NUCLEAR FACILITY SECURITY SYSTEM AND METHOD FOR MAKING SUCH A FACILITY SECURE

Abstract

A nuclear installation safety system comprising a reactor hall (4) equipped with a nuclear reactor (1) in a cooling pool (3) for said reactor, characterized in that it comprises a ventilation system, a system for regulating the ambient conditions of technical rooms and the reactor hall, and a filtration system for the protection of the environment from gaseous radioactive releases, equipped with: at least one variable-section air inlet (201a) in a wall of a building (10), said air inlet opening into an inlet treatment room (201) equipped with a first air outlet (201b) in the reactor hall, a water / air exchanger (5) in the pool comprising an inlet duct (5a) open in the reactor hall (4), an air outlet duct (5b) from said exchanger (5), said duct being adapted to carry the air exiting said water / air exchanger to an outlet filtration device (203) in communication with a stack (210),of at least one extractor (202) disposed at the outlet of the air outlet duct (5b) in a room containing said outlet filtration device (203), and wherein, with the extractor off, said water / air heat exchanger and the cross-section of said at least one air inlet (201a) are configured so that, with the reactor off, the rise in the temperature of the pool water heats the air in said heat exchanger relative to the outside temperature at the chimney outlet and creates an airflow drawing in outside air at said air inlet and expelling it at the chimney outlet through said filtration device (203). Figure 10,

Description

Title of the invention: SECURITY SYSTEM FOR A NUCLEAR FACILITY AND METHOD FOR MAKING SUCH A FACILITY SECURE technical field

[0001] 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. Previous technique

[0002] The fight against climate change leads to a reduction in the carbon footprint of all energy sectors. Housing is one of these sectors and is still very dependent on fossil fuels. Highly incentivizing public policies to move towards decarbonized solutions are being implemented throughout Europe.

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

[0004] 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.

[0005] According to the International Atomic Energy Agency, approximately 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.

[0006] However, the coupling of constraints related 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 any concrete realization to date.

[0008] One example is the Thermos reactor, from the Energy Commission French nuclear (CEA) design, conceived 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.

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

[0010] 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 event of a stoppage of the cooling fluid circulation pumps of a secondary circuit.

[0011] Document FR2 314 560 Al 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 provided with a connection element provided for this purpose, in the form of a gas obturator tube.

[0012] US document 4,363,780 relates to a steam-generating reactor which includes valves allowing, in the event of overheating, the evacuation of steam and the introduction of water from a pool into the reactor core container. Technical problem

[0013] It is desirable to produce low power reactors and therefore to 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. Description of the invention

[0014] In view of this situation, the present disclosure proposes several improvements for a nuclear reactor installation adapted to 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 operator intervention, a cooling solution capable of covering all its operating states, normal or accidental. This reactor will therefore require no human intervention to ensure safety for an extended period, typically at least one week, and does not rely on any electrical source to ensure cooling of the heart.

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

[0017] More specifically, the present disclosure proposes a safety system for a nuclear installation, comprising a reactor hall equipped with a nuclear reactor in a cooling pool of said reactor, which includes a ventilation system, control of ambient conditions in technical rooms and the reactor hall, and filtration for the protection of the environment from gaseous radioactive releases, 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 equipped with a first air outlet in the reactor hall, b. of a water / air heat exchanger in the pool comprising an inlet duct open into the reactor hall, c. of an air outlet duct from said exchanger, said duct being adapted to carry the air exiting said water / air exchanger to an outlet filtration device in communication with a chimney, d. at least one extractor fan located at the outlet of the air outlet duct in a room containing said outlet filtration device, and for which, with the extractor off, said water / air exchanger and the section of said at least one air inlet are configured so that, with the reactor off, the rise in the temperature of the pool water heats the air in said exchanger relative to the outside temperature at the chimney outlet and creates an airflow drawing in outside air at the level of said air inlet to expel it at the chimney outlet through said filtration device.

