Modular pressurized water core reactor (PRPR) with pressure holder, without water spray

Description

technical field

[0001] The present invention relates to pressurized water nuclear reactors, in particular those of the integrated modular reactor type, known as small or medium power or SMR in English (acronym for "Small Modular Reactor").

[0002] The invention aims to overcome a major drawback of manufacturing complexity, associated with a high cost, and complexity of piloting and operating such an SMR reactor, in particular its pressurizer.

[0003] For the purposes of this invention, "SMR reactor" means the usual technological meaning, namely a nuclear fission reactor, smaller in size and power than conventional SLR reactors, of which a block is manufactured in a factory and transported to a nuclear site for installation.

[0004] For the purposes of this invention, "reactor block" means the reactor vessel, as well as all the components and parts of the fluidic circuit, including the reactor core which generates heat through nuclear fission reactions, and which is housed inside the reactor vessel.

[0005] For the purposes of this invention, "heat-producing reactor" refers to a nuclear installation, nuclear power plant, or nuclear reactor whose power output is primarily dedicated to heat production. A heat-producing reactor may be entirely dedicated to heat production. However, a small portion of its power output may also be used to generate electricity.

[0006] For the purposes of this invention, "power-generating" refers to a nuclear installation, nuclear power plant, or nuclear reactor whose power output is primarily dedicated to electricity generation. A power-generating reactor may be 100% dedicated to electricity production. However, a small portion of its power output may also be used to generate heat. Previous technique

[0007] A significant portion of the current fleet of pressurized water reactor (PWR) nuclear power plants is nearing the end of its operating period for which the reactors were designed and licensed, in a context where the energy transition with the decarbonization of uses will increase the need for electricity (non-intermittent, high availability and competitive electricity).

[0008] Documents DE 20 26 217 A1 and US 2018 / 190391 A1 disclose examples of prior art PRE.

[0009] A pressurized water reactor (PWR) comprises three cycles (fluidic circuits) whose general principle of normal operation is as follows.

[0010] High-pressure water from a primary circuit extracts the energy supplied, in the form of heat, by the fission of uranium nuclei, and where applicable plutonium, in the reactor core.

[0011] Next, this water, under high pressure and high temperature (typically 155 bar and 300 °C), circulates through a steam generator (SG) where it exchanges its heat with a secondary circuit, which also uses pressurized water as a heat transfer fluid. This water, in the form of steam at high pressure (typically around 70 bar), is then expanded via an expansion valve, transforming the change in the fluid's enthalpy into mechanical work and then electrical work in the presence of an electric generator.

[0012] The water from the secondary circuit is then condensed via a condenser using a third cycle, the cooling cycle, as a cold source.

[0013] The design principles of PWR reactors according to these three cycles have been essentially the same since the beginning of the commissioning of the first ones operated.

[0014] The main components of a PWR primary circuit are shown in the figure 1 : a reactor building 1 providing various functions including a contribution to the containment safety function, a reactor vessel 20, located in the center of building 1, housing the reactor core C, a primary circuit 2 in pressurized water including vessel 20.

[0015] These main elements are therefore common, their composition and the number of components varying according to the power of the reactor.

[0016] Typically, the building envelope of reactor 1 can consist of several layers. For example, as illustrated in the figure 1 , a reactor building 1 can consist of an outer wall of reinforced concrete 12, an inner wall of prestressed concrete 10 separated from the outer wall 12 by an annular space 13 devoid of material, and a metallic skin 11 on the inside of the prestressed concrete wall 10, for a 1650 MWe reactor.

[0017] As illustrated on the figure 2 , taken from publication [1], the primary circuit 2 consists of the following main components: a reactor vessel 20, primary loops 21 each comprising a primary pump 22 and a steam generator 23, a single pressurizer 24.

[0018] Furthermore, we can distinguish on this figure 2 , the reactor core control rod mechanisms and control rod clusters 25.

[0019] Depending on the reactor power, the number of loops can be three for a 900MWe reactor or four for a 1300MWe reactor and above.

[0020] The reactor 1 building is therefore sized, among other things, to house all the components of the primary circuit 2.

[0021] There figure 3 illustrates the energy transfer cycle (heat then electricity) of a PWR reactor. On this figure 3 In particular, we can distinguish the distribution of the positioning of the components in relation to the building of reactor 1, which provides the function of a third containment barrier.

[0022] The fluid connections between the inside and outside of the reactor 1 building are provided by lines 30, 31 of the external circuit of the steam generators 23 to the secondary circuit 3 comprising a turbine 32 connected to the electric generator 33, a condenser 34, a feed pump 35 and a heater not shown.

[0023] More specifically, for a given steam generator 23, the reactor building 1 is traversed by a line called the hot line 30 which evacuates the steam from the steam generator 23 to extract the power and brings it to the turbine 32, and by a line called the cold line 31 which supplies liquid water to the steam generator 23.

