Installation comprising at least one nuclear reactor and a thermal storage pit at least partly arranged above the reactor and connected to a heat network

Abstract

A nuclear installation comprising at least one modular nuclear reactor (SMR) and a reactor vessel well defining a water basin in which the SMR reactor block is immersed. The invention essentially consists of positioning a PWR-type reactor at the bottom of a thermal storage pit defining a thermocline and thermally coupling them to supply heat to a network.

Description

Domaine technique

[0001] The present invention relates to the field of nuclear power plants, in particular those comprising pressurized water nuclear reactors (PWRs). More particularly, it relates to the field of small reactors known as small or medium power reactors or SMRs in English (acronym for "Small Modular Reactor"), for calogenic purposes.

[0002] The main objective of the invention is therefore to greatly simplify the operation of a PWR reactor for calogenic purposes which operates at low pressure, typically less than 15 bar and which is intended to provide relatively low thermal power, of the order of a few tens of MWth.

[0003] In particular, the invention aims to achieve a coupling between a nuclear reactor and a heat storage facility to smooth the reactor's heat production, to the point where it can operate all year round at 100% of its power by using the storage facility to manage the intermittent consumption of a low-temperature heat network, typically below 100°C, for which the reactor is intended. This may be an urban, rural or industrial heat network.

[0004] By "SMR reactor" is meant here and within the scope of the invention, the usual technological meaning, namely a nuclear fission reactor, of smaller size and power than those of conventional PWR reactors, which is manufactured in a factory and transported to a nuclear installation site for installation there.

[0005] Although described with reference to a calogenic SMR reactor with natural convection of fluids in the primary and secondary circuits, the invention applies to any type of PWR reactor capable of being immersed in a water basin with its primary and secondary circuits with natural or forced convection.

[0006] By "SMR reactor" is meant here and within the framework of the invention, the usual technological meaning, namely a nuclear fission reactor, of smaller size and power than those of conventional REL reactors, a block of which is manufactured in a factory and transported to a nuclear installation site for installation there.

[0007] By "reactor block" is meant here and in the context, the vessel, called the reactor vessel as well as all the components and part of the fluid circuit, in particular the core of the reactor creating heat by nuclear fission reactions, which is housed inside the reactor vessel.

[0008] By "heat-generating" is meant here and within the scope of the invention, a nuclear installation, a nuclear power plant or a nuclear reactor whose power is mainly dedicated to the supply of heat. The power of a heat-generating reactor can be 100% to supply heat. A small part of its power can still be used to supply electricity.

[0009] By "electrical purpose" is meant here and within the scope of the invention, a nuclear installation, a nuclear power plant or a nuclear reactor whose power is mainly dedicated to the supply of electricity. The power of a reactor with an electric purpose can be 100% to supply electricity. A small part of its power can still be used to supply heat. Technique antérieure

[0010] One of the current developments for nuclear reactors concerns so-called calogenic reactors intended to provide a thermal power level of a few tens of MWth, mainly for the purpose of providing so-called urban heat, i.e. in urban networks, for towns / agglomerations of several hundred thousand inhabitants.

[0011] Among the various technological solutions that provide heat from nuclear fission, it is commonly accepted that to date, pressurized water reactors (PWRs) are the most suitable for providing heat at relatively low temperatures.

[0012] In fact, boiling water reactors (BWR) are designed to produce steam in a primary circuit directly used in a turbo-alternator group in order to produce electricity.

[0013] Fourth generation fast neutron reactors (FNRs) provide heat at temperatures beyond those required, and their main disadvantages are the cost of construction and operation, which makes them incompatible with the exclusive supply of district heat.

[0014] Similarly, graphite-moderated and gas-cooled reactors are designed to provide heat at relatively high temperatures.

[0015] Finally, emerging concepts such as molten salt reactors do not have sufficient technological maturity for deployment in the relatively short term.

[0016] It should be remembered that a pressurized water nuclear reactor (PWR) comprises three cycles (fluid circuits) whose general principle of normal operation is as follows.

[0017] The high-pressure water in a primary circuit extracts the energy supplied, in the form of heat, by the fission of uranium nuclei, and where appropriate plutonium, in the reactor core.

[0018] Then, this water under high pressure and high temperature, typically 155 bars and 300 °C, enters a steam generator (SG) and transmits its energy to a secondary circuit, also using pressurized water as a heat transfer fluid. This water in the form of steam, at high pressure, typically around 70 bars, is then expanded via an expansion device transforming the variation in enthalpy of the fluid into mechanical and then electrical work in the presence of an electric generator.

[0019] The water from the secondary circuit is then condensed via a condenser using a third cycle, the tertiary or cooling cycle, as a cold source.

[0020] The design principles of PWR reactors according to these three cycles have been essentially the same since the first ones were put into service.

[0021] The main elements of a PWR primary circuit are shown in figure 1 : a reactor building 1 providing various functions including in particular a contribution to the containment safety function, a reactor vessel 20, located in the center of building 1, housing the core C of the reactor, a primary circuit 2 in pressurized water including the vessel 20.

[0022] These main elements are therefore common, their composition and the number of components varying according to the power of the reactor.

[0023] Typically, the envelope of the reactor building 1 may consist of several layers. For example, as shown in figure 1 , a reactor building 1 may 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 metal skin 11 on the inside of the prestressed concrete wall 10, for a 1650 MWe reactor.

[0024] As illustrated in the figure 2 , 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.

[0025] Furthermore, we can distinguish on this figure 2 , the reactor core control rod and control cluster mechanisms 25.

[0026] Depending on the reactor power, the number of loops can be three for a 900 MWe reactor or 4 for a 1300 MWe reactor and above.

[0027] The reactor 1 building is therefore sized, among other things, to house all the components of primary circuit 2.

[0028] There figure 3 illustrates the energy transfer cycle (heat then electricity) of a PWR reactor. On this figure 3 , we can notably distinguish the distribution of the positioning of the components in relation to the reactor 1 building, which ensures the function of third containment barrier.

[0029] The fluid connections between the inside and the outside of the reactor building 1 are provided by the 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.

[0030] More precisely, for a given steam generator 23, the reactor building 1 is crossed by a line called the hot line 30 which evacuates the steam from the steam generator 23 to evacuate the power and bring it to the turbine 32, and by a line called the cold line 31 which supplies liquid water to the steam generator 23.

[0031] Among the solutions already studied for PWR reactors for calogenic purposes, three main categories can be distinguished corresponding to the main design architectures of the primary circuit, namely respectively the so-called "pool" type reactors, those with primary loop(s), and those of the integrated type generally illustrated in small power reactors, designated by the English acronym SMR ("Small Modular Reactor"), for electrogenic purposes. Reference may be made to publications [2] and [3].

[0032] A pool reactor has generally been implemented experimentally with a low power, typically 10 MWth: the pressure in the primary circuit can be close to atmospheric pressure, involving both moderate neutron fluences within the core, and the temperature of the primary circuit is limited and close to 100°C maximum. In this pool reactor, the height of liquid water above the core, however, allows the primary pressure within the fissile core to be slightly increased, while remaining within the order of magnitude of a few bar. The advantage of such a reactor lies in the simplicity of design, the reactor vessel not being considered as a pressure vessel, the concrete reactor pit and its internal sealed casing forming the reactor vessel surrounding the primary circuit.It is therefore mainly the maintenance of radiological containment, as in the case of a fuel storage pool, which governs the design quality of this "sandwich" component consisting of the concrete reactor pit and its internal watertight casing. The reactor vessel itself is assembled on site, and must of course withstand all extreme external attacks, including major earthquakes and aircraft crashes. In addition, the concrete must be subjected to temperatures compatible with maintaining its mechanical characteristics over time. This type of design is no longer used due to the strong constraints of demonstrating the quality of construction and on-site inspection for the construction of the second containment barrier of a nuclear boiler, rules set by the RCC-M, which is the French code defining the rules for the design and construction of mechanical equipment for the nuclear islands of PWR reactors.A self-supporting metal tank, manufactured in the factory, is preferred, and also allows for higher primary pressures.