[0018] The safety system is designed to maintain ventilation of the reactor hall directed towards the stack even if the ventilation system fan(s) stop due to 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.

[0019] Said at least one air inlet may include a diaphragm or a flap having a position of full opening of said air inlet and a position of partial closing of said air inlet.

[0020] 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.

[0021] Said electromagnetic force can be generated by an electrical device powered in parallel with the power supply of said extractor.

[0022] Preferably, the airflow maintains the reactor hall under negative pressure, with the extractor off. This prevents emissions through the building's air intake.

[0023] The inlet filtration room may 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, each of which may be provided with a third air outlet to the reactor hall. In this way, a converging air circuit to the reactor hall is created.

[0024] At least one of the rooms of the auxiliary systems, heat transfer room and loading / unloading hall is advantageously maintained under negative pressure.

[0025] The system advantageously includes one or more filters upstream of said stack and downstream of said extractor. This ensures that the air exiting the stack is free of 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 stack with the extractor stopped.

[0026] Said at least one air inlet advantageously comprises a diaphragm or a flap comprising a position of full opening of said air inlet under the action of an electromagnetic force and a position of partial closure of said air inlet in the absence of said electromagnetic force.

[0027] Thus a depression remains present at the level of the air intake of the building.

[0028] Said electromagnetic force can be generated by an electrical device powered in parallel with said extractor.

[0029] According to this disclosure, the reactor hall is maintained under negative pressure with the extractor stopped.

[0030] The inlet filtration room may 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.

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

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

[0033] This disclosure further proposes a method for securing a nuclear installation equipped with a safety system according to this disclosure, whereby: a. In normal operation, the variable section of said air inlet in the building wall is configured in the fully open position, and said extractor draws air from the reactor hall, creating negative pressure in the reactor hall; and b. In the event of a power outage to the installation, a reactor shutdown sequence is initiated, the variable section of said air inlet in the building wall is configured in the reduced open position, and the water / air heat exchanger produces a convection current that draws air from the hall. reactor to vent it through the chimney.

[0034] According to a particular embodiment, a loss of electrical power to the systems of the nuclear installation also activates a passive fluidic 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. Brief description of the drawings

[0035] 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:

[0036] [Fig-1] is a schematic cross-sectional view of a nuclear installation of this disclosure;

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

[0038] [Fig.3] is a variant of the reactor of [Fig.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 side view in cross-section of a lower part of a reactor;

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

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

[0045] [Fig.8C] is a top view of a lower 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 this disclosure;

[0048] [Fig. 11] shows a diagram of a district heating network comprising an ins nuclear settlement of this disclosure. Description of implementation methods

[0049] The drawings and description below contain elements that can not only serve to better understand the present invention, but also contribute to its definition, if necessary.

[0050] Figure 1 is a schematic view of a nuclear installation comprising a nuclear reactor vessel 1 in which is located a nuclear reactor core with a removable lid 11. The vessel is situated in a well 2 of a pool 3 filled with a heat transfer fluid for cooling and radiation blocking 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 cover 11, one or more fuel storage racks 31 within the pool, a refueling bench 32, and a water / air heat exchanger 5. The pool 3 is located in a building 4 which includes a bridge 41 and its sling 42, and a refueling machine 43.

[0051] The nuclear installation of the present disclosure is in particular adapted to serve as a heat generator for a heating network such as an urban heating network and [Fig.1 1] proposes a schematic diagram of the implementation of the nuclear installation of [Fig.1] in a heat network.

[0052] The reactor core in vessel 9 produces heat, which is transferred to a primary heat exchanger 91 by means of a primary circuit 92. The heat is then transferred to an intermediate circuit 8 equipped with one or more circulation pumps 101, which in turn supplies heat to a main distribution network 105. This network may also include other production facilities 106, which can be of various technologies, either conventional or nuclear. This arrangement ensures a continuous supply of heat to tertiary heat exchangers that supply heating circuits 115, 125 for buildings 107, 108, 109. The presence of several production facilities makes it possible to provide heating even during maintenance periods or in the event of an incident affecting one of the production components.