[0024] A currently emerging technology is that of small modular reactors (SMRs). The main advantages of these SMRs compared to existing PWRs are the simplification of systems, primarily for safety purposes, and increased modularity through the large-scale manufacturing of components in factories for transport to the construction site.

[0025] Furthermore, SMRs are flexible due to their low power level and their ability to be integrated into local areas.

[0026] They thus appear as a competitive solution for the future. To date, approximately 70 SMR projects have been identified worldwide at various stages of development, a quarter of which use mature, generation 3 (Gen-III) technologies, such as those used in the French fleet.

[0027] Among the SMRs currently under development, some offer a configuration based on integrating the steam generator, or even all the components of the primary circuit, including the pressurizer and primary pumps, inside the reactor vessel. These SMRs are called integrated SMRs.

[0028] In addition to the gain in compactness, integrated SMRs have the advantage of no longer requiring overhead pressurized water fluidic lines, with the exception of control circuit lines of very limited diameter, typically a few cm, which considerably reduces the risks of accidents and associated consequences related to the rupture of primary circuit lines.

[0029] As an example, the nuclear power plant project with the acronym NUWARD™<, is a plant consisting of two integrated SMRs, with a unit power of 170MWe, with all the primary circuit components inside the reactor vessel.

[0030] Other integrated SMR projects are under development or have been studied, including the SCOR project with a power of 150 to 200 MWe on behalf of the Applicant or the ACP100 project with a power of 100 MWe.

[0031] The gain in compactness of integrated-type SMRs complicates operations in operation, compared to those carried out in a conventional PWR.

[0032] Indeed, the main structural operability and maintainability operations on the architecture for a reactor primary circuit are as follows: fuel loading / unloading operations which require, under appropriate radiation protection conditions, access to the inside of the reactor vessel, maintenance operations on equipment which require accessibility to the equipment.

[0033] If we refer to the figure 2, we see that the loops of a primary circuit 2 of a conventional PWR are designed to allow maintenance on each component without impacting, or in a very limited way, the other components and that the fuel handling operations are carried out by opening the lid of the tank 20 without impacting the primary loops 21.

[0034] Conversely, due to the integration of components in an integrated SMR, access to the fuel area for loading / unloading operations may require removing functional parts of the primary circuit, which is more significant than handling the tank cover.

[0035] We represented at the figure 4An example of an integrated SMR currently under development. Such an integrated SMR reactor, generally designated by reference 4, comprises a fixed compartment 40 and a removable compartment 41 in the form of a cover, for fuel handling or maintenance phases of the reactor internals.

[0036] The inventors analyzed that integrated SMR reactors as currently envisaged have several disadvantages, particularly related to the complexity of their pressurizer.

[0037] Indeed, just as with a conventional PWR reactor, the pressurizer of an integrated SMR reactor includes an electric heating part which produces steam and thereby increases the pressure within the pressurizer, and a steam cooling and condensation part which causes a drop in pressure.

[0038] Currently, this part of the steam cooling and condensation consists of a liquid water spraying / injection device by pumping, from a sample of the primary circuit.

[0039] In addition to the inherent complexity of manufacturing, controlling, and operating such a spraying system within an SMR reactor, the inventors chose not to include a primary pumping unit in the reactor vessel, as proposed in the patent application filed today and entitled "Nuclear installation comprising at least one modular nuclear reactor (SMR) and a reactor vessel shaft delimiting a water basin in which the SMR reactor and the heat exchangers between the primary and secondary circuits are immersed."

[0040] There is therefore a need to find a solution that addresses the aforementioned disadvantages associated with a liquid water spraying / injection device as part of steam cooling and condensation in SMR type pressurized water reactors (PWRs).

[0041] The aim of the invention is to meet at least part of this need. Description of the invention

[0042] To this end, the invention relates, in one of its aspects, to a pressurized water reactor (PWR), of the modular reactor (SMR) type, comprising: a central axis reactor vessel (X) comprising a dome-shaped cover housing at least part of the primary circuit; a primary circuit pressurizer whose steam cooling and condensation part comprises: two walls of the dome, separated from each other forming a space (E) sealed against the primary circuit and within which liquid cooling water can circulate from the bottom to the top of the dome forming a central exhaust stack to the outside of the vessel and the dome, so as to condense the steam in the saturation state of the primary circuit inside the vessel and thus reduce the pressure within the vessel; a liquid water flow control valve in the space, arranged in the central exhaust stack.

[0043] The term "primary circuit" refers to the usual meaning: the fluid circuit that removes the heat generated in the reactor core through the circulation of pressurized water, known as primary coolant. The reactor vessel, which houses part of the primary circuit, is the second of the three containment / safety barriers that prevent the release of radioactive materials.