[0033] An example of a pool reactor is represented by the SLOWPOKE project carried out in the 1970s-1980s by Canada (AECL) concerning NHP reactors (an English acronym for "Nuclear Heating Plants"). The figure on page 13 of the publication [3] illustrates this demonstration reactor, with a thermal power output of 10 MWth, whose water-filled reactor pit contains the entire primary circuit and the exchangers between the primary circuit and the secondary circuit, the whole being closed by a slab at ground level. In this pool reactor, the circulation of the primary fluid is achieved by natural convection, which simplifies electrical loss transients and general system maintenance.The reactor also has significant thermal inertia, due to the importance of the water inventory in the primary circuit in relation to the reactor power, which also provides a gain in general safety in relation to incidental transients, and operation. Finally, the thickness of the reactor pit and its absence of lateral or lower crossings ensure by design the impossibility of draining the primary circuit, nor its depressurization. Such a basin reactor therefore meets many criteria of simplicity of design, safety in design, and ease of control.

[0034] A more recent example of implementation with the Chinese DHR 400 project of the CNNC company seems to show its reproducibility, since the reactor vessel is made up of a sandwich component with a thick prestressed concrete shell, typically of the order of one meter, partly coated internally with a 5mm thick stainless steel liner, the whole being enclosed in an external cylinder of 10mm thick carbon steel. However, this reactor can only be reasonably retained on the condition that the criteria of modularity and maximization of manufacturing of components in the factory, with transport to the site, which are preponderant, are respected.

[0035] An example of a pool reactor, with a traditional reactor pit and associated casing, is presented in the Russian RUTA-70 reactor project of the NIKIET company.

[0036] An example of a heat-generating loop reactor is the Chinese HAPPY 200 reactor from SPIC, dedicated to district heating in the city of Beijing. With a unit capacity of 200 MWth, it is designed in batches of two units, thus equaling the performance of the aforementioned DHR400 project from CNNC. Unlike the latter, the reactor vessel is a self-supporting steel structure, designed in a factory and assembled on site, although the primary loop requires complete welding, thus requiring heavy construction work. The thermal power is removed from the core via two primary loops feeding plate heat exchangers, by forced convection using two pumps. A water pool surrounds the entire reactor vessel, but without direct contact due to a double jacket. In an accidental transient situation, this double jacket is flooded by the cold water present in the pool.This loop reactor configuration thus has the advantage of being able to supply heat at a temperature relatively close to that leaving the core, thanks to forced convection in the primary circuit, which facilitates the extraction of thermal power, and due to the presence of plate exchangers between the primary and secondary circuits, which also allows a low thermal gradient between the temperatures of the primary circuit and the secondary circuit.

[0037] The last category is that of so-called integrated reactors, which comprise a block delimited by a reactor vessel entirely manufactured in the factory and transported to the site, and which houses the primary circuit in its entirety, and in particular the exchangers between the primary and secondary circuits. This type of integrated reactor has the same configuration as the main concepts of so-called SMR reactors currently existing, with an electrogenic function, namely a configuration based on the integration of the steam generator, or even all the components of the primary circuit, in particular the pressurizer and the primary pumps, inside the reactor vessel. These SMRs are called integrated SMRs.

[0038] The main advantages of SMR reactors over existing PWRs are that they allow for system simplification, mainly for safety reasons, and increased modularity through significant factory manufacturing of components for transport to the construction site.

[0039] In addition to the gain in compactness, integrated SMRs have the advantage of no longer requiring overhead fluid lines with pressurized water, which considerably reduces the risk of accidents and associated consequences linked to the rupture of the primary circuit lines. On-site installation is thus greatly facilitated by limiting itself to secondary piping connections, except for the tappings of the volumetric and chemistry system of the primary circuit, which are of small diameter.

[0040] For example, the nuclear power plant project with the acronym NUWARD ™<, is a power plant with an electrogenic purpose, consisting of two integrated SMRs, with a unit power equal to 170MWe, each of which includes a block housing all the components of the primary circuit inside the reactor vessel.

[0041] Other integrated SMR projects for electricity generation 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.

[0042] It has been represented on the figure 4 an example of an integrated SMR currently in the planning stage. Such an integrated SMR reactor, the block of which is generally designated by the reference numeral 4, comprises a fixed compartment 40 and a removable compartment 41 in the form of a cover, for the fuel handling or maintenance phases of the reactor internals.

[0043] A pioneering concept for an integrated calogenic reactor is the THERMOS project jointly conducted by the Applicant and Technicatome: [4]. The reactor according to this project had a thermal power of 100 MWth and was intended to supply district heat to the city of Grenoble. According to this project, the reactor vessel integrates the entire primary circuit, thus allowing operation under a pressure higher than that of a basin reactor, necessary to provide district heat of around 120°C. With a diameter of 5m and a height of 9m, the reactor vessel that was proposed was thus entirely factory-assembled, and housed in particular the exchangers between the primary and secondary circuits in its upper part. The low temperature gradient in the core required the presence of primary fluid pumping units, arranged in the hot part, which is not ideal in terms of safety and ease of maintenance.In addition, the thermal inertia of the primary and secondary circuits is relatively limited, the basin in which the reactor is immersed being thermally disconnected from the normal operation of the reactor, which is detrimental to operational safety and the smoothing of power call transients from the customer's heating network.

[0044] Recent studies are dedicated to calogenic reactors in Finland, carried out mainly by the VTT organization, for the district heating of the city of Helsinki:[5]. In particular, a reactor with a thermal power of 50 MWTh of the integrated tank type was studied, containing the exchangers between primary and secondary circuits in its upper part, the reactor tank itself being contained in a nearby containment enclosure. The circulation of the primary fluid is by natural convection in nominal operation, and the secondary circuit is a liquid loop circuit including an exchanger with the tertiary circuit which operates by forced convection by means of a pump. The tank and containment enclosure assembly is immersed in a pool which is the fuel handling pool (IRWST Anglo-Saxon acronym for " In-containment Refueling Water Storage Tank ”) .More specifically, the IRWST pool includes a pit forming the reactor pit, inside which the reactor vessel and containment building are partially submerged. This IRWST pool also serves as a heat sink for design-basis accidents. This type of integrated reactor offers the same advantages of factory constructability and modularity as those of integrated SMRs with a generator function, notably from NuScale Power.

[0045] As with other energy sources, there would be considerable advantage in coupling a calogenic nuclear reactor with a heat storage system in order to smooth the reactor's heat production, to the point where it could operate all year round at 100% of its power, using storage to manage the intermittent consumption of a heat network.

[0046] Here, we can cite the patent applications CN210069996U, CN113790469A, CN205388908U, US20120314829A1, JP2003270383A, US4294311, FR2322242B2, FR2257869B1, CN210123170U and CN105509121B which evoke the possibility of such a coupling.

[0047] In particular, patent US4294311 discusses the possible storage of heat from a PWR nuclear reactor in a pit filled with a PTES (Pit Thermal Energy Storage) media with layers of different temperatures separated by polyurethane membranes. Not only does such a pit seem expensive to implement, but no real details are given on the coupling between the reactor and the PTES pit.

[0048] Document US 2022 / 084697 A1 relates to a system for storing and converting thermal energy from an integral molten salt reactor into usable energy.

[0049] There is therefore a need to improve the reactor installations envisaged as a calogenic reactor and a thermal storage system for the heat produced by the reactor, in order to overcome the drawbacks mentioned above.

[0050] The aim of the invention is therefore to meet this need at least in part. Exposé de l'invention

[0051] To this end, the invention relates, in one of its aspects, to a nuclear installation comprising: at least one calogenic nuclear reactor comprising a reactor vessel; a reactor pit housing the reactor vessel, a primary circuit and a part of a secondary circuit; a tertiary circuit comprising: a thermal storage pit, dug into the ground, filled with water and in the bottom of which the reactor pit is excavated or laid, the pit being configured to achieve vertical thermal stratification resulting in the formation of a thermocline delimited between the bottom of the pit at a so-called cold temperature and the top of the pit at a so-called hot temperature, at least one heat exchanger with the secondary circuit of which at least one inlet is fluidically connected to a zone of the pit below the thermocline and at least one outlet is fluidically connected to a zone of the pit above the thermocline so that the water in the pit circulates by natural or forced convection between the inlet and the outlet of the heat exchanger;a heat network of which at least one inlet is fluidically connected to an area of ​​the pit below the thermocline and at least one outlet is fluidically connected to an area of ​​the pit above the thermocline.;

[0052] By "tank well placed in the bottom of the pit" is meant here and within the scope of the invention, that the pit is not excavated as such to modify the altimetry of the various components of the reactor and the related exchangers so as to allow circulation by natural convection of the tertiary water. Thus, a tank well placed on the ground implies circulation by forced convection of the tertiary water.

