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).

Abstract

Method for 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), replaced by at least one integrated modular nuclear reactor (SMR). 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 LWR reactor vessel, which is emptied of all material and neutralized, then replacing in place of a part of these components with sub-assemblies each consisting of an integrated SMR reactor and a mixed concrete / metal structure which acts both as a reactor vessel for the SMR reactor, advantageously filled with water, as an anchor for the SMR inside the reactor building and advantageously as a contribution to the third containment barrier, while minimally modifying the infrastructure of the reactor building. Figure for abstract: Fig. 14D

Description

Description Title of the invention: A method for renovating a nuclear power plant initially comprising at least one light water reactor (LWR), in particular a pressurized water reactor (PWR) or a boiling water reactor (BWR), replaced by at least one modular nuclear reactor (SMR) integrated. technical field

[0001] The present invention relates to the field of nuclear power plants, in particular the nuclear power plant fleet including light water reactors (LWRs), particularly with pressurized water and boiling water.

[0002] = The invention thus aims to overcome a major drawback of cost and capacity to renew the fleet of nuclear power plants with REL reactors.

[0003] — 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 boiling water nuclear reactor, or more generally any power plant nuclear with light water reactor (LWR). Previous technique

[0004] A significant portion of the current fleet of water-cooled nuclear power plants pressurized water reactor (PWR) is nearing the end of its operating period, for which the reactors were designed and licensed, in a context where the energy transition with Decarbonizing consumption will increase electricity needs (non-in- termittent, with high availability rate and competitive).

[0005] — Figure 1, taken from publication 1, traces the chronology of the introduction of global reactor operation. Most of the 440 reactors that make up The global park was commissioned in the 1970s to 1990s.

[0006] — With a programmed operating life of 40 to 60 years, depending on the country, rules for authorization or extension of operation, all these reactors will be halted no later than between the 2030s and 2050s. Countries with a fleet nuclear power for their electricity supply and wishing to maintain this park will therefore having to cope with a significant investment.

[0007] — PWR reactors represent more than 60% of the 440 reactors in the nuclear fleet global.

[0008] — A pressurized water reactor (PWR) comprises three cycles (circuits fluidics) whose general principle of normal operation is as follows.