[0044] By "secondary circuit" we mean either the usual meaning of a closed loop circuit, or an open environment including a secondary water basin, contained within the space of the reactor vessel well forming the third containment barrier.

[0045] Preferably, both walls of the dome are metallic, preferably stainless steel.

[0046] According to an advantageous embodiment, the inner wall forms the pressure-resistant enclosure of the primary circuit. Preferably, the thickness of the inner wall is between 10 and 20 mm.

[0047] According to an advantageous embodiment, the reactor includes a passive heat sink arranged inside the dome to cool the steam from the primary circuit.

[0048] According to this method, and an advantageous embodiment variant, the heat sink comprises a plurality of cooling fins arranged inside the inner wall, preferably distributed uniformly over the surface of the latter.

[0049] Advantageously, the fins are welded or brazed to the inner wall.

[0050] Preferably, the space between the inner and outer walls is between 0.5 and 5 cm.

[0051] According to another advantageous embodiment, the external wall of the dome is covered with a cap housing thermal insulation within it.

[0052] Preferably, the pressurizer liquid water intended to circulate between the two walls of the dome is at least 10°C lower than the primary circuit liquid water temperature.

[0053] According to another advantageous embodiment, the heating part of the pressurizer comprises a plurality of electrical resistors wrapped in electrical insulation, arranged inside the dome.

[0054] According to an advantageous configuration, the liquid water from the pressurizer intended to circulate between the two walls of the dome is the water from the secondary circuit of the reactor.

[0055] According to an advantageous embodiment, the secondary circuit includes a water-filled basin contained within the reactor vessel well space and supplying the bottom of the space between the dome walls.

[0056] According to this method, and an advantageous embodiment variant, the water basin is configured so that, when the SMR reactor is in normal operation, it achieves a vertical thermal stratification resulting in the formation of a thermocline delimited between the bottom of the basin at a so-called cold temperature in which the SMR reactor vessel is immersed and the top of the basin at a so-called hot temperature, the thermocline being positioned above the dome so that the circulation of liquid water from the basin into the space between the walls of the pressurizer is achieved by natural convection.

[0057] The invention also relates to a nuclear installation comprising at least one SMR nuclear reactor as described above.

[0058] Thus, the invention essentially consists of a pressurizer whose cooling and vapor condensation part is integrated directly into the cover in the form of a double-walled dome inside which liquid water can circulate in a sealed manner with respect to the primary circuit, preferably by natural convection, from the bottom to the top of the dome to be evacuated through a central chimney housing a valve regulating the flow of the circulating water and thus the cooling of the primary water vapor inside the dome.

[0059] Thus, the liquid water circulates in an independent circuit, separate from the reactor's primary water circuit, and which is integrated into the primary vessel dome.

[0060] Cooling is therefore achieved by thermal conduction through the double wall of the dome and the temperature cooled on the inner wall of the dome condenses the primary water vapor inside the dome, without the need for spraying or injecting water as according to the state of the art, moreover from a liquid water circuit independent of the primary circuit.

[0061] In conclusion, the nuclear reactor with a pressurizer integrated within the reactor vessel head according to the invention offers numerous advantages, including: the elimination of water spraying / injection devices as part of steam cooling and condensation for an integrated SMR reactor pressurizer; improved safety through efficient cooling solely by natural convection; significant simplification of the design of SMR reactors with a heat-generating function; and a significant reduction in associated costs, both capital expenditure (CAPEX) and operating expenditure (OPEX).

[0062] Other advantages and features of the invention will become clearer upon reading the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0063] [ Fig 1 ] there figure 1 is a schematic perspective and partial cross-sectional view of an existing PWR-type nuclear reactor. Fig 2 ] there figure 2is a schematic view of a state-of-the-art primary circuit of a PWR-type nuclear reactor in a three-primary-loop configuration. Fig 3 ] there figure 3 is a schematic view of the three cycles of a PWR-type nuclear reactor according to the state of the art. Fig 4 ] there figure 4 is a schematic perspective view of an integrated SMR-type reactor as currently envisaged. Fig 5 ] there figure 5 is a schematic perspective view of a calogenous SMR reactor incorporating a pressurizer according to the invention. Fig 5A ] there figure 5A is an exploded perspective view of the parts of the SMR reactor vessel according to the invention. Fig 6 ] there figure 6 is a longitudinal cross-sectional view of the reactor according to the figure 5 , and which illustrates the natural convection circulation of water in the primary and secondary circuits. Fig 6A ] there figure 6Ais a schematic detail view of a control rod assembly, control rod, and rod control mechanism of a carbon-heating SMR reactor according to the invention. Fig 7] [Fig 8 ] THE figures 7 and 8 These are detailed perspective views showing the primary circuit flow control valve in an intermediate position and the fully open position, respectively. Fig 9 ] there figure 9 is a perspective view of part of the reactor according to the figure 5 , and which shows in detail the installation of a valve to regulate the water flow of the secondary circuit. Fig 8 ] ] Fig 10A ] ] Fig 10B ] ] Fig 10C ] THE Figures 10A, 10B And 10CThese are perspective views showing an example of a secondary circuit water flow control valve and its integration into a heat exchanger outlet manifold, with the valve respectively in the fully open position, an intermediate position, and the fully closed position. Fig 11 ] there figure 11 is a partial longitudinal cross-sectional view of the upper part of an SMR reactor according to the Figures 5 And 6 showing a pressurizer according to the invention. Detailed description

