Method for producing a light water nuclear reactor core, in particular a pressurized water reactor (PWR) with batch operation at low volumetric power and temperature, comprising fuel assemblies of the UOX type that are low in 235U enriched and highly poisoned.
A PWR core design with uranium-235 enriched fuel assemblies and gadolinium poisons, combined with a core control scheme, addresses the need for long-cycle operation without soluble boron, achieving efficient and cost-effective heat production for industrial use.
Patent Information
- Authority / Receiving Office
- FR · FR
- Patent Type
- Applications
- Current Assignee / Owner
- COMMISSARIAT A LENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES
- Filing Date
- 2024-12-18
- Publication Date
- 2026-06-19
AI Technical Summary
Current nuclear reactors designed for heat generation lack a design that allows for long cycle operation without soluble boron, which complicates the core design and increases costs, and do not meet the specific thermal power and temperature requirements for industrial heat production.
A method for designing a pressurized water reactor (PWR) core with a batch operation that includes a loading plan for fuel assemblies arranged in a symmetrical three-dimensional pattern, using uranium oxide fuel rods enriched to less than 5% in uranium-235 and neutron poisons like gadolinium, along with a core control scheme to maintain reactivity and power limits, allowing a 10-year cycle without refueling.
The solution enables a PWR core to operate at low thermal power and temperature, providing consistent heat for industrial use with reduced operating costs and simplified fuel handling by eliminating the need for a fuel building, achieving a 10-year cycle with minimal residual power after shutdown.
Smart Images
Figure 00000000_0000_ABST
Abstract
Description
Title of the invention: Method for making a light water nuclear reactor core, in particular pressurized water (PWR) operating in batches at low volumetric power and temperature, comprising fuel assemblies of the UOX type that are low in 235U enriched and highly poisoned. technical field
[0001] The present invention relates to the general field of light water reactors (LWRs), in particular pressurized water reactors (PWRs). More specifically, it relates to the field of so-called small or medium power reactors or SMRs (Small Modular Reactor), designed for heat generation.
[0002] By "SMR reactor", we mean here and within the framework of the invention, the usual technological meaning, namely a nuclear fission reactor, of smaller size and power than conventional REL reactors, of which a block is manufactured in a factory and transported to a nuclear site for installation.
[0003] By “reactor block”, we mean here and within the framework of the invention, the vessel, called reactor vessel as well as all the components and part of the fluidic circuit, in particular the reactor core which creates heat by nuclear fission reactions, which is housed inside the reactor vessel.
[0004] In general, the invention applies to a nuclear reactor core of the family of so-called second, third, fourth generation (GEN IV) reactors.
[0005] The main objective of the invention is to propose a new design for a nuclear reactor core intended for heat generation.
[0006] For the purposes of this invention, "heat-producing reactor" refers to a nuclear installation, nuclear power plant, or nuclear reactor whose core power is primarily dedicated to heat production. A heat-producing reactor may be 100% dedicated to heat production. However, a small portion of its power may also be used to generate electricity. Previous technique
[0007] In the context of climate and energy transition, the nuclear industry must meet several challenges for the future. Indeed, to address tomorrow's energy and societal challenges, it will be necessary to design nuclear reactors that enable:
[0008] - to limit the need for a so-called "environmental" liquid cold source (rivers, rivers, sea) and associated discharges into the environment;
[0009] - to be more flexible and therefore more complementary to other so-called energies renewables (RES), to meet fluctuating electricity demand and the intermittency of RES;
[0010] - to decarbonize processes by supplying heat to industries energy consumers (desalination, heat networks, hydrogen...) while increasing energy efficiency;
[0011] - to capture atmospheric CO2 to limit the effects of warming climate and contribute to closing the carbon cycle as a source of carbon for industrial processes;
[0012] and this without degrading the profitability of the installation, either by taking economic advantage of the new services provided, or by significantly increasing the amount of electricity produced during the day.
[0013] In this current general context of energy decarbonization, heat production presents a significant market to be decarbonized.
[0014] One of the current development topics for nuclear reactors concerns so-called heat-generating reactors intended to provide a thermal power level of a few tens of MWth, for the purpose of producing decarbonized hot water at the temperature required by industrial processes, mainly for the supply of so-called urban heat, i.e. in urban networks, for cities / agglomerations of several hundred thousand inhabitants.