[0009] The water under high pressure in a primary circuit absorbs the energy supplied, in the form of heat, by the fission of uranium nuclei, and where applicable plutonium, in the reactor core. Next, this high-pressure, high-temperature water, typically 155 bar and 300 °C, enters a steam generator (SG) and transfers its energy to a secondary circuit, which also uses pressurized water as a heat transfer fluid. This water, in the form of steam at high pressure, typically around 70 bar, is then expanded via an expansion valve, transforming the change in the fluid's enthalpy into mechanical work and then electrical work in the presence of an electric generator. The water from the secondary circuit is then condensed via a condenser using a third cycle, the cooling cycle, as a cold source. The design principles of PWR reactors according to these three cycles have been essentially the same since the beginning of the commissioning of the first ones operated. The main elements of a PWR primary circuit are shown in Figures 2A to 2C+ - a building of reactor 1 performing various functions, including a contribution to the containment safety function, - a reactor vessel 20, located in the center of building 1, housing the reactor core C, - a primary circuit 2 in pressurized water including tank 20. These main elements are therefore common, their composition and the number of components varying according to the power of the reactor. Typically, the building envelope of reactor 1 can consist of several layers. Thus, depending on the configuration, a reactor building | can be made up of: - of a prestressed concrete wall 10, as an interface with the outside, the inside of which is lined with a metallic skin 11, with a sealing function for containment, for a 900 MWe reactor ([Fig.2A]); - of an outer wall in reinforced concrete 12, and an inner wall in pre-stressed concrete 10 separated from the outer wall 12 by an annular space 13 devoid of material, for a reactor of 1300 / 1450 MWe ([Fig.2B]); - of an outer wall of reinforced concrete 12, of an inner wall of prestressed concrete 10 separated from the outer wall 12 by an annular space 13 devoid of material, and of a metallic skin 11 on the inside of the prestressed concrete wall 10, for a 1650 MWe reactor ([Fig.2C]). As illustrated in [Fig.3], from publication [2], 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 generator steam generator 23, - a single pressurizer 24, In addition, we can distinguish on this [Fig.3], the mechanisms of reactor core control rods and control rod clusters 25. Depending on the reactor power, the number of loops can be three for a 900MWe reactor ([Fig.3]) or 4 for a 1300MWe reactor and above. The reactor 1 building is therefore sized, among other things, to house all the components of the primary circuit 2. Figure 4 illustrates the energy transfer cycle (heat then electricity) of a PWR reactor. In this Figure 4, we can distinguish in particular the distribution of the positioning of the components in relation to the building of reactor 1, which provides the function of a third containment barrier. The fluid connections between the inside and outside of the reactor 1 building are provided by lines 30, 31 of the external circuit of the steam generators 23 to the secondary circuit 3 comprising a turbine 32 connected to the electric generator 33, a condenser 34, a feed pump 35 and a heater not shown. More specifically, for a given steam generator 23, the reactor building 1 is traversed by a line called the hot line 30 which evacuates the steam from the steam generator 23 for the evacuation of power and brings it to the turbine 32, and by a line called the cold line 31 which supplies liquid water to the steam generator 23. To date, all existing pressurized water reactor (PWR) technologies are based on the principle of a power plant whose operating life is based on the operating life of the non-replaceable component(s) with the shortest operating life. These are mainly the components of the primary circuit and critically the reactor vessel which are the components which dictate the operating life of the plant, due to the consequences of the activation of materials, and the aging of some. It is therefore primarily the age of the reactor vessel that will dictate the overall operating life of the power plant and which initially led to planning for the operation of existing power plants over a period of 40 to 60 years, depending on the country and in particular on safety reassessments. The other structural elements of the reactor also age. Among these, two classes are distinguished: those that can be replaced and those that cannot be replaced during the operating life. Among the replaceable parts are the steam generators, the primary pumps, and the pressurizer. Among the irreplaceable components, beyond the primary circuit mentioned above, there are In particular, civil engineering, whose aging must be analyzed in relation to the safety requirements assigned to it. For a PWR reactor with primary circuit lines operating in pressurized water and arranged in an overhead configuration, an accident on the primary circuit requires specific sizing for the reactor building, which must ensure the safety function of containing nuclear materials. One example is the Loss of Coolant Accident (LOA), studied in the safety reports for Pressurized Water Reactors (PWRs), which is a hypothetical accident caused by a breach in the primary circuit containment. There is therefore a direct link between the operating life of the concrete structure of the building and the safety functions assigned to it. A currently emerging technology is that of small modular reactors (SMRs). The main advantages of these SMRs compared to existing PWRs are the simplification of systems, primarily for safety purposes, and increased modularity through the large-scale manufacturing of components in factories for transport to the construction site. Furthermore, SMRs are flexible due to their low power level and their ability to be integrated into the territory. They thus appear as a competitive solution for the future. To date, approximately 70 SMR projects have been identified worldwide at various stages of development, a quarter of which use mature, generation 3 (Gen-IIT) technologies, such as those used in the French fleet. Among the SMRs currently under development, some offer a configuration based on integrating the steam generator, or even all the primary circuit components, including the pressurizer and primary pumps, inside the reactor vessel. These SMRs are called integrated SMRs. Besides the gain in compactness, integrated SMRs have the advantage of eliminating the need for overhead pressurized water lines, which significantly reduces the risk of accidents and associated consequences related to primary circuit line ruptures. As an example, the nuclear power plant project with the acronym NUWARD"M, is a power plant consisting of two integrated SMRs, with a unit power of 170MWe, with all the components of the primary circuit inside the reactor vessel. Other integrated SMR projects are under development or have been studied, including the SCOR project with a power of 150 to 200 MWe on behalf of the Applicant or the ACP100 project with a power of 100 MWe. The gain in compactness of integrated-type SMRs complicates operational activities, compared to those carried out in a conventional PWR. Indeed, the main structural operability and maintainability operations on The architecture for a reactor primary circuit is as follows: - fuel loading / unloading operations which require, under appropriate radiation protection conditions, access to the inside of the reactor vessel, - maintenance operations on equipment that require accessibility to the equipment. Referring to [Fig.3], we see that the loops of a primary circuit 2 of a conventional PWR are designed to allow maintenance on each component without impacting, or only very minimally impacting, the other components, and that fuel handling operations are carried out by opening the lid of the tank 20 without impacting the primary loops 21. 