[0053] In order to better thermally insulate the pit from the ambient air, the pit is advantageously covered with a thermally insulating blanket.

[0054] According to an advantageous embodiment, the pit is artificially dug into the ground and comprises a sealing coating, preferably thermally insulating, covering its bottom and its side walls.

[0055] Preferably, the liner is a membrane adapted to fit the shape of the bottom and side walls of the pit.

[0056] The thermally insulating blanket or sealing coating may comprise one or more thermal insulating materials made of material(s) selected from stainless steel, a polymer such as high density polyethylene (HDPE), or an elastomer.

[0057] According to an advantageous embodiment, the installation comprises: at least one pipe connected to the inlet of the tertiary circuit of the heat exchanger, the end of which forming a draw-off collector opens directly into the area of ​​the pit below the thermocline, at least one pipe connected to the outlet of the tertiary circuit of the heat exchanger, the end of which forming an injection collector opens directly into the area of ​​the pit above the thermocline,

[0058] According to another advantageous embodiment, the installation comprises: at least one pipe connected to the inlet of the heat network, the end of which forms an injection collector and opens directly into the area of ​​the pit below the thermocline, at least one pipe connected to the outlet of the heat network, the end of which forms a withdrawal collector and opens directly into the area of ​​the pit above the thermocline. each pipe manifold including a hydraulic flow distribution device adapted to broaden the flow of water entering or leaving the areas of the pit below or above the thermocline.

[0059] According to yet another advantageous embodiment, the installation comprises: a pipe directly connecting the secondary circuit part housed in the tank well to an inlet of the exchanger with the tertiary circuit; a pipe directly connecting the secondary circuit part housed in the tank well to an outlet of the exchanger with the tertiary circuit.

[0060] The excavated or installed tank well is preferably closed by a removable metal cover forming the separation wall with the volume of the pit.

[0061] The excavated or installed tank well may include a concrete wall forming the separation wall with the ground and the volume of the pit.

[0062] The height He of the tank well excavated below the bottom of the pit is advantageously at least equal to 15m.

[0063] Advantageously, the reactor is a calogenic SMR type reactor.

[0064] Thus, the invention essentially consists of positioning a PWR type reactor at the bottom of a thermal storage pit delimiting a thermocline and thermally coupling them.

[0065] To do this, the tertiary circuit of the reactor is made up of the pit itself and one or more exchangers with the secondary circuit whose inlet is fluidically connected in a cold temperature zone below the thermocline and the outlet is fluidically connected in a hot temperature zone above the thermocline so as to ensure natural or forced convection of the water in said tertiary circuit and this whatever the season of the year and the power demand of the heat network whose inlets and outlets are preferably connected respectively to the cold and hot temperature zones.

[0066] The heat network itself includes a circuit for pumping and rejecting water and for heat exchange with the network.

[0067] The advantages of the invention are numerous, among which we can cite: due to the coupling between the calogenic reactor 4 and the PTES type thermal storage pit, the possibility of operating the reactor at 100% of its power all year round, with the exception of the core reloading / unloading phases, during which the pit allows the heat network to continue to be supplied. The economic efficiency of the installation is therefore maximum. The adaptation of production to the intermittency of heat consumption of an urban type heat network is managed by the thermal storage in the water within the pit, the dimensioning of which is adapted to manage this intermittency, in particular during seasonal changes. The thermal storage pit allows the time shifting of a large quantity of heat, typically several GW.h, produced in the summer by the reactor to consumption during the winter,increased safety of the reactor against external attacks due to its positioning below the thermal storage pit, typically under several meters of water, the elimination of incident situations of the "loss of cooling" type with the presence of a passive and quasi-infinite cold source constituted by the tertiary water within the pit, above the reactor, increased radiological protection by the presence of a significant height of tertiary water above the reactor, typically under a few meters of water, preferably at least 4 meters of water, the positioning of the nuclear reactor at least partly below the thermal storage pit with a significant height of water above the reactor allows, in the event of an extreme event leading to a prolonged loss of electrical resources and normal cold source, of the type of the accident that occurred at Fukushima Daishi,to have the direct proximity of a water reservoir (pit filled with tertiary water) unlimited with regard to the cooling needs of the reactor in residual power operation. For example, water at 1 bar pressure has an enthalpy of vaporization of 2257 kJ / kg, i.e. for a volume of water in the pit of the order of 800,000 m 3 < , a thermal energy potentially evacuable by evaporation of 500 GW.h. As a first approximation, considering a residual power of the reactor of 1% of MWth, i.e. 200 kWth, the potential energy of 500 GW.h offers a cooling capacity lasting approximately 280 years. More reasonably, considering a residual power law penalizing the reactor core whose nominal power is 20 MWTh, the total thermal energy released by the nuclear fission products over a period of 1000 years is approximately 28 GW.h,or 6% of the available reserve of a pit filled with boiling water. the possibility of sharing the excavation and civil engineering costs between the thermal storage pit and the reactor vessel pit, and of minimizing the total footprint of the installation, an increase in the driving height available for circulation by natural convection of all the reactor circuits (primary, secondary and tertiary), which minimizes or even cancels out the pumping power required in these circuits, the demonstration of the practical elimination of the core meltdown accident of the PWR reactor, the origin of which would come from a prolonged lack of means of cooling the reactor core, because it is arranged at least partly under a thermal storage pit filled with water, the optimization of the neutron operation of the reactor and its control thanks to its possible operation at 100% of its power all year round,which makes it possible to eliminate the intermediate power levels that are necessary in the event of reduced and then restarted operation. In particular, there is no longer any xenon effect to control, no more constraints linked to taking into account the Pellet Cladding Interaction (PCI) phenomenon in the control of the reactor. a minimization of thermal leaks from the reactor since some of them can be recovered in the thermal storage pit, which optimizes the overall energy efficiency of the installation.

[0068] The installation as planned with its calogenic reactor is intended to supply urban heat networks, supply industrial processes such as desalination, drying, and food product processing.

[0069] Advantageously, a complementary electrical production unit can also be added, using an organic Rankine cycle, to obtain local emergency electrical production in the event of an ultimate need, in addition to the conventional battery bank solutions generally used. For reasons of simplicity of design and maintenance, the installation of active means classified as emergency, such as diesel combustion, is not planned.

[0070] Other advantages and characteristics of the invention will become more apparent 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. Brève description des dessins

[0071] [ Fig 1 ] there figure 1 is a schematic perspective and partial sectional view of an existing PWR nuclear reactor. Fig 2 ] there figure 2 is a schematic view of a state-of-the-art PWR nuclear reactor primary circuit in a three-primary loop configuration. Fig 3 ] there figure 3 is a schematic view of the three cycles of a state-of-the-art PWR nuclear reactor. Fig 4 ] there figure 4 is a schematic perspective view of an integrated type SMR reactor as currently envisaged. Fig 5 ] there figure 5 is a schematic perspective view of a calogenic SMR reactor according to the invention. Fig 5A ] there figure 5A is an exploded perspective view of the parts of the SMR calogenic reactor vessel according to the invention. Fig 6 ] there figure 6 is a longitudinal sectional view of the reactor along the figure 5 , and which illustrates the circulation by natural convection of the water in the primary circuit and the secondary circuit. Fig 6A ] there figure 6A is a detailed schematic view of a control cluster assembly, the control rod, and the rod control mechanism of a calogenic SMR reactor according to the invention. Fig 7] [Fig 8 ] THE figures 7 et 8 are detailed perspective views showing the circuit water 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 water flow control valve in the secondary circuit. Fig 10A] [Fig 10B] [Fig 10C ] THE figure 10A, 10B et 10C are perspective views showing an example of a secondary circuit water flow control valve and its integration into an exchanger outlet manifold, the valve being respectively in the fully open position, an intermediate position and the fully closed position. Fig 11 ] there figure 11 is a partial longitudinal sectional view of the upper part of the reactor according to the figures 5 And 6 showing a pressurizer according to the invention. [ Fig 12 ] there figure 12 is a schematic view in longitudinal section and in transparency of a nuclear installation according to the invention with a calogenic reactor according to the figures 5 And 6 in a watertight tank well arranged below the bottom of a PTES type thermal storage pit filled with water which constitutes the water for the tertiary circuit. Fig 13 ], [ Fig 14 ] THE figures 13 And 14 illustrate alternative embodiments of the flow distribution device within the thermal storage pit of the installation according to the invention. Fig 15 ] there figure 15 is a schematic perspective and transparency view of a nuclear installation according to the invention with a calogenic reactor according to the figures 5 And 6in a watertight tank well, arranged below the bottom of a thermal storage pit whose water constitutes part of the tertiary circuit in natural convection. Fig 16 ] there figure 16 is a schematic perspective and transparency view of a nuclear installation according to the invention with a calogenic reactor according to the figures 5 And 6 in a sealed tank well, arranged on the bottom of a thermal storage pit whose water constitutes part of the tertiary circuit in forced convection. Fig 16A ] there figure 16A is a detail view of the figure 16 showing the bottom of the tank well with the installation of a circulation pump for the water from the thermal storage pit. Description détaillée

