A method for upgrading a nuclear power plant that initially includes at least one light water reactor (LWR), particularly a pressurized water reactor (PWR) or a boiling water reactor (BWR), to at least one integrated modular reactor (SMR).

Abstract

The present invention essentially comprises the steps of disassembling and removing all components of the primary circuit except for the LWR reactor vessel, which is essentially emptied and neutralized, and subsequently replacing some of these components with sub-assemblies each composed of an integrated SMR reactor and a concrete / metal hybrid structure, where this concrete / metal hybrid structure is also used as a reactor pit for the SMR reactor, which is preferably filled with water, and fixing the SMR inside the reactor building and contributing to a third containment barrier, preferably while minimizing damage to the infrastructure of the reactor building. The present invention is related to a method for retrofitting a nuclear power plant.

Description

Technical Field

[0001] The present invention relates to the field of nuclear power plants, particularly to the installation bases of nuclear power plants including light water reactors (LWRs), particularly pressurized water reactors and boiling water reactors.

[0002] Therefore, an object of the present invention is to reduce the significant drawbacks in cost and capacity for modifying the installation base of an LWR nuclear power plant.

[0003] The present invention will be described by referring to a nuclear power plant including at least one pressurized water reactor, but the present invention is applicable to any boiling water reactor nuclear power plant or, more generally, to any light water reactor (LWR) nuclear power plant.

Background Art

[0004] In a situation where the demand for electricity (competitive, highly available, non-intermittent electricity) is increasing due to energy conversion by decarbonization of use, most of the current installation bases of pressurized water reactor (PWR) nuclear power plants are approaching the end of their operating life for which the reactors were designed and approved soon.

[0005] FIG. 1 is excerpted from Non-Patent Document 1 and shows the time series of the start of operation of nuclear reactors on a global scale. Most of the 440 reactors constituting the installation base worldwide started operation from 1970 to 1990.

[0006] Depending on the country, extension of approval rules, or operation rules, an operating life of 40 to 60 years is planned, and all of these reactors will be closed by at least between 2030 and 2050. Countries that are using the nuclear power installation base for electricity energy supply and want to maintain that installation base will therefore have to face many investments.

[0007] PWR reactors account for more than 60% of the 440 reactors in the nuclear power installation base worldwide.

[0008] A pressurized water reactor (PWR) consists of three cycles (fluid circuits), and the general principle of its normal operation is as follows:

[0009] The high-pressure water in the primary circuit absorbs the energy supplied in the form of heat by the fission of uranium nuclei and, where appropriate, plutonium nuclei in the reactor core.

[0010] This water, under high pressure and high temperature, typically 155 bar and 300°C, then enters a steam generator (SG), transferring its energy to a secondary circuit, which also uses water under pressure as a heat transfer fluid. This water, in steam form under high pressure, typically around 70 bar, then expands through an expansion member, converting the change in the enthalpy of this fluid into mechanical work, which is then converted into electricity by a generator.

[0011] The water in the secondary circuit then condenses via a condenser, using the third cycle, the cooling cycle, as the cool source.

[0012] The design principles of PWR reactors within these three cycles are essentially the same as those of the first one used since it came into operation.

[0013] The main elements of the PWR primary circuit are, namely, In particular, Reactor Building 1, which has various functions including contributions to containment safety, The reactor vessel 20, located in the center of building 1, contains the reactor core C. Primary pressurized water circuit 2 including reactor vessel 20, This is shown in Figures 2A to 2C.

[0014] These main elements are therefore shared, and their composition and the number of components vary depending on the reactor's output.

[0015] The casing of reactor building 1 can typically consist of multiple thicknesses.

[0016] Therefore, depending on the configuration, reactor building 1 is In a 900 MWe reactor, a prestressed concrete wall 10 (Figure 2A) functions as an interface to the outside world, and its interior is covered with a metal outer shell 11 that has a containment and sealing function. In a 1300 / 1450 Mwe reactor, a reinforced concrete outer wall 12 and a prestressed concrete inner wall 10 separated from the outer wall 12 by a material-free annular space 13 (Figure 2B) In a 1650 MWe reactor, a reinforced concrete outer wall 12, a prestressed concrete inner wall 10 separated from the outer wall 12 by a materialless annular space 13, and a metal outer shell 11 on the interior of the prestressed concrete wall 10 (Figure 2C), It can consist of.

[0017] As shown in Figure 3, which is quoted from Non-Patent Document 2, the primary circuit 2 has the following main components: reactor vessel 20, A primary loop 21, which includes a primary pump 22 and a steam generator 23, Single pressurizer 24, It consists of.

[0018] Figure 3 also shows the reactor core's control rod mechanism and control rod 25.

[0019] Depending on the reactor's output, the number of loops can be three for a 900 MWe reactor (Figure 3), or four for reactors of 1300 MWe or more.

[0020] Reactor building 1 is therefore sized to accommodate, in particular, all the components of the primary circuit 2.

[0021] Figure 4 shows the energy transfer cycle (heat, then electricity) of a PWR reactor. In particular, Figure 4 shows the distribution of the positions of the components relative to the reactor building 1, which has a third containment barrier function.

[0022] The fluid connection between the inside and the outside of the nuclear power plant building 1 is provided by lines 30, 31 of the external circuit of the steam generator 23, which lead to the secondary circuit 3 including the turbine 32 connected to the generator 33, the condenser 34, the feed water pump 35, and a heater not shown.

[0023] More precisely, for a given steam generator 23, the nuclear power plant building 1 passes through it the so-called hot line 30 that discharges steam from the steam generator 23 for output discharge and supplies the steam to the turbine 32, and the so-called cold line 31 that supplies liquid water to the steam generator 23.

[0024] Currently, all existing PWR reactor technologies are based on the principle of power plants whose operating life depends on the operating life of the non-replaceable components with the shortest operating life.

[0025] These are mainly components of the primary circuit, and decisively the reactor vessel, which are components that determine the operating life of the power plant due to the activation of materials and the influence of the aging of some of them.

[0026] Therefore, in response to national and especially safety reassessments, when trying to determine the operating life of the entire power plant, it is mainly the age of the reactor vessel that leads to the predicted operation of existing power plants over a period of 40 to 60 years at first.

[0027] Other structural elements of the reactor also age. Among these, it is possible to distinguish two classes: elements that can be replaced during the operating life and elements that cannot be replaced.

[0028] The replaceable elements include the steam generator, the primary pump, and the pressurizer.

[0029] In addition to the primary circuitry mentioned above, non-replaceable elements include civil engineering structures in particular, and their deterioration must be analyzed according to the safety requirements assigned to them. In PWR reactors, which operate with pressurized water and have a primary circuitry line located overhead, a specific size of the reactor building is required, which must provide the function of safely containing nuclear material in the event of an accident in the primary circuitry. In particular, one can cite the loss of primary coolant accident, a hypothetical accident caused by damage in the primary circuitry casing, which has been studied in reports on the safety of pressurized water reactors (PWRs). Thus, there is a direct relationship between the service life of the concrete structure of the building and the safety function assigned to it.

