order of 20 cm to 30 cm for a reactor as defined above.

[0068] This second annular envelope forms a hot plenum of the intermediate circuit 8 in communication with the outlet 91 b of said secondary circuit of the primary exchanger 91 in the upper part of said exchanger.

[0069] The intermediate circuit further comprises a first pipe 81, for the arrival of a cold heat transfer liquid, for example cold water, opening into the first casing 1a in 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.

[0070] In operation, the intermediate circuit is designed to operate with a temperature difference of around 20°C to 30°C between the cold heat transfer liquid inlet pipe and the hot heat transfer liquid outlet pipe.

[0071] The first and second annular envelopes constitute a cylindrical interface box, which envelops a large part of the primary tank 9, at the level of the main exchanger and which is organized as follows:

[0072] - The inlet pipes 81 and outlet pipes 82 of the intermediate circuit are connected to this box. a. - The inlet plenum or cold plenum 83 guides the water from the intermediate circuit to inlets in the lower part of the exchanger 91 integrated in the primary tank 9, b. - In the exchanger 91, regularly distributed plates ensure the distribution of the flow, the efficiency of the heat exchange and the vibratory maintenance of the exchanger tubes, c. - In the upper part, the hot water leaves the exchanger 91 to be injected into the upper plenum from where it joins the hot pipes of the intermediate circuit.

[0073] This box also recovers heat leaks from the primary tank to inject them into the useful heat production system.

[0074] 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.

[0075] Figure 3 depicts one aspect of the present disclosure wherein valves 85, 86 are incorporated in the casings 1a and 1b between said casings and the pool 3.

[0076] These valves are configured to isolate the intermediate circuit of the swimming pool when the pump(s) 101 are in operation but configured to put the hot and cold plenums in communication with the heat transfer liquid contained in the swimming pool when the pump(s) 101 of the intermediate circuit are stopped, which allows on the one hand the entry of the water from the swimming pool into the cold plenum of the interface box and on the other hand the exit of the heated water from the hot plenum towards the swimming pool.

[0077] Indeed, with the pump stopped, for example in the case of a loss of electrical power to the pump(s) or by decision of the operator, for example in the absence of a heat demand or during a maintenance operation, the device is designed so that, on the cold plenum side, the cold water from the swimming pool descends to the inlet vent 91 a in the primary exchanger 91 , where it rises in heating up. The difference in density between the descending and rising water provides sufficient driving force for the establishment of natural circulation.

[0078] It should be emphasized that when circulation in the primary circuit stops, the reactor is designed so that the control rods descend into the core and so that the residual power of the core is only a few percent of its nominal power. In the shutdown configuration of the pump(s) 101, the flow rate of the natural circulation between the pool and the exchanger will be only a small fraction of the nominal flow rate, pumping through the intermediate circuit, leading to very significantly lower water velocities.

[0079] Figure 4A shows 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 Figure 4B shows the inlet valve 85 in the position where the water inlet from the pool is open while the water inlet from the intermediate circuit is closed. According to the example, the valve is a gravity valve comprising a flap 85 for closing an opening 83a for connecting the cold plenum 83 with the pool above the inlet of the pipe 81 in the cold plenum. This valve comprises a counterweighted vane 86 at the level of the arrival of the pipe 81 in the casing 1a forming the cold plenum 83. In figure 4A the flow of the heat transfer liquid F1 circulating in the intermediate circuit 8 and arriving via the inlet pipe 81 pushes the counterweighted vane which pushes the flap 85 and closes the opening 83a.In Figure 4B, the heat transfer liquid 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 to enter the cold plenum while the outlet of the pipe 81 in the cold plenum is closed.

[0080] Figure 5A shows the outlet valve 87 in the closed position while Figure 5B shows the outlet valve 87 in the open position.

[0081] In Figure 5A, the flow F1 leaving the primary exchanger is sucked in by the conduit 82 under the action of the intermediate circuit pump. This flow of liquid forces the valve 87 into the closed position, which does not allow the water from the hot plenum to exit into the pool 3.