[0015] Historically, the use of nuclear energy for purely power generation purposes has constrained the design of nuclear facilities.
[0016] Indeed, nuclear reactors which are generally used to produce electricity have a reactor core with fuel elements which undergo chain fission reactions to heat the water and steam circuits which supply a thermal energy-to-electricity conversion system, such as a Rankine cycle.
[0017] For a heat-producing reactor, the thermal power produced in the core allows for the heating of water circuits, including a downstream circuit that goes to the end users, i.e., factories that use superheated water.
[0018] In general, a low thermal power output from the core is sufficient for industrial heat requirements. Typically, 50 MWth is enough to supply several factories operating in the same geographical area. The core of the nuclear reactor will then be smaller compared to the cores of power-generating reactors, which have much higher thermal power outputs, typically exceeding 2700 MWth in France.
[0019] Nuclear reactors operate according to cycles which are generally 2 years, with part of the fuel being renewed between each cycle.
[0020] More specifically, the cores of power-generating reactors are designed to be fractional, that is to say to operate in short cycles, typically between 1 and 2 years, with a partial refueling of the fuel at each irradiation cycle in order to consume it to the maximum, that is to say to increase the burn-up rate ("bum-up" in Anglo-Saxon language) for the unloading of the fuel.
[0021] In a fractionated core, part of its fuel assemblies is new, that is to say, has not undergone an irradiation cycle; part has already undergone a first irradiation cycle, with its fuel assemblies therefore being partially worn and their positions in the core changing with each cycle; another part has already undergone two cycles, ...
[0022] This splitting implies an increase in operating costs due to frequent reactor shutdowns for refueling and the construction of a building called a fuel building in which the irradiated assemblies are stored before their transfer to other plants.
[0023] To reduce these costs, batch reactor cores have already been considered, i.e. designed so that all the assemblies loaded new at the beginning of a cycle are all unloaded from the reactor vessel and sent to a fuel reprocessing plant at the end of the cycle.
[0024] Batch cores have already been described in the literature but their thermal power does not correspond to the specificities of a reactor intended for heat generation.
[0025] Thus, a batch reactor core makes it possible to reduce costs, because the cycle time is extended.
[0026] Typically, in France, a target cycle length can be 10 years so that reactor shutdowns coincide with the ten-year inspections by the Nuclear Safety Authority (ASN). In other words, a batch core operates continuously for a 10-year cycle. Furthermore, during the shutdown between cycles, all fuel assemblies are replaced.
[0027] These long cycle times are rarely mentioned in the literature. Nevertheless, reactors operating with cycles of 6.5, 10 or 20 years have already been mentioned, but their technology, power or application do not correspond to the specifications of a heat-producing reactor.
[0028] Among the various technological solutions that provide heat from nuclear fission, it is generally accepted that to date, pressurized water reactors (PWRs) are the most suitable for providing heat at relatively low temperatures.
[0029] In a PWR reactor, the fuel assemblies are immersed in water, also called a moderator, which has a dual function, namely to slow down the neutrons to to allow the fission reaction on fuel enriched in 235U and to cool the fuel elements.
[0030] For this type of reactor, the reactivity of the core is generally controlled by the addition of soluble boron in the moderator and by the insertion by vertical translation from the top of absorbing clusters in the active zone of the core, i.e. the zone of presence of the nuclear fuel.
[0031] The use of soluble boron complicates the design of the core and therefore increases costs.
[0032] In the literature, some concepts without soluble boron propose to control the reactivity of the core with absorbing clusters; other concepts exist but whose power or application does not correspond to the configuration targeted by the invention.
[0033] We therefore aim to design a core that integrates the various constraints mentioned above.
[0034] There is therefore a need for a solution for a PWR type reactor with a heat-generating purpose, of low volumetric power with a batch core which can operate in the absence of soluble boron and for a cycle typically of at least 10 years.