[0030] The emerging technology is the Small Modular Reactor (SMR). These SMR reactors offer fundamental advantages over existing PWRs, primarily for safety reasons, by simplifying the system and improving modularity by manufacturing many components at off-site plants and transporting them to the construction site.

[0031] SMR is also flexible due to its low power level and its regional insertion capability.

[0032] Therefore, it appears that there are competitive future solutions. As of this writing, approximately 70 SMR projects have been identified as being more or less in progress worldwide, a quarter of which are using mature third-generation (Gen-III) technologies, such as the installation infrastructure in France.

[0033] Some SMRs under development propose configurations based on integrating the steam generator, or even all components of the primary circuit, particularly the pressurizer and primary pump, inside the reactor vessel. These SMRs are called integrated SMRs. Apart from improved compactness, integrated SMRs have the advantage of no longer requiring overhead pressurized water fluid lines, which significantly reduces the risk of accidents and their consequences associated with the rupture of primary circuit lines.

[0034] For example, the NUWARD® nuclear power plant project consists of two integrated SMRs, each with a power rating of 170 MWe, and is a power plant where all components of primary circuit 1 are located inside the reactor vessel.

[0035] Other integrated SMR projects are under development or research, including the SCOR project under the applicant's name with an output rating of 150 MWe to 200 MWe, and the ACP100 project with an output rating equal to 100 MWe.

[0036] As integrated SMRs become smaller, their operation is more complex compared to standard PWRs.

[0037] In fact, the main work on the operability and structural maintainability of the architecture for the reactor primary circuit is as follows: Fuel loading / unloading operations requiring access to the inside of the reactor vessel under appropriate radiation protection conditions. Maintenance work for equipment that requires access to the equipment. That is the case.

[0038] Referring to Figure 3, it can be seen that the loops of the primary circuit 2 in a standard PWR are designed to allow maintenance of each component with little to no impact on other components, and fuel handling operations are performed by opening the lid of the reactor vessel 20 without affecting the primary loop 21.

[0039] On the other hand, for the integration of components, accessing the fuel zone for loading / unloading operations in an integrated SMR may require the removal of functional components of the primary circuit, which is a more challenging task than handling the reactor vessel lid.

[0040] In integrated SMR designs, accessibility to certain components varies depending on their configuration and the arrangement and functional assembly of these components. For example, in some SMR reactor designs, the fuel loading operation may require the removal of several components of the primary circuit.

[0041] Similarly, in some SMR reactor designs, the placement of inlet / outlet tapping for steam and feedwater replenishment lines may vary between the fixed lower compartment and the removable upper compartment in the SMR reactor. If tapping is located in the removable upper compartment, it is required to disconnect the steam and water replenishment lines at the steam generator inlet before any fuel handling operations.

[0042] The differences in these configurations, depending on the design, are mainly: The selection of technology for internal components, particularly the types of exchangers, pressurizers, pumps, etc. Principles of the arrangement and reassembly of the internal architecture of the reactor vessel (location and type of steam generators), particularly in the reassembly of the so-called critical path, They are related. For example, in the SCOR project, the steam generator is located on the vertical critical path.

[0043] In summary, the main structural design criteria for integrated SMR reactors, with a view to architectural integration within the reactor building, are: Requirements for vertical and / or axial access for fuel handling and component maintenance, A methodology for removing / replacing upper functional components located in the removable compartment of the SMR to access the fuel. Arrangement of tapping for steam and / or feedwater fluid connections in removable compartments, That is the case.

[0044] Pressurized water reactor (PWR) nuclear power plants must be dismantled once their planned operating life ends.

[0045] In France, as of the time of writing, PWR reactor power plants have not yet been dismantled.

[0046] Globally, the number of PWR reactor power plants being dismantled is extremely limited.

[0047] Nevertheless, France's first nuclear power plant, the Fessenheim plant, was shut down in 2021, and the dismantling of the Fessenheim plant is about to begin. The operator of this nuclear power plant, EDF, is developing a dismantling plan (Non-Patent Document 2). In particular, please refer to the page of this plan that details the timeline of the various stages expected before, during, and after dismantling.

[0048] Before the dismantling itself can begin, the process must be stopped and work must be carried out to optimize the power plant. These preparations for the dismantling work are To reduce the risks and shortcomings of the facility, namely the discharge of spent and new fuel, waste and liquid waste, circuit drainage, and decontamination of some circuits, from which 99.9% of the radioactivity is being discharged at the time of writing. Preparing the power plant for demolition work, namely, arranging access and routes, support functions, especially ventilation, adapting power distribution and handling, and exhausting specific equipment to create space. To refine knowledge of the equipment's condition, that is, inventory of hazardous materials, identification of asbestos, and collection of samples for radiation analysis. The purpose is to achieve this.

[0049] The intended final state upon completion of the demolition is a nuclear-free site where all buildings have been demolished to a depth of 1 meter below the ground surface.

[0050] Figures 5A to 5D reproduce the four consecutive steps of the demolition process envisioned in Plan [3] (Non-Patent Document 3) and depicted on page 5 of that plan.

[0051] Step 1: This is electromechanical dismantling, which involves removing and cutting all equipment / components present in reactor building 1, especially those in the primary loop 21 (reactor vessel 20, pumps 22, steam generators 23, etc.), and packaging them as waste, which will be monetized where possible (Figure 5A). Only the equipment necessary to carry out the decontamination work in Step 2 will be left in place.

[0052] Step 2: Decontamination of the nuclear building structures involves removing radioactive contaminants accumulated on the inside of the buildings, particularly on the inner walls of reactor building 1 and the infrastructure 4 within it (Figure 5B).

[0053] Step 3: Demolition of the buildings, including reactor building 1 and machine room 5. Conventional buildings can be demolished and dismantled as soon as they are no longer useful. In nuclear buildings, demolition cannot begin until the structure is decontaminated in Step 2. The cavity below the ground surface is filled with backfill material consisting of rubble generated by the demolition (Figure 5C).

[0054] Step 4: Site restoration. This involves ensuring compatibility between the ground condition and future use. Any zone where the buried portion 40 of the initial infrastructure 4 is chemically or radiologically marked is subject to the soil management plan (Figure 5D).

[0055] With the demand for electricity increasing due to the electrification of numerous energy uses resulting from energy decarbonization, most of the electronic nuclear infrastructure will reach the end of its operational lifecycle within the next 20 years.

[0056] Nuclear power plants, therefore, have already been shut down or are scheduled to be shut down, even though they have not recovered most of the initial investment made during their operating life, and nuclear operators face significant investments in upgrading some or all of the infrastructure of their nuclear power plants.

[0057] The authors of Non-Patent Document 5 examine the feasibility of power plant design based on Pb-Bi FNR type reactor technology in the form of a reactor module.

[0058] The authors of Non-Patent Document 6 mention the theoretical possibility of inserting a Pb-Bi SVBR 75 / 100 FNR reactor unit into nuclear power plants, including light water reactors that have reached the end of their reactor lifespan.