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. Depending on the integrated SMR design, accessibility to certain components varies according to their configuration, positioning, and functional assembly. For example, in some SMR reactor designs, fuel loading operations may require removing certain primary circuit components. Similarly, depending on the integrated SMR design, the positioning of the inlet / outlet connections for the steam and feedwater lines can vary between the fixed lower compartment and the removable upper compartment of an SMR reactor. If the connections are located on the removable upper compartment, fuel handling operations require the steam and feedwater lines to be disconnected beforehand at the steam generator inlet. These differences in configuration depending on the concepts are mainly related to: to technological choices regarding internal components, in particular the type of heat exchanger, pressurizer, pumps,… to principles of layout and reassembly of internal architecture to the tank (position and type of steam generators), particularly of reassembly of so-called critical paths. For example, for the SCOR project, the Steam generators are on the vertical critical path. In summary, the main structural design criteria for an integrated SMR reactor, with a view to its architectural integration within a reactor building, are: - the vertical and / or axial accessibility required for fuel handling and component maintenance, - the procedures for dismantling / reassembling the upper functional parts posi- assembled in the removable compartment of the SMR in order to access the fuels, - the positioning of the connections for the fluidic links of steam and / or food-grade water in the removable compartment. At the end of the programmed operating life of pressurized water reactor (PWR) nuclear power plants, a dismantling process must take place. In France, to date, no PWR reactor power plant has yet been dismantled. Worldwide, the number of PWR reactor power plants that have been decommissioned is extremely limited. However, the first nuclear power plant in France was shut down in 2021 and is preparing to begin its decommissioning process: the Fessenheim plant. EDF, the nuclear operator of this plant, has established a decommissioning plan: [2]. The page of this plan provides a timeline of the various stages envisaged before, during, and after decommissioning. Before the actual dismantling, operations to shut down the processes and organize the power plant will be necessary. These preparatory operations for dismantling aim to: - Reduce the risks and drawbacks present at the installation: removal of spent and new fuel, waste and effluents, draining of circuits, decontamination of certain circuits. At this stage, 99.9% of the radioactivity is removed. - prepare the plant for dismantling operations: organization of access and traffic areas, adaptation of support functions including ventilation, electrical distribution and handling, evacuation of certain equipment to free up space; - refine knowledge of the state of the installation: inventory of hazardous materials, asbestos identification, sampling for radiological analyses. Following dismantling, the intended final state is a non-nuclear site, in which all buildings are demolished to a depth of one meter below ground level. Figures 5AA to 5D show the four successive stages of the dismantling process as envisaged in plan [3] and illustrated on page 5 of that plan. Step 1: This involves carrying out electromechanical dismantling, which consists of removing and cutting up all the equipment / components present, in particular in the reactor 1 building, notably 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 ([Fig. 5A]). Only the equipment necessary for carrying out the remediation work according to step 2 is left on site. Step 2: The decontamination of nuclear building structures consists of removing any radioactive contamination deposited inside the buildings, by... ticular on the inner wall of the reactor building 1 and infrastructure 4 within it ([Fig.SB]). Step 3: Demolition of the buildings, including Reactor Building 1 and Machine Hall 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 decontaminated according to Step 2. Cavities below ground level are filled with backfill, consisting of rubble from the demolition ([Fig.SC]). Step 4: Site rehabilitation takes place. This involves ensuring the compatibility between the soil conditions and the future use. Any areas where the buried portion 40 of the original infrastructure 4 shows chemical or radiological contamination are subject to a soil management plan ([Fig.5D]). Most of the nuclear power plant fleet will reach the end of its operational life in the next two decades, and this in a context of increasing electricity demand due to the electrification of a significant number of energy uses resulting from the decarbonization of our energy. Nuclear power plants are or will therefore be shut down while a significant portion of the initially planned investment has not reached its operational lifespan, leaving nuclear operators facing a substantial investment to ensure the renewal of all or part of the nuclear power plant fleet. There is therefore a need to find a solution that can reduce the investment in nuclear power plants with light water reactors (LWR), particularly pressurized (PWR) or boiling water (BWR), which is linked to their shutdown, particularly their dismantling as currently planned. The aim of the invention is therefore to meet at least part of this need. Description of the invention To this end, the invention relates, in one of its aspects, to a "retrofit" process, i.e., renovation, of 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). In a pressurized water reactor (PWR) configuration comprising a reactor building housing a reactor vessel, a primary circuit and a reactor pool, a fuel building, a nuclear fuel handling system to move nuclear fuel assemblies from the fuel building to the reactor building inside the vessel and vice versa, a turbine hall, a control room and a nuclear auxiliary building; the process includes the following steps, for each reactor: a / shutdown of the reactor including the removal, outside the reactor building, of all fuel assemblies present in the reactor vessel and the complete draining of the primary circuit; b / partial electromechanical dismantling of the reactor including the removal and evacuation, outside the reactor building, of the primary circuit components 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 primary circuit components removed during step a / , of at least one mixed structure, removably closed on itself, consisting of a double metallic skin and concrete poured in the space between the two metallic walls constituting the double skin; d / installation and maintenance within 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. For the purposes of this invention, "nuclear island" means the usual meaning in the technology, namely an assembly encompassing the nuclear boiler and fuel-related installations, as well as the equipment necessary for the operation and safety of this assembly. By "reactor building" we mean in the usual sense, namely a building which contains the reactor itself and all the components of the pressurized primary circuit as well as part of the circuits ensuring the operation and safety of the reactor. The term "fuel building" is used in its usual sense, namely a building in which are located the storage facilities (fuel assembly storage pools) and the handling facilities for new fuel (awaiting loading into the reactor) and irradiated fuel (awaiting transfer to a reprocessing plant). The term "nuclear auxiliary building" is used in its usual sense, namely a building that houses the auxiliary circuits necessary for the normal operation of the reactor. 