[0064] Throughout this application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to an SMR nuclear reactor, as provided for in a vertical operating configuration and arranged in a water basin according to the invention.

[0065] THE figures 1 to 4have already been detailed in the preamble, so they will not be commented on below.

[0066] For the sake of clarity, the same element according to the invention and according to the prior art is designated by the same numerical reference throughout the figures 1 to 11 .

[0067] We describe with reference to the figure 5 , a pressurized water type 4 nuclear reactor, according to an integrated SMR type primary circuit configuration, according to the invention.

[0068] This reactor 4 has a unit power of 20 MW thermal, designed for heating, i.e., dedicated to supplying hot water at 90°C. Its unit power can, however, vary up or down, within a range of approximately 10 MWth to 100 MWth, and the hot water supply temperature can also vary up to approximately 150°C.

[0069] Reactor 4 with central X-axis figure 5comprises a block delimited by a reactor vessel body 40, an intermediate body 45, and a metallic dome 41, preferably made of stainless steel, with a thickness of approximately 10 to 20 mm, and formed of a hemispherical bottom and a vertical cylinder. This reactor vessel consists of a fixed compartment 40 and a removable compartment 45 and 41 as shown figure 5A , above the reactor core for fuel handling or maintenance of reactor internals. The compartment formed by bodies 41 and 45 is removable to allow handling of the assemblies contained in the reactor vessel body 40. The removable compartment 41 is a dome-shaped cover 6 whose central chimney incorporates a valve 64, adapted for cooling the reactor pressurizer as detailed later.

[0070] The reactor core C comprises a set of fuel assemblies similar to those conventionally used in PWR-type reactors, but with a fissile height adapted to achieve the desired total thermal power. Each fuel assembly has several missing fuel rod locations, replaced by absorber rods that can move up or down within the assembly to control the reaction and form the control rods 42. Data from preliminary studies conducted by the Applicant consider a number of 52 assemblies and a cycle life of 10 years, with a fissile height of 1.5 m.

[0071] The reactor vessel body 40 houses in its lower part a cylinder 43, supporting an assembly basket usually referred to as the "core support basket", dedicated to holding the fuel assemblies, and a separation envelope 40 with its peripheral neutron reflector 440 intended to ensure the maintenance of the neutron flux in the core.

[0072] A set of flanges is bolted between the fixed compartment 40 and the removable compartment 45, and the dome 41. The seal between the flanges of compartments 40 and 45 on the one hand, and 45 and 41 on the other, is advantageously ensured by a metal gasket. Removing the bolted flange located between compartments 40 and 45 allows for the complete handling of the fuel assemblies during core refueling phases. The block formed by compartments 45 and 41, with the heat exchangers 49 attached, is completely removed to provide direct access to the reactor core during handling. Removing the bolted flange connecting compartments 45 and 41 allows access to the upper internals of the core, including the control rod mechanisms and the flow control ring. The pressurizer associated with compartment 41 and its internal components can also be separated from the rest of the reactor vessel block for intervention and maintenance.

[0073] The studies carried out by the Applicant foresee scheduled fuel refueling stops during ten-year outages, without intervention on the core between these periods.

[0074] Above the core C, control rods 46 of the control rod assembly allow the insertion of nuclear reactivity control rods 42, in a manner similar to that commonly found in conventional PWR reactors. The control rods 42 are made of neutron-absorbing material.

[0075] The free space above the reactor core C allows for the fully extended positioning of the control rods 42, as well as the standby position of the so-called emergency fuel rods, dedicated to the safe shutdown of the nuclear reaction. The control rods 46 are individually steered vertically by means of the rod control mechanisms 47. Above the control mechanisms 47, a plate 48 with holes 480 is fixed, allowing the passage of the hot primary fluid exiting the core into the central section called the "riser." Peripheral holes also allow the passage of the control rods for the throttling valve 481.

[0076] At the periphery of plate 48 is arranged a flow control valve 481 for the primary circuit water, called a flow control valve. This valve is in the form of a flow control ring 481 which follows the inner periphery of compartment 42 of the tank and extends over a height sufficient to cover the primary water outlet openings, thus allowing the flow rate of this primary water to be regulated.