[0035] The object of the invention is to meet at least partially this need. Description of the invention
[0036] To this end, the invention relates, in one of its aspects, to a method for producing a light water nuclear reactor core, in particular a pressurized water reactor (PWR), comprising the following steps:
[0037] i / definition of a loading plan for the fuel assemblies constituting the core, according to four zones arranged in a three-dimensional pattern symmetrical with respect to two orthogonal planes, comprising the following sub-steps:
[0038] he / choose a total number of assemblies,
[0039] i2 / define, as a function of the total number chosen in substep il / , the position of the Assemblies, which yields a number n of zoning types,
[0040] i3 / define for each of the n zoning types and for each of the four zones, the nature of the assemblies defined by a percentage of neutron poison, the number of pencils containing this percentage of neutron poison and an enrichment percentage, considering that:
[0041] each of the four zones is made up of assemblies having the same initial isotopic composition, and that
[0042] The fuel assemblies each comprise uranium oxide (UOx) fuel rods enriched to less than 5% in uranium-235 (235U), fuel rods containing the defined percentage of neutron poison, and guide tubes adapted to house by inserting each one of the number of neutron absorber rods grouped in a cluster,
[0043] i4 / determine the possible combinations between the number of assemblies, the number n types of zoning and the nature of the assemblies,
[0044] i5 / select from the possible combinations those for which the coefficient the effective multiplication (keff) is greater than one, which corresponds to a state where the heart's reactivity is not subcritical at the beginning of life, then those leading to a predetermined lifespan T,
[0045] ii / once the loading plan has been defined according to step i / , define a core control scheme which consists of:
[0046] i1 / consider at least three groups of neutron-absorbing pencil clusters (PI, P2, P3) whose number and / or neutron-absorbing material differs from one group of clusters to another,
[0047] ii2 / first insert one of the groups into the core areas exhibiting the greatest reactivity in DDV then modify the positions of the clusters of each group, the number of clusters in a group, the total number of groups, their order of insertion in the core, their overlap in the core, and the nature of their neutron absorber until a value is obtained that respects a power limit (Fxy) to avoid boiling of the water, over the entire duration of the reactor cycle, and this while keeping the effective multiplication coefficient of the core (keff) equal to 1 throughout the cycle.
[0048] Preferably, the assemblies according to step i3 / include pencils containing a percentage of neutron poison from gadolinium pencils of composition UO2-Gd2O3 and / or erbium (Er2O3).
[0049] Preferably, step i3 / consists of defining four types of assemblies having different enrichments and a number of pencils containing a different percentage of neutron poison as follows:
[0050] - assemblies enriched to 1.5% in 235U and eight gadolinium-coated pencils,
[0051] - assemblies enriched with 2% in 235U and eight gadolinium-coated pencils,
[0052] - assemblies enriched with 3% in 235U and twenty-four gadolinium-coated pencils,
[0053] - assemblies enriched with 4% in 235U and sixteen gadolinium-coated pencils,
[0054] According to an advantageous embodiment, the at least two criteria of step i5 / are at least two criteria of peak power in evolution, and / or combustion rate and / or reactivity at the beginning or end of the cycle.
[0055] According to this mode, the at least two criteria of step i5 / are respectively that the maximum two-dimensional core power peak reached during the evolution (Fxy) and the effective multiplication coefficient of the core (keff) at Early Life (DL) calculated by considering all clusters inserted into the core to be minimal.
[0056] According to an advantageous embodiment, the fuel assemblies each comprise a number equal to 17x17 uranium oxide (UOX) fuel rods enriched to less than 5% in uranium 235 (235U) and whose fissile column height is less than or equal to 1.60 m.
[0057] The invention also relates to a computer program comprising instructions which, when the program is executed by a computer, lead the computer to implement step i / of the process described above.
[0058] The invention also relates to a light water nuclear reactor core, in particular pressurized water (PWR), of batch type, obtained by implementing the process as defined above.
[0059] The intended operation is preferably at a thermal power, less than or equal to 300MWth, typically equal to 50 MWth.
[0060] The invention finally relates to the use of the reactor core described above to provide heat at a temperature below 200°C, typically equal to 150°C.
[0061] Thus, thanks to the process according to the invention, a PWR nuclear reactor core is obtained which can operate with a single cycle for 10 years and in batch, without partial refueling, i.e. with at the beginning of the cycle, all the assemblies new, and at the end of the cycle all the assemblies unloaded to be reprocessed or permanently stored.
[0062] This core operates at low thermal power and low temperature imposed by the target markets, in particular that of industrial heat.
[0063] Despite the long cycle duration, the low thermal power, combined with the number of assemblies, results in a low operating power volume throughout the cycle. This leads to a low residual power of the fuel assemblies after reactor shutdown, which is consistent with the objective: eliminating the fuel building since it is no longer necessary to cool them before transport.