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 electricity in a circuit, and then to cool the circuits. By "machine room" we mean the usual meaning, namely a building which houses the turbogenerator unit, whose role is to transform the steam produced in the nuclear island into electricity, and its auxiliaries. By "reactor vessel neutralization" we mean the process of sealing in a hermetically sealed and radio-protected manner in order to render the reactor vessel, left in place in its original vessel well, permanently unusable, without any combustible material inside and filled with an inert fluid ensuring its maintenance in this state. According to an advantageous embodiment, the process comprises, after step d / , a step e / of fluid and / or electrical connections of each reactor to the control room and the machine room, of installation of auxiliary circuits and of fluid and / or electrical connections to the nuclear auxiliary building. According to an advantageous embodiment, the installation according to step c / and the installation according to step d / include 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 / . According to an advantageous embodiment, the removal and disposal according to step b / comprises the following successive substeps: bl / removal of the primary lines arranged between the steam generators and the reactor vessel; b2 / removal and disposal of steam generators; b3 / removal and disposal of primary pumps; b4 / removal and disposal of the pressurizer; b5 / removal of primary lines initially at the outlet of steam generators up to the point where they pass through the reactor building wall. According to another advantageous embodiment, the neutralization of the reactor vessel according to step b / comprises the following successive substeps: b6 / watertight sealing of the hydraulic connections of the tank; b7 / closing the tank by reassembling its lid, with the necessary installation of a radiation protection cover; b8 / filling the reactor vessel with water or inert gas via a connection and level or pressure monitoring device. 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 / includes, after the neutralization of the reactor vessel, a step of decontamination of the reactor building to eliminate any radioactive contamination deposited inside said building. According to another advantageous embodiment, step c / includes cutting and removing the parts of the walls and / or floors and, where applicable, the foundation slab of the reactor building infrastructure that initially support the components of the primary circuit. According to another advantageous embodiment, step c / includes fixing each composite structure, preferably by means of a fixing plate itself integral with 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. According to another advantageous embodiment, step c / comprises, 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 section of the shell separating the reactor vessel well from the REL reactor, forming part of the reactor pool, from each mixed structure; - the installation of a horizontal connecting pipeline between each mixed structure and the tank well. According to one embodiment, the process includes, once the 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 reactor vessel well side. The invention also relates to a nuclear power plant obtained using the retrofit process described above, comprising: - a reactor building housing a neutralized REL reactor vessel 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, hybrid structures arranged around the decommissioned reactor vessel, each hybrid structure housing an integrated SMR reactor and a fuel building, each integrated SMR reactor being positioned for accessibility by the fuel handling system. The final number of hybrid structures, each housing an integrated SMR, will depend in particular on the desired power upgrade for the plant undergoing retrofitting. According to an advantageous embodiment, the power plant further comprises a horizontal connecting pipeline between each composite structure and the reactor vessel well and at least one isolation valve 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 unit fuel assemblies from horizontal to vertical to allow their transfer via the connecting pipeline. According to an advantageous construction variant, each mixed structure includes a base configured to support an integrated SMR reactor. Advantageously, each mixed structure can be at least partially filled with water. Even more advantageously, each hybrid 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. 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. Thus, the invention essentially consists of a method for retrofitting a nuclear power plant which consists of removing and evacuating all the components of the primary circuit with the exception of the reactor vessel (RV), which is emptied of all material and neutralized, and then replacing in place of a part of these components with sub-assemblies each consisting of an integrated SMR reactor and a mixed concrete / metal structure which plays the role of both a vessel well for the SMR reactor, advantageously filled with water, an anchoring of the SMR inside the reactor building and advantageously a contribution to the third containment barrier and this with minimal modification of the reactor building infrastructure. A hybrid structure according to the invention acts in a way as the reactor vessel well of an integrated SMR reactor, and therefore performs the following functions: - a solidity function through its anchorages with the existing civil engineering infrastructure (foundation slab, walls and intermediate floors) of the PWR reactor, in order to ensure compliance with seismic resistance requirements; - a sealing function ensured by its double-walled metal casing which allows respectively: to fill with water and ensure the biological protection function, to ensure crossings to the main pool above the tank shaft existing RFP reactor and to connect to the handling chain com- existing fuel, to position the entirety of an integrated SMR reactor within a volume uniform water volume, which is defined by the internal volume of the mixed structure contributing to the safety function of residual power evacuation, - a contribution to the safety function of containing nuclear materials: with a closure by a removable cover on top of the mixed structure, the integrated SMR reactor which is housed and maintained there is in an enclosure which takes up all or part of the requirements related to the safety function of guaranteeing the containment of nuclear materials (third barrier); - advantageously, a constructability function because a mixed structure can be created from prefabricated modules, which allows: a modularity that guarantees modular integration into the building The reactor, similar to an integrated SMR reactor, allows for the adaptation of the points anchoring and connection with existing civil engineering structures to ensure the resumption of effort and allows adaptation to all configurations of PWR reactors; a welded assembly that guarantees great flexibility under the conditions assembly, a small footprint, and watertightness contribute to the containment function: an absence of formwork as such and of formwork support allowing for optimal integration into the existing infrastructure, reduced the impact of retrofitting according to the invention to best meet the need and optimizes the lifespan of the construction site. In fact, the process according to the invention is in some way a break with all the dismantling processes envisaged. In essence, compared to a PWR decommissioning plan as envisaged in [3], the invention is distinguished by the fact that: no building whatsoever on the nuclear island (reactor building, building of the Fuel building, auxiliary building) or the conventional island (room of machines) is not subject to deconstruction, the reactor vessel is not removed from the reactor building; No actual site rehabilitation needs to be carried out. In other words, the inventor has overcome a universally widespread prejudice among nuclear experts according to which the complete dismantling of a nuclear power plant must be carried out up to the destruction of all buildings and the rehabilitation of the site, whereas the technical reality is that only a few non-replaceable components of the primary circuit of a reactor have reached their regulatory operating life. And, even if this is accompanied by a decrease in power, the retrofit process according to the invention makes it possible to give a second phase of operation to a 900 / 1300 