[0077] The function of this throttling valve 481 is to regulate the natural flow rate of water in the primary circuit passing through the openings 400, which constitute the inlets of the primary water manifolds of the heat exchangers 49 between the primary and secondary circuits. The positioning of this regulating valve is controlled by a motor or rod control mechanism, with the control rod 482 linked to the ring 481, which is advantageously similar to those used for the control rods of the reactivity control rods 42.

[0078] In an intermediate position, as illustrated in the figure 7 , the rolling ring 481 leaves the openings 400 partially clear, which determines the flow of primary water that passes through them towards the exchangers 49.

[0079] In the event of a power failure or emergency shutdown, the gravitational fall of the control rods 46 also triggers the gravitational fall of the primary fluid control valve. In the lowest position, as illustrated in the figure 8 This control valve 481 allows all water from the primary circuit to pass through the openings 400, thus maximizing the flow rate in the heat exchangers 49, in order to remove residual heat and cool the primary circuit. This gravity-fed operation of the control valve ensures reliability and safety in the event of a power outage or reactor failure.

[0080] In reactor 4 of Figures 5 And 6In normal operation, the thermal power generated by the nuclear chain reaction within the reactor core is dissipated by the primary circuit fluid, which rises by natural convection to the upper section. There, it flows through the various outlet openings 400, corresponding to the inlet manifolds of the heat exchangers 49 between the primary and secondary circuits, and into a central upper portion of the core, in the form of a riser. This central riser, not detailed here, contains, in addition to the control rod mechanisms, the sensors for monitoring core parameters.

[0081] Thus, the separation jacket 44 of core C allows the water, the fluid of the primary circuit, to be separated into its so-called cold and hot temperatures. The cold primary water surrounds core C inside jacket 44, while the hot primary water, heated by circulating upwards within core C, is found in the upper central portion of the core.

[0082] Above the outlet openings 400, within reactor 4, a separation plate 7 separates the interior of the dome 41 from the vessel containing a pressurizer, and from the riser. This separation plate 7 is a plate with through holes, providing thermal insulation and pressure differentials for the integrated pressurizer. This separation plate may be of the type already described in patent application WO2012 / 158929 A3.

[0083] The upper section incorporating the reactor pressurizer will be detailed later with reference to the figure 11 .

[0084] After cooling through the heat exchangers 49 in a downward direction, the primary circuit water passes through the openings 401, which constitute the primary water outlet manifolds of the heat exchangers 49, and then returns in a closed loop to the lower part of the reactor core for another heating phase. The closed-loop circulation P, driven solely by natural convection of the primary water, is symbolized by the white arrows in figure 6 The driving force of the primary circuit in natural convection is controlled by the difference in height between the position of the exchangers 49, and the average height of the fissile zone of the core defined by the assemblies 42.

[0085] As previously mentioned, the primary pressure drop, and therefore the flow rate, is regulated by the throttling valve 481, whose control mechanisms are housed in one of the holes 480 in the plate 48. The inlet and outlet temperatures of the primary water are regulated by the neutron flux conditions, i.e., the thermal power of the core, by the positions of the reactivity control rods 42 in the core, and by the saturation temperature and pressure conditions in the pressurizer. Here, due to circulation being solely by natural convection, i.e., in the absence of any active primary water pumping system, it is the primary water flow throttling valve and the thermo-hydraulic parameters (hot and cold temperatures) that determine the circulation and heat exchange conditions between the primary and secondary circuits in relation to the power produced in the core.In fact, the operation of the heat-generating reactor can be controlled simply by adjusting the core power using all the control rods 42, in addition to this primary water pressure setting.

[0086] The heat exchangers 49 between the primary and secondary circuits are preferably plate heat exchangers, advantageously made of stainless steel, and designed to withstand the water pressure of the primary circuit. Advantageously, these heat exchangers 49 are manufactured by stacking grooved metal plates assembled together either by hot isostatic compression (HIC) or by hot uniaxial compression (HUC) to achieve diffusion welding between the metal plates, or by brazing.

[0087] Within an exchanger 49, the flow is downward for primary water, and upward for secondary water.

[0088] As depicted in figure 6The secondary circuit of this reactor 4 is not a closed-loop circuit like in conventional PWR reactors, but includes a water basin B, as schematically shown in the figure 5 . This basin B is contained within the space of the reactor vessel well forming the third containment barrier, and the reactor vessel 4 is immersed in it.

[0089] This secondary circuit with liquid water basin B is an open environment delimited by the tank well, without a circulation pump.

[0090] With such a liquid water basin B for the secondary circuit, the heat exchangers 49 are not integrated into the reactor vessel 40, 41 but are arranged and fixed to its exterior. This arrangement is possible because the unlikely event of a rupture of the primary water inlet or outlet pipes, causing a large-diameter breach, would not have significant accidental consequences for the reactor, thanks in particular to the primary and secondary pressures. Indeed, the liquid water basin B completely surrounds the reactor vessel 4, and such an accident cannot lead to a risk of core uncovering, endangering the physical integrity of the reactor core.

