Method for refurbishing a nuclear power plant initially comprising at least one light-water nuclear reactor (LWR), in particular a pressurised water reactor (PWR) or a boiling water reactor (BWR), with at least one integrated modular nuclear reactor (SMR)

Abstract

The invention consists essentially in a method for retrofitting a nuclear power plant which consists in dismantling and removing all the components of the primary circuit apart from the LWR reactor vessel, which is essentially emptied of all material and neutralised, subsequently replacing a part of these components with subassemblies that are each made up of an integrated SMR reactor and a mixed concrete / metal structure, which mixed concrete / metal structure is also used as a reactor pit for the SMR reactor, which reactor pit is advantageously filled with water, anchoring the SMR to the inside of the reactor building and advantageously contributing to the third confinement barrier while ensuring minimal disruption to the infrastructure of the reactor building.

Description

[0001] Description

[0002] Title: Method for renovating a nuclear power plant initially comprising at least one light water nuclear reactor (LWR), in particular pressurized water reactor (PWR) or boiling water reactor (BWR), replaced by at least one integrated modular nuclear reactor (SMR).

[0003] Technical field

[0004] The present invention relates to the field of nuclear power plants, in particular the fleet of nuclear power plants comprising light water nuclear reactors (LWRs), in particular pressurized water and boiling water reactors.

[0005] The invention thus aims to overcome a major drawback in terms of cost and capacity to renew the fleet of REL nuclear power plants.

[0006] Although described with reference to a nuclear power plant comprising at least one pressurized water nuclear reactor, the invention applies to any nuclear power plant with a boiling water nuclear reactor, or more generally to any nuclear power plant with a light water nuclear reactor (LWR).

[0007] Prior art

[0008] A significant portion of the current fleet of pressurized water reactor (PWR) nuclear power plants is soon reaching 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 electricity needs (non-intermittent, high availability and competitive electricity).

[0009] Figure 1, taken from the publication [1], traces the chronology of the commissioning of reactors on a global scale. Most of the 440 reactors that make up the global fleet were commissioned in the decades 1970 to 1990.

[0010] With a planned operating life of 40 to 60 years, depending on the country and the rules governing authorization or extension of operation, all these reactors will be shut down at the latest between the 2030s and 2050s. Countries with nuclear power plants for their electricity supply and wishing to maintain these plants will therefore have to face a significant investment.

[0011] PWR reactors represent more than 60% of the 440 reactors in the world's nuclear fleet. A pressurized water nuclear reactor (PWR) comprises three cycles (fluid circuits) whose general principle of normal operation is as follows.

[0012] 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.

[0013] 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.

[0014] The water from the secondary circuit is then condensed via a condenser using a third cycle, the cooling cycle, as a cold source.

[0015] The design principles of PWR reactors according to these three cycles have been essentially the same since the first ones were put into service.

[0016] The main elements of a REP primary circuit are shown in Figures 2A to 2C:

[0017] - a reactor 1 building providing various functions, including a contribution to the containment safety function,

[0018] - a reactor vessel 20, located in the center of building 1, housing the reactor core C,

[0019] - a primary circuit 2 in pressurized water comprising tank 20.

[0020] These main elements are therefore common, their composition and the number of components varying according to the power of the reactor.

[0021] Typically, the envelope of the reactor building 1 can be made of several layers.

[0022] Thus, depending on the configurations, a reactor 1 building can be made up of:

[0023] - a prestressed concrete wall 10, as an interface with the exterior, the interior of which is coated with a metal skin 11, with a sealing function for confinement, for a 900 MWe reactor (figure 2A); - an outer wall of reinforced concrete 12, and an inner wall of prestressed concrete 10 separated from the outer wall 12 by an annular space 13 devoid of material, for a 1300 / 1450 MWe reactor (figure 2B);

[0024] - 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 (figure 2C).

[0025] As illustrated in Figure 3, from publication [2], the primary circuit 2 consists of the following main components:

[0026] - a reactor vessel 20,

[0027] - primary loops 21 each comprising a primary pump 22 and a steam generator 23,

[0028] - a single 24 pressurizer.

[0029] In addition, this figure 3 shows the control rod mechanisms of the reactor core and control clusters 25.

[0030] Depending on the reactor power, the number of loops can be three for a 900 MWe reactor (figure 3) or 4 for a 1300 MWe reactor and above.

[0031] The reactor 1 building is therefore sized, among other things, to house all the components of primary circuit 2.

[0032] Figure 4 illustrates the energy transfer cycle (heat then electricity) of a PWR reactor. In this figure 4, we can see in particular the distribution of the positioning of the components in relation to the reactor building 1, which acts as the third containment barrier.

[0033] 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.

[0034] 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.

[0035] To date, all existing pressurized water reactor (PWR) technologies are based on the principle of a plant whose operating life is based on the operating life of the non-replaceable component(s) which has / have the shortest operating life.

[0036] These are mainly the components of the primary circuit and critically the reactor vessel which are the components that dictate the operating life of the plant, due to the consequences of the activation of materials, and the aging of some.

[0037] It is therefore mainly the age of the reactor vessel which will dictate the operating life of the plant as a whole and which initially led to the planning of operating existing plants for a period of 40 to 60 years, depending on the country and in particular the safety reassessments.

[0038] The reactor's other structural elements also age. Among these, two classes are distinguished: replaceable elements and those that cannot be replaced during the operating period.

[0039] Among the replaceables are the steam generators, the primary pumps, and the pressurizer.

[0040] Among the non-replaceable elements, beyond the primary circuit mentioned above, there is in particular the civil engineering whose aging must be analyzed according to the safety requirements assigned to it. For a PWR reactor with primary circuit lines that operate in pressurized water and are arranged in an aerial manner, an accident on the primary circuit requires a specific dimensioning for the reactor building which must ensure the safety function of confining nuclear materials. We can notably cite the Loss of Primary Coolant Accident (LOCA) studied in the safety reports of Pressurized Water Reactors (PWR) which is a hypothetical accident caused by a breach in the primary circuit envelope. There is therefore a direct link between the operating life of the concrete structure of the building and the safety functions assigned to it.

[0041] A currently emerging technology is that of small power reactors, known by the English acronym SMR (Small Modular Reactor). These SMR reactors have the primary advantages over existing PWRs: they allow for simplification of systems, mainly for safety reasons, and an increased modularity capacity through significant factory manufacturing of components for transport to the construction site.

[0042] In addition, SMRs are flexible due to their low power level and their territorial insertion capacity.

[0043] They thus appear to be a competitive solution for the future. To date, around 70 SMR projects have been identified worldwide at varying stages, a quarter of which use mature, generation 3 (Gen-III) technology lines, like those in the French fleet.