MWe type PWR pressurized water reactor nuclear power plant by replacing the PWR reactor with its three or four steam generators with integrated SMR reactors. Typically, an integrated SCOR 200 SMR reactor can be designed to deliver 200 MWe of power. Therefore, replacing a 900 MWe PWR with three SCOR-type SMR reactors would result in a plant whose power output in the second phase of operation would be equal to 3 x 200 / 900 = 67% of its initial power, representing a 33% power reduction. For a 1300 MWe PWR, the power reduction would be 38%. Another power assessment with an integrated SMR reactor planned in the NUWARD project would give an equivalent order of magnitude. In conclusion, the retrofit process according to the invention offers numerous advantages, including: reducing the initial investment in a nuclear power plant through reuse integration of a significant portion of the equipment, covering almost the entire block conventional and part of the nuclear island, including civil engineering. In addition to the partial dismantling according to the invention, only a rearrangement of the machine room is necessary to adapt the sizing of the energy conversion cycle equipment with reduced power output related to retrofit; the absence of a new nuclear site to be found, which implies a reduction significant environmental and land impact, with the continuity of the local, economic and social environment surrounding the various nu- sites existing nuclear power plants whose facilities would be transformed by the process of retrofit. Furthermore, from a societal point of view, the acceptability of nude sites- Since existing clarities can be considered acquired, it is likely that it the same applies to the retrofitting of these sites; the significant reduction in construction times: the transformation of a power plant nuclear power, thanks to the invention, is produced according to an optimized operating procedure. with a large number of operations prepared in advance in the workshop, which allows at least some of the deadlines to be parallelized, such as the completion in prefabricated modules of mixed structures; significant waste reduction: by reusing as many buildings as possible / of the power plant's equipment for a second phase of operation, the quantity of waste generated, including very low-level nuclear waste, decreases sharply; modularity regarding the share of nuclear power in the energy mix: with the power reduction 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 park, this allows for planning within a temporal dynamics of the desired share of nuclear power in the energy mix; an improvement in the reputation of nuclear power because with the retrofit according to The invention makes this form of energy sustainable: a circular economy of materials and materials, obsolescence, … a reduction in the carbon footprint of nuclear energy, the most important part of which load-bearing capacity is linked to the construction of the facilities. By increasing the duration operating a facility (nuclear power plant), we reduce the balance carbon reduced to the MWhe it actually produces. Other advantages and features of the invention will become clearer upon reading the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings [Fig.1] [Fig.1] illustrates in the form of a histogram the temporal evolution of the number of commissioning and shutdown of nuclear reactors in the world, according to the publication [1]. [Fig.2A], [Fig.2B), [Fig.2C] Figures 2A, 2B, 2C are schematic perspective and partial cross-section views of an existing PWR nuclear reactor in different configurations. [Fig.3] [Fig.3] is a schematic view of a state-of-the-art PWR nuclear reactor primary circuit in a three-primary-loop configuration. [Fig.4] [Fig.4] is a schematic view of the three cycles of a PWR type nuclear reactor according to the state of the art. [Fig.5A], [Fig.5B], [Fig.5C], [Fig.5D] Figures 5A to 5D illustrate the different stages of the dismantling plan of a PWR nuclear reactor as foreseen in publication [2]. [Fig.6], [Fig.6A], [Fig.6B] Figures 6, 6A, 6B are schematic views respectively in perspective and transparency, in perspective and section, and from above of a mixed structure according to the invention, in which is housed an integrated SMR reactor with the example for illustration of the SCOR integrated SMR model. [Fig.7] [Fig.7] is a schematic top view illustrating a mixed structure according to the invention, with an integrated SMR reactor as exemplified by the SCOR model, the upper removable compartment of which is removed from its fixed compartment, the two compartments being housed side by side in the mixed structure. [Fig.8] [Fig.8] is a perspective and transparency view illustrating a variant of a mixed structure according to the invention, which includes a removable upper closing cover closing said structure over the integrated SMR reactor. [Fig.9] [Fig.9] is a perspective and cross-sectional view showing the interior of the metallic part of a composite structure according to the invention. [Fig.10] [Fig.10] is a perspective and cross-sectional view showing the interior of the metallic part of a variant of a mixed structure according to the invention, made up of prefabricated modules. [Fig. 11] [Fig. 11] is a view of a steam generator as installed in the PWR 900 1300MWe type power plants of the pressurized water reactor (PWR) nuclear power plant fleet. [Fig.12] [Fig.12] is a perspective and partial sectional view of another integrated SMR project, called SCOR, in a configuration with the removable compartment fixed on top of the fixed compartment. [Fig.13] [Fig.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. [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. [Fig.15] [Fig.15] is a schematic perspective and cross-sectional view of a nuclear power plant transformed according to the retrofit process according to the invention. Detailed description Throughout this application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a reactor building of a nuclear power plant and an integrated SMR nuclear reactor, as provided for in vertical operating configuration and arranged in the reactor building according to the retrofit method of the invention. Figures 1 to 5D have already been detailed in the preamble, so they will not be commented on below. 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. It should be noted that the various figures do not show all the fluid, electrical, and control connections, nor all the instrumentation required for the operation of a nuclear power plant converted according to a process of the invention. In particular, the steam fluid lines, with their associated piping, are not shown because there is no requirement for first-order integration into the architecture for these lines. Specifically, the steam and water supply fluid lines to and from an integrated SMR reactor, which require penetrations in the composite structure, are not shown. Prior to describing the process of 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. Figures 6, 6A, and 6B show a mixed structure according to the invention, glo- The unit designated by reference 6 houses an integrated SMR reactor, designated by reference 7, and is intended to replace a sub-assembly consisting of a primary pump and a steam generator in an existing PWR reactor primary circuit. In Figures 6, GA, and 6B, the SCOR integrated SMR model was chosen for the illustrations. A mixed structure 6 plays in a 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) 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 of the fuel or maintenance of the internals of the fixed compartment 70 of the SMR. A mixed structure 6 consists of a double-metallic skin, i.e. two metal walls 60, 61 spaced apart, the space between these two walls 60, 61 being filled with concrete 62. A supporting floor 63 is arranged substantially horizontally as a base inside the inner wall 61 to support the integrated SMR reactor 7. The sealed internal 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 according to the configurations of the integrated SMR 6, it contributes to the function of removing residual power from the SMR. Furthermore, as shown in [Fig. 7], the