[0091] The internal circuit within a heat exchanger 49, which is part of the secondary circuit of reactor 4, therefore sees a flow of liquid water as a secondary fluid. This water is heated by contact with the primary water within the heat exchanger 49, through natural suction from its inlet manifold 490 at the bottom to its outlet manifold 491 at the top. The secondary water then creates a volume greater than that at a so-called hot temperature. The separation layer between the so-called cold temperature and the so-called hot temperature of the secondary water is designated as a thermocline, as symbolized by the term thermocline in figure 6 .

[0092] In other words, when the SMR reactor is operating normally, water basin B is configured to achieve vertical thermal stratification, resulting in the formation of a thermocline delimited between the bottom of the basin, at a cold temperature (where reactor vessel 4 is immersed), and the top of the basin, at a hot temperature. The height of this thermocline layer determines the cooling flow rate of the secondary circuit through the heat exchangers 49. The closed-loop circulation S, driven solely by natural convection of the secondary water, is symbolized by the gray arrows in figure 6 .

[0093] The natural convection flow rate of the secondary water is regulated by control valves 5 integrated into each of the outlet manifolds 491 of the heat exchangers 49, as illustrated in the figure 9The secondary water cold temperature is governed by the temperature conditions of the secondary water basin B. The hot temperature is set by the control valves 5 in the outlet manifolds 491, and by the heat exchange within the heat exchangers 49.

[0094] An example of integrating a control valve 5 in the form of a butterfly valve 50 into an outlet manifold 491 is shown in Figures 10A, 10B , 10C which show the valve in a position that is respectively fully open, allowing the maximum flow of secondary water from the basin, intermediate, and fully closed, allowing no flow. The butterfly valve 5 is driven in rotation by the output shaft 51 of an electric motor 52.

[0095] Advantageously, the end of the shaft 51 opposite to that connected to the butterfly 50 is connected to an offset weight 53. As shown in Figure 10AIn the event of an electrical failure or emergency stop being triggered, the gravity fall of the weight 53 puts the valve 5 in its fully open position so as to circulate the maximum flow of secondary water from basin B.

[0096] During the reactor's startup phase, the thermocline coincides completely with the upper free level of the secondary water basin B. The driving head of the secondary water circulation is then at its maximum due to the maximum weight of the cold water column supplying the inlets of heat exchangers 49. The thermal power demand on the primary circuit is therefore at its maximum, and the average temperature of the primary water decreases. This decrease in primary water temperature leads to average cooling of the moderator in the core, consequently increasing the core's reactivity and thus its thermal power. These maximum thermal heating conditions of the secondary water volume are accompanied by a natural increase in core power; therefore, reactor 4 is naturally stable.As already mentioned, the position of the primary water shunt valve, combined with the positions of the 42 pilot rods for reactivity, makes it possible to limit the increase in core reactivity, to remain within the temperature rise range of the entire reactor block 4 and its reactor vessel well.

[0097] Conversely, when the thermocline level drops, this implies a rise in the secondary hot water layer, and therefore a decrease in the driving head of secondary water through the heat exchangers 49, since the height of the cold water column decreases. Consequently, the circulation of secondary water by natural convection decreases, thereby reducing the heat exchange between the primary and secondary circuits. In core C, the decrease in power output leads to an increase in the average temperature of the primary water, and therefore to an increase in the average temperature of the moderator in the core. There is thus a decrease in reactivity due to moderator expansion, and the neutron and thermal power produced decreases. The reactor is therefore naturally stable for heat output and thermal storage to the secondary water volume defined by basin B.Typically, a difference in altitude between the median plane of core C and the median plane of the exchangers 49 of about 4 meters allows the development of the natural circulation of a cold primary fluid at 80°C at the outlet of exchanger 49, and hot at 120°C at the outlet of core C, generating about 1100 Pascals of driving pressure necessary to overcome the pressure losses of core C, the exchangers 49, and the rest of the primary circuit including the additional adjustable pressure loss formed by the rolling ring 480.

[0098] The secondary water volume is determined by the dimensions of the tank well on the one hand, and by the height dedicated to the cold and hot zones of the secondary water on the other. Typically, the secondary water volumes are on the order of 200 to 300 m³ for the cold zone, and 100 to 150 m³ for the hot zone, i.e. a total volume for basin B of between 300 and 450 m³.Typically, a difference in altitude of about 4 meters between the median plane of the exchangers 49 and the position of the secondary thermocline separating a secondary cold water layer at 65°C from the hot water layer at 105°C allows the development of the natural circulation of a secondary cold fluid at 65°C in the inlet of exchanger 490, and hot at 105°C at the outlet of exchanger 491, generating about 1000 Pascals of driving pressure necessary to overcome the pressure losses due to the passage through the exchangers 49, from the suction 490 to the outlet 491, including the pressure losses adjustable by means of the secondary flow control valves 5.