[0044] Among the SMRs currently under development, some propose a configuration based on the integration of the steam generator, or even all the components of the primary circuit, including the pressurizer and the primary pumps, inside the reactor vessel. These SMRs are called integrated SMRs. In addition to the gain in compactness, integrated SMRs have the advantage of no longer requiring overhead fluid lines carrying pressurized water, which considerably reduces the risks of accidents and associated consequences linked to the rupture of the primary circuit lines.

[0045] For example, the nuclear power plant project, acronym NUWARD™, is a plant consisting of two integrated SMRs, with a unit power of 170MWe, with all the components of the primary circuit inside the reactor vessel.

[0046] Other integrated SMR projects are under development or have been studied, including the SCOR project with a capacity of 150 to 200 MWe on behalf of the Applicant or the ACP100 project with a capacity of 100 MWe.

[0047] The gain in compactness of integrated type SMRs complicates operations in operation, compared to those carried out in a conventional REP.

[0048] Indeed, the main operability and maintainability operations structuring the architecture for a primary reactor circuit are the following:

[0049] - fuel loading / unloading operations which require, under suitable radiation protection conditions, access to the inside of the reactor vessel,

[0050] - maintenance operations on equipment that require accessibility to the equipment. If we refer to Figure 3, we see that the loops of a primary circuit 2 of a usual 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 cover of the tank 20 without impacting the primary loops 21.

[0051] 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.

[0052] Depending on the integrated SMR concepts, accessibility to certain components differs depending on their configuration and due to the positioning of the components and their functional assembly. For example, depending on some SMR reactor concepts, fuel loading operations may require the removal of certain components from the primary circuit.

[0053] Similarly, depending on the integrated SMR concepts, the positioning of the inlet / outlet taps for the steam and feedwater make-up lines may vary between the lower compartment, which is fixed, and the upper compartment, which is removable, of an SMR reactor. In the case of a tap arrangement on the removable upper compartment, fuel handling operations require the steam and feedwater make-up lines at the steam generator inlet to be disconnected beforehand.

[0054] These configuration differences according to the concepts are mainly linked to:

[0055] - technological choices on internal components, in particular the type of exchanger, pressurizer, pumps, etc.,

[0056] - to principles of arrangement and reassembly of internal architecture in the tank (position and type of steam generators), particularly reassembly of so-called critical paths. For example, for the SCOR project, the steam generators are on the vertical critical path.

[0057] In summary, the main structural design criteria for an integrated SMR reactor with a view to its architectural integration within a reactor building are:

[0058] - the vertical and / or axial accessibility requirements for handling fuel and maintaining components, - the methods of dismantling / reassembling the upper functional parts positioned in the removable compartment of the SMR in order to access the fuels,

[0059] - the positioning of the connections for the steam and / or feed water fluid connections in the removable compartment.

[0060] At the end of the scheduled operating life of pressurized water reactor (PWR) nuclear power plants, a process of dismantling them must take place.

[0061] In France, to date, no PWR reactor power plant has been dismantled.

[0062] Worldwide, the number of decommissioned PWR reactors is extremely limited.

[0063] However, the first power plant was shut down in 2021 in France and is preparing to begin its dismantling process: the Fessenheim power plant. The EDF company, the nuclear operator of this power plant, has drawn up a dismantling plan: [2]. You can refer in particular to the page of this plan for the chronology of the different stages envisaged, before, for and after dismantling.

[0064] Before the actual dismantling, operations to shut down processes and put the plant in order will have to be carried out. These dismantling preparation operations aim to:

[0065] - reduce the risks and disadvantages present on the installation: evacuation of spent and new fuel, waste and effluents, emptying of circuits, decontamination of certain circuits. At this stage, 99.9% of the radioactivity is evacuated,

[0066] - prepare the plant for dismantling operations: organization of access and circulation areas, adaptation of support functions, in particular ventilation, electrical distribution and handling, evacuation of certain equipment to free up space;

[0067] - refine knowledge of the state of the installation: inventory of hazardous materials, asbestos identification, samples for radiological analyses.

[0068] Following dismantling, the intended end state is a non-nuclear site, in which all buildings are demolished to a depth of one meter below ground level.

[0069] Figures 5A to 5D show the four successive stages of the dismantling process as envisaged in plan [3] and illustrated on page 5 of this plan. This involves carrying out electromechanical dismantling, which consists of removing and cutting out all the equipment / components present in particular in the reactor building 1, in particular those of the primary loops 21 (reactor vessel 20, pumps 22, steam generators 23, etc.) and packaging them as waste, which will be recovered where possible (Figure 5A). Only the equipment necessary for the clean-up work according to step 2 is left on site.

[0070] Step 2: The remediation of nuclear building structures involves eliminating any radioactive contamination deposited inside the buildings, particularly on the inner wall of reactor building 1 and infrastructure 4 within it (Figure 5B).

[0071] Step 3: The demolition of the buildings, including the reactor building 1 and the engine room 5, is carried out. For conventional buildings, demolition can take place as soon as they are no longer needed for dismantling. For nuclear buildings, it can only begin once the structures have been cleaned up according to step 2. The cavities below ground level are filled with backfill, made up of rubble from the demolition (Figure 5C). The rehabilitation of the site takes place. This consists of ensuring compatibility between the state of the soil and the future use. Any areas, including the buried part 40 of the initial infrastructure 4, which have chemical or radiological markings, are subject to a soil management plan (Figure 5D).

[0072] Most of the nuclear power plant fleet will reach the end of its service life within the next two decades, in a context of increasing demand for electricity due to the electrification of a significant number of energy uses resulting from the decarbonization of our energy.

[0073] Nuclear power plants are or will therefore be shut down while a significant part of the initially planned investment has not reached its operating life, leaving nuclear operators facing a significant investment to ensure the renewal of all or part of the nuclear power plant fleet.

[0074] The authors of the publication [5] are interested in the relevance of a power plant concept based on the technology of Pb-Bi RNR type reactors in the form of reactor modules.

[0075] The authors of the publication [6] discuss the theoretical potential of inserting an RNR Pb-Bi SVBR 75 / 100 reactor block into a nuclear power plant containing light water reactors whose reactors have reached the end of their life. The publication [7] discusses the renovation of VVER type power plants with RNR Pb-Bi SVBR 75 / 100 unit blocks. The authors briefly and exclusively discuss the economic aspects to be taken into account in carrying out such a renovation, which includes a spatial insertion of the reactor blocks into the original reactor building.

[0076] There is therefore a need to find a solution that can reduce the investment in nuclear power plants with light water reactors (LWRs), particularly pressurized reactors (PWRs) or boiling water reactors (BWRs), which is linked to their shutdown, particularly their dismantling as currently planned.

[0077] The aim of the invention is therefore to meet this need at least in part.