[0053] 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 interfaces with the main distribution network via a heat exchanger.

[0054] In this context, the present disclosure relates to a cooling solution for a pool-immersed reactor with a core outlet temperature in the range of 70°C to 10°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 to provide core cooling, thus providing inherent safety and the possibility of installing the reactor near urbanized areas without risk.

[0055] Firstly, this disclosure proposes to optimize heat recovery from the core. Furthermore, unlike conventional boiler rooms 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, which is the subject of the invention, is immersed in a pool, a solution commonly used for research reactors, as shown in [Fig. 1]. This provides a water reserve ensuring a long period of autonomy without requiring any human intervention. This pool is located in a reactor hall which has a controlled atmosphere according to nuclear standards and is protected against all external hazards that are always considered for nuclear facilities. The reactor hall is located in a building whose other rooms contain all the equipment necessary for the operation of the installation.

[0057] 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.

[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 water-cooled power reactors, or of plates, as in research reactors.

[0059] To clarify, the present disclosure relates to a reactor with a thermal power 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.

[0060] Also located in the reactor vessel 9 are 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.

[0061] 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, and an annular space for the return flow of the heat transfer fluid to the bottom of the tank below the core, thus creating a closed circuit for the circulation of the heat transfer fluid through the core 6. The annular return space incorporates a Counter-current heat exchanger or primary exchanger 91 comprising a primary circuit in 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.

[0062] 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.

[0063] 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.

[0064] According to [Fig.2], a lower part of the cylindrical wall 94 of the tank 9 is surrounded by a first annular envelope 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.

[0065] 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.

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

[0067] 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.

[0068] 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.

[0069] The intermediate circuit further comprises a first tube 81, for the inlet of a cold heat transfer fluid, for example cold water, opening into the first envelope la in the upper part of said first envelope la while a second tube for the outlet of hot heat transfer fluid exits in the upper part of said second envelope 1b.

[0070] 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 hot heat transfer fluid outlet pipe.

[0071] The first and second annular envelopes constitute a cylindrical interface housing, which encloses 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 case. a. - 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, b. - In the heat exchanger 91, regularly distributed plates ensure flow distribution, heat exchange efficiency, and vibration damping of the heat exchanger tubes. c. - 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.

[0073] This unit 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 describes one aspect of the present disclosure in which valves 85, 86 are incorporated into the envelopes 1a and 1b between said envelopes and pool 3.

[0076] 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.

[0077] 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, for example in the absence of a heat demand or during maintenance, the device 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.

[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 core power is only a few percent of its power nominal. In the stop configuration of 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, when 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 a position where the water inlet from the pool is open and 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 above the inlet of the tubing 81 into the cold plenum. This valve has a counterweighted paddle 86 at the point where the tubing 81 enters the casing forming the cold plenum 83. In [Fig. 4A] the flow of the heat transfer fluid Fl circulating in the intermediate circuit 8 and arriving through the inlet tubing 81 pushes the counterweighted paddle, which pushes the flap 85 and closes the opening 83a. In [Fig.4B], 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.

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

[0081] In [Fig. 5A], the flow Fl exiting the primary heat exchanger is drawn through conduit 82 by the pump of the intermediate circuit. This flow of liquid forces the valve 87 into the closed position, which prevents the hot plenum water from exiting into pool 3.

[0082] In [Fig.5B], the flow Fl is stopped when the pump 101 of [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.

[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 housing 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 opening by gravity, as they are weighted sufficiently 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.

[0085] It should be noted that because the heat transfer fluid and the pool fluid are identical, the valves 85, 87 do not necessarily need a qualified seal 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 specifically sized to ensure passive cooling of the residual power of the core 6 in the event of a power supply problem to the pumps 101 and to maintain the heat transfer fluid of the primary circuit below a fixed temperature for a determined period.

[0088] 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.