[0099] The thermocline can only be maintained at a fixed position if a quantity of secondary water at its hot temperature is continuously drawn off and replaced with the same quantity of secondary water at its cold temperature. Therefore, an adjustable pumping system is in place to transport the power corresponding to customer demand—that is, the power required by the district heating network—to a district heating network. In the event of an unexpected interruption of this heat transfer, or an unforeseen failure of the pumping system, the stability conditions described above allow for the temporary storage of power produced by the reactor core by altering the ratio between the secondary water at its cold and hot temperatures, and by lowering the thermocline level.After several minutes of operation, the continuous removal of the thermal power produced by the reactor, without an external escape route, necessitates shutting down the reactor to remove only the residual power through specifically dedicated residual power removal systems. Typically, a continuous thermal power output of 20 MW, with secondary water supplied at 90°C and returning at 45°C, requires pumping 123 liters per second, or 442 m³ per hour, from the hot water layer above the thermocline and returning the same quantity to the bottom of the reactor vessel. Preferably, this pumping and return can be implemented using piping from the top of the reactor vessel to avoid lateral connections that could cause leaks or lateral structural integrity issues, thus limiting expansion and seismic resistance.

[0100] The heat transfer of 20 MW from the pumped secondary water layer to the customer's tertiary water circuit is achieved using one or more heat exchangers sized to transfer 20 MW with a hot pinch point of 15°C (from 105°C to 90°C) and a cold pinch point of 20°C (from 65°C to 45°C). This secondary water is pumped using pumping units, preferably installed in parallel, to provide operational redundancy in case of failure or maintenance.

[0101] The presence of this piping must not hinder the transport of the entire reactor block, as detailed later for its removal from the reactor vessel well using heavy handling equipment.

[0102] As previously mentioned, the reactor's primary circuit operates solely by natural convection, i.e., without a pumping unit.

[0103] Consequently, the inventors faced a problem in designing a pressurizer whose primary fluid vapor cooling and condensation section could not be designed with a liquid water spraying / injection device, using a sample from the primary circuit as per the state of the art.

[0104] The inventors then thought of modulating the heat losses by conduction through the dome 6, to control the depressurization of the primary steam of the pressurizer, taking advantage of the fact that the metal casing 60 of the cover 41 is of low thickness, typically between 10 and 20 mm.

[0105] Thus, as illustrated in the figure 11, the steam cooling and condensation part includes a double-walled dome 6 60, 61 spaced apart forming a space E inside which liquid water from basin B can flow from the bottom to the top of the dome forming a central exhaust chimney 62. Typically, space E has a constant height of the order of 0.5 to 2 cm.

[0106] In normal operation, the thermocline level is set sufficiently above the pressurizer, specifically so as to be above the central exhaust stack 62, as illustrated in the figure 11 .

[0107] Thus, as illustrated in the figure 11The liquid water circulating solely by natural convection within space E, delimited by the two walls 60 and 61, from a cold temperature below the thermocline, condenses the saturated steam of the primary circuit inside the tank, thus reducing the pressure within the tank. Typically, the cold temperature of the liquid water entering space E at the bottom of dome 6 is around 65°C, which allows for efficient and rapid cooling of dome 6, and in particular of the inner wall 60 forming the pressure-resistant enclosure of the primary circuit, and consequently of the underlying primary steam, to a primary pressure saturation temperature of around 3.5 bar, or approximately 140°C. Typically, such a device allows for the extraction, by natural circulation, of approximately 0.3 MW of thermal energy, and thus the condensation of about 0.15 kg / s of saturated steam.The saturated steam inventory in the pressurizer, under normal operating conditions, is on the order of a few kilograms, depending on the required pressurizer volume. The primary depressurization capacity is therefore fully compatible with the primary pressure control requirements.

[0108] The central chimney 62 incorporates a regulating valve 64, or in other words, a laminating valve, which allows the flow rate of secondary liquid water circulating in space E to be adjusted, thus regulating the liquid cooling process. Indeed, in a fully closed position of valve 64, the water layer is trapped and stratified in space E. Conversely, in an open position, particularly a fully open position, the hot water rises naturally in space E and then through the central chimney 62 to join the upper hot water layer of basin B, while the cold water from basin B is drawn in through the lower inlet of the double wall 60, 61.

[0109] Valve 64 can be a butterfly valve like secondary flow valve 5 illustrated in the diagram. Figures 10A, 10B , 10C .

[0110] The two walls 60, 61 of the dome 6 are metallic, preferably stainless steel.