[0078] Statement of the invention

[0079] To this end, the invention relates, in one of its aspects, to a method of "retrofitting", i.e. renovating, a nuclear power plant initially comprising at least one light water nuclear reactor (LWR), in particular a pressurized water reactor (PWR) or boiling water reactor (BWR).

[0080] In a configuration of a pressurized water reactor (PWR) comprising a reactor building housing a reactor vessel, a primary circuit and a reactor pool, a fuel building, a nuclear fuel handling line for bringing nuclear fuel assemblies from the fuel building to the reactor building inside the vessel and vice versa, a machine room, a control room and a nuclear auxiliaries building; the method comprises the following steps, for each reactor: a / shutdown of the reactor including the evacuation, outside the reactor building, of all the fuel assemblies present in the reactor vessel and the complete emptying of the primary circuit;b / partial electromechanical dismantling of the reactor comprising the removal and evacuation, outside the reactor building, of the components of the primary circuit with the exception of the reactor vessel left in its location in the reactor building, the removal of all material from inside the nuclear vessel followed by the neutralization of the latter; c / installation, in place of part of the components of the primary circuit evacuated during step a / , of at least one mixed structure, removably closed on itself, consisting of a double metal skin and concrete poured into the space between the two metal walls constituting the double skin; d / installation and maintenance inside each mixed structure, installed according to step c / , of at least one nuclear reactor, called an integrated modular reactor (SMR); the integrated SMR reactor(s) being arranged in a position of accessibility by the fuel handling chain.;

[0081] In the context of the invention, the term "nuclear island" means the usual meaning of the technology, namely an assembly including the nuclear boiler and the fuel-related installations, as well as the equipment necessary for the operation and safety of this assembly.

[0082] The term "reactor building" has the usual meaning, namely a building which contains the reactor itself and all the components of the primary pressurized circuit as well as part of the circuits ensuring the operation and safety of the reactor.

[0083] The term "fuel building" means the usual meaning, namely a building in which the storage facilities (fuel assembly storage pools) and handling of new fuel (awaiting loading into the reactor) and spent fuel (awaiting transfer to a reprocessing plant) are located.

[0084] The term "nuclear auxiliary building" is used in the usual sense, namely a building which houses the auxiliary circuits necessary for the normal operation of the reactor.

[0085] By "conventional island" we mean the usual meaning, namely a set which brings together all the equipment which makes it possible to transform the heat released by nuclear fission into an electrical circuit, then to cool the circuits.

[0086] By "engine room" we mean the usual meaning, namely a building which houses the turbo-alternator group, whose role is to transform the steam produced in the nuclear island into electricity, and its auxiliaries.

[0087] By "neutralization of the reactor vessel" is meant the fact of closing in a watertight and radio-protected manner in order to make the reactor vessel, left in place in its original reactor pit, permanently unusable, without any combustible material inside and filled with an inert fluid ensuring its maintenance in this state.

[0088] According to an advantageous embodiment, the method comprises, after step d / , a step e / of fluidic and / or electrical connections of each reactor to the control room and to the machine room, of installation of the auxiliary circuits and of fluidic and / or electrical connection to the nuclear auxiliary building.

[0089] According to an advantageous embodiment, the installation according to step c / and the setting up according to step d / comprise the passage respectively of each mixed structure in the form of prefabricated modules and of each integrated SMR reactor, through the same access airlock to the outside from the evacuation reactor building through which each of the components in their entirety is evacuated according to step b / .

[0090] According to an advantageous embodiment variant, the removal and evacuation according to step b / comprise the following successive sub-steps: bl / removal of the primary lines arranged between steam generators and the reactor vessel; b2 / removal and evacuation of the steam generators; b3 / removal and evacuation of the primary pumps; b4 / removal and evacuation of the pressurizer; b5 / removal of the primary lines initially at the outlet of the steam generators until they pass through the wall of the reactor building.

[0091] According to another advantageous embodiment variant, the neutralization of the reactor vessel according to step b / comprises the following successive sub-steps: b6 / watertight sealing of the hydraulic connections of the vessel; b7 / closing of the vessel by reassembling its cover, with, where appropriate, installation of a radiation protection blanket; b8 / filling of the reactor vessel with water or inert gas by a connection and level or pressure monitoring device.

[0092] Preferably, step b6 / consists of placing a solid plug in each hydraulic connection and then sealing the plug, the welds preferably being checked by gammagraphy. According to another advantageous embodiment, step b / comprises, after neutralization of the reactor vessel, a step of cleaning up the reactor building to eliminate any radioactive contamination deposited inside said building.

[0093] According to another advantageous embodiment variant, step c / comprises the cutting and removal of the parts of the walls and / or floors and, where appropriate, the base of the infrastructure of the reactor building which initially support the components of the primary circuit.

[0094] According to another advantageous embodiment variant, step c / comprises the fixing of each mixed structure, preferably by means of a fixing plate itself integral or fixed to one and / or the other of the metal walls of the double skin, to the base of the infrastructure of the reactor building.

[0095] According to another advantageous embodiment variant, step c / comprises, once the positioning and, where appropriate, the fixing to the raft of the mixed structure(s) have been carried out, the following successive sub-steps:

[0096] - the cutting and removal of the part of the veil separating the reactor vessel well REL, forming part of the reactor pool, from each mixed structure;

[0097] - the installation of a horizontal connecting pipe between each mixed structure and the tank well.

[0098] According to an alternative embodiment, the method comprises, once the integrated SMR reactor has been installed and maintained according to step d / , the installation of at least one isolation valve on the pipeline, preferably two isolation valves, one on the mixed structure side and the other on the vessel well side.

[0099] The invention also relates to a nuclear power plant obtained according to the retrofit method described above, comprising:

[0100] - a reactor building housing a neutralized REL reactor vessel and a reactor pool;

[0101] - a nuclear fuel handling chain for bringing nuclear fuel assemblies from the fuel building to the reactor building inside the vessel and vice versa; - at least one, preferably three or four mixed structures, arranged around the neutralized reactor vessel, each mixed structure housing an integrated SMR reactor in a fuel building, each integrated SMR reactor being arranged in a position accessible by the fuel handling chain. The final number of mixed structures each housing an integrated SMR will depend in particular on the power adaptation desired for the plant being retrofitted.

[0102] According to an advantageous embodiment, the power plant further comprises a horizontal connecting pipe between each mixed structure and the reactor pit and at least one isolation valve on the pipe, preferably two isolation valves, one on the mixed structure side and the other on the reactor pit side; the fuel handling chain comprising at least one device for tilting fuel assemblies from horizontal to vertical individually to allow their transfer via the connecting pipe.

[0103] According to an advantageous construction variant, each mixed structure comprises a bottom configured to support an integrated SMR reactor.

[0104] Advantageously, each mixed structure can be filled at least partly with water.