[0089] The containment of radioactive materials, according to the usual method for nuclear reactors, is ensured both statically and dynamically. Static containment relies on the physical barriers interposed 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 leak-proof and designed to maintain this leak-proofness even in the event of any internal or external attacks on the installation. In particular, the third barrier, the building, must maintain sufficient integrity in all circumstances to contain all the nuclear materials it contains.

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

[0092] The nuclear part of the reactor is thus immersed in the pool, a solution commonly used for research reactors. This provides a water reserve ensuring a long period of 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.

[0093] Building 10 comprises watertight structures – walls, roof and access vestibule – with Reinforced concrete walls ensure their integrity in the event of 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: firstly, to ensure ambient conditions that permit human activity, where necessary, and the proper functioning of equipment; and secondly, to filter the facility's air before its release when it may be contaminated by radioactive materials. To achieve this, and with reference to [Fig. 10], outside air is drawn in through dedicated, calibrated external vents 201a, protected by an intake module 201.Extraction fans 202, powered by an external electrical network, maintain a slight negative pressure in the room(s) likely to contain fission products in gaseous or aerosol form during normal operation. The air is then discharged through an atmospheric stack 210 after passing through adsorption or filtration devices 203 that retain any such fission products in gaseous or aerosol form. To ensure that radioactive products that might be present in the facility's air can only exit through stack 210, after inlet filtration in the inlet filtration room 201a, a slight negative pressure is maintained in the rooms where nuclear materials are present by means of one or more extractors 202 that draw air from the reactor hall 4.Thus, releases of radioactivity in gaseous form, through small leaks such as at door seals, are ruled out because the negative pressure ensures an inflow and not an outflow. Any residual particles that might escape filtration are released upwards through chimney 210 and are therefore extremely diluted, thus avoiding any impact on the surrounding population.

[0094] 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 the present 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 action from an operator 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 the passive cooling of the reactor after the intermediate circuit coolant circulation has been stopped.

[0095] The safety of the installation of this disclosure includes a nuclear ventilation system that ensures the containment of radioactive gases and particles that may to be emitted within the facility. To enable staff to work there, it purifies the air and regulates temperature and humidity within specified ranges. It therefore protects the public, the environment, and staff.

[0096] 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.

[0097] In [Fig. 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 201a each having a calibrated opening; b. 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.

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

[0099] 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 has a filtration function, 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.

[0100] The other rooms in building 10, which may contain radioactive elements, such as here auxiliary systems room 12, a heat transfer room 13 and loading / unloading hall 14, are supplied with air from the inlet filtration room 201 via an air duct 201c 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 or equivalent traps in a filtration device 203 before being discharged through a stack 210. The heat exchanger 5, when the reactor is in normal operation, 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 a possible auxiliary water temperature control system that maintains the water at 50°C. However, its main 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 no longer ensured, resulting in the loss of the exhaust fan 202 and the auxiliary temperature control circuit. Pool temperature. In such an accident sequence, the reactor is immediately shut down by the fall of control rods into the core. In addition, the loss of electrical power causes a damper to fall or a diaphragm to close at the air inlet(s) 201a.

[0102] Indeed, in normal operation, the damper or diaphragm is held open by an electromagnetic field and said air inlet 201a in the wall of the 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.

[0103] In the reactor core, fission reactions are stopped, 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 pool, which gradually heats up to over 50°C, as its cooling system is not operational. The pool water mass 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 has only the stack as an outlet, particularly because the air inlets have been reduced to increase the negative pressure in the reactor hall. The heat exchanger then ensures the stack draft, as the outside air has 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 a near 100% extent, trapped in the outlet filtration device 203, and the releases therefore no longer present a risk to the environment.

[0104] In the same vein, this disclosure relates to means intended to prevent the control of core reactivity from not being guaranteed, which contributes to an inherent safety of the installation and strengthens the possibility of installing the reactor in urbanized areas by eliminating a whole family of risks.

[0105] 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.