[0111] The outer wall 61 of the dome 6 is advantageously covered with a cap 63 housing within it thermal insulation to prevent the cooling of the dome when the valve 64 is in the closed position.

[0112] According to an advantageous variant, reactor 4 comprises, as a passive heat sink, a plurality of cooling fins 65 arranged inside the inner wall, preferably uniformly distributed over its surface, and preferably welded or brazed. These cooling fins increase the vertical condensation surfaces inside the pressurizer, thus enabling improved gravity flow of the steam condensate film. In this way, a significant condensation flow rate of the primary steam is maintained by removing and replacing the liquid film created on the cold surface.

[0113] These fins 65 also increase the total contact area with the primary circuit steam, thus improving heat exchange by conduction between said steam and the dome 5. In the illustrated example, these fins 65 are straight and extend over most of the dome's height. These fins 65 are preferably made of the same material as the walls 60, 61 of the dome 6, and are typically a few centimeters thick and a few tens of centimeters long along the inside of the wall 60.

[0114] Furthermore, the heating element of the pressurizer comprises a plurality of electrical resistors 8 wrapped in electrical insulation and powered by electrical cables, arranged inside the dome, preferably on the separating plate 7, which in its center includes a perforated portion 70 to provide thermal insulation and pressure differential functions for the integrated pressurizer. Such a perforated portion 70 is, for example, as described in the device in patent application WO 2012 / 158929A3.

[0115] The electrical resistors 8 can be of the type described in US patent 4135552.

[0116] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0117] Other variants and embodiments may be envisaged without departing from the scope of the invention as defined by the claims. List of cited references

[0118] [1]: The World Nuclear Industry Status Report 2017. https: / / www.worldnuclearreport.org / IMG / pdf / 20170912wnisr2017-en-lr.pdf

Claims

1. Pressurized water reactor (PWR) (4) of the modular reactor (SMR) type, comprising: - a reactor vessel of central axis (X), comprising a cover (41) in the form of a dome (6) and accommodating at least a portion of the primary circuit; - a pressurizer of the primary circuit, the steam cooling and condensing portion of which comprises: • two walls (60, 61) of the dome, spaced apart from each other and thus forming a space (E) which is sealed with respect to the primary circuit and inside which cooling water in liquid form can flow from the bottom to the top of the dome forming a central discharge chimney (62) to the outside of the vessel and dome, in such a way as to condense the saturated steam of the primary circuit inside the vessel and thus reduce the pressure inside the vessel; • a control valve (64) for controlling the flow of liquid water flowing through the space, the control valve being arranged in the central discharge chimney.

2. Reactor according to Claim 1, the two walls of the dome being metallic, preferably made of stainless steel.

3. Reactor according to Claim 1 or 2, the inner wall (60) of the dome forming the enclosure that mechanically withstands the pressure in the primary circuit.

4. Reactor according to Claim 3, the thickness of the inner wall being comprised between 10 and 20 mm.

5. Reactor according to one of the preceding claims, comprising a passive heat sink arranged inside the dome for cooling the steam of the primary circuit.

6. Reactor according to Claim 5, the heat sink comprising a plurality of cooling fins (65) arranged inside the inner wall, preferably being evenly distributed over the surface thereof.

7. Reactor according to Claim 6, the fins being welded or brazed to the inner wall of the dome.

8. Reactor according to one of the preceding claims, the space between inner and outer walls of the dome being comprised between 0.5 and 5 cm.

9. Reactor according to one of the preceding claims, the outer wall of the dome being covered by a cap (63) which accommodates a thermal insulator within it.

10. Reactor according to one of the preceding claims, the liquid water of the pressurizer that is intended to flow between the two walls of the dome being at least 10°C lower than the temperature of liquid water of the primary circuit.

11. Reactor according to one of the preceding claims, the heating portion of the pressurizer comprising a plurality of electrical resistors (8) encased in an electrical insulator and arranged inside the dome.

12. Reactor according to one of the preceding claims, comprising a secondary water circuit, the liquid water that is intended to flow between the two walls of the dome being the water of the secondary circuit of the reactor.

13. Reactor according to one of the preceding claims, comprising a secondary water circuit which comprises a tank (B) filled with water, contained in the space of the pit of the reactor vessel and feeding the bottom of the space between the walls of the dome.

14. Reactor according to Claim 13, the water tank being configured to, when the SMR reactor is operating normally, implement a vertical thermal stratification resulting in the formation of a thermocline that is delimited between the bottom of the tank at what is referred to as a cold temperature in which the SMR reactor vessel is immersed and the top of the tank at what is referred to as a hot temperature, the thermocline being positioned above the dome in such a way that the liquid water flows from the tank into the space between walls of the pressurizer by natural convection.

15. Nuclear installation comprising at least one SMR nuclear reactor according to one of the preceding claims.