structure 6 is dimensioned so that this internal volume can accommodate, underwater alongside the fixed compartment 70 of the integrated SMR reactor 7, the removable compartment 71, which is removed from the top of the fixed compartment 70. This ensures safe fuel handling and / or maintenance operations of the fixed compartment 70, as both compartments 70 and 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 [Fig. 8], the composite structure 6 is preferably equipped with a metal cover 64 in a sealed and removable connection to one or both of the metal walls 60, 61 of the composite structure. When this cover 64 is in its installed configuration, the structure 6 itself contributes to the safety function of controlling the containment of nuclear materials, the first being constituted by the metal cladding that encloses the fuel within the integrated SMR reactor 7, the second by the shell that constitutes the vessel of the integrated SMR reactor 7. Furthermore, the mixed structure 6 has a through opening P. As As detailed later, this opening P is intended to be connected to a pipeline for the transfer of fuel assemblies to and from the interior of the SMR7 reactor. As shown in more detail in [Fig. 9], the composite structure 6 first comprises a metal anchor plate 65 which is fixed to the foundation slab 41 of the reactor building, thus anchoring the composite structure 6 to the existing infrastructure 4 of the reactor building 1. In the illustrated example, this anchor plate 65 is welded to the inner metal wall 60. It is obviously possible to consider another anchor plate welded to the outer metal wall 61, in addition to the plate 65 welded to the inner metal wall 60. The connection to the foundation slab 41 can be achieved using various methods specific to civil engineering techniques and dependent on the strength requirements derived from structural studies, particularly seismic loads. In the space 62 internal to the double metal skin, support bars between the walls 60, 61 are welded to them to maintain the spacing between them. To reinforce the concrete in 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. Furthermore, although not shown, other plates may be welded to one or both of the metal walls 60, 1, in particular for: to serve as a support for base 63 intended to serve as a support for the SMR reactor integrated 7; support the pipes and other auxiliary equipment necessary for operation of the integrated SMR reactor 7; link the composite structure 6 to the intermediate floors 42 and walls 43 the existing infrastructure 4 of reactor building 1 so as to reinforce mecha- simply the whole thing and restore overall solidity to the structure less equivalent to that before the implementation process of the structure mixed 6. Figure 10 shows an embodiment of a composite structure 6 made of factory-prefabricated modules M1, M2, M3, and M4, which are then assembled on-site, i.e., inside the reactor building 1. In this embodiment, the structure 6 may include mechanical stiffeners 69 arranged in an upper part of the structure. This embodiment is advantageous because, depending on the type of reactor building 1 and its existing airlock, originally designed to replace the steam generators 23, the size of each module can be adapted to achieve the largest dimension that can be accommodated through the airlock. This further optimizes the time and cost of the retrofit according to the invention. Structures have already been produced mixed for nuclear installations internationally and projects currently being qualified for France: see [4]. Although not imperative, the retrofit process according to the invention, as detailed below, is advantageously implemented when all the components involved in the transformation are handled without or minimally impacting the infrastructure 4 of reactor building 1. 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. The possibility of handling the structures 6 before pouring concrete within them has been demonstrated by the above. The inventor therefore also verified beforehand that integrated SMR reactors 7 could also be handled in a single block, i.e. once fully assembled, by the same handling route, i.e. through the entrance airlock of the reactor building. 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 entrance airlock of the reactor building 10 already provided to allow its replacement with dimensions of approximately 22m high by 5m wide. Figure 12 shows an integrated SMR reactor 7, according to the SCOR project: its overall dimensions H2*L2 are less than H1*L1 of a steam generator 23. The maximum overall dimensions of less than 22m x 5m for this example of an integrated SMR reactor project 7 allow it to be introduced through the airlock into reactor building 1 via the steam generator handling system 23, as originally designed. The handling system, which involves horizontal positioning followed by vertical tilting, is also compatible with this example. Similarly, its mass is compatible with the load capacities of the handling system equipment. Therefore, the introduction of an integrated SMR reactor in reactor building 1 without impacting its infrastructure 4 is achieved. The inventor then had to consider the optimal location for the mixed structures 6 with the integrated SMR reactors 7 within the reactor building 1. To optimize the implementation and costs of the retrofit process, the inventor selected the following integration criteria: - limit the impact on infrastructure 4 of anchoring the integrated SMR reactors 7, - reuse as much of the existing infrastructure as possible: functionalities of the various barriers, biological protection, - to 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 power evacuation to the machine room. Based on these criteria, the siting of the integrated SMR reactors was carried out by an analysis of the three-dimensional critical paths. The inventor concluded that the optimal positioning was: in altimetry (z), according to an alignment on the handling chain com- fuel / biological protection, in the (x,y) plane, top view, in place of the steam generators 23 in axial symmetry, In addition to these two positioning parameters, the inventor analyzed that, furthermore, 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 purpose of fuel loading / reloading and / or maintenance of internal components. With a nuclear power plant converted 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 as there are reactors. By analyzing the spatial configuration of the primary circuit of a PWR, as it currently exists, the inventor found this optimal location. Indeed, on each primary loop 21, the primary pump 22 is spatially adjacent to the steam generator 23 to which it is connected. 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 for the removable compartments 71. This optimal configuration is shown schematically in [Fig. 13]: it ultimately allows each SMR to be allocated a dedicated space for its removable upper section and makes it possible to consider an operating configuration of the installation requiring the simultaneous opening of all the SMRs. With reference to figures 14AA to 14E, we now describe 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. Step a: the PWR reactor is shut down. This first step aims to enable the power plant in a site configuration for the retrofit. The evacuation of all as- fuel assemblies present in reactor vessel 20. Then the primary circuit 1 is completely drained. Pre-construction safety analyses can determine whether the fuel assemblies can remain in the fuel building pools for the duration of the project. If so, the retrofit work (steps b and c) could begin without waiting for the fuel assemblies to reach a residual power level compatible with nuclear material transport regulations, thus saving time on the overall project schedule. Step b / A partial electromechanical dismantling of the PWR reactor is carried out. This involves removing and evacuating the components of the primary circuit 2 outside the reactor building (stage 1), preferably according to the following successive sub-steps: b1 / Removal of the primary lines 21 arranged between steam generators 23 and reactor vessel 20: b2 / removal