[0105] Advantageously, each mixed structure is configured to contain the fixed compartment of the SMR reactor and the removable compartment of the latter when it is removed from the fixed compartment.

[0106] According to an advantageous variant, each mixed structure is equipped with a removable cover contributing to the safety function of controlling the containment of nuclear materials.

[0107] Thus, the invention essentially consists of a method for retrofitting a nuclear power plant which consists of removing and removing all the components of the primary circuit with the exception of the REL reactor vessel, which is emptied of all material and neutralized, then replacing some of these components with subassemblies each consisting of an integrated SMR reactor and a mixed concrete / metal structure which acts as both a reactor vessel for the SMR reactor, advantageously filled with water, anchoring the SMR inside the reactor building and advantageously contributing to the third containment barrier, while minimally modifying the infrastructure of the reactor building. A mixed structure according to the invention acts as a kind of reactor vessel for an integrated SMR reactor, and therefore performs the following functions:

[0108] - a solidity function through its anchors with the existing civil engineering infrastructure (base, walls and intermediate floors) of the REP reactor, in order to ensure compliance with earthquake resistance requirements;

[0109] - a sealing function ensured by its double metal skin which allows respectively:

[0110] • to fill with water and ensure the biological protection function,

[0111] • to provide crossings to the main pool above the existing PWR reactor vessel shaft and to connect to the existing fuel handling chain,

[0112] • to position the entirety of an integrated SMR reactor within a uniform volume of water which is defined by the internal volume of the mixed structure, contributing to the safety function of evacuating residual power,

[0113] - a contribution to the nuclear material containment safety function: with a closure by a removable cover on top of the mixed structure, the integrated SMR reactor housed and maintained there is found in an enclosure which takes up all or part of the requirements linked to the nuclear material containment guarantee safety function (third barrier);

[0114] - advantageously, a constructability function because a mixed structure can be made from prefabricated modules, which allows:

[0115] • modularity which guarantees insertion by modules into the reactor building like an integrated SMR reactor, allows the anchoring and connection points to be adapted with the existing civil engineering to ensure load transfer and allows adaptation to all PWR reactor configurations;

[0116] • a welded assembly which guarantees great flexibility in assembly conditions and a small footprint and sealing to contribute to the containment function;

[0117] • an absence of formwork as such and formwork support allowing for best integration into the existing infrastructure, reduces the impact of the retrofit according to the invention to the most exacting need and optimizes the duration of the work. In fact, the process according to the invention is in some way a break with all the dismantling processes envisaged.

[0118] In substance, compared to a plan for dismantling a REP as envisaged in [3], the invention is distinguished by the fact that:

[0119] - no building of the nuclear island (reactor building, fuel building, auxiliary building) or of the conventional island (engine room) is subject to deconstruction,

[0120] - the reactor vessel is not evacuated from the reactor building;

[0121] - no rehabilitation of the site itself has to be carried out.

[0122] In other words, the inventor has overcome a universally held prejudice among experts in the nuclear field, according to which the complete dismantling of a nuclear power plant must be carried out until the destruction of all buildings and the rehabilitation of the site, while the technical reality is that only a few non-replaceable components of the primary circuit of a reactor have reached their regulatory operating life.

[0123] And, even if this is accompanied by a reduction in power, the retrofit process according to the invention makes it possible to give a second phase of operation to a nuclear power plant with pressurized water reactors of the REP 900 / 1300 MWe type by replacing the REP reactor with its three or four steam generators with integrated SMR reactors.

[0124] Typically, an integrated SMR reactor of the S COR 200 model can be designed to deliver a power of 200 MWe. Also, replacing a 900 MWe PWR reactor with three SCOR type SMR reactors would lead to a plant whose power in the second phase of operation would be equal to 3x200 / 900 = 67% of its initial power, or a 33% power reduction. For a 1300 MWe PWR reactor, the power reduction would be 38%. Another evaluation of the power with an integrated SMR reactor planned in the NUWARD™ project would give an equivalent order of magnitude.

[0125] Ultimately, the retrofit process according to the invention has many advantages, including:

[0126] - the reduction of the initial investment of a nuclear power plant by the reuse of a significant part of the equipment, of almost the entire conventional island and of a part of the nuclear island including in particular the civil engineering. In addition to the partial dismantling according to the invention, only a redevelopment of the machine room is necessary to adapt the sizing of the equipment of the energy conversion cycle to the reduction in power linked to the retrofit;

[0127] - the absence of a new nuclear site to be found, which implies a significant reduction in the environmental and land impact, with the continuity of the local, economic and social environment around the various existing nuclear sites whose power plants would be transformed by the retrofit process. Furthermore, from a societal point of view, since the acceptability of existing nuclear sites can be considered as acquired, it is likely that the same will apply to the retrofitting of these sites;

[0128] - the significant reduction in construction times: the transformation of a nuclear power plant using the invention is carried out using an optimized operating method, with a large number of operations prepared in advance in the workshop, which makes it possible to parallelize at least part of the timescales, such as the production of mixed structures in prefabricated modules;

[0129] - significant reduction in waste: by reusing as many of the plant's buildings / equipment as possible for a second phase of operation, the quantity of waste generated, including very low-level nuclear waste, is significantly reduced;

[0130] - modularity in the share of nuclear power in the energy mix: with the reduction in power and the modularity over time provided by the number of reactors to be transformed according to the invention and their positioning on the territory, particularly for the French fleet, this makes it possible to plan in a temporal dynamic the desired share of nuclear power in the energy mix;

[0131] - an improvement in the reputation of nuclear power because with the retrofit according to the invention, this form of energy becomes sustainable: circular economy of materials, obsolescence, etc.

[0132] - a reduction in the carbon footprint of nuclear energy, the largest part of which is linked to the construction of the installations. By increasing the operating life of an installation (nuclear power plant), the carbon footprint is reduced in relation to the MWhe it actually produces.

[0133] 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.

[0134] Brief description of the drawings [Fig 1] Figure 1 illustrates in the form of a histogram the temporal evolution of the number of commissioning and shutdowns of nuclear reactors in the world, according to the publication [1].

[0135] [Fig 2A], [Fig 2B], [Fig 2C] Figures 2A, 2B, 2C are schematic perspective and partial sectional views of an existing PWR nuclear reactor in different configurations.

[0136] [Fig 3] Figure 3 is a schematic view of a state-of-the-art PWR nuclear reactor primary circuit in a three-primary loop configuration.

[0137] [Fig 4] Figure 4 is a schematic view of the three cycles of a state-of-the-art PWR nuclear reactor.

[0138] [Fig 5A], [Fig 5B], [Fig 5C], [Fig 5D] Figures 5A to 5D illustrate the different stages of the dismantling plan for a PWR nuclear reactor as provided for in the publication [2].