[0064] Other advantages and features of the invention will become clearer from 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
[0065] [Fig.1] [Fig.1] illustrates in cross-sectional view a fuel assembly with UOx rods, standard as currently used in a PWR nuclear reactor.
[0066] [Fig.2] [Fig.2] illustrates five configurations with a total number of different assemblies implemented in the step of defining the loading plan of the process according to the invention.
[0067] [Fig.3] [Fig.3] illustrates an example of the number of zoning types for a number total number of assemblies fixed.
[0068] [Fig.4] [Fig.4] illustrates a 3D power peak with a quarter-core mesh for calculations performed with a deterministic code according to the method of the invention.
[0069] [Fig. 5] [Fig. 5] illustrates, with the same mesh as [Fig. 4], the 2D power peak in DDV with pilot clusters partially inserted into the core.
[0070] [Fig.6] [Fig.6] illustrates the arrangements of the four types of assemblies selected in an example according to the invention, with UOx and UO2-Gd2O3 pencils implemented in the process according to the invention.
[0071] [Fig.7] [Fig.7] shows a core diagram according to an optimal configuration obtained according to the process of the invention.
[0072] [Fig.8] [Fig.8] illustrates in the form of curves the evolution of the 2D and 3D power peaks of the core during the cycle.
[0073] [Fig.9] Figure [Fig.9] illustrates the evolution of the residual power in the form of curves of a core assembly according to an optimal configuration of the invention and in comparison those of different residual powers of PWR reactor according to the state of the art.
[0074] [Fig. 10] [Fig. 10] is an enlargement of [Fig. 9], at the beginning of the curves. Detailed description
[0075] Fig. 1 shows the radial dimensions of a standard PWR assembly. The assemblies used are standard PWR UOx assemblies (17x17 rods) but with a lower height: 160 cm active height (i.e., fuel height).
[0076] Step i / : In accordance with the invention, we begin by defining a loading plan for the fuel assemblies constituting the core, according to four zones arranged according to a three-dimensional pattern symmetrical with respect to two orthogonal planes.
[0077] In substep 1 / 2, a total number of fuel assemblies is chosen. Figure 2 shows five different core sizes depending on the total number.
[0078] In substep i2, the position of the assemblies is then defined according to the total number chosen in substep i11, resulting in a number n of zoning types. Figure 3 illustrates five zoning types corresponding to a total number of assemblies equal to 45.
[0079] Substep i3 / For each of the n zoning types and for each of the four zones, the nature of the assemblages is defined by a percentage of poison neutron, the number of neutron poison pencils and an enrichment percentage, considering that:
[0080] each of the four zones is made up of assemblies having the same initial isotopic composition, and that
[0081] The fuel assemblies each comprise uranium oxide (UOx) rods enriched to less than 5% in uranium 235 (235U) and guide tubes adapted to house by insertion each one of the number of neutron-absorbing rods grouped in a cluster.
[0082] In the illustrated example, the core contains 4 types of assemblies having enrichments and pencils containing a different percentage of neutron poison as follows:
[0083] -assemblies enriched to 1.5% in 235U and having 8 gadolinium-coated pencils,
[0084] - assemblies enriched with 2% in 235U and possessing 8 gadolinium-coated pencils,
[0085] - assemblies enriched with 3% in 235U and possessing 24 gadolinium-coated pencils,
[0086] - assemblies enriched to 4% in 235U and having 16 gadolinium-coated pencils.
[0087] Substep i4 / : the possible combinations between the number are then determined of assemblies, the number n of zoning types and the nature of the assemblies.
[0088] A combination is not retained if the heart is subcritical at the beginning of life, that is to say for an effective multiplication coefficient keff less than 1.
[0089] Then we retain the combinations giving a heart cycle duration between 9 and 11 years.
[0090] Sub-step i5 / : we then select from the possible combinations those for which:
[0091] A / the maximum two-dimensional (X, Y) core power peak reached during the (FXy) evolution is minimal, which allows the reactor power sheet to be flattened,
[0092] B / the effective multiplication coefficient of the heart (keff) at the beginning of life (DDV) calculated by considering all the clusters inserted in the heart is minimal.