[0059] Non-patent document 7 refers to the repair of an older NPP-type power plant with a Pb-Bi SVBR 75 / 100 FNR integrated block. The authors briefly and exclusively refer to the economic aspects that should be considered when carrying out such repairs, including the spatial insertion of the reactor block into the original reactor building.

[0060] Therefore, there is a need to find solutions that can reduce the investment associated with shutting down light water reactors (LWRs), particularly pressurized water reactors (PWRs) or boiling water reactors (BWRs). In particular, there is a need to find solutions that can dismantle them as currently planned. [Prior art documents] [Non-patent literature]

[0061] [Non-Patent Document 1] https: / / www.worldnuclearreport.org / IMG / pdf / 20170912wnisr2017-en-lr.pdf (World Nuclear Industry Status Report 2017) [Non-Patent Document 2] http: / / www.centrale-energie.fr / spip / IMG / pdf / 20200115_centrale_energies_final_.pdf [Non-Patent Document 3] https: / / www.edf.fr / sites / default / files / contrib / groupe-edf / producteur-industriel / nucleaire / Notes%20d%27information / dem_fessenheim_p3.pdf [Non-Patent Document 4] https: / / csti-groupe.com / 2019 / 01 / 07 / [Non-Patent Document 5] Zrodnikov et al. “Nuclear power development in market conditions with use of multi-purpose modular fast reactors SVBR-75 / 100”, Nuclear Engineering and Design, Amsterdam, Vol.235, N 14-16, 1st August 206, pages 1490-1502 [Non-Patent Document 6] Dragunov Yu G et al. “Project of SVBR-75 / 100 reactor plant with improved safety for nuclear sources of small and medium power”, 5th International Conference on Nuclear Option in Countries with Small and Medium Electricity Grids, Dubrovnik, 16 May 2014, pages 1-13, XP09003576 [Non-Patent Document 7] Zrodnikov AV et al: “Renovation of the “Old” NPP units as an Economically Effective Way of Nuclear Power Development”, Proceedings of GLOBAL 2005 Tsukuba, 9 October 2005, pages 1-6, XP09003571 [Overview of the project] [Problems that the invention aims to solve]

[0062] The objective of the present invention is therefore to address this need at least partially. [Means for solving the problem]

[0063] For this purpose, one aspect of the present invention relates to a method for modifying, or refurbishing, a nuclear power plant that initially includes at least one light water reactor (LWR), in particular a pressurized water reactor (PWR) or a boiling water reactor (BWR).

[0064] One configuration of a pressurized water reactor (PWR) includes a reactor building housing the reactor vessel, primary circuitry, and reactor pool; a fuel building; a nuclear fuel handling system for supplying nuclear fuel assemblies from the fuel building to the reactor building, into the reactor vessel, and vice versa; a machine room; a control room; and a nuclear auxiliary building. This method is followed for each reactor by the following steps, namely: a / A step of shutting down the reactor, which includes draining all fuel assemblies present in the reactor vessel to the outside of the reactor building and completely draining the primary circuit, b / A step of partially electromechanically dismantling a reactor, comprising removing and discharging the components of the primary circuit to the outside of the reactor building, with the exception of leaving the reactor vessel inside the reactor building, the step of neutralizing the reactor vessel after removing all materials from inside the reactor vessel, c / Step a / In place of some of the components of the primary circuit discharged during step a / , install at least one removable closed hybrid structure consisting of a metal double skin and concrete injected into the space between the two metal walls constituting the double skin, Step d / involves placing and holding at least one reactor, called an integrated small modular reactor (SMR), inside each hybrid structure installed in step c / , wherein the integrated SMR reactor is positioned in a location accessible by a fuel handling system. Includes.

[0065] In the context of this invention, the term "nuclear island" has the usual meaning of this technology, namely, a combination of a nuclear boiler and fuel-related equipment, as well as the equipment necessary for the operation and safety of that combination.

[0066] The term "reactor building" has its usual meaning, referring to the building that houses the reactor itself, all the components of the primary pressurization circuit, and some of the circuits for the operation and safety of the reactor.

[0067] The term "fuel building" has its usual meaning, specifically referring to a building that houses storage facilities (fuel assembly storage pools) and handling facilities for new fuel (awaiting loading into the reactor) and irradiated fuel (awaiting transfer to the processing plant).

[0068] The term "nuclear auxiliary building" has its usual meaning, referring to a building that protects the auxiliary circuits necessary for the normal operation of a nuclear reactor.

[0069] The expression "conventional island" has its usual meaning, referring to the combination of all the equipment that enables the conversion of heat generated by nuclear fission into electrical circuits, and then the cooling of those circuits.

[0070] The term "machine room" has its usual meaning, referring to the building that houses the turbine generator unit, which is responsible for converting steam generated on the nuclear island into electricity, and its auxiliary equipment.

[0071] "Neutralizing the reactor vessel" means sealing the vessel in a sealed, radiation-protective manner to permanently render the reactor unusable, leaving it in its original state as the reactor vessel, free of fuel material, and filled with an inert fluid to preserve it.

[0072] An advantageous embodiment of this method includes, after step d / , step e / , which involves fluidly and / or electrically connecting each reactor to a control room and a machine room, arranging auxiliary circuits, and fluidly and / or electrically connecting them to a nuclear auxiliary building.

[0073] In one advantageous embodiment, the installation in step c / and the arrangement in step d / involve passing each hybrid structure in the form of a prefabricated module and each integrated SMR reactor, respectively, through the same access airlock from the reactor building to the outside, from which the entirety of each component is discharged in step b / .

[0074] In a favorable variation, the removal and discharge in step b / are followed by the next successive substeps, i.e., b1 / Substep of removing the primary line located between the steam generator and the reactor vessel, b2 / Substep of removing and discharging the steam generator, b3 / Substep of removing and discharging the primary pump, b4 / Substep of removing and discharging the pressurizer, b5 / Initially, a substep to remove the primary line from the steam generator outlet through the reactor building shell, Includes.

[0075] In another favorable variation, the neutralization of the reactor vessel in step b / is followed by the next successive substep, namely, b6 / Substep of sealing and blocking the hydraulic connection of the reactor vessel, b7 / Substep of closing the reactor vessel by replacing the lid and attaching the radiation shielding cover if necessary, b8 / Substep of filling the reactor vessel with water or inert gas by connection and level or pressure monitoring device, Includes.

[0076] Step b6 / preferably involves placing a solid plug at each hydraulic connection and subsequently sealing the plug by welding, the weld preferably verified by gamma graphics.

[0077] In another advantageous embodiment, step b / includes, after neutralization of the reactor vessel, decontaminating the reactor building to remove any radioactive contaminants accumulated inside the building.

[0078] In another advantageous variant, step c / includes cutting and removing the shell and / or floor of the reactor building infrastructure that initially supports the components of the primary circuit, as well as a portion of the raft if necessary.

[0079] In another advantageous variant, step c / preferably involves securing each hybrid structure to the raft of the reactor building infrastructure by fastening plates that are themselves firmly attached or fixed to one and / or the other of the metal walls of the double skin.