[0139] [Fig 6], [Fig 6A], [Fig 6B] Figures 6, 6A, 6B are schematic views respectively in perspective and in transparency, in perspective and in section, and from above of a mixed structure in accordance with the invention, in which an integrated SMR reactor is housed with the example for illustration of the SCOR integrated SMR model.

[0140] [Fig 7] Figure 7 is a schematic top view illustrating a mixed structure in accordance with the invention, with an integrated SMR reactor according to the example of the SCOR model for illustration, the upper removable compartment of which is removed from its fixed compartment, the two compartments being housed next to each other in the mixed structure.

[0141] [Fig 8] Figure 8 is a perspective and transparent view illustrating a variant of a mixed structure according to the invention, which comprises a removable upper closing cover closing said structure above the integrated SMR reactor.

[0142] [Fig 9] Figure 9 is a perspective and cross-sectional view showing the interior of the metal part of a mixed structure according to the invention.

[0143] [Fig 10] Figure 10 is a perspective and cross-sectional view showing the interior of the metal part of a variant of a mixed structure according to the invention, consisting of prefabricated modules.

[0144] [Fig 11] Figure 11 is a view of a steam generator as installed in the REP 900 1300MWe type power plants of the pressurized water reactor (PWR) nuclear power plant fleet. [Fig 12] Figure 12 is a perspective and partial sectional view of another integrated SMR project, called SCOR, in configuration with the removable compartment fixed on top of the fixed compartment.

[0145] [Fig 13] Figure 13 is a schematic view illustrating the physical possibility of integrating three mixed structures according to the invention in place of the pumps and steam generator of an existing PWR nuclear reactor primary circuit.

[0146] [Fig 14A], [Fig 14B], [Fig 14C], [Fig 14D], [Fig 14E] Figures 14 to 14E illustrate the different stages of a retrofit process for a nuclear power plant initially comprising a PWR reactor.

[0147] [Fig 15] Figure 15 is a schematic perspective and sectional view of a nuclear power plant transformed using the retrofit method according to the invention.

[0148] Detailed description

[0149] Throughout the present application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a reactor building of a power plant and an integrated SMR nuclear reactor, as provided in a vertical operating configuration and arranged in the reactor building according to the retrofit method of the invention.

[0150] Figures 1 to 5D have already been detailed in the preamble, so they will not be commented on below.

[0151] For the sake of clarity, the same element according to the invention and according to the state of the art and designated by the same numerical reference in all of figures 1 to 15.

[0152] It is specified that the various figures do not represent all the fluid, electrical and control connections, as well as all the instrumentation that will be necessary for the operation of a nuclear power plant that has been transformed according to a method of the invention. In particular, the fluid lines for steam, with the associated piping, are not mentioned because there is no first-order integration requirement in the architecture for these lines. In particular, the fluid lines for steam and water supply from and to an integrated SMR reactor, which require crossings in the mixed structure, are not represented. Before describing the method for retrofitting a nuclear power plant according to the invention, the essential means implemented are described, as well as the feasibility of integrating these different means within an existing reactor building.

[0153] Figures 6, 6A, and 6B show a mixed structure according to the invention, generally designated by the reference 6, housing within it an integrated SMR reactor generally designated by the reference 7, the whole being intended to be installed in place of a subassembly consisting of a primary pump and a steam generator of an existing PWR reactor primary circuit. In these figures 6, 6A and 6B, the SCOR integrated SMR model was chosen for the representations.

[0154] A mixed structure 6 plays in some way the role of a reactor vessel well of an integrated SMR reactor 7 and therefore has as its main functions the housing and support of such a reactor, the civil engineering functions (anchoring, solidity, sealing, constructability) which are attached to it as well as the advantageous possibility of being able to store underwater the removable compartment 71 of the integrated SMR reactor 7, for the phases of handling the fuel or maintenance of the internals of the fixed compartment 70 of the SMR.

[0155] A mixed structure 6 consists of a double metal skin, i.e. two metal walls 60, 61 spaced apart from each other, the space between these two walls 60, 61 being filled with concrete 62.

[0156] A support floor 63 is arranged substantially horizontally as a bottom inside the inner wall 61 to support the integrated SMR reactor 7.

[0157] The sealed interior volume of the mixed structure 6 is therefore delimited by the internal wall 61 and the bottom 63. It is intended to be filled with water to serve as a biological barrier, and depending on the configurations of the integrated SMR 6, it contributes to the residual power evacuation function of the SMR.

[0158] Furthermore, as shown in Figure 7, the structure 6 is dimensioned so that this interior volume can house underwater next to the fixed compartment 70 of the integrated SMR reactor 7, the removable compartment 71 removed from above the fixed compartment 70. This makes it possible to ensure the handling of the fuel and / or the maintenance operations of the fixed compartment 70 in safety because the two compartments 70, 71 are in a uniform volume of water. The removable compartment 71 is handled by the large component handling equipment used for the insertion of the integrated SMRs 7. As shown in Figure 8, the mixed structure 6 is preferably provided with a metal cover 64 in sealed and removable connection on one and / or the other of the metal walls 60, 61 of the mixed structure.When this cover 64 is in the installed configuration, the structure 6 constitutes in itself a contribution to the safety function of controlling the confinement of nuclear materials, the first being constituted by the metal sheath which envelops the fuel within the integrated SMR reactor 7, the second by the envelope which constitutes the vessel of the integrated SMR reactor 7.

[0159] Furthermore, the mixed structure 6 has a through opening P. As detailed below, this opening P is intended to be connected to a pipe for the transfer of fuel assemblies from and to the interior of the SMR 7 reactor.

[0160] As shown in more detail in Figure 9, the mixed structure 6 firstly comprises a metal anchoring plate 65 which is fixed to the foundation 41 of the reactor building, which makes it possible to anchor the mixed structure 6 to the existing infrastructure 4 of the reactor building 1. In the example illustrated, this anchoring plate 65 is welded to the inner metal wall 60. It is obviously possible to envisage another anchoring plate welded to the outer metal wall 61, in addition to the plate 65 welded to the inner metal wall 60. The connection device with the foundation 41 can be ensured according to different methods specific to civil engineering techniques and dependent on the strength requirements resulting from structural studies, in particular loadings in earthquake conditions.

[0161] In the space 62 internal to the double metal skin, retaining bars between the walls 60, 61 are welded to them to maintain the spacing between them.

[0162] To reinforce the concrete in the space 62, reinforcements 67 are arranged by being welded by metal studs 66, 68 themselves welded to one and / or the other of the metal walls 60, 61.