[0072] Throughout the present application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to an SMR nuclear reactor, as provided in a vertical operating configuration and whose reactor pit is on the bottom or excavated below the thermal storage pit according to the invention.

[0073] By "primary water", "secondary water", "tertiary water" we mean the water which constitutes the fluid of the primary, secondary and tertiary circuits respectively.

[0074] THE figures 1 à 4 have already been detailed in the preamble, they will therefore not be commented on below.

[0075] Please note that the different temperatures, powers, volumes, flow rates, etc. indicated are for information purposes only. For example, other temperatures may be considered depending on the configurations, in particular the SMR reactor power, the volume of water in the secondary basin, the power requirement for the heating network, etc.

[0076] It is also noted that the figures are not necessarily to scale. In particular, in the figure, the reactor pit 100 and the reactor 4 housed there are in reality much smaller relative to the thermal storage pit 9.

[0077] It is described with reference to the figures 5 And 6 , a pressurized water type nuclear reactor 4, according to an integrated SMR type primary circuit configuration according to one embodiment of the invention.

[0078] This reactor 4 has a unit power of 20 MW thermal, for calogenic purposes, i.e. dedicated to the supply of hot water, typically at 90°C. Its unit power can however vary upwards or downwards, in a range of approximately 10 MW to 100 MW, and the hot water supply temperature can also change.

[0079] The reactor 4 with central axis X comprises a block delimited by a metallic tank 40, 41 preferably made of stainless steel, with a thickness of the order of 10 to 20 mm, and formed of a hemispherical tank bottom and a vertical cylinder. This reactor tank consists of a fixed compartment 40 and a removable compartment 41, above the reactor core for the fuel handling or maintenance phases of the reactor internals. This removable compartment 41 is a cover in the form of a dome 5 whose central chimney integrates a throttling valve 64 adapted for cooling the reactor pressurizer as detailed below.

[0080] The reactor core C comprises a set of 42 fuel assemblies such as those conventionally used in PWR type reactors but with a fissile height adapted to obtain the desired total thermal power. Each fuel assembly has several locations lacking fuel rods, replaced by absorber rods which can rise or fall in the assembly to control the reaction. Data from preliminary studies carried out by the Applicant consider a number of 52 assemblies and a lifetime of 10 years, with a fissile height of 1.5 m.

[0081] The metal casing 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 42, and a separation casing 40 with its peripheral neutron reflector 440 intended to ensure the maintenance of the neutron flux in the core.

[0082] Two flanges 45 are bolted between the fixed compartment 40 and the dome 41. The seal between a flange 45 and each of the two compartments 40, 41 is advantageously ensured by a metal seal. The dismantling of this flange 45 allows the complete handling of the fuel assemblies during the core reloading phases. The studies carried out by the Applicant provide for shutdowns for fuel reloading scheduled during ten-yearly inspections, without intervention on the core between these periods.

[0083] Above core C, rod cluster guides 46 allow the insertion of nuclear reactivity control rods 42, in a manner similar to that usually found in conventional PWR reactors. The control rods 42 are rods made of neutron absorbing material.

[0084] The free volume above the reactor core C allows the control rods 42 to be fully extended, as well as the standby position of so-called emergency absorber rods, dedicated to the safety shutdown of the nuclear reaction.

[0085] Above the control bars 42, a perforated plate 48 is fixed, allowing the free passage of the hot primary fluid from the core through the holes 480, as well as the rods controlling the movement of the rolling crown 481.

[0086] At the periphery of the plate 48, a flow control valve 481 for regulating the flow of the water in the primary circuit, also called a flow control valve, is arranged. This valve is in the form of a throttling ring 481 which follows the inner periphery of the compartment 45 of the tank and extends over a height between the plate 48 and the primary water outlet openings, making it possible to adjust the flow of the latter.

[0087] This throttle valve 481 has the function of regulating the natural circulation flow of the water in the primary circuit passing through the openings 400 which constitute the inlets of the primary water collectors of the exchangers 49 between the primary and secondary circuits. The positioning control of this regulating valve is carried out by an electric motor controlling the vertical movement of the control rod 482 linked to the crown 481, which is advantageously identical to those used by the control rods of the reactivity control bars 46.

[0088] In an intermediate position, as shown in the figure 7 , the rolling crown 481 leaves the openings 400 partially clear, thus regulating the flow of primary water intended for the exchangers 49.

[0089] In the event of a power failure or emergency shutdown, the gravitational fall of the bars 46 mechanically coupled to the absorbent rods 42 also triggers the gravitational fall of the primary fluid control valve 481. In the extreme low position, as illustrated in figure 8 , this control valve 481 allows the water from the primary circuit to pass completely through the openings 400, thus maximizing the heat exchange in the exchangers 49 between the primary and secondary circuits, in order to evacuate the residual power and cool the primary circuit. This operation by gravity fall of this control valve guarantees reliability and safety in the event of electrical loss or reactor failure.

[0090] In reactor 4 of the figures 5 And 6, in normal operation, the thermal power created by the nuclear chain reaction within the reactor core is evacuated by the fluid of the primary circuit which rises by natural convection in an upward manner, to arrive in the upper part, where it can then flow according to the different outlet openings 400 corresponding to the inlet collectors of the exchangers 49 between the primary and secondary circuits and in an upper central portion of the core, in the form of a chimney. This central chimney, not detailed, called a riser, contains, in addition to the control rod piloting mechanisms, the instrumentation sensors of the core parameters, and above the movement motors 47 of the reactivity control rods 42.

[0091] Thus, the separation envelope 44 of the core C makes it possible to separate the water, the fluid of the primary circuit, in its so-called cold and hot temperatures. Thus, the primary water at cold temperature surrounds the core C inside the envelope 40, while the primary water at hot temperature, heated by circulating upwards in the core C, is found in the upper central portion of the core.

[0092] Above the outlet openings 400, within the reactor 4, a separation plate 7 separates the interior of the dome 41 of the tank containing a pressurizer, from the riser. This separation plate 7 contains a plate with holes 70 opening out to ensure the thermal insulation and pressure difference functions of the integrated pressurizer. This separation plate may be of the type already described in patent application WO2012 / 158929 A3.

[0093] The upper part integrating the reactor pressurizer will be detailed later with reference to the figure 11 .

[0094] After cooling through the exchangers 49 in a downward manner, the water in the primary circuit passes through the openings 401 which constitute the primary water outlet collectors of the exchangers 49 then returns in a closed circuit to the lower part of the reactor core. First at the periphery of the shell 40, in an area conventionally called "down comer", then returns to the bottom of the vessel, passes through the core support basket 43, and will heat up through the core C. The circulation in a closed circuit P solely by natural convection of the primary water is symbolized by the white arrows in figures 5 And 6The driving force of the primary circuit in natural convection is controlled by the difference in height between the average altimetric position of the exchangers 49 (primary cold source), and the average height of the fissile zone of the core C (primary hot source).