[0082] In Figure 5B, the flow F1 is stopped when the pump 101 of Figure 2 stops, the valve opens under the action of its counterweight 88 and the water heated in the exchanger 91 escapes into the swimming pool according to the flow F3.

[0083] The outlet valve does not need to close the outlet to the intermediate circuit, which is also preferred to avoid problems.

[0084] As previously stated, when the pump(s) are stopped, the natural circulation that occurs in the interface box is only a very small fraction of the nominal circulation with the pumps. In this configuration, the water velocities are low and insufficient to prevent the valves from being opened by gravity, as they are ballasted with sufficient weight 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 maintain it in this position.

[0085] It should be noted that since the heat transfer fluid and the pool fluid are identical, the valves 85, 87 do not necessarily require qualified sealing in the closed position, a slight tolerance on leaks being acceptable.

[0086] Due to the heat released by the core in the 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.

[0087] The volume of the pool 3 is in particular sized so as to guarantee passive cooling of the residual power of the core 6 in the event of a problem with the electrical supply of the pump 101 and to maintain the heat transfer liquid of the primary circuit below a fixed temperature for a determined duration.

[0088] This fixed temperature may in particular be a boiling temperature of the heat transfer liquid 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.

[0089] The containment of radioactive materials, as is customary in nuclear reactors, is ensured both statically and dynamically. Static containment relies on physical barriers between the nuclear materials and the environment: the fuel cladding, the primary circuit, and the building in which the reactor is located.

[0090] These three barriers are watertight and designed to maintain this watertightness even in the event of possible internal and external attacks on the facility. In particular, the third barrier, the building, must maintain sufficient integrity in all situations to contain all the nuclear materials it contains.

[0091] Returning to Figure 1, the radioactive materials present in the reactor installation described here will be located in pool 3 which contains the primary vessel of reactor 1, the reactor fuel reloading bench 32, the racks 31 for storing new or spent fuel assemblies, the core reloading zone 33. The pool as shown in Figure 10 is located in reactor hall 4 of building 10.

[0092] 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 a large autonomy without requiring any human intervention. This pool is located in the reactor hall which has a controlled atmosphere according to nuclear criteria and protected against all external aggressions always considered for nuclear installations. Reactor hall 4 is located in building 10 whose other rooms contain all the equipment necessary for the operation of the installation.

[0093] Building 10 has structures - walls, roof and airlock - that are watertight with reinforced concrete walls to ensure their resistance in the event of internal or external aggression. According to this disclosure, dynamic containment is ensured 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 classically fulfills two missions: ensuring ambient conditions allowing human activities, where necessary, and the proper functioning of the equipment on the one hand, and on the other hand ensuring the filtration of the air in the installation before its discharge when it may be contaminated by radioactive bodies. To do this and with reference to Figure 10, the outside air is sucked in through an air inlet 201 a in the form of dedicated and protected calibrated external vents at the level of an inlet treatment room 201 providing in particular filtration of the incoming air.Extraction fans 202, powered by an external electrical network, maintain, during normal operation, a slight depression in the room(s) 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 the radioactive products which could be present in the air of the installation can only exit through the chimney 210, after inlet filtration in the inlet treatment room 201, a slight depression is maintained in the rooms where nuclear materials are present by means of one or more extractors 202 which suck in the air contained in the reactor hall 4.Thus, the release of radioactivity in gaseous form, through small leaks such as door seals, is excluded because the depression guarantees an inflow and not an outflow. Any residues that escape filtration are released upwards through stack 210 and are therefore extremely diluted, thus avoiding any stress on the surrounding population.

[0094] Usually in nuclear reactors, in the event of an accident, the rooms likely to contain radioactive elements are isolated and maintain their depression for a certain time, depending on the tightness of the isolation means. In the present case, an important point of the present disclosure is to ensure that a depression is maintained over time in the rooms likely to contain radioactive isotopes and in the reactor hall without requiring any action by an operator or the use of active means, such as electric fans, the aim being to guarantee the maintenance of the filtration of gaseous discharges over a period consistent with the 7 days typically considered for the passive cooling of the reactor after stopping the circulation of the heat transfer liquid in the intermediate circuit 8.