[0163] Furthermore, although not shown, other plates can be welded to one and / or the other of the metal walls 60, 1, in particular for:

[0164] - serve as support for the bottom 63 intended to serve as support for the integrated SMR reactor 7;

[0165] - support the pipelines and other auxiliaries necessary for the operation of the integrated SMR reactor 7;

[0166] - link the mixed structure 6 to the floors 42 and intermediate walls 43 of the existing infrastructure 4 of the reactor building 1 so as to mechanically reinforce the whole and restore an overall solidity of the structure at least equivalent to that before the process of implementing the mixed structure 6.

[0167] Figure 10 shows an alternative embodiment of a mixed structure 6 made of factory-prefabricated modules M1, M2, M3, M4 which are then assembled on site, i.e. inside the reactor building 1. In this alternative, the structure 6 may include mechanical stiffeners 69 arranged in an upper part of the structure. This alternative is advantageous because, depending on the type of reactor building 1, and its existing inlet airlock initially designed to replace the steam generators 23, the size of each module can be adapted so as to have the largest dimension that can be fitted through the inlet airlock. This further optimizes the time and cost of the retrofit according to the invention. Mixed structures have already been produced for nuclear installations internationally and projects currently being qualified for France: reference may be made to [4].

[0168] Although not imperative, the retrofit method according to the invention, as detailed below, is advantageously implemented when all the components involved in the transformation are handled without or at least minimally impacting the infrastructure 4 of the reactor building 1.

[0169] The inventor analyzed that this implies being able to remove all the components (primary pumps 22, steam generators 23, and pressurizer 24) from the primary circuit of the existing PWR reactor through the airlock provided for this purpose and in the modification phase, to introduce all the largest components (mixed structures 6, integrated SMR reactors 7) of the new primary circuit through the same airlock.

[0170] The possibility of handling the structures 6 before pouring the concrete into them has been demonstrated by the above.

[0171] The inventor therefore also verified beforehand that integrated SMR reactors 7 could also be handled in a single block, i.e. once completely assembled, by the same handling route, i.e. by the entrance airlock of the reactor building.

[0172] Figure 11 relates to an existing steam generator 23 of a PWR reactor. The overall dimensions H1*L1 of such a steam generator 23 allow it to be introduced through an inlet airlock of the reactor building 10 already planned to allow its replacement with dimensions of the order of 22m in height by 5m in width. Figure 12 shows an integrated SMR reactor 7, according to the SCOR project: its overall dimensions H2*L2 are smaller than H1*L1 of a steam generator 23.

[0173] The maximum overall dimensions of less than 22m*5m for this example of integrated SMR reactor projects 7, therefore allow it to be introduced through the entry airlock into reactor building 1 by the steam generator handling chain 23, as originally designed. The handling chain providing for horizontal positioning then tilting to the vertical is also compatible with this example. Similarly, its mass is compatible with the load capacities of the equipment in the handling chain.

[0174] Therefore, the introduction of an integrated SMR reactor in reactor building 1 without impacting its infrastructure 4 is a given.

[0175] The inventor then has to think about the optimal location that the mixed structures 6 with the integrated SMR reactors 7 should have within the reactor building 1.

[0176] To optimize the implementation and costs of the retrofit process, the inventor retained the following integration criteria:

[0177] - limit the impact on infrastructure 4 of the anchoring of integrated SMR reactors 7,

[0178] - reuse existing infrastructure elements as much as possible: functionalities of the different barriers, biological protection, etc.

[0179] - functionally integrate the integrated SMR reactors with optimized connections to the two existing functional chains, namely the one dedicated to fuel handling, and the one dedicated to the evacuation of power to the engine room.

[0180] Based on these criteria, the implementation of integrated SMR reactors was carried out by an analysis of three-dimensional critical paths.

[0181] The inventor came to the conclusion that the optimal positioning was:

[0182] - in altimetry (z), according to an alignment on the fuel handling chain / biological protection,

[0183] - in the (x,y) plane in top view, in place of the steam generators 23 in axial symmetry,

[0184] In addition to these two positioning parameters, the inventor analyzed that, in addition, the operating phase of an integrated SMR reactor required additional space to store its removable compartment 71 which must be removed from its fixed compartment 70, for the purposes of loading / reloading the fuel and / or maintenance of the internal components.

[0185] With a converted nuclear power plant with a number of 3 or 4 integrated SMR reactors, it is preferable to be able to consider as many storage locations for removable compartments 71 as there are reactors.

[0186] By analyzing the spatial configuration of the primary circuit of a PWR, as it exists, the inventor found this optimal location. Indeed, on each primary loop 21, the primary pump 22 is just spatially attached to the steam generator 23 with which it is associated. Thus, if an integrated SMR reactor 7 is installed in place of a steam generator 23, it is possible to reserve the space occupied by the primary pumps 22 by the removable compartments 71. This optimal configuration is shown diagrammatically in Figure 13: it ultimately allows each SMR to be allocated a location dedicated to its removable upper part and makes it possible to consider an operating configuration of the installation requiring the simultaneous opening of all the SMRs.

[0187] We will now describe with reference to figures 14 A to 14E the different stages of the retrofit process according to the invention of an existing PWR nuclear power plant, which takes into account the analyses mentioned above. proceeds to shut down the REP reactor.

[0188] This first step aims to enable the plant to be retrofitted in site configuration.

[0189] All fuel assemblies present in reactor vessel 20 are evacuated outside the reactor building 1. Then the primary circuit 1 is completely drained.

[0190] Safety analyses prior to the completion of the work can indicate whether the fuel assemblies can remain in the fuel building pools for the duration of the work. In this case, the retrofit work (steps b / and c / ) could start without waiting for the fuel assemblies to have a residual power compatible with the rules for transporting nuclear materials and therefore save time on the complete schedule of the operation. Step b / involves a partial electromechanical dismantling of the PWR reactor.The components of the primary circuit 2 are therefore removed and evacuated outside the reactor building 1, preferably according to the following successive sub-steps: bl / removal of the primary lines 21 arranged between the steam generators 23 and the reactor vessel 20; b2 / removal and evacuation of the steam generators 23; b3 / removal and evacuation of the primary pumps 22; b4 / removal and evacuation of the pressurizer 24; b5 / removal of the primary lines 21 initially at the outlet of the steam generators 23 until they pass through the wall of the reactor building.

[0191] Only the reactor vessel 20 is left in its location in the reactor building 1 (Figure 14A). Indeed, keeping the reactor vessel 20 in place does not hinder the retrofit configuration of the facility. Furthermore, the inventor believes that by leaving the vessel in place during the operating phase of the nuclear power plant once retrofitted with the integrated SMR reactors, the activation materials, in particular Co 60 , will have time to decrease.

[0192] On the other hand, all material inside nuclear vessel 20 is removed and then neutralized.