[0080] In another advantageous variation, step c / involves placing the hybrid structure in the raft and fixing it if necessary, followed by the next successive substep, i.e., A substep of cutting and ejecting the shell portion that separates the reactor vessel well of the LWR reactor, which forms part of the reactor pool of each hybrid structure, A substep involves installing horizontal connecting pipes between each hybrid structure and the reactor vessel well, Includes.

[0081] One variation of this method includes, after the placement and holding of the integrated SMR reactor in step d / , placing at least one shut-off valve, preferably two, in the pipe, one on the hybrid structure side and the other on the reactor vessel well side.

[0082] The present invention further aims to provide a nuclear power plant obtained by the above-described modification method, which is, The reactor building, which houses the neutralized LWR reactor vessel and reactor pool, A nuclear fuel handling system for supplying nuclear fuel assemblies from the fuel building to the reactor building, into the reactor vessel, and vice versa, A hybrid structure comprising at least one, preferably three or four, arranged around a neutralized reactor vessel, each hybrid structure housing an integrated SMR reactor, a fuel building, and each integrated SMR reactor positioned in a location accessible by a fuel handling system, Includes.

[0083] The final number of hybrid structures housing each integrated SMR reactor will depend particularly on the power adaptations required for the power plants being modified.

[0084] An advantageous embodiment of this power plant further includes a horizontal connecting pipe between each hybrid structure and the reactor vessel well, and at least one shut-off valve on the pipe, preferably two shut-off valves, one on the hybrid structure side and the other on the reactor vessel well side, and the fuel handling system includes at least one device for tilting the fuel assemblies one by one from horizontal to vertical and enabling their transfer through the connecting pipe.

[0085] In one advantageous structural variation, each hybrid structure includes a bottom configured to support an integrated SMR reactor.

[0086] Each hybrid structure is advantageously filled with water, at least partially.

[0087] Each hybrid structure is advantageously configured to house a fixed compartment for the SMR reactor and a removable compartment for the SMR reactor when removed from the fixed compartment.

[0088] In one advantageous deformation, each hybrid structure is provided with a removable lid that contributes to the secure containment of nuclear material.

[0089] Therefore, the present invention is essentially a method for modifying a nuclear power plant, which involves removing and discharging all components of the primary circuit except for the LWR reactor vessel, all of which is emptied and neutralized; and then replacing some of these components with an integrated SMR reactor and a concrete / metal hybrid structure that serves as a reactor vessel well for the SMR reactor and is advantageously filled with water, respectively, and rearranging subassemblies that fix the SMR inside the reactor building and advantageously contribute to a third containment barrier, all of which is done with minimal changes to the reactor building infrastructure.

[0090] The hybrid structure according to the present invention functions, to some extent, as a reactor vessel well in an integrated SMR reactor, and therefore has the following functions, namely, - To meet seismic requirements, it must be securely fixed to the existing civil engineering infrastructure of the PWR reactor (raft, shell, and intermediate floor). • To be placed in water and to be biologically protected, - To provide a through connection to the main pool above the existing reactor vessel well of a PWR reactor, and to connect it to the existing fuel handling system. • By placing the entire integrated SMR reactor in a uniform volume of water defined by the internal volume of the hybrid structure, it contributes to the safe discharge of residual power. -The sealing function provided by its metallic double casing makes this possible. - Contribution to nuclear material containment safety function, namely, by closing with a removable lid at the top of the hybrid structure, the integrated SMR reactor contained and held within is placed within an enclosure that meets some or all of the requirements related to nuclear material containment assurance safety function (third barrier), - Advantages include the ability to manufacture hybrid structures from prefabricated modules, The modularity, which ensures that modules are inserted one by one in the reactor building, as in an integrated SMR reactor, allows for the adaptation of anchor points and connection points to existing civil engineering structures, ensuring that forces are absorbed, and enabling compatibility with all PWR reactor configurations. • Welded assembly, ensuring great flexibility in assembly conditions; sealing for small footprint and contribution to containment function. Because there is no shutter or shutter support, optimal insertion into existing infrastructure is possible, and the impact of modifications according to the present invention is limited to only those areas where it is necessary, optimizing the survival of the land. Buildability features that enable this It has.

[0091] In fact, the method according to the present invention is, to some extent, different from all conceivable demolition methods.

[0092] In effect, compared to the plan for dismantling a PWR as envisioned in Non-Patent Document 3, the present invention is Whether it's a building on the nuclear island (reactor building, fuel building, or auxiliary building) or a conventional building on the island (machine room), the buildings will not be demolished. The reactor vessel will not be removed from the reactor building. It is not necessary to carry out restoration work on the site itself. They are distinguished by this fact.

[0093] In other words, the technical reality is that only a few irreplaceable components in the reactor's primary circuit have reached the end of their statutory operating life, but the inventors have overcome the common prejudice among experts in the nuclear field that the complete dismantling of a nuclear power plant must be carried out to the extent of destroying all buildings and restoring the site.

[0094] Furthermore, even if it involves a reduction in output, the modification method according to the present invention makes it possible to provide a second operating phase to a 900 / 1300 MWe pressurized water reactor (PWR) nuclear power plant by replacing the PWR reactor and its three or four steam generators with an integrated SMR reactor.

[0095] The SCOR 200 integrated SMR reactor can typically be designed to produce 200 MWe. Replacing a 900 MWe PWR reactor with three SCOR-type SMR reactors would result in a power plant with an output equal to 3 × 200 / 900 = 67% of its initial output in the second operating phase, representing a 33% power reduction. For a 1300 MWe PWR reactor, the power reduction would be 38%. Other power assessments for the integrated SMR reactor planned for the NUWARD(trademark) project would likely yield similar figures.

[0096] Ultimately, the modification method according to the present invention has numerous advantages, including the following:

[0097] Reduction of initial investment in nuclear power plants through the reuse of almost all conventional islands and parts of nuclear islands, including most of the equipment, particularly civil engineering structures. In addition to the partial dismantling according to the present invention, only the remodeling of the machine rooms is required to adapt the dimensions of the equipment in the energy conversion cycle to match the power reduction associated with the modifications.

[0098] This eliminates the need to find new nuclear power plant sites, which suggests a significant reduction in environmental and financial impacts, and this conversion method maintains continuity in the surrounding regional, economic, and social environments of the different existing nuclear power plant sites being transformed. Furthermore, from a social perspective, the acceptability of existing nuclear power plant sites can be considered achieved, and the same can be applied to the conversion of these sites.

[0099] Significant reduction in construction time. Modifications to nuclear power plants using this invention are carried out according to optimized work procedures, and many tasks are prepared in advance off-site, which makes it possible to carry out at least some of the work in parallel, such as the manufacturing of prefabricated modules of hybrid structures.

[0100] Significant reduction in waste. By using the maximum capacity of the power plant buildings / equipment for the second operating phase, the amount of waste generated, including waste with exceptionally low levels of nuclear activity, is drastically reduced.