[0095] The safety of the facility of this disclosure includes a nuclear ventilation system that ensures the containment of radioactive gases and particles that may be emitted in the facility. To allow personnel activity, it cleans the air, regulates temperature and humidity within specified ranges. It therefore protects the public, the environment and personnel.

[0096] The ventilation system includes a water / air exchanger 5, installed in the pool of reactor hall 4 on the ventilation circuit, upstream of the extractor(s) 202.

[0097] In Figure 10, the arrows indicate the air movements between the different rooms: a. fresh air is continuously introduced into the installation through one or more air inlets 201 a, each having a calibrated opening; b. this air is purified and treated (heated or cooled) in an inlet treatment room 201 before being distributed to the different rooms requiring nuclear ventilation.

[0098] A first circuit comprises an air outlet 201 b in the reactor hall 4 comprising the reactor 1 in a cooling pool 3 of said reactor.

[0099] In reactor hall 4, the air passes through an air / water heat exchanger 5 placed in the pool. It heats up there because the pool water is, in normal operation, maintained at 50°C by a circuit 220 dedicated to the management of the pool water located in the auxiliary systems room, this circuit, having in addition a filtration function, being configured to recover the heat from the pool water by heat pumps to reinject it into the production system. An electrically powered air extractor 202, downstream of the exchanger, ensures that the reactor hall is kept under negative pressure.

[0100] The other rooms of 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 treatment room 201 provided with an air duct 201 c and each 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.

[0101] The air extracted by the extractor 202 then passes through filters of different mesh sizes and activated carbon traps or equivalent in a filtration device 203 before being evacuated by a chimney 210. The exchanger 5, when the reactor is in normal operation, can contribute to the cooling of the pool water, heated by the thermal leaks from the primary vessel and by any spent fuel assemblies present in the storage racks 31. It thus relieves a possible auxiliary system for regulating the water temperature which maintains it at 50°C. But its main function, according to the present disclosure, is its contribution to maintaining dynamic confinement in an accidental situation where the electrical supply of the reactor by the network is no longer ensured, involving the loss of the extraction fan 202 and the auxiliary circuit for regulating the temperature of the pool.In such an accident sequence, the reactor is immediately shut down by the fall of control rods into the core. In addition, the absence of electrical power causes a shutter to fall or a diaphragm to close at the air inlet(s) 201 a.

[0102] Indeed, in normal operation, the shutter or diaphragm is held open by an electromagnetic field and said air inlet 201 a in the wall of the building 10 is configured in the maximum open position and, when said extractor stops, the released shutter or diaphragm reduces the surface area of ​​the air inlet of the nuclear ventilation in order to adapt it to a configuration where the electric extractors are no longer in operation.

[0103] In the reactor core, fission reactions are stopped, but the residues of previous fissions continue to emit heat at a low level: a few percent of the nominal power in the first hours, then a few percent in the first days. This residual heat from the fuel is evacuated into the pool, which gradually rises in temperature, beyond 50°C, its cooling circuit being non-operational. The mass of water in the pool is sized for this temperature remains below 80°C. after a week of reactor insulation. At exchanger 5, the air heated by the pool water is filtered and has only the chimney as an outlet, in particular because the air inlets have been reduced to increase the depression in the reactor hall. The exchanger then ensures the chimney draft, the outside air having a lower temperature than the air heated by the pool water. The radioactive elements present in the air in the reactor hall are therefore, almost 100%, trapped in the outlet filtration device 203 and the discharges therefore no longer present a risk to the environment.

[0104] In the same vein, this disclosure concerns means intended to prevent the control of the reactivity of the core from being guaranteed, which contributes to the inherent safety of the installation and reinforces the possibility of installing the reactor in urbanized areas by eliminating a whole family of risks.

[0105] In the reactor, all the core reactivity variation requirements 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.