[0095] As already mentioned, the primary water pressure is adjusted by the throttling valve 481 whose control mechanisms are housed in one of the holes 480 of the plate 48. The primary water inlet and outlet temperature is adjusted using the neutron fluence conditions, i.e. the thermal power of the core, by the positions of the reactivity control rods 42 in the core, and the saturation temperature and pressure conditions in the pressurizer. Here, due to the circulation being solely by natural convection, i.e. in the absence of an active means of pumping primary water, it is the primary water flow throttling valve which sets the exchange circulation conditions between the primary and secondary circuits in relation to the power produced in the core.In fact, the control of the operation of the calogenic reactor can be carried out simply with a control of the pressure and temperature at the pressurizer using the primary steam cooling system 6 and the heaters 8, an adjustment of the primary natural circulation flow rate evacuating the thermal power from the core via the rolling crown 481, and the adjustment of the core power by the set of control rods 42.

[0096] The exchangers 49 between the primary and secondary circuits are preferably plate exchangers, advantageously made of stainless steel, and designed to withstand the pressure of the water in the primary circuit. Advantageously, these exchangers 49 are manufactured by stacking consisting of grooved metal plates assembled together either by hot isostatic pressing (HIP) or by hot uniaxial pressing (HUC) so as to obtain diffusion welding between the metal plates, or by brazing.

[0097] Within a 49 exchanger, the flow is downward for primary water, and upward for secondary water.

[0098] As represented in figure 6 , the secondary circuit of this reactor 4 is not a closed loop circuit as in conventional PWR reactors, but includes a water basin B, as shown diagrammatically in figure 11 This basin B is contained in the space of the reactor vessel well forming the third containment barrier, and the reactor vessel 4 is immersed there.

[0099] This secondary circuit with liquid water basin B is an open environment delimited by the tank well, without a circulation pump.

[0100] With such a liquid water basin B for the secondary circuit, the exchangers 49 are not integrated into the reactor vessel 40, 41 but arranged and fixed outside it. Such an arrangement is possible because the unlikely event of rupture of the primary water inlet or outlet pipes, then causing a large diameter breach of loss of the latter, does not have significant accidental consequences for the reactor. Indeed, the liquid water basin B completely surrounds the reactor vessel 4, and an accident of this type cannot lead to a risk of dewatering of the core, endangering the physical integrity of the reactor core.

[0101] The internal circuit within an exchanger 49 which is part of the secondary circuit of the reactor 4 therefore sees a flow of liquid water pass as a secondary fluid which heats up in contact with the primary water within the exchanger 49, by natural suction from its inlet collector 490 at the bottom to their outlet collector 491 at the top of it. The secondary water then creates a volume greater than a so-called hot temperature. The separation layer between a so-called cold temperature and the so-called hot temperature of secondary water is designated as a thermocline, as symbolized by the name thermocline 1 in figure 6 .

[0102] In other words, when the SMR reactor is in normal operation, the 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 in which the reactor vessel 4 is immersed and the top of the basin at the hot temperature. It is the height of the thermocline layer which will fix the cooling flow rate of the secondary circuit through the exchangers 49. The circulation in closed circuit S solely by natural convection of the secondary water is symbolized by the gray arrows in figures 5 And 6 .

[0103] The flow rate by natural convection of the secondary water is regulated by control valves 5 integrated in each of the outlet collectors 491 of the exchangers 49, as illustrated in figure 9 The secondary cold water temperature is governed by the temperature conditions of the secondary water basin B. The hot temperature is set by the control valves in the outlet manifolds 491, and by the heat exchange within the exchangers 49 themselves.

[0104] An example of integrating a control valve 5 in the form of a butterfly valve 50 in an outlet manifold 491 is shown in figures 10A, 10B, 10C which show the valve in a position respectively fully open in which the maximum secondary water flow from the basin can pass, intermediate, and fully closed in which no flow can pass. The butterfly 5 is rotated by the output shaft 51 of an electric motor 52.

[0105] Advantageously, the end of the shaft 51 opposite that linked to the butterfly 50 is linked to an offset weight 53. As shown in figure 10A , in the event of a power failure or emergency stop triggering, the gravitational 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.

[0106] During the nuclear reactor start-up phase, thermocline 1 is completely merged with the upper free level of secondary water basin B. The driving height of secondary water circulation is then maximum due to the maximum weight of the cold water column feeding the inlets of exchangers 49. The thermal power demand towards the primary circuit is then maximum, and the average temperature of the primary water drops. The lowering of the primary water temperature leads to an average cooling of the moderator in the core, inducing an increase in the reactivity of the core, and therefore an increase in its thermal power. Maximum conditions of thermal heating of the secondary water volume are accompanied by a natural increase in the power of the core, reactor 4 is therefore naturally stable.As already mentioned, the position of the primary water throttling valve, combined with the positions of the reactivity control bar 42, however, makes it possible to limit the increase in the reactivity of the core, to remain within the temperature rise range of the entire reactor block 4 and its reactor pit.

[0107] Conversely, when thermocline 1 drops in level, this implies a rise in the secondary hot water layer, and therefore a drop in the secondary water driving height through exchangers 49 since the height of the cold water column decreases. Then, the circulation of secondary water by natural convection decreases, thereby reducing the heat exchange between the primary and secondary circuits. At constant neutron power in core C, the drop in power removal leads to an increase in the average temperature of the primary water, and therefore an increase in the average temperature of the moderator in the core. There is therefore a drop in reactivity due to expansion of the moderator, and the neutron and thermal power produced decreases. The reactor is therefore naturally stable for removal and thermal storage towards the volume of secondary water defined by basin B.

[0108] The secondary water volume is sized 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 hand. Typically, the secondary water volume is in the order of 300 to 400 m 3< for the cold zone, and 100 to 150 m 3< for the hot zone, i.e. a total volume for basin B of between 400 and 550 m 3< .

[0109] The position of thermocline 1 can only be maintained at a fixed position on condition that there is a continuous withdrawal of a quantity of secondary water at its warm temperature and a replacement by the same quantity of secondary water at its cold temperature. This continuous withdrawal is detailed later in relation to the figure 12 , so that there is a balance between the thermal power produced by the reactor core, and its extraction by this sampling and replacement.

[0110] There is therefore an adjustable pumping system making it possible to transport, to a heating network, the power corresponding to the customer's demand, i.e. required by the heating network.

[0111] According to the invention, as detailed below, the heat evacuated by the reactor can be stored, before its transport to a heat network, by a thermal storage pit.

[0112] In the event of an untimely shutdown of this heat removal, or an accidental shutdown of the pumping system, the stability conditions described above make it possible to temporarily store the power produced by the reactor core by modifying the ratio between the secondary water at its cold temperature and at its hot temperature, and by lowering the level of thermocline 1. After several minutes of operation, the continuous removal of the thermal power produced by the reactor without external escape, leads to the shutdown of the reactor, to remove only the residual power, through specifically dedicated residual power removal systems.Typically, a continuous thermal power of 50 MW, with a secondary water supply at 90°C and a return at 45°C, requires pumping 270 liters per second, or 970 m 3 < per hour from the hot water layer above thermocline 1 and a return of the same quantity to the bottom of the tank pit. Preferably, this discharge and return can be implemented by means of piping coming from the upper part of the tank pit, in order to avoid lateral connections likely to cause leaks or problems of lateral mechanical strength restricting expansion or earthquake resistance.

[0113] The presence of this piping must not hinder the transport of the entire reactor block, as detailed below for its removal from the reactor pit using heavy handling equipment.

[0114] As previously stated, the primary circuit of the reactor operates solely by natural convection, i.e. without a pumping unit.

[0115] Consequently, the inventors were faced with the problem of producing a pressurizer whose cooling and condensation part of the primary fluid vapors cannot be designed with a liquid water spraying / injection device, from a sample from the primary circuit as according to the state of the art (due to the absence of primary pumping units).

[0116] The inventors then thought of modulating the thermal losses by conduction through the dome 6, to control the depressurization of the primary steam of the pressurizer, by taking advantage of the fact that the metal casing 60 of the cover 41 is of low thickness, typically between 10 and 20 mm.

[0117] So, as illustrated in the figure 11 , the steam cooling and condensation part comprises a dome 6 with double walls 60, 61 spaced apart from each other forming a space E inside which liquid water from basin B can circulate from the bottom to the top of the dome forming a central evacuation chimney 62. Typically, space E has a constant height of the order of 1 to 3 cm.

[0118] In normal operation, the level of thermocline 1 is set sufficiently above the pressurizer, in particular so as to be located above the central exhaust chimney 62, as illustrated in figure 11 .