[0101] Modularization of the nuclear component of the energy mix. Modularization of power output and time, achieved by the number of reactors to be converted according to the present invention, and particularly by their arrangement within a region in the case of the French infrastructure, makes it possible to plan the nuclear component required in the energy mix on a dynamic time scale.

[0102] The modifications according to the present invention make this form of energy more durable, thereby improving the reputation of nuclear energy, namely the circular economy of materials and equipment, aging, etc.

[0103] Reducing the carbon balance of nuclear energy. The majority of this is related to the construction of facilities. Increasing the operating hours of facilities (nuclear power plants) reduces the carbon balance relative to the efficiently produced MWe.

[0104] Other advantages and features of the present invention will become clearer by referring to the following figures and reading the detailed description of embodiments of the present invention given as non-limiting examples. [Brief explanation of the drawing]

[0105] [Figure 1] The histogram shows a temporary change in the number of nuclear reactors started up and decommissioned worldwide, according to Non-Patent Document 1. [Figure 2A] These are schematic perspective and partial cross-sectional views of various configurations of existing PWR type reactors. [Figure 2B] These are schematic perspective and partial cross-sectional views of various configurations of existing PWR type reactors. [Figure 2C] These are schematic perspective and partial cross-sectional views of various configurations of existing PWR type reactors. [Figure 3] This is a schematic diagram of a prior art PWR reactor primary circuit with a configuration comprising three primary loops. [Figure 4] This is a schematic diagram of the three cycles of a prior-generation PWR type reactor. [Figure 5A] This document illustrates the various steps involved in the planned decommissioning of a PWR reactor, as described in Non-Patent Document 2. [Figure 5B] This document illustrates the various steps involved in the planned decommissioning of a PWR reactor, as described in Non-Patent Document 2. [Figure 5C] This document illustrates the various steps involved in the planned decommissioning of a PWR reactor, as described in Non-Patent Document 2. [Figure 5D] This document illustrates the various steps involved in the planned decommissioning of a PWR reactor, as described in Non-Patent Document 2. [Figure 6] This is a schematic perspective view of a hybrid structure according to the present invention, which houses an integrated SMR reactor using the SCOR integrated SMR design to illustrate an example. [Figure 6A] This is a schematic cross-sectional perspective view of a hybrid structure according to the present invention, which houses an integrated SMR reactor using the SCOR integrated SMR design for illustrative purposes. [Figure 6B] This is a schematic top view of a hybrid structure according to the present invention, which houses an integrated SMR reactor using the SCOR integrated SMR design for illustrative purposes. [Figure 7] This is a schematic top view illustrating a hybrid structure according to the present invention, which has an integrated SMR reactor using the SCOR design used in the illustrative example, with the removable upper compartment removed from its fixed compartment and the two compartments housed side by side within the hybrid structure. [Figure 8]This is a perspective view showing one variation of the hybrid structure according to the present invention, which includes a removable upper closing lid that closes the structure at the top of an integrated SMR reactor. [Figure 9] This is a cross-sectional perspective view showing the interior of the metal portion of the hybrid structure according to the present invention. [Figure 10] This is a cross-sectional perspective view showing the interior of a modified metal portion of the hybrid structure according to the present invention, which consists of prefabricated modules. [Figure 11] This is a diagram of a steam generator, similar to those installed in 900 / 1300 MWe PWR power plants, which are electronic nuclear bases for pressurized water reactors (PWRs). [Figure 12] This is a partial cross-sectional perspective view of another integrated SMR project, namely the SCOR project, in which a removable compartment is fixed to the top of a fixed compartment. [Figure 13] This schematic diagram illustrates the physical possibility of integrating the three hybrid structures according to the present invention in place of the pumps and steam generators in the primary circuit of an existing PWR reactor. [Figure 14A] First, we will outline the various steps involved in modifying nuclear power plants, including PWR reactors. [Figure 14B] First, we will outline the various steps involved in modifying nuclear power plants, including PWR reactors. [Figure 14C] First, we will outline the various steps involved in modifying nuclear power plants, including PWR reactors. [Figure 14D] First, we will outline the various steps involved in modifying nuclear power plants, including PWR reactors. [Figure 14E] First, we will outline the various steps involved in modifying nuclear power plants, including PWR reactors. [Figure 15] This is a schematic cross-sectional perspective view of a nuclear power plant converted by the modification method according to the present invention. [Modes for carrying out the invention]

[0106] Throughout this application, the terms “vertical,” “lower,” “upper,” “lower,” “higher,” “downward,” and “upward” should be understood in reference to an integrated SMR reactor located in a reactor building of a power plant and in a vertical operating configuration, and which is installed in the reactor building using the modification method according to the present invention.

[0107] Figures 1 through 5D have already been explained in detail in the introduction, so they will not be explained below.

[0108] For clarity, the same elements in the present invention and the prior art are designated by the same reference numerals in all of Figures 1 through 15.

[0109] The various diagrams do not represent all of the fluid, electrical, control and command connections or instrumentation systems required for the operation of a nuclear power plant converted by the method according to the present invention. In particular, fluid lines for steam and associated piping are not mentioned because there are no requirements for the primary integration of these lines in the architecture. In particular, steam and water fluid supply lines from and to the integrated SMR reactor, which need to pass through the hybrid structure, are not shown.

[0110] As a preliminary step to describing the method according to the present invention for modifying nuclear power plants, the essential means used and the feasibility of integrating these various means into existing reactor buildings are described.

[0111] Figures 6, 6A, and 6B show a hybrid structure according to the present invention, shown overall by reference numeral 6, which houses an integrated SMR reactor shown overall by reference numeral 7, and is intended to be installed in place of a subassembly consisting of the primary pump and steam generator of an existing PWR reactor primary circuit. In these Figures 6, 6A, and 6B, the SCOR type integrated SMR design is selected as representative.

[0112] The hybrid structure 6 functions to some extent as the reactor vessel well of the integrated SMR reactor 7, and therefore, as its primary function, has the advantage of housing and supporting such a reactor, the associated civil engineering functions (fixing, strength, sealing, constructability), and the ability to handle fuel or store the removable compartment 71 of the integrated SMR reactor 7 in water for maintenance phases of those inside the fixed compartment 70 of the SMR.

[0113] The hybrid structure 6 consists of a double metal skin, i.e., two spaced metal walls 60 and 61, with the space between these two walls 60 and 61 filled with concrete 62.

[0114] The support floor 63 is positioned substantially horizontally as the bottom inside the inner wall 61, supporting the integrated SMR reactor 7.

[0115] The sealed internal volume of the hybrid structure 6 is therefore demarcated by an inner wall 61 and a bottom 63. This is intended to be filled with water to function as a biological barrier and, depending on the configuration of the integrated SMR 6, contributes to the function of discharging residual output from the SMR.

[0116] Furthermore, as shown in Figure 7, the structure 6 is sized such that this internal volume can be submerged in water next to the fixed compartment 70 of the integrated SMR reactor 7, and the removable compartment 71 is removed from the top of the fixed compartment 70. This allows for safe fuel handling and / or maintenance work in the fixed compartment 70, as the two compartments 70 and 71 are submerged in a uniform volume of water. The removable compartment 71 is handled by large component handling equipment used for inserting the integrated SMR 7.