and disposal of steam generators 23; b3 / removal and disposal of primary pumps 22; b4 / removal and disposal of pressurizer 24; b5 / removal of primary lines 21 initially at the outlet of steam generators 23 until the crossing of the reactor building wall. Only the reactor vessel of reactor 20 is left in its location within the reactor 1 building ([Fig. 14A]). Indeed, keeping the reactor vessel of reactor 20 in place does not hinder the retrofit configuration of the installation. Furthermore, the inventor believes that by leaving the vessel in place during the operational phase of the nuclear power plant after retrofitting it with the integrated SMR reactors, the activation materials, particularly Co°, will have time to decay. On the other hand, all material is removed from inside nuclear tank 20, and then the latter is neutralized. To do this, we carry out the following successive sub-steps: b6 / watertight sealing of the hydraulic connections of the tank. This sealing may consist of placing a solid plug in each hydraulic connection and then welding the plug watertight, the welds preferably being checked by gamma radiography; b7 / sealing of the tank by reassembling its cover, after all control bar crossings have been plugged, with the necessary installation of a radiation protection cover; b8 / Filling the reactor vessel with water or inert gas via a connection and pressure and / or liquid level control device. The filling and level control device will be located within the reactor building. can notably be connected to the tank by using one or more lid penetrations to ensure fluid connection. The lid of tank 20 may, if necessary, undergo modifications in particular to improve its sealing and / or to optimize the neutralization of the tank. After the neutralization of reactor vessel 20, if necessary, the interior of the reactor building 1 is cleaned to eliminate any radioactive contamination that may have accumulated. Taking into account radiation protection considerations, steps a / and b / are implemented by human intervention or remotely operated. Step c / : the installation of the mixed structures 6 is carried out. Prior to this step c / , studies of solidity can be carried out, in particular of seismicity of the nuclear island in its overall configuration to define all the links of the infrastructure 4 of the reactor building | 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 dowels and connecting rods between walls 60, 61, the methods of anchoring to the foundation slab 41 and the connection with the floors 43 and walls 42 in connection with the mixed structures 6. This step c / includes the cutting and removal of parts of the walls 42 and / or floors 43 and, where applicable, the foundation slab 41 of the infrastructure 4 of the reactor building l This allows us to prepare the footprint of the mixed structure 6 and to plan all the anchoring devices to the infrastructure 4. Furthermore, step c / consists of opening the shell 42 into the existing reactor pool above for connection to the fuel handling system. The coring technique will be advantageously used for this operation. [Fig. 14C] shows: - the required footprint E for the installation of a composite structure 6 to be cleared, - the circular opening O towards the pool above the reactor vessel 20. All cutting operations can be carried out using concrete sawing equipment, already widely used in nuclear conditions. Preparation operations on the existing infrastructure 4 can be performed. Once these cutting operations and the removal of the cut sections of the infrastructure have been completed, the prefabricated modular composite structures are brought in through the entrance airlock of reactor building 1. Typically, modules can be introduced in the form of horizontal sections, each 5 meters high. Then, the actual installation of the composite structures 6 takes place. This installation includes anchoring them to the infrastructure 4 of the building. Reactor 1. In particular, each composite structure 6 is fixed to the foundation slab 41 by means of a fixing plate 65. The prefabricated modules are welded together and anchorage connections are made with the walls 42 and floors 43. In addition, sealing connections are made with the compartment above the reactor vessel 20. Once each fixing of a composite structure 6 has been installed and secured, a horizontal metal connecting pipe 80 is installed between each composite structure and the reactor vessel well 20, preferably by welding it tightly to the two metal walls 60, 61 of the double shell. On the pool side above the reactor vessel 20, to ensure the pipe 80 is watertight, it is welded to the pool liner. This pipe 80 is a transfer pipe through which a fuel assembly can be handled using the handling chain. Step d: We proceed to the installation and maintenance within each mixed structure 6, installed according to step c, of an integrated SMR nuclear reactor 7. As mentioned above, the integrated SMR 7 reactor is arranged in a position accessible by the existing fuel handling chain. 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. Finally, isolation valves 81, 82 are installed at the ends of each pipe 80 (figures 14D, 14F). Step e / : the fluid and / or electrical connections of each integrated SMR reactor 7 to the control room and the machine room are then made. We install the auxiliary circuits and make the fluid and / or electrical connections to the nuclear auxiliary building. Fig. 15 illustrates the internal architecture of a reactor building 1 of a PWR reactor plant initially, which has been transformed with the retrofit process of the invention with three mixed structures 6 each housing and supporting an integrated SMR reactor 7. The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants. Other variants and embodiments may be considered without departing from the scope of the invention. While in the illustrated example, the mixed structures are sized to optimize the integration of the integrated SMR reactors 7 and their removable compartments 71 during operational use, a smaller size can also be considered for mixed structures, i.e. with a shared location solution. for all removable compartments 71, once removed from their respective fixed compartments 70. Within the framework of the invention, it is possible to consider handling the removable compartment of an integrated SMR reactor, at the bottom of a mixed structure, at the very least next to and under the same water as the fixed compartment of the SMR. In the illustrated example, the transfer method from an integrated SMR reactor 7 to the pool above the reactor vessel 20 is limited to a single pipeline 80. This allows the different water volumes (internal volume of the composite structure 6, pool above the reactor vessel 20) to be isolated using valves 81 and 82. This choice necessitates a horizontal transfer of a fuel assembly and therefore requires a vertical / horizontal tilting device. Once the fuel assembly is extracted vertically from inside the integrated SMR reactor 7, it must be inserted horizontally into the pipeline 80. This horizontal position can be maintained until it exits the reactor building 1, as it is in this position that the assembly is transferred to the fuel building. One alternative solution is to replace the 80 transfer pipes with an open-channel water channel, possibly equipped with a cofferdam to isolate the water volumes. The cofferdam acts as a valve, providing hydraulic isolation between the two compartments it separates. This arrangement eliminates the need for a horizontal / vertical tipping mechanism. 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 of a BWR reactor, subject to adaptations related to the particular configuration of this type of reactor compared to a PWR, these modifications being accessible to a person skilled in the field of nuclear reactors. List of cited references [1]: The World Nuclear Industry Status Report 2017. https: / / www.worldnuclearreport.org / IMG / pdt / 20170912wnisr2017-en-lr.pdf [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-industriel / nucleaire / Notes%20d%27information / dem_fessenheim_p3.pdf [4]: https: / / csti-groupe.com / 2019 / 01 / 07 /