[0106] One of the most severe accidents that can occur in a nuclear reactor is an uncontrolled runaway chain reaction that would cause an exponential excursion in power. To avoid such a situation, prevention and mitigation measures are conventionally implemented in reactors to avoid what is called prompt criticality. They reduce the consequences of such an accident to an acceptable level.

[0107] In the present low-power installation, for example from 15 to 80 MWth, the very low pressure difference between the reactor vessel and the surrounding pool makes it possible to exclude control rod ejection type accidents, which are penalizing on most power reactors. Furthermore, the maximum speed of rod extraction by the control mechanisms is very slow, rapid power variations not being necessary in district heating given the high thermal inertia of the networks. The increase in power due to an unexpected withdrawal of control rods will leave a large time for the protection system to react and cause all the rods to drop.

[0108] The core of a nuclear reactor is the site of a chain reaction that must be able to be maintained over time. It therefore has an excess of reactivity which is 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 which maintain this factor equal to 1. These absorbers are gradually removed to compensate for the increase in neutron capture of the bodies created and residues of nuclear fission, until the reserve of reactivity is no longer sufficient to maintain the critical core. At this time, worn assemblies are removed from the core and replaced by new assemblies.

[0109] Nuclear regulations impose underreactivity criteria when the reactor is shut down, whether the fuel assemblies are still in the core, being moved for loading and unloading operations, or in storage at the reactor site. The usual criterion is to guarantee in all cold shutdown configurations keff < 0.95.

[0110] In the proposed solution, the shutdown, reloading - unloading or storage configurations are defined in such a way that a return to criticality is physically impossible and that the regulatory criteria are naturally respected, making it possible to exclude any uncontrolled reactivity excursion.

[0111] 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.

[0112] Figure 6 gives the example of a core 6, in top view, equipped with a plate fuel 600, but the invention is just as relevant when it comes to a rod fuel, similar to that of pressurized water power reactors. In the case presented, the core comprises 45 assemblies and 32 control rods 601 in the shape of a cross and regularly distributed in the core. The neutron absorbing material of the rods can be boron carbide, hafnium or an alloy of particular metals containing silver, indium and cadmium, according to the state of the art. These 32 rods 601 are organized in several groups, each managed autonomously by means of control bars 7 shown diagrammatically in particular in figure 2.

[0113] The control rods can be organized into groups: a. - A group dedicated to regulating the reactor power. b. - A group dedicated to compensating for fuel assemblies wear. c. - Two shutdown groups, each capable of shutting down the reactor and providing the regulatory anti-reactivity margin to carry out unloading and reloading operations.

[0114] The movement of each of the rods is ensured by traditional electromagnetic mechanisms acting on the control rods, and, in the absence of electric current, the rods fall into the core, according to the usual principles in nuclear reactors: a parameter leaving the normal operating range induces the cutting of the electrical power supply to the mechanisms, and the rods stop the chain reaction and ensure regulatory subcriticality.

[0115] The heart is shown diagrammatically in Figure 7 in a side section view.

[0116] To allow the core to be extracted, the latter is installed in a chimney surrounded by the annular exchanger 91. The control rods are composed of cross-shaped plates 601 also visible from above in figure 8A and are inserted between the fuel assemblies 600. The control rods are attached to rods 7 as seen previously.

[0117] Core 6 is particularly small compared to that of a power reactor, which leads to very significant peripheral and axial neutron leaks. Usually, in research reactors, neutron economy is preserved by surrounding the core with light materials that capture neutrons poorly, such as heavy water, beryllium or graphite, which reflect neutrons towards the fissile material and increase performance, particularly that of neutron flux, an essential parameter for this type of reactor. In this disclosure, the core is surrounded according to figures 6 and 7 by four corner reflectors 62 and four peripheral reflectors 61 under the exchanger 91.