[0119] So, as illustrated in the figure 11 , the liquid water which thus circulates solely by natural convection in the space E delimited by the two walls 60, 61, from a cold temperature below the thermocline 1, will condense the saturated vapor of the primary circuit inside the tank and thus reduce the pressure within the tank. Typically the cold temperature of liquid water entering the space E at the bottom of the dome 6 is around 50°C, which allows efficient and rapid cooling of the dome 6, and in particular of the internal wall 60 forming the enclosure of mechanical resistance to the pressure of the primary circuit, and thereby of the primary water vapor which is underlying it.

[0120] The central chimney 62 incorporates within it a regulating valve 64 or in other words a throttling valve which makes it possible to regulate the flow rate of secondary liquid water which circulates in the space E and therefore to regulate the liquid cooling as such. Indeed, in a position of total closure of the valve 64, the layer of water is trapped and stratified in the space E. Conversely, in an open position, in particular total, of the valve 64, the hot water rises naturally in the space E then through the central chimney 62 and will join the upper layer of hot water of the basin B, while the cold water of the basin B is sucked in by the inlet in the lower position of the double wall 60, 61.

[0121] Valve 64 may be a butterfly valve like the secondary flow valve 5 shown in figures 10A, 10B, 10C .

[0122] The two walls 60, 61 of the dome 6 are metallic, preferably stainless steel.

[0123] The external wall 61 of the dome 6 is advantageously covered with a cap 63 housing a thermal insulator within it.

[0124] According to an advantageous variant, the reactor 4 comprises, as a passive heat sink, a plurality of cooling fins 65 arranged inside the inner wall 60, preferably being distributed uniformly over the surface of the latter, preferably being welded or brazed. These fins 65 thus increase the contact surface with the steam of the primary circuit and therefore make it possible to improve the heat exchange by conduction between said steam and the dome 6. In the example illustrated, these fins 65 are rectilinear and extend over a major part of the height of the dome. These fins 65 are preferably made of the same material as the walls 60, 61 of the dome 6, and typically have a thickness of a few cm and a length of a few tens of cm along the inside of the wall 60.

[0125] Furthermore, the heating part of the pressurizer comprises a plurality of electrical resistors 8 wrapped in an electrical insulator and powered by electrical cables, arranged inside the dome, preferably on the separation plate 7 which in its center comprises a portion with holes 70 making it possible to ensure the thermal insulation and pressure difference functions of the integrated pressurizer. Such a portion with holes 70 is for example as according to the device described in patent application WO 2012 / 158929A3. The electrical resistors 8 may be of the type described in patent US4135552.

[0126] According to the invention, in order to smooth the heat production of the reactor 4, to the point of being able to operate it all year round at 100% of its power, the inventors have provided a thermal coupling between the latter and a thermal storage pit 9 filled with water delimiting a thermocline which will make it possible to manage the intermittency of consumption of a heat network.

[0127] In addition, the inventors planned to position the reactor pit 100 of the reactor 4 below the bottom of an artificially dug thermal storage pit 9, of the PTES type (an acronym for “Pit Thermal Energy Storage”), filled with water. As illustrated in figure 12 , the positioning of the excavated tank well 100 is in the middle of the pit 9 which is symmetrical in shape. Reference may be made to publication [6] which describes the construction of such a pit 9.

[0128] The bottom 90 and the side walls 91 of the pit are coated with a waterproofing coating in the form of a membrane (liner). For example, this membrane may be made of high-density polyethylene (HDPE).

[0129] Pit 90 is covered with a thermally insulating cover 92 to limit heat losses into the atmospheric air.

[0130] Such a pit 9 of the PTES type makes it possible to achieve vertical thermal stratification resulting in the formation of a thermocline delimited between the bottom 90 of the pit at a so-called cold temperature and the top of the pit at a so-called hot temperature. This thermocline is designated as thermocline 2 in figure 12 .

[0131] As an indicative example, we can consider a volume of the order of 800,000m 3< for a WPS of pyramid geometry with a truncated rectangular base with a small base (bottom of the storage) of 100m * 100m, a large base (top of the storage) of 180m * 180m and a water height of 40m. The cold and hot temperatures of the heat network can be respectively equal to 45°C and 90°C.

[0132] The main advantages inherent in such a thermal storage pit 9 can be summarized as follows: rapid thermal charge and discharge dynamics, when filled with water, significant stratification and high storage density (64 kWh / m 3< for example for hot and cold temperatures of 90°C and 35°C respectively). low construction cost when the geological characteristics are met.

[0133] Within pit 9 filled with water, the position of thermocline 2 changes depending on the season and the related atmospheric and ground temperatures. More precisely, the position of thermocline 2 changes rather to the first order depending on the energy voluntarily loaded and discharged from reactor 4 and / or to the heating network 300. Then, to the second order, it naturally degrades due primarily to thermal conduction between the hot layer and the cold layer of thermocline 2, and also due to thermal losses to the outside which can indeed depend on the season and atmospheric temperatures. It is specified here that the ground temperature varies very little and has only a marginal effect.

[0134] Reference may be made to publication [7] which details the advancement of a thermocline in a thermal storage tank. Thus, at the end of the summer period, the layer of cold water within pit 9 is minimal and the thermocline is at its lowest, as symbolized by the dotted line on the figure 12 .

[0135] According to the invention, as shown in the figure 9 , the tank well 100 excavated below the bottom 90 of the thermal storage pit 9 comprises a peripheral wall 101 made of prestressed concrete coated internally with a metal coating, preferably in the form of a stainless steel membrane. Such a coating has the function of serving as a rigid structure for the reactor vessel 40, 41, 42 forming the third containment barrier, and the prestressed concrete wall 101 has the function of meeting the pressure and temperature conditions of the secondary water, and ensuring the physical integrity of the third barrier.

[0136] As also shown in the figure 12 , the assembly consisting of the reactor vessel 40, 41, 42 forming a reactor block 4 to which the exchangers 49 are fixed is supported by a metal base 102, preferably made of stainless steel, or black steel coated with an anti-corrosion deposit. Preferably, this base is held in the bottom of the reactor pit 100 by a mechanical locking system, not shown, adapted to prevent its movement and lifting in the event of an earthquake. However, during fuel handling or component replacement phases, the base 102 and the reactor block 4 that it supports can be lifted and brought to the top of the reactor pit. To do this, the mechanical locking system of the base must be able to be unlocked simply by tools accessible from the top of the reactor pit 100.

[0137] The base 102 which supports the reactor block 4 is rigidly connected to a foundation raft 103, and the metal cladding with which the prestressed concrete wall 101 is coated is rigidly and tightly fixed to the raft 103. As already mentioned, the reactor 4 has a total physical impossibility of occurrence of a serious accident, with significant melting of the core and piercing of the primary circuit vessel. There is therefore no specific device for recovering the corium at the bottom of the reactor pit.

[0138] The tank well 100 is closed by a removable and watertight cover plug 104, adapted to withstand the pressure of the secondary circuit. Preferably, the cover plug 104 is a metal slab, mechanically welded, preferably still honeycomb or formed of boxes.

[0139] A gas ceiling 105, preferably nitrogen, is delimited by the free level of the secondary water volume in the tank well 100. This gas ceiling 105 makes it possible to control the secondary water pressure, to adapt the variations in free level and expansion of secondary water, and to prevent the presence of oxygen. The mechanical connection between the metal coating of the tank well and the cover plug is constituted by a metal seal or a high-temperature elastic seal, in order to maintain a total seal.

[0140] If necessary, as a cooling system for the tank well 100, a pipe, not shown, with water circulating therein, can be embedded in the concrete wall 101, at a distance close to the interior and the metal coating forming the third barrier.

[0141] According to the invention, the tertiary circuit of the reactor is constituted by the pit 9 itself and at least one exchanger 200 with the secondary circuit whose inlet 201 is fluidically connected in a cold temperature zone below the thermocline and whose outlet 202 is fluidically connected in a hot temperature zone above the thermocline.

[0142] And a heat network 300 has at least one inlet 301 which is fluidically connected to a zone of the pit below the thermocline and at least one outlet 302 fluidically connected to a zone of the pit above the thermocline. These tappings make it possible to guarantee stability of the layers of tertiary water at cold and hot temperatures and therefore of the thermocline within the pit 9.