[0117] As shown in Figure 8, the hybrid structure 6 is preferably provided with a metal lid 64 that is removably connected in a sealed manner to one and / or the other of the metal walls 60, 61 of the hybrid structure. When this lid 64 is installed, the structure 6 itself contributes to the safe and controlled containment of nuclear material, consisting first of a metal sheath enclosing the fuel in the integrated SMR reactor 7, and second of a casing that constitutes the reactor vessel of the integrated SMR reactor 7.

[0118] Furthermore, the hybrid structure 6 has an opening P through it. As will be described in detail below, this opening P is intended to be connected to a pipe for transporting fuel assemblies from and into the SMR reactor 7.

[0119] As shown in more detail in Figure 9, the hybrid structure 6 first includes a metal anchor plate 65 that is fixed to the raft 41 of the reactor building, thereby enabling the hybrid structure 6 to be fixed to the existing infrastructure 4 of the reactor building 1. In the illustrated example, this anchor plate 65 is welded to the internal metal wall 60. In addition to the plate 65 welded to the internal metal wall 60, another anchor plate welded to the external metal wall 61 can also obviously be envisioned. The connection to the raft 41 can be provided using various specific civil engineering techniques, depending on the strength requirements derived from structural studies, particularly regarding loads under seismic conditions.

[0120] In space 62, retaining bars are welded between walls 60 and 61 inside a double metal outer shell to maintain the spacing between them.

[0121] To reinforce the concrete within the space 62, reinforcing bars 67 are welded to metal studs 66, 68 which are welded to one and / or the other of the metal walls 60, 61.

[0122] Furthermore, although not shown in the illustration, in particular, To serve as a support for the bottom 63, which is intended to serve as a support for the integrated SMR reactor 7, To support the pipes and other auxiliary items necessary for the operation of the integrated SMR reactor 7, To mechanically reinforce the entire structure and achieve an overall structural strength at least equivalent to that before the implementation of the hybrid structure 6, the hybrid structure 6 is connected to the intermediate floor 42 and shell 43 of the existing infrastructure 4 of the reactor building 1, Other plates can be welded to one and / or the other of the metal walls 60, 61.

[0123] Figure 10 shows a variation of the hybrid structure 6, which consists of modules M1, M2, M3, and M4 that are manufactured off-site in advance and then assembled on-site, i.e., inside the reactor building 1. In this variation, the structure 6 may include mechanical reinforcements 69 positioned on top of the structure. This variation is advantageous because, depending on the type of reactor building 1 and its existing inlet airlock, which was initially designed to replace the steam generator 23, the size of each module can be adapted to have the maximum dimensions that will allow it to be inserted through the inlet airlock. This further optimizes the time and cost of the modification according to the present invention. Hybrid structures have already been introduced internationally for nuclear facilities, and a project is currently approved in France. See Non-Patent Literature 4.

[0124] Although not mandatory, the modification method according to the present invention is advantageous when all components related to the modification are handled with no or minimal impact on the infrastructure 4 of the reactor building 1, as will be detailed below.

[0125] The inventors analyzed that this suggests the ability to remove all components of the primary circuit of an existing PWR reactor (primary pump 22, steam generator 23, and pressurizer 24) through an airlock provided for this purpose, and to insert all the bulkiest components of the new primary circuit (hybrid structure 6, integrated SMR reactor 7) through the same airlock during the conversion phase.

[0126] The possibility of handling structure 6 before pouring concrete has been demonstrated by the above.

[0127] The inventors therefore also verified in advance that the integrated SMR reactor 7, after being fully assembled, can also be handled as a single block via the same handling route, namely the entrance airlock of the reactor building.

[0128] Figure 11 shows an existing steam generator 23 inside a PWR reactor. The overall dimensions of this type of steam generator 23 are approximately H1*L1, which is about 22m high and 5m wide, making it possible to insert it through the existing entrance airlock of the reactor building 10 for replacement.

[0129] Figure 12 shows the integrated SMR reactor 7 of the SCOR project. Its overall dimensions H2*L2 are smaller than the H1*L1 of the steam generator 23.

[0130] The maximum overall dimensions for this example of the Integrated SMR Reactor 7 project are less than 22m*5m, allowing for insertion through the entrance airlock into the reactor building 1 via a handling system for the steam generator 23, as initially designed. A handling system that positions horizontally and then tilts vertically is also suitable for this example. Similarly, its mass is suited to the load capacity of the equipment in the handling system.

[0131] As a result, the integrated SMR reactor is successfully inserted into reactor building 1 without affecting its infrastructure 4.

[0132] The inventors then considered the optimal arrangement that the hybrid structure 6 should have while the integrated SMR reactor 7 is located in the reactor building 1.

[0133] To optimize the layout and cost of the modification method, the inventors adopted the following integrated criteria: To limit the impact on the infrastructure 4 that secures the integrated SMR reactor 7, To maximize the reuse of existing infrastructure elements, the functionality of various barriers, biological protection, etc. To functionally integrate an integrated SMR reactor by optimizing its connections to two existing functional systems: one dedicated to fuel handling and another dedicated to exhausting power into the machine room. We adopted it.

[0134] Based on the above criteria, the layout of the integrated SMR reactor was determined through 3D critical path analysis.

[0135] The optimal placement is, At height (z), according to the arrangement of fuel handling / biological protection systems, In the (x,y) plane viewed from above, instead of the axisymmetric steam generator 23, The inventors concluded that...

[0136] In addition to these two placement parameters, the inventors have analyzed that during the operating phases of an integrated SMR reactor, additional space is needed for removable compartments 71 that must be removed from its fixed compartments 70 for fuel loading / unloading and / or maintenance of internal components.

[0137] In a converted nuclear power plant with three or four integrated SMR reactors, it is preferable that the removable compartment 71 and the reactor layout be given equal consideration.

[0138] By analyzing the spatial configuration of the current PWR primary circuit, the inventors found this optimal layout. In fact, in each primary loop 21, the primary pump 22 is spatially back-to-back with the steam generator 23 to which it is associated. Therefore, if an integrated SMR reactor 7 is installed in place of the steam generator 23, the space occupied by the primary pump 22 can be freed up by a removable compartment 71. This optimal configuration is schematically shown in Figure 13. Ultimately, it becomes possible to allocate a dedicated space for its removable upper part to each SMR, and to consider installation configurations that require opening all SMRs simultaneously.

[0139] Next, with reference to Figures 14A to 14E, various steps of the method according to the present invention for modifying an existing PWR reactor nuclear power plant will be described, taking the above analysis into consideration.

[0140] Step a: The PWR reactor is shut down.

[0141] This initial step aims to enable the configuration of the site for the remodeled power plant.

[0142] All fuel assemblies present in the reactor vessel 20 are discharged to the outside of the reactor building 1. The primary circuit 2 is then completely drained.