[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 power surge. To avoid such a situation, prevention and mitigation measures are conventionally implemented in reactors to prevent what is called prompt criticality. They reduce the consequences of such an accident to a acceptable level.

[0107] In the present low-power installation, for example from 15 to 80 MWth, the very small pressure difference between the reactor vessel and the surrounding pool makes it possible to rule out control rod ejection accidents, which are detrimental to most power reactors. Furthermore, the maximum speed at which the control rods can be extracted by the control mechanisms is very slow, as rapid power variations are not necessary in district heating given the high thermal inertia of the networks. The power increase due to an unexpected withdrawal of control rods will allow the protection system ample time to react and lower all the rods.

[0108] The core of a nuclear reactor is the site of a chain reaction that must be sustained over time. It therefore exhibits 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 that maintain this factor equal to 1. These absorbers are progressively removed to compensate for the increasing neutron capture of the created bodies and residues of nuclear fission, until the reactivity reserve is no longer sufficient to maintain the core at criticality. At this point, spent fuel assemblies are removed from the core and replaced with new ones.

[0109] 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 in storage on the reactor site. The usual criterion is to guarantee keff < 0.95 in all cold shutdown configurations.

[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, stop the chain reaction and provide the regulatory anti-reactivity margin.

[0112] Figure 6 shows an example 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 used in 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 special metal alloy containing silver, indium, and cadmium, according to the prior art. These 32 rods 601 are organized into several groups, each managed autonomously by means of control bars 7 schematically represented in [Fig.2].

[0113] The control bars can be organized into groups: a. - A group dedicated to regulating the reactor's power. b. - A group dedicated to compensating for wear in assemblies fuels. c. - Two shutdown groups, each with the capacity to shut down the reactor and provide the regulatory anti-reactivity margin to carry out the unloading-reloading operations.

[0114] 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: an exit from a parameter of 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.

[0115] The heart is schematically represented in [Fig.7] in cross-sectional side view.

[0116] To allow the extraction of the core, the latter is implanted 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. 8A], and are inserted between the fuel assemblies 600. The control rods are attached to rods 7 as seen previously.

[0117] The core 6 is particularly small compared to that of a power reactor, which leads to very significant peripheral and axial neutron leakage. Typically, in research reactors, neutron economy is maintained by surrounding the core with lightweight materials that do not readily capture neutrons, such as heavy water, beryllium, or graphite. These materials reflect neutrons back towards the fissile material and increase performance, particularly the neutron flux, a crucial parameter for this type of reactor. In the present disclosure, the core is surrounded, as shown in Figures 6 and 7, by four corner reflectors 62 and four peripheral reflectors 61 under the heat exchanger 91.

[0118] According to an important aspect, the core is arranged in a basket 60. The basket, shown more precisely in [Fig. 9], comprises, as an example, 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 [Fig. 8A]; an upper plate 73 provided with holes 74 for the passage of control rods 7 and water passage holes 72, shown in [Fig. 8B]; and a lower plate 75 having water passage openings 76 allowing in addition to locating the fuel assemblies. 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.

[0119] This basket has side uprights provided with angle brackets 60a for passage of corner reflectors 62 and is a metallic structure which can be made of steel and / or of a neutron-transparent material such as zircaloy or aluminium.

[0120] The basket 60 allows the core to be completely removed from its tank 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 beam connected to the overhead crane and itself equipped with slings. Once in the reloading bench, the walls of which 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 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 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 the vessel 11 closed.

[0122] 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.

[0123] The core exhibits its highest intrinsic reactivity when new assemblies have just been introduced into it, at the beginning of the cycle. Minimizing this reactivity directly translates into a reduction in the available antireactivity margin when the core is in its various operational configurations, both inside and outside the reactor, and thus facilitates the achievement of regulatory antireactivity margins.

[0124] The first way to reduce this intrinsic reactivity, while maintaining the energy produced during the cycle, may be 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.

[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 refueling, while maintaining the same energy produced during the fuel cycle. This therefore allows to lower the intrinsic initial reactivity of the heart, when it is no longer swaddled by its reflector blankets.