[0118] According to an important aspect, the core is arranged in a basket 60. The basket shown more precisely in Figure 9 comprises, according to the example, an openwork bottom supporting the fuel assemblies and two removable openwork plates: an intermediate plate 70 covering the fuel assemblies 600, comprising cross-shaped slots 71 for the passage of the control rods and water passage holes 72 shown in Figure 8A, an upper plate 73 provided with holes 74 for the passage of the control rods 7 and water passage holes 72 shown in Figure 8B and a lower plate 75 comprising water passage openings 76 also making it possible to locate the fuel assemblies. The control rods are mechanically disconnectable from the control rods to allow their removal while maintaining the rods in the core and therefore in the basket.

[0119] This basket has side uprights provided with angles 60a for passing the corner reflectors 62 and is a metal structure which can be made of steel and / or a material transparent to neutrons such as zircaloy or aluminum.

[0120] The basket 60 allows the core to be completely removed from its vessel and placed on a reloading bench 32 in the pool by means of an overhead crane 41 and for example a sling 42 or a spreader connected to the overhead crane and itself comprising slings. Once in the reloading bench, the walls of which are for example composed 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-reloading operations. A reloading machine positioned above the pool can ensure the removal of worn assemblies and the insertion of new assemblies into the core and the storage of the assemblies in the adjacent racks 31.

[0121] Once the core is reconfigured for a new cycle, with the control rods inserted, the reverse operations to 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 tank 11 closed.

[0122] The unloaded fuel assemblies are temporarily stored in racks before being transferred by the reloading machine to transport casks which will be removed from the reactor hall by truck.

[0123] The core exhibits its highest intrinsic reactivity when new assemblies have just been introduced, at the beginning of the cycle. Minimizing this reactivity has a direct impact on the available anti-reactivity margin when the core is in its various operational configurations, inside or outside the reactor, and therefore facilitates the achievement of regulatory anti-reactivity margins.

[0124] The first way to reduce this intrinsic reactivity, while maintaining the energy produced during the cycle, can be the use of consumable poisons. To do this, bodies whose nuclei capture neutrons are introduced into the fuel, creating new isotopes that are themselves non-absorbent. They are commonly used in the nuclear industry; among the solutions that can be considered for the present disclosure, we can cite cadmium wires or gadolinium oxide, integrated into the fuel part of the assemblies.

[0125] The use of reflectors 61, 62 also makes it possible to significantly reduce the number of new assemblies to be introduced into the core at each reloading, while maintaining the same energy produced during the fuel cycle. This therefore makes it possible to lower the initial intrinsic reactivity of the core, when it is no longer swaddled by its reflector covers.

[0126] This minimization of initial reactivity, bare core, without control rods, allows a greater margin of anti-reactivity when these are inserted, and therefore facilitates compliance with regulatory criteria. During unloading - reloading operations, the core, in its entirety, is extracted from its reflective environment with the bars inserted and therefore sees its margin of anti-reactivity increased until its deposit in the reloading bench.

[0127] The reactor can be recharged every year, during the summer period, when the demand for heat is lowest, for a period of a few days during which additional means of production on the heat network are activated. This is also the time when maintenance is carried out and the systems tested.

[0128] The shutdown and unloading operations can be carried out according to the following sequence: a. - Shutdown of the reactor and insertion of all the control rods 601 into the core; b. - Equalization of the reactor and pool pressures; c. - Opening of the primary vessel in the pool, removal and removal of the cover 11 in a dedicated area 33; d. - Dismantling and removal of the upper internals of the vessel, leaving the control rods inserted in the core; e. - movement, by an overhead crane 41 from the reactor hall 4 of the basket containing the core with the control rods inserted between the bottom of the vessel 9 and a reloading bench 32 in the pool 3 containing the vessel 9.

[0129] The core, in the reloading bench, can be surrounded by neutron-absorbing materials (boron steel for example, or other neutron-absorbing materials), which provide sufficient antireactivity to manipulate the control rods and insert new assemblies.

[0130] Important features include:

[0131] - A set of measures are taken on the reactor, when it is in operation to prohibit a rapid excursion of reactivity.

[0132] - The core, in operation, is surrounded by neutron-reflecting materials, making it possible to limit their leakage, which is very important given its small size, and to reduce its intrinsic reactivity as much as possible.

[0133] Consumable poisons, which gradually disappear during the fuel cycle, also contribute to the reduction of the intrinsic reactivity of the core.