[0143] A safety analysis conducted before opening the site can indicate whether the fuel assemblies can remain in the fuel building pool for the duration of the site's existence. In this case, the modification site (steps b / and c / ) can be opened without waiting for the fuel assemblies to reach a residual output that complies with the regulations for the transport of nuclear material, thus saving time required to complete the work plan.

[0144] Step b: Partial electromechanical dismantling of the PWR reactor is performed. The components of the primary circuit 2 are therefore preferably carried out in the following successive substeps, namely, b1 / Dismantling of the primary line 21 located between the steam generator 23 and the reactor vessel 20. b2 / Dismantling and discharge of steam generator 23, b3 / Dismantling and discharge of primary pump 22, b4 / Dismantling and discharge of pressurizer 24, b5 / First, the dismantling of the primary line 21 from the outlet of steam generator 23 through the shell of the reactor building, It is then dismantled and discharged outside of reactor building 1.

[0145] Only the reactor vessel 20 remains in the reactor building 1 (Figure 14A). In fact, leaving the reactor vessel 20 as is does not prevent the achievement of the modified installation configuration. Furthermore, the inventors believe that by leaving the reactor vessel as is during the operation phase of a nuclear power plant modified with an integrated SMR reactor, the activating material, particularly Co 60 The opinion is that there are times when it decreases.

[0146] Meanwhile, all material is removed from inside the reactor vessel 20, and then the reactor vessel 20 is neutralized.

[0147] For this purpose, the following substeps are taken: b6 / If possible, solid plugs are placed at each hydraulic connection, and the plugs are then sealed and welded to seal and block the hydraulic connections of the reactor vessel, and the welds are preferably verified by gamma graphics. b7 / Reattach the reactor vessel lid, seal all passages for the control rods in advance, and close the reactor vessel by installing radiation shielding covers if necessary. b8 / The reactor vessel will be filled with water or inert gas by connection and pressure control and / or liquid level control devices, and the filling and liquid level control devices will be located inside the reactor building and, in particular, may be connected to the reactor vessel by again using one or more passages through the cover to provide fluid connections. The action will be carried out.

[0148] If necessary, the lid of the reactor vessel 20 can be modified to ensure perfect sealing and / or to optimize the neutralization of the reactor vessel.

[0149] After neutralizing the reactor vessel 20, if necessary, the interior of the reactor building 1 will be decontaminated to remove any radioactive contaminants that may have accumulated there.

[0150] Taking radiation protection into consideration, steps a / and b / are performed either by human intervention or remotely.

[0151] Step c / : Hybrid structure 6 is installed.

[0152] Prior to step c / , a seismic strength survey of the entire configuration of the nuclear island can be performed to define, in particular, all connections between the infrastructure 4 of the reactor building 1 and the hybrid structure 6, the dimensions of the hybrid structure 6, usually the thickness of the plates for the walls 60, 61, the density and dimensions between the walls 60, 61, as well as the studs and connecting rods, the method of fastening to the raft 41, and the connections with the floor 43 and shell 42 that are connected to the hybrid structure 6.

[0153] Step c / includes cutting and removing a portion of the shell 42 and / or floor 43 of the infrastructure 4 of the reactor building 1, as well as the raft 41 if necessary.

[0154] This allows for the curing of the hybrid structures 6 and the provision of all the necessary equipment for securing them to the infrastructure 4.

[0155] Furthermore, step c / involves creating an opening within the shell 42 that connects to the existing lower pool of the reactor in order to connect to the fuel handling system. Core drilling technology is advantageously used for this task.

[0156] Figure 14B shows Space E that needs to be opened for the installation of hybrid structure 6. A circular opening O leads to a pool above the reactor vessel 20. This indicates.

[0157] All cutting operations can be carried out using concrete cutting equipment already widely used in nuclear applications. Preparation work for existing infrastructure 4 can also be performed.

[0158] Once these operations, which involve cutting the infrastructure and removing the severed sections, are completed, a hybrid structure consisting of prefabricated modules enters the entrance airlock of reactor building 1. The modules can typically be introduced in the form of horizontal sections with a unit height of 5 meters.

[0159] Next, the hybrid structures 6 are appropriately positioned. This positioning involves fixing them to the infrastructure 4 of the reactor building 1. In particular, each hybrid structure 6 is fixed to the raft 41 by fixing plates 65. The prefabricated modules are welded to each other and anchored to the shell 42 and floor 43. Furthermore, sealing is performed for the compartments beneath the reactor vessel 20.

[0160] Once each fixing device of the hybrid structure 6 is positioned and secured in place, a metal horizontal connecting pipe 80 is installed between each hybrid structure and the reactor vessel well 20, preferably by sealing welding to the two metal walls 60, 61 of the double casing. On the side of the pool above the reactor vessel 20, the pipe 80 is welded to the pool liner to ensure sealing of the pipe 80. This pipe 80 is a transfer pipe that allows fuel assemblies to be handled by a handling system.

[0161] An integrated SMR reactor 7 is placed and held in each hybrid structure 6 installed in steps d / and c / .

[0162] As described above, the integrated SMR reactor 7 is positioned in a location accessible by the existing fuel handling system.

[0163] Each integrated SMR reactor 7, initially manufactured off-site, is brought into the reactor building by an existing handling system and placed directly into the bottom 63 of a hybrid structure 6 designed for this purpose.

[0164] Finally, shut-off valves 81 and 82 are installed at the ends of each pipe 80 (Figures 14D and 14E).

[0165] Step e: This is followed by fluid and / or electrical connections to the control and machine rooms of each integrated SMR reactor 7.

[0166] Auxiliary circuits are installed, and fluid and / or electrical connections are made to the nuclear auxiliary building.

[0167] Figure 15 shows the internal architecture of the reactor building 1 of an original PWR power plant after conversion using the modification method of the present invention, in which three hybrid structures 6 each house and support an integrated SMR reactor 7.

[0168] The present invention is not limited to the examples described herein, and in particular, the features of the illustrated examples can be combined with each other in variations not shown.

[0169] Other modifications and embodiments can be envisioned without departing from the scope of the present invention.

[0170] In the illustrated example, the hybrid structure is sized to optimize the integration of the integrated SMR reactor 7 and its removable compartments 71 during operation. However, it is also possible to envision a solution for reducing the dimensions of the hybrid structure, i.e., a solution for the mutual layout of all removable compartments 71 after their respective fixed compartments 70 have been removed.

[0171] In the context of the present invention, it can be assumed that the removable compartments of an integrated SMR reactor are handled at the bottom of the hybrid structure, or at least next to the fixed compartments of the SMR, under the same water.

[0172] In the illustrated example, the means of transferring fuel assemblies from the integrated SMR reactor 7 to the pool above the reactor vessel 20 is limited to a single pipe 80, so that valves 81 and 82 can isolate different volumes of water (the internal volume of the hybrid structure 6 and the pool above the reactor vessel 20). This choice necessitates the horizontal transfer of fuel assemblies and, therefore, the introduction of a vertical / horizontal tilting device, as fuel assemblies must be introduced horizontally into the pipe 80 once they have been vertically extracted from the interior of the integrated SMR reactor 7. This horizontal position can be maintained until the assemblies exit the reactor building 1, as they are then transferred to the fuel building.