[0093] The two-dimensional power peak Fxy is defined according to the following equation [1]:
[0094] [Equation 1]: [00951 F xy = i^P(x,y,z).dz
[0096] where H represents the active height, that is to say the area of presence of the nuclear fuel or in other words the height of the fissile column, and P(x,y,z) the power at a given point in space in the core.
[0097] Step i / is preferably performed entirely by a deterministic calculation code. This step can advantageously be implemented by the calculation code deterministic implemented by the software called "APOLLO" as described in the publications.
[0098] Figure 4 illustrates a quarter-core mesh for calculations performed with this Deterministic code for a reactor core with 45 assemblies, a homogeneous radial mesh of 2x2 cells per assembly, and a cell height of 10 cm. In [Fig. 4], the color scale represents the 3D power peak in DDV with driver clusters partially inserted into the core. Note that due to the symmetries of the core pattern, a calculation performed on a quarter of the core is sufficient.
[0099] Figure 5 illustrates, with the same mesh, the 2D power peak in DDV with driver clusters partially inserted into the core. Thus, the maximum core power peak in two dimensions (X, Y) represents the axial power integral in a channel delimited by the computational mesh.
[0100] For the application of criterion A / , a high value of Fxy represents a high power released in the channel, and therefore a high water outlet temperature.
[0101] In the process, the limit has been set, in order to avoid the boiling of the water at the outlet of a channel, it is calculated with the following equation 1:
[0102] [Equation 1]: ■Rg" = -”rE (î»! -
[0103] with:
[0104] Qt denotes the total core power, equal in this example to 50MWth,
[0105] m' designates the total flow rate in the core, which depends on each configuration.
[0106] Cp denotes the specific heat which depends on the pressure / temperature in the core,
[0107] Tin denotes the core inlet temperature,
[0108] Tsat denotes the saturation temperature which depends on the pressure.
[0109] For the application of criterion B / , in order to guarantee an initial core reactivity moderate to facilitate its piloting, and to respect the safety criterion, the value of keff in DDV is set less than or equal to 0.95.
[0110] Step ii / : once the loading plan is defined according to step i / , a core control scheme is defined.
[0111] Substep ii 1 / : three groups of neutron-absorbing pencil clusters are considered (PI, P2, P3) whose number and / or neutron-absorbing material is different from one group of clusters to another.
[0112] Substep ii2 / : one of the groups is first inserted into the core areas exhibiting the greatest reactivity in DDV, then the positions of the clusters in each group, the number of clusters in a group, and the total number of groups are modified. their order of insertion in the heart, their overlap in the heart, and the nature of their neutron absorber.
[0113] This is carried out until a value is obtained which respects a power limit (Fxy) to avoid boiling of the water, over the entire duration of the reactor cycle, and this while keeping the effective multiplication coefficient of the core (keff) equal to 1 throughout the cycle.
[0114] In the example with 45 fuel assemblies, an optimal configuration is obtained with arrangements of the four types of assemblies previously defined shown in [Fig.6], a core scheme as shown in [Fig.7].
[0115] In the optimal configuration, as illustrated in [Fig. 7], the neutron absorber clusters are inserted in the following order: P1, then P2, then P3. The overlap between these three groups of clusters is 12.5% of the active height. This percentage represents the distance traveled by one group of clusters before the next group begins to be inserted. The stop clusters are designated by PA.
[0116] The inventor has made calculations of evolution to guarantee compliance with the duration of the cycle, i.e. until exhaustion of the heart.
[0117] In the assumptions, the core is considered exhausted when the effective multiplication coefficient of the core without any inserted clusters becomes very close to 1, typically keff is equal to 1.001. At each step of the combustion rate, a calculation is performed, considering that the clusters are inserted in such a way as to ensure that the coefficient keff is equal to 1.
[0118] The duration of the cycle obtained is equal to 11 years.
[0119] To maintain a margin of maneuverability at the end of the cycle, it is always possible to stop the cycle before 11 years. This allows sufficient flexibility to move the fuel assemblies to vary the power output for load following. Indeed, at the beginning of their life, the assemblies are firmly inserted into the core to compensate for the core's initial reactivity. This reactivity increases at the beginning of the cycle as the gadolinium is consumed. Maximum insertion of the assemblies occurs at the gadolinium peak. Subsequently, fuel degradation reduces the reactivity, and the