[0134] - 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 shutdown rods.

[0135] - For the operations of extracting spent fuel and inserting new fuel, the basket containing the core is placed in a reloading bench composed of neutron-absorbing materials, thus allowing the handling of the control and shutdown rods.

[0136] - The core thus respects the regulatory anti-reactivity margins even in the event of mechanical failure or operator error. Industrial application

[0137] The invention is not limited to the examples described above, only by way of example, but it encompasses all the variants that may be envisaged by those skilled in the art within the framework of the protection sought. The configuration described allows the installation of the nuclear installation presented in urbanized areas.

Claims

Claims

1. Safety system for a nuclear installation, comprising a reactor hall (4) provided with a nuclear reactor (1) in a pool (3) for cooling said reactor, characterized in that it comprises a ventilation system, for adjusting the ambient conditions of technical rooms and the reactor hall and for filtration for the protection of the environment from gaseous radioactive discharges provided with: a. at least one air inlet (201 a) with variable section in a wall of a building (10), said air inlet opening into an inlet treatment room (201) provided with a first air outlet (201 b) in the reactor hall, b. a water / air exchanger (5) in the pool comprising an inlet duct (5a) open in the reactor hall (4), c. of an air outlet duct (5b) from said exchanger (5), said duct being adapted to bring the air leaving said water / air exchanger to an outlet filtration device (203) in communication with a chimney (210), d.of at least one extractor (202) arranged at the outlet of the air outlet duct (5b) in a room comprising said outlet filtration device (203), e. and for which, with the extractor stopped, said water / air exchanger and the section of said at least one air inlet (201 a) are configured so that, with the reactor stopped, the rise in the temperature of the swimming pool water heats the air in said exchanger relative to the outside temperature at the chimney outlet and creates an air flow sucking in the outside air at the level of said air inlet to spit it out at the chimney outlet through said filtration device (203).

2. Security system according to claim 1 wherein said at least one air inlet (201 a) comprises a diaphragm or a flap comprising a fully open position of said air inlet and a partially closed position of said air inlet (201 a).

3. A safety system according to claim 2 wherein said fully open position of said air inlet is maintained by a force electromagnetic and the partially closed position of said air inlet (201 a) is obtained in the absence of said electromagnetic force.

4. A security system according to claim 3 wherein said electromagnetic force is generated by an electrical device powered in parallel with the electrical supply of said extractor (202).

5. Safety system according to any one of the preceding claims, for which said air flow maintains the reactor hall (4) under depression, with the extractor stopped.

6. A safety system according to any preceding claim wherein the input processing room (201) is provided with a second air outlet (201c) to an auxiliary systems room (12) and / or a heat transfer room (13) and / or a loading / unloading hall (14) forming part of the technical rooms and each provided with a third air outlet to the reactor hall (4).

7. Security system according to claim 6 for which at least one of the rooms of the auxiliary systems (12), heat transfer room (13) and loading / unloading hall (14) is maintained under vacuum.

8. A safety system according to any preceding claim comprising one or more filters (203) upstream of said chimney (210) and downstream of said extractor (202).

9. Method for securing a nuclear installation provided with a safety system according to any one of the preceding claims, for which: a. in normal operation, the variable section of said air inlet (201 a) in the wall of the building (10) is configured in the maximum opening position and said extractor (202) sucks air from the reactor hall (4), putting the reactor hall under negative pressure and, b. during a loss of electrical power to the installation, a reactor shutdown sequence is initiated, the variable section of said air inlet (201 a) in the wall of the building (10) is configured in the reduced opening position and the water / air exchanger produces a convection current sucking air from the reactor hall to evacuate it into the chimney (210).

10. Method according to claim 9 for which a loss of electrical power supply to the systems of the nuclear installation further activates a passive fluid circuit for cooling the reactor by the pool so as to put the reactor into self-sufficient operation for a period compatible with the restart of said electrical power supply.

11. Nuclear installation comprising a safety system according to any one of claims 1 to 8.

Images