[0126] 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 deposited in the reloading bench.

[0127] 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.

[0128] The stopping and unloading operations can be carried out according to the following sequence: a. - Shutdown of the reactor and insertion of all the 601 control rods into the core; b. - Equalization of the pressures of the reactor and the pool; c. - Opening of the primary tank in the pool, removal and placement of the cover 11 in a dedicated area 33; d. - Dismantling and removal of the upper internals of the tank, leaving the control rods inserted in the core; e. - movement, by a rolling 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.

[0129] 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 the control bars and insert new assemblies.

[0130] Important features are as follows:

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

[0132] - The core, in operation, is surrounded by neutron-reflecting materials, allowing to limit their escape, 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 reducing the intrinsic reactivity of the core.

[0134] - The heart is always installed in a basket which allows its extraction and its insertion into the reactor while maintaining the presence of all the rods control and shutdown.

[0135] - For the operations of extracting spent fuel and inserting fuel nine, the basket containing the core is placed in a reloading bench made of neutrophilizing materials, thus allowing the manipulation of the control and stop bars.

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

[0137] The invention is not limited to the examples described above, which are given only by way of example, but encompasses all the variants that a person skilled in the art could consider within the framework of the desired protection. The described configuration allows the installation of the nuclear installation presented in urban areas.

Claims

Demands

1. A safety system for a nuclear installation, comprising a reactor hall (4) equipped with a nuclear reactor (1) in a cooling pool (3) for said reactor, characterized in that it comprises a ventilation system, a system for regulating the ambient conditions of technical rooms and the reactor hall, and a filtration system for the protection of the environment from gaseous radioactive releases, equipped with: a. at least one variable cross-section air inlet (201a) in a wall of a building (10), said air inlet opening into an inlet treatment room (201) equipped with a first air outlet (201b) in the reactor hall, b. of a water / air heat exchanger (5) in the pool comprising an inlet duct (5a) open in the reactor hall (4), c. of an air outlet duct (5b) of said exchanger (5), said duct being adapted to bring the air exiting said water / air exchanger to an outlet filtration device (203) in communication with a chimney (210), d. at least one extractor (202) located at the outlet of the air outlet duct (5b) in a room containing said outlet filtration device (203), e. and wherein, with the extractor off, said water / air exchanger and the section of said at least one air inlet (201a) are configured so that, with the reactor off, the rise in the temperature of the pool water heats the air in said exchanger relative to the outside temperature at the chimney outlet and creates an airflow drawing in outside air at said air inlet to expel it at the chimney outlet through said filtration device (203).

2.

3. Safety system according to claim 1 wherein said at least one air inlet (201a) comprises a diaphragm or flap comprising a fully open position of said air inlet and a partially closed position of said air inlet (201a). A safety system according to claim 2, wherein said fully open position of said air inlet is maintained by an electromagnetic force and the partially closed position of said inlet air (201a) is obtained in the absence of said electromagnetic force.

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

5. Safety system according to any one of the preceding claims wherein said airflow maintains the reactor hall (4) under negative pressure, extractor off.

6. A safety system according to any one of the preceding claims wherein the inlet 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. A security system according to claim 6 wherein at least one of the rooms of the auxiliary systems (12), heat transfer room (13) and loading / unloading hall (14) is maintained under negative pressure.

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

9. A method for making a nuclear installation equipped with a safety system according to any one of the preceding claims safe, wherein: a. in normal operation, the variable section of said air inlet (201a) in the wall of the building (10) is configured in the maximum open position and said extractor (202) draws air from the reactor hall (4) putting the reactor hall into negative pressure and, b. in the event of a loss of power supply to the installation, a reactor shutdown sequence is initiated, the variable section of said air inlet (201a) in the wall of the building (10) 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 stack (210).

10. A method according to claim 9, wherein a loss of electrical power to the nuclear installation systems further activates a passive fluidic 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 power supply.

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

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