[0193] To do this, the following successive sub-steps are carried out: b6 / watertight sealing of the hydraulic connections of the vessel. This sealing may consist of placing a solid plug in each hydraulic connection and then sealing the plug, the welds preferably being checked by gammagraphy; b7 / closing the vessel by reassembling its cover, after which all the control rod penetrations will have been plugged, with, if necessary, the installation of a radiation protection blanket; b8 / filling the reactor vessel with water or inert gas using a pressure and / or liquid level connection and control device. The filling and level control device will be positioned within the reactor building. It may in particular be connected to the vessel by using one or more cover penetrations to ensure the fluid connection.

[0194] The cover of the tank 20 may, if necessary, undergo modifications, in particular to improve its sealing and / or to enable optimization of the neutralization of the tank.

[0195] After neutralization of reactor vessel 20, if necessary, the interior of reactor building 1 is cleaned to remove any radioactive contamination that may have settled.

[0196] Taking into account radiation protection considerations, steps a / and b / are implemented by human intervention or remotely.

[0197] Step c / : the installation of mixed structures 6 is carried out.

[0198] Prior to this step c / , solidity studies can be carried out, in particular the seismicity of the nuclear island in its overall configuration to define all the connections of the infrastructure 4 of the reactor building 1 with the mixed structures 6, the dimensioning of the mixed structures 6, typically the thickness of the sheets for the walls 60, 61, the density and dimensioning of the studs and connecting rods between the walls 60, 61, the anchoring methods to the raft 41 and the connection with the floors 43 and walls 42 in connection with the mixed structures 6.

[0199] This step c / includes the cutting and removal of the parts of sails 42 and / or floors 43 and where applicable of the base 41 of the infrastructure 4 of the reactor building 1.

[0200] This makes it possible to prepare the footprint of the mixed structure 6 and to plan all the anchoring devices to the infrastructure 4.

[0201] Furthermore, step c / consists of opening the veil 42 towards the existing reactor pool above for connection to the fuel handling chain. The coring technique will be advantageously used for this operation.

[0202] Figure 14C shows:

[0203] - the necessary footprint E for the installation of a mixed structure 6 to be cleared,

[0204] - the circular opening O towards the pool above the reactor vessel 20. All cutting operations can be carried out using concrete sawing devices, already widely used in nuclear conditions. Operations to prepare the existing infrastructure 4 can be carried out.

[0205] Once these cutting operations and the removal of the cut parts of the infrastructure have been carried out, the mixed structures are brought in by prefabricated modules through the entrance airlock of reactor building 1. Typically, modules can be introduced in the form of horizontal sections with a unit height of 5 metres.

[0206] Then, the actual installation of the mixed structures 6 is carried out. This installation is accompanied by anchoring of the latter to the infrastructure 4 of the reactor building 1. In particular, each mixed structure 6 is fixed by means of a fixing plate 65 to the base 41. The prefabricated modules are welded together and anchoring connections are made with the walls 42 and floors 43. In addition, sealing connections are made with the overlying compartment of the reactor vessel 20.

[0207] Once the installation and fixing of each fixing of a mixed structure 6 has been carried out, a horizontal connecting metal pipe 80 is installed between each mixed structure and the reactor pit 20, preferably by welding it in a sealed manner to the two metal walls 60, 61 of the double jacket. On the pool side above the reactor vessel 20, to ensure the sealing of the pipe 80, the latter is welded to the pool liner. This pipe 80 is a transfer pipe in which a fuel assembly can be handled by means of the handling chain.

[0208] Step d / : we proceed to the installation and maintenance inside each mixed structure 6, installed according to step c / , of an integrated SMR nuclear reactor 7.

[0209] As specified above, the integrated SMR reactor 7 is arranged in a position of accessibility by the existing fuel handling chain.

[0210] Each integrated SMR reactor 6, manufactured entirely in the factory, is introduced into the reactor building using the existing handling chain and positioned directly on the bottom 63 of the mixed structure 6 provided for this purpose.

[0211] Finally, isolation valves 81, 82 are installed at the ends of each pipe 80 (figures 14D, 14E). The fluid and / or electrical connections of each reactor are then made.

[0212] Integrated SMR 7 in the control room and engine room.

[0213] The auxiliary circuits are set up and the fluid and / or electrical connections made to the nuclear auxiliary building.

[0214] Figure 15 illustrates the interior architecture of a reactor building 1 of an initially PWR reactor power plant, which was transformed with the retrofit method of the invention with three mixed structures 6 each housing and supporting an integrated SMR reactor 7.

[0215] 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.

[0216] Other variants and embodiments may be envisaged without departing from the scope of the invention.

[0217] If in the example illustrated, the mixed structures are sized to optimize the integration of the integrated SMR reactors 7 and their removable compartments 71 during operating operations, it is also possible to envisage a smaller sizing for the mixed structures, that is to say with a shared location solution for all the removable compartments 71, once removed from their respective fixed compartments 70.

[0218] Within the framework of the invention, it is possible to envisage handling the removable compartment of an integrated SMR reactor, at the bottom of a mixed structure, at least next to and under the same water as the fixed compartment of the SMR.

[0219] In the example illustrated, the transfer means from an integrated SMR reactor 7 to the pool above the reactor vessel 20 is limited to a single pipe 80 so as to be able to isolate by means of the valves 81, 82 the different water volumes (internal volume of the mixed structure 6, pool above the reactor vessel 20). This choice requires carrying out a horizontal transfer of a fuel assembly and therefore providing a vertical / horizontal tilting device since once the fuel assembly has been extracted from the interior of the integrated SMR reactor 7 vertically, it must be introduced horizontally into the pipe 80. This horizontal position can be maintained until the exit from the reactor building 1, because it is in this position that the assembly passes towards the fuel building.A variant may consist of replacing the transfer pipes 80 with a free-surface water channel, possibly equipped with a cofferdam to ensure the isolation of the water volumes. The cofferdam fulfills the function of a valve in the sense that it allows hydraulic isolation between the two compartments that it separates. Such a device makes it possible to do without a horizontal / vertical tilting device.

[0220] The illustrated example of the retrofit process relates to a PWR reactor. Such a process can also serve as a basis for a retrofit process for a REB reactor, subject to adaptations linked to the particular configuration of this type of reactor compared to a PWR, these modifications being accessible to a person skilled in the art of nuclear reactors.

[0221] List of cited references

[0222] [1]: The World Nuclear Industry Status Report 2017. https : / / www . worldnuclearreport.org / IM G / pdf / 20170912 wnisr2017 -en-lr .pdf

[0223] [2]: http: / / www.centrale-energie.fr / spip / IMG / pdf / 20200115 centrale energies final .pdf [3]: https: / / www.edf.fr / sites / default / files / contrib / groupe-edf / producteur- indu s triel / nucleaire / N otes %20d % 27 information / dem fe s senheim p3.pdf

[0224] [4]: https : / / csti-groupe.com / 2019 / 01 / 07 /

[0225] [5]: Zrodnikov et al. « Nuclear power development in market conditions with use of multipurpose modular fast reactors SVBR-75 / 100», Nuclear Engineering and Design, Amsterdam, Vol.235, N°14-16, 1er Août 206, pages 1490-1502.