Claims

Demands

1. A method for retrofitting a nuclear power plant initially comprising at less a light water nuclear reactor (LWR), in particular a reactor pressurized water (PWR) or boiling water (BWR) systems, including a reactor building (1) housing a reactor vessel (20), a circuit primary (2) and a reactor pool, a fuel building, a nuclear fuel handling chain to bring as- nuclear fuel assemblies from the fuel building up to the reactor building inside the vessel and vice versa, an engine room (5), a control room and a building of nuclear auxiliaries; the process comprising the following steps, for each reactor: a / shutdown of the reactor including evacuation, 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 components (21, 22, 23, 24) of the primary circuit, excluding the tank of the reactor (20) left in its location in the reactor building, the removal of all material from inside the nuclear reactor vessel followed of the neutralization of the latter: c / installation, in place of part of the circuit components primary evacuated during step a / , from at least one mixed structure (6), closed in a removable manner upon itself, consisting of a double- metallic skin (60, 61) and concrete poured into the space (62) between the two metallic walls forming the double skin; d / implementation and maintenance within each mixed structure, installed according to step c / , of at least one nuclear reactor (7), said integrated modular reactor (SMR); the integrated SMR reactor(s) being arranged in a position accessible by the handling chain of fuels.

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, for commissioning location of auxiliary circuits and fluidic connections and / or electrical systems in the nuclear auxiliary building

3. Retrofit method according to claim 1 or 2, installation according to step c / and the implementation according to step d / including the res- passage respectively of each mixed structure in the form of pre-modules manufactured and of each integrated SMR reactor, through the same access airlock to the exterior from the reactor escape building through which Each component is removed in its entirety according to step b / .

4. Retrofit method according to any one of the preceding claims, removal and evacuation according to step b / comprising the successive sub-steps following: bl / deposit of primary lines (21) arranged between generators of steam (23) and the reactor vessel (20); b2 / removal and disposal of steam generators (23); b3 / removal and disposal of primary pumps (22); b4 / removal and disposal of the pressurizer (24); b5 / removal of primary lines (21) initially at the output of ge- steam generators up to the point where they pass through the building's hull reactor.

5. Retrofit method according to any one of the preceding claims, the neutral- reactor vessel modification according to step b / including the sub- next successive steps: b6 / watertight sealing of the hydraulic connections of the tank; b7 / closing the tank by replacing its lid, with the case if necessary, implementation of radiation protection coverage; 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 the installation of a solid plug in each hydraulic connection then by sealing the cap, the welds preferably verified by gamma radiography.

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

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

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

10. Retrofit method according to any one of the preceding claims, step c / including, once the positioning and, where applicable, the fixing to the foundation slab of the composite structure(s), the following sub-steps following cessives: - the cutting and removal of the section of the wall separating the well from REL reactor vessel, forming part of the reactor pool, of each mixed structure; the installation of a horizontal connecting pipeline (80) between each mixed structure and the tank 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 pipe- installation, preferably of two isolation valves, one on the mixed structure and on the other side a tank well.

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

13. Nuclear power plant according to claim 12, further comprising a horizontal connecting pipe (80) between each mixed structure and the tank well and at least one isolation valve (81, 82) on the cana- installation, preferably of two isolation valves, one on the mixed structure and the other side tank well; the handling chain of fuels including at least one switching device horizontal to vertical arrangement of individual fuel assemblies for to allow their transfer via the connecting pipeline.

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

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

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

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

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