[0143] More specifically, as illustrated in the figure 12 , a pipe 210 is connected to the inlet of the tertiary circuit of the heat exchanger 200 and its end 211 forming a draw-off collector opens directly into the area of ​​the pit below the thermocline.

[0144] A pipe 220 is connected to the outlet of the tertiary circuit of the heat exchanger and its end 221 forming an injection collector opens directly into the pit area above the thermocline.

[0145] Considering the altitude of the draw-off collector 211 in the area below the thermocline, i.e. the lowest cold layer of the water in pit 9, regardless of the season, a minimum driving height of water is guaranteed, necessary for the establishment of natural convection of water in pit 9, i.e. in the tertiary circuit of the installation. The circulation in closed circuit T solely by natural or forced convection of the tertiary water is symbolized by the arrows in dots drawn in figure 12 . It is specified that this circulation T is shown in pit 9 for clarity purposes but that in reality the rising branch of the convection passes through pipe 220 and that the descending branch does not exist as such, since it is a natural piston which advances over the entire section of thermocline 2.

[0146] Furthermore, a pipe 310 is connected to the inlet 301 of the heat network and its end 311 forming an injection collector opens directly into the area of ​​the pit below the thermocline.

[0147] A pipe 320 is connected to the outlet 302 of the heat network and its end 321 forming a draw-off collector opens directly into the pit area above the thermocline.

[0148] For the part of the secondary circuit which is connected to exchanger 200, the installation includes: a pipe 230 opens directly into the secondary water of the tank well 100 and is connected to an inlet 203 of the exchanger 200; a pipe 240 connected to an outlet 204 of the exchanger 200 opens directly into the secondary water of the tank well 100.

[0149] The reactor installation configuration in storage pit 9 as shown in figures 12 And 15allows circulation by natural convection. To do this, as shown, the tank well 100 is installed partly below the bottom 90 of the pool 9, that is to say with a tank well 100 excavated to a height He such that a pump can be dispensed with, the pipe 210 for supplying water from the pit 9 at its cold temperature opening into the bottom 90 of the pit.

[0150] For a reactor with a power equal to 20MW, with a thermal storage pit 9, the excavation height He can be between 16 and 18m.

[0151] We can also consider a configuration of installation of the reactor in pit 9 as shown in figures 16 et 16A according to which the tank well is in some way placed on the bottom 90 of the thermal storage pit 9. Such a configuration involves circulation by forced convection of the tertiary water by means of pumps 270, advantageously arranged in the concrete wall 101 of the tank well 100.

[0152] The inventors carried out thermo-hydraulic pre-sizing calculations on all the primary, secondary and tertiary circuits of the installation according to the figure 12 from thermomechanical calculation software. This can be a classic thermo-hydraulic software known under the name CATHARE:[8].

[0153] Considering a pinch of 15°C for exchangers 49 between primary and secondary circuits and an identical pinch for secondary / tertiary exchanger 200, and a supply of hot water to network 300 at 90°C, the calculations give an average temperature at the outlet of reactor core C of 120°C.

[0154] It is recalled here that an exchanger pinch 49 is the minimum temperature difference between the primary water and the secondary water at a given point of the exchanger.

[0155] With a temperature gradient of 40°C between the inlet and outlet of the reactor core, we obtain the thermo-hydraulic operating parameters allowing us to pre-size the exchangers 49, and an estimate of their positioning with respect to the core.

[0156] Table 1 below summarizes these different parameters for a 20 MWTh reactor. [Table 1] Paramètres Valeurs Pression de l'eau primaire 4 bars Température en entrée du cœur C 80°C Température en sortie du cœur C 120°C Débit de l'eau primaire 118.6 kg / s Secondary water pressure 5 bars Temperature T1 at the inlet of exchangers 49 65°C Temperature T2 at the outlet of exchangers 49 105°C Secondary water flow 119.1 kg / s Water balance driving height between the core and the exchangers 49 3,16 m

[0157] From these data, an estimate of the volume of the 49 exchangers can be established, with an installation at a minimum height of 3.16 m in relation to the average altitude of the fissile zone. The primary water pressure of 4 bars is determined so that the boiling margin with respect to the average temperature at the core outlet is 20°C.

[0158] The reactor vessel 40, 41 of the reactor block 4 has an overall height of approximately 9 m, with an overall diameter of 3 m incorporating the main shell 42 with a diameter of the lower compartment 40 of the vessel of 2.74 m and a diameter of the upper compartment 41 of 2.15 m.

[0159] A number of three identical exchangers 49 is retained, each with a useful heat exchange volume of approximately 1.2 m 3 < and an exchange height between primary and secondary circuits of 2 m. The three exchangers 49 are fixed with their collectors 400, 401, 490, 491 at 120° from each other to the compartments 40, 42 of the reactor vessel.

[0160] In fact, the thickness of the reactor vessel is not dictated by considerations of pressure resistance since it is immersed in the secondary water of basin B, but rather by constraints of mechanical rigidity, buckling resistance and core support. An average thickness of approximately 20 mm is therefore retained. It should be remembered here that the primary pressure is lower than the secondary pressure.

[0161] The material considered for the reactor vessel 4 and the exchangers 49 is stainless steel.

[0162] Computer-aided design (CAD) gives a secondary water volume of approximately 200 m 3< for its cold temperature T1 and approximately 100 m 3< for its hot temperature T2.

[0163] The average temperature of secondary water is around 85°C, which is lower than the boiling temperature of water in the atmosphere.

[0164] The volume of primary water is around 30 m 3 < , i.e. a factor of approximately 10 less than that of the volume of secondary water.

[0165] This therefore results in a very significant thermal inertia which contributes greatly to the operational safety of the reactor block 4, 102, both in terms of absorption of any heating of the primary water, but also with regard to the rise in pressure of the primary water in the event of a failure to evacuate the residual power.

[0166] In addition, the consequences of a Primary Cooling Loss Accident (PCLA) are greatly reduced by the configuration of the secondary water completely surrounding the primary circuit.

[0167] Table 2 below illustrates the characteristics of the thermo-hydraulic dimensioning of secondary and tertiary circuits. The CAD indicates a number of three 200 exchangers, which could be reduced to two, for reasons of economic optimum. [Table 2] Settings Values Temperature T1 at the inlet of exchangers 49 65°C Temperature T2 at the outlet of exchangers 49 105°C Secondary water flow 119.1 kg / s Secondary water pressure 5 bars Temperature T3 at inlet 201 of exchangers 200 35°C Temperature T4 at outlet 202 of exchangers 200 90°C Tertiary water flow rate 86.9 kg / s Natural convection equilibrium driving height S of secondary water 2,7 m

[0168] The operating conditions are therefore substantially identical to those governing the circulation of primary water.

[0169] A number of three identical 200 exchangers is retained, each with a useful heat exchange volume of approximately 1.1 m 3 and an exchange height between secondary and tertiary circuits of 1.7 m.

[0170] The pinch in a 200 exchanger is increased compared to that of a 49 exchanger due to the temperature difference between the cold water layer at 35°C in pit 9, and the hot water layer at 90°C.

[0171] The driving height of natural convection of the secondary water is at least 2.7 m, which sets the maximum altitude position of thermocline 1. A height of approximately 3.5 m can be considered in order to take dimensioning margins concerning the pressure losses in the piping, and to have adjustment margins by modulating the flow rate of the tertiary water.

[0172] The minimum driving height necessary for the establishment of natural convection of the tertiary water corresponds to the difference in altimetry between the median position of an exchanger 200, and the position of the separation thermocline between the cold layer and the hot layer in the pit 9. This thermocline evolves seasonally and at the end of the summer period, before the start of the heat supply to the network 300, the cold water layer is minimal and as is already clear from the above, the conservative value chosen is the altitude of the tertiary water draw-off collector 211. The pre-dimensioning carried out is based on the minimization of the pressure losses allowing the establishment of natural convection at moderate driving height.The minimum calculated height is around 2 m, which allows for a compact installation, despite the cumulative conditions of natural convection of the primary, secondary and tertiary circuits which lead to a relatively deep burial of reactor block 4.

[0173] The level of the invert 103 of the tank pit is approximately 50 m above the ground, when the tank pit 100 is fully excavated, i.e. buried completely below the bottom 91 of pit 9. In the CAD design, the bottom of pit 9 is 32 m above the ground surface. In the case of a deeper pit 9, for example 40 m, the depth of the invert would then be closer to 60 m, due to the need to establish natural convection of the tertiary water.