[0226] [6]: Dragunov Yu G et al.« Project of SVBR-75 / 100 reactor plant with improved safety for nuclear sources of small and medium power », 5 thInternational Conference on Nuclear Option in Countries with Small and Medium Electricity Grids Dubrovnik, 16 mai 2014, pages 1-13, XP09003576. [7]: Zrodnikov AV et al: « Renovation of the “Old” NPP units as an Economically Effective

[0227] Way of Nuclear Power Development », Proceedings of GLOBAL 2005 Tsukuba, 9 Octobre 2005, pages 1-6, XP09003571.

Claims

Demands 1. A method for retrofitting a nuclear power plant initially comprising at least one light water nuclear reactor (LWR), in particular a pressurized water reactor (PWR) or a boiling water reactor (BWR), comprising a reactor building (1) housing a reactor vessel (20), a primary circuit (2) and a reactor pool, a fuel building, a nuclear fuel handling system for bringing nuclear fuel assemblies from the fuel building to the reactor building inside the vessel and vice versa, a turbine hall (5), a control room and a nuclear auxiliary building; the method comprising the following steps, for each reactor: a / shutdown of the reactor comprising the removal, outside the reactor building, of all fuel assemblies present in the reactor vessel (20) and the complete draining of the primary circuit (2);b / partial electromechanical dismantling of the reactor including the removal and evacuation, outside the reactor building, of the components (21, 22, 23, 24) of the primary circuit with the exception of the reactor vessel (20) left in its location in the reactor building, the removal of all material from inside the nuclear vessel followed by the neutralization of the latter; d installation, in place of part of the components of the primary circuit evacuated during step a / , of at least one mixed structure (6), removably closed on itself, consisting of a metallic double skin (60, 61) and concrete poured in the space (62) between the two metallic walls constituting the double skin; d / installation and maintenance inside each mixed structure, installed according to step c / , of at least one nuclear reactor (7), called an integrated modular reactor (SMR);the integrated SMR reactor(s) being arranged in a position accessible by the fuel handling chain.

2. Retrofit method according to claim 1, comprising after step d, a step e of fluid and / or electrical connections of each reactor to the control room and the machine room, installation of auxiliary circuits and fluid and / or electrical connections to the nuclear auxiliary building 3. Retrofit method according to claim 1 or 2, installation according to step c and placement according to step d, comprising the passage of each composite structure respectively in the form of prefabricated modules and each integrated SMR reactor, through the same access airlock to the outside from the evacuation reactor building through which each of the components in their entirety is evacuated according to step b / .

4. Retrofit method according to any one of the preceding claims, the removal and disposal according to step b / comprising the following successive substeps: b1 / removal of the primary lines (21) arranged between steam generators (23) and the reactor vessel (20); b2 / removal and disposal of the steam generators (23); b3 / removal and disposal of the primary pumps (22); b4 / removal and disposal of the pressurizer (24); b5 / removal of the primary lines (21) initially at the outlet of the steam generators up to the point of passing through the reactor building wall.

5. Retrofit method according to any one of the preceding claims, the neutralization of the reactor vessel according to step b / comprising the following successive substeps: b6 / hermetically sealing the hydraulic connections of the vessel; b7 / closing the vessel by reassembling its cover, with the appropriate installation of a radiation protection cover; b8 / filling the reactor vessel with water or inert gas by a connection and level or pressure monitoring device.

6. Retrofit method according to claim 5, step b6 / consisting of placing a solid plug in each hydraulic connection and then sealing the plug, the welds preferably being checked by gammagraphy.

7. Retrofit method one of the preceding claims, step b / comprising, after neutralization of the reactor vessel, a step of decontamination of the reactor building to eliminate any radioactive contamination deposited inside said building.

8. Retrofit method according to any one of the preceding claims, step c / comprising cutting and removing parts of the walls (42) and / or floors (43) and where applicable of the foundation slab (41) of the infrastructure (4) of the reactor building which initially support the primary circuit components.

9. Retrofit method according to any one of the preceding claims, step c / comprising fixing each composite structure, preferably by means of a fixing plate (65) itself attached or fixed to one and / or the other of the metallic walls of the double-skin, to the foundation slab of the reactor building infrastructure.

10. Retrofit method according to any one of the preceding claims, step c / comprising, once the positioning and, where applicable, the fixing to the foundation slab of the composite structure(s) has been carried out, the following successive sub-steps: - cutting and removal of the part of the shell separating the reactor vessel well from the REL reactor, forming part of the reactor pool, from each mixed structure; installation of a horizontal connecting pipe (80) between each mixed structure and the reactor vessel well.

11. Retrofit method according to claim 10, comprising once the installation and maintenance of the integrated SMR reactor according to step d / , the installation of at least one isolation valve (81, 82) on the pipeline, preferably two isolation valves, one on the mixed structure side and the other on the reactor vessel well side.

12. Nuclear power plant obtained according to the retrofit process according to any one of claims 1 to 11, comprising: - a reactor building (1) housing a neutralized reactor vessel (20) REL and a reactor pool; - a nuclear fuel handling chain to bring nuclear fuel assemblies from the fuel building to the reactor building inside the vessel and vice versa; - at least one, preferably three or four, mixed structures (6), arranged around the neutralized reactor vessel, each mixed structure housing an integrated SMR reactor (7), a fuel building, each integrated SMR reactor being arranged in a position of accessibility by the fuel handling chain.

13. Nuclear power plant according to claim 12, further comprising a horizontal connecting pipeline (80) between each composite structure and the reactor vessel well and at least one isolation valve (81, 82) on the pipeline, preferably two isolation valves, one on the composite structure side and the other on the reactor vessel well side; the fuel handling chain comprising at least one device for tilting individual fuel assemblies from horizontal to vertical to allow their transfer through the connecting pipeline.

14. Nuclear power plant according to claim 12 or 13, each mixed structure comprising a bottom (63) configured to support an integrated SMR reactor (7).

15. A nuclear power plant according to any one of claims 12 to 14, each composite structure being at least partially filled with water.

16. A nuclear power plant according to any one of claims 12 to 15, each composite structure being configured to contain the fixed compartment (70) of the SMR reactor and the removable compartment (71) thereof when removed from the fixed compartment.

17. Nuclear power plant according to any one of claims 12 to 16, each mixed structure being provided with a removable cover (64) contributing to the safety function of controlling the containment of nuclear materials.

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