[0173] As a variation, the transfer pipe 80 can be replaced with a free-surface channel equipped with a weir or enclosure to isolate its volume of water. The weir functions as a valve in the sense that it allows for hydraulic isolation between the two compartments it separates. This type of device makes it possible to omit horizontal / vertical tilting devices.

[0174] The illustrated example of modification methods relates to a PWR reactor. This type of method can also serve as the basis for modifying a BWR reactor, subject to adaptations related to the specific configuration of that type of reactor compared to a PWR, and these modifications are obvious to those skilled in the art of reactor technology. [Explanation of symbols]

[0175] 1. Reactor building 10 Inner wall 12 Exterior Walls 13 Ring Space 20 Reactor vessel 22 Primary pump 23 Steam generator 4. Infrastructure 41 Raft 42 shells 43 beds 6 Hybrid structure 60 internal metal wall 61 Exterior metal wall 62 Space 63 Bottom 64 Lid 65 Anchor Plate 66 metal studs 67 Reinforcement bar 69 Reinforcement material 7. Integrated SMR reactor 70 fixed compartments 71 Removable compartments 80 pipes 81, 82 Shut-off valves

Claims

1. A method for modifying a nuclear power plant that initially includes at least one light water reactor (LWR), particularly 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 supplying nuclear fuel assemblies from the fuel building to the reactor building, into the reactor vessel, and vice versa; a machine room (5); a control room; and a nuclear auxiliary building, wherein for each reactor, a / A step of shutting down the reactor, which includes discharging all of the fuel assemblies present in the reactor vessel (20) to the outside of the reactor building and completely draining the primary circuit (2), b / A step of partially electromechanically dismantling the reactor, comprising removing and discharging the components of the primary circuit (21, 22, 23, 24) to the outside of the reactor building, with the exception of leaving the reactor vessel (20) of the reactor inside the reactor building, the step of neutralizing the reactor vessel after removing all materials from inside the reactor vessel, Step c / Step a / In place of some of the components of the primary circuit discharged in step a / , install at least one removable closed hybrid structure (6) consisting of a double metal skin (60, 61) and concrete injected into the space (62) between the two metal walls constituting the double skin, Step d / Step c / involves placing and holding at least one reactor (7) called an integrated small modular reactor (SMR) inside each hybrid structure installed in step c / , wherein the integrated SMR reactor is positioned in a location accessible by the fuel handling system, Methods that include...

2. The modification method according to claim 1, further comprising step e / after step d / , which involves fluidly and / or electrically connecting each reactor to the control room and the machine room, arranging auxiliary circuits, and fluidly and / or electrically connecting to the nuclear auxiliary building.

3. The modification method according to claim 1, wherein the installation in step c / and the arrangement in step d / respectively involve passing each hybrid structure in the form of a prefabricated module and each integrated SMR reactor through the same access airlock from the reactor building to the outside, from which the entirety of each component is discharged in step b / .

4. The dismantling and discharge in step b / are carried out in the following successive substeps, namely, b1 / A substep of dismantling the primary line (21) located between the steam generator (23) and the reactor vessel (20), b2 / A substep of dismantling and discharging the steam generator (23), b3 / Substep of dismantling and discharging the primary pump (22), b4 / Substep of dismantling and discharging the pressurizer (24), b5 / Firstly, a substep of dismantling the primary line (21) from the outlet of the steam generator to the shell of the reactor building, The modification method according to claim 1, including the modification method described in claim 1.

5. The neutralization of the reactor vessel in step b / is carried out in the following successive substeps, namely, b6 / A substep of sealing and blocking the hydraulic connection of the reactor vessel, b7 / A substep of closing the reactor vessel by returning the lid of the reactor vessel to its original position and, if necessary, attaching a radiation shielding cover, b8 / The step of filling the reactor vessel with water or inert gas by connection and level or pressure monitoring device, The modification method according to claim 1, including the modification method described in claim 1.

6. Step b6 / is to place a solid plug at each hydraulic connection, and then seal-weld the plug, the welding of which is preferably verified by gamma graphics, the modification method according to claim 5.

7. Step b / includes, after neutralizing the reactor vessel, decontaminating the reactor building to remove any radioactive contaminants accumulated inside the building, the modification method according to claim 1.

8. The modification method according to claim 1, step c / comprising cutting and removing a portion of the shell (42) and / or floor (43) of the reactor building infrastructure (4) that initially supports the components of the primary circuit, and, if necessary, a portion of the raft (41).

9. The modification method according to claim 1, wherein step c / preferably includes securing each hybrid structure to the raft of the reactor building infrastructure by fixing plates (65) which are themselves firmly attached or fixed to one and / or the other of the metal walls of the double skin.

10. Step c / involves placing the hybrid structure on the raft and securing it if necessary, followed by the next series of substeps, i.e., A substep of cutting and discharging the shell portion that separates the reactor vessel well of the LWR reactor, which forms part of the reactor pool of each hybrid structure, A substep involves installing a horizontal connecting pipe (80) between each hybrid structure and the reactor vessel well, The modification method according to claim 1, including the modification method described in claim 1.

11. The modification method according to claim 10, comprising, after the arrangement and holding of the integrated SMR reactor in step d / , arranging at least one shut-off valve (81, 82), preferably two shut-off valves, one on the hybrid structure side and the other on the reactor vessel well side, in the pipe.

12. A reactor building (1) housing a neutralized LWR reactor vessel (20) and reactor pool within a reactor vessel well, A nuclear fuel handling system for supplying nuclear fuel assemblies from the fuel building to the reactor vessel inside the reactor building, and vice versa, At least one, preferably three or four, hybrid structures (6) are arranged around the neutralized reactor vessel, each hybrid structure housing an integrated SMR reactor (7), a fuel building, and each integrated SMR reactor is positioned in a location accessible by the fuel handling system. A nuclear power plant obtained by the modification method described in claim 1, which includes the modification method described in claim 1.

13. A nuclear power plant according to claim 12, further comprising a horizontal connecting pipe (80) between each hybrid structure and the reactor vessel well, and at least one shut-off valve (81, 82) on the pipe, preferably two shut-off valves, one on the hybrid structure side and the other on the reactor vessel well side, wherein the fuel handling system includes at least one device for tilting the fuel assemblies one by one from horizontal to vertical to enable the transfer of the fuel assemblies through the connecting pipe.

14. The nuclear power plant according to claim 12, wherein each hybrid structure includes a bottom (63) configured to support an integrated SMR reactor (7).

15. The nuclear power plant according to claim 12, wherein each hybrid structure is at least partially filled with water.

16. The nuclear power plant according to claim 12, wherein each hybrid structure is configured to house a fixed compartment (70) of the SMR reactor and a removable compartment (71) of the SMR reactor when removed from the fixed compartment.

17. The nuclear power plant according to claim 12, wherein each hybrid structure is provided with a removable lid (64) that contributes to the nuclear material containment control safety function.

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