[0174] Water injections into thermal storage pit 9 and withdrawal to reactor 4 are planned to have constant flow rates throughout the year, corresponding to reactor operation at 100% power.

[0175] The flow rates for the 300 heat network vary depending on the network's power demand. Variations in hydrostatic head in pit 9 during the year may require the addition of a device to regulate the withdrawal flow rate to the reactor to keep it constant.

[0176] The flow differential between the injection / withdrawal of reactor 4 and the back and forth of the heat network 300 in the pipes 310, 320 leads to the movement of the thermocline upwards or towards the bottom of pit 9, as follows: if the flow rate to the heat network 300 is lower than the injection / withdrawal flow rate of the reactor 4, then the thermocline moves towards the bottom 90 and the thermal storage is in charging mode, i.e. the thermal energy stored there increases, if the flow rate to the heat network 300 is higher than the injection / withdrawal flow rate of the reactor 4 then the thermocline moves upwards and the storage is in discharging mode, i.e. the thermal energy stored there decreases, if the flow rate to the heat network 300 is equal to the injection / withdrawal flow rate of the reactor 4, then the thermocline does not move, and the stored thermal energy remains constant.

[0177] To optimize the stability of thermocline formation, the various collectors for withdrawal 211 or injection 221 of tertiary water or withdrawal 321 and injection 311 of the heat network 300 can be produced according to a preferred configuration.

[0178] It may advantageously be a hydraulic flow distribution device adapted to widen the incoming or outgoing water flow, of the type known under the designation of a pavilion as illustrated in figures 13 And 14 for tertiary water injection at hot temperature.

[0179] The invention is not limited to the examples which have just been described; in particular, it is possible to combine characteristics of the illustrated examples within non-illustrated variants.

[0180] Other variants and embodiments may be envisaged without departing from the scope of the invention.

[0181] On the figure 12, for the sake of clarity, a single exchanger 200 has been illustrated. It goes without saying that the number of exchangers 49 between primary and secondary circuits and of exchangers 200 between secondary and tertiary circuits is illustrated for information purposes only and will have to be adapted according to the configurations of the installation required for the supply of hot water to a heat network 300.

[0182] Also, for the sake of simplification and clarity, the exchanger 200 between the secondary and tertiary circuits is shown in figure 12 inside the pit 9. Preferably, the exchanger(s) 200 is (are) arranged outside the secondary water volume, more preferably behind the tank well 100 in the concrete wall 101, as illustrated in Figures 16 and 16A , with a 212 isolation valve on each inlet and outlet pipe, in order to be able to isolate the secondary circuit and complete the sealing of the third radioactive containment barrier.

[0183] Furthermore, if the calogenic reactor 4 described is provided with circulation of primary water and secondary water solely by natural convection, any PWR reactor with water flow rates in the primary and secondary circuits generated by pumps, i.e. by forced convection or by a thermosiphon, may be suitable for coupling by its tertiary circuit in natural convection with a thermal storage pit filled with water as described previously. List of cited references

[0184] [1]: The World Nuclear Industry Status Report 2017. https: / / www.worldnuclearreport.org / IMG / pdf / 20170912wnisr2017-en-lr.pdf. [2]: IAEA-TECDOC-397 «Potential of Low-temperature Nuclear Heat Applications». [3]: IAEA-TECDOC-463 «Small Reactors for Low-temperature Nuclear Heat Applications». [4]: https: / / inis.iaea.org / collection / NCLCollectionStore / _Public / 10 / 494 / 10494922.pdf [5]:https: / / www.ecosmr.fi / wp-content / uploads / 2021 / 06 / Leppanen_EcoSMR_15062021.pdf [6]: « Design Aspects for Large Scale Aquifer and Pit Thermal Energy Storage for District Heating and Cooling». International Energy Agency Technology Collaboration Program on District Heating and Cooling including Combined Heat and Power - Integrated Cost-Effective large-scale thermal energy storage for smart district heating and cooling. Draft - September 2018. [7]: Kaloudis E. & al., «Large eddy simulations of turbulent mixed convection in the charging of a rectangular thermal storage tank».International Journal of Heat and Fluid Flow 44 (2013) 776-791. [8]: G. Geffraye et al. " CATHARE 2 V2.5 2: A single version for various applications" Nuclear Engineering and Design 241 (2011) 4456-4463.

Claims

1. Nuclear installation comprising: - at least one heat-generating nuclear reactor (2) comprising a reactor vessel (40, 41, 42); - a reactor pit (100) housing the reactor vessel, a primary circuit and part of a secondary circuit, and; - a tertiary circuit; characterized in that the tertiary circuit comprises: • a thermal storage pit (9), dug into the ground, filled with water and in the bottom of which the reactor pit is excavated or placed, the thermal storage pit being configured to produce vertical thermal stratification leading to the formation of a thermocline (thermocline 2) that is delimited between the bottom of the thermal storage pit at what is referred to as a cold temperature and the top of the thermal storage pit at what is referred to as a hot temperature, • at least one heat exchanger (200) with the secondary circuit, at least one inlet (201) of which is fluidically connected to a zone of the thermal storage pit below the thermocline and at least one outlet (202) of which is fluidically connected to a zone of the thermal storage pit above the thermocline such that the water in the thermal storage pit circulates by natural or forced convection between the inlet and the outlet of the heat exchanger; - a heat network (300), at least one inlet (301) of which is fluidically connected to a zone of the thermal storage pit below the thermocline and at least one outlet (302) of which is fluidically connected to a zone of the thermal storage pit above the thermocline.

2. Nuclear installation according to Claim 1, the thermal storage pit being covered by a thermally insulating cover (92).

3. Nuclear installation according to Claim 1 or 2, the thermal storage pit being artificially dug into the ground and comprising a sealing coating, which is preferably thermally insulating, covering its bottom (90) and its lateral walls (91).

4. Nuclear installation according to Claim 3, the coating being a membrane adapted to conform to the shape of the bottom and of the lateral walls of the thermal storage pit.

5. Nuclear installation according to one of Claims 2 to 4, the thermally insulating cover or the sealing coating comprising one or more pieces of thermal insulation made of material(s) selected from among stainless steel, a polymer such as high-density polyethylene (HDPE), or an elastomer.

6. Nuclear installation according to one of the preceding claims, comprising: - at least one piping system (210) connected to the inlet of the tertiary circuit of the heat exchanger, the end of which forming a withdrawal manifold (211) opens out directly in the zone of the thermal storage pit below the thermocline, - at least one piping system (220) connected to the outlet of the tertiary circuit of the heat exchanger, the end of which forming an injection manifold (221) opens out directly in the zone of the thermal storage pit above the thermocline.

7. Nuclear installation according to one of the preceding claims, comprising: - at least one piping system (310) connected to the inlet of the heat network, the end of which forming an injection manifold (311) opens out directly in the zone of the thermal storage pit below the thermocline, - at least one piping system (320) connected to the outlet of the heat network, the end of which forming a withdrawal manifold (321) opens out directly in the zone of the thermal storage pit above the thermocline.

8. Nuclear installation according to Claim 5 or 6, each piping system manifold comprising a hydraulic flow distribution device adapted to increase the flow of water entering or exiting the zones of the thermal storage pit below or above the thermocline.

9. Nuclear installation according to one of the preceding claims, comprising: - a piping system (230) directly connecting the secondary circuit part housed in the reactor pit to an inlet (203) of the exchanger with the tertiary circuit; - a piping system (240) directly connecting the secondary circuit part housed in the reactor pit to an outlet (204) of the exchanger with the tertiary circuit.

10. Nuclear installation according to one of the preceding claims, the excavated or placed reactor pit being closed by a removable metallic cover (104) forming the separation wall with the volume of the thermal storage pit.

11. Nuclear installation according to one of the preceding claims, the excavated or placed reactor pit comprising a concrete wall (101) forming the separation wall with the ground and the volume of the thermal storage pit.

12. Nuclear installation according to one of the preceding claims, the height He of the excavated reactor pit below the bottom of the thermal storage pit being at least 15 m.

13. Nuclear installation according to one of the preceding claims, the reactor being a heat-generating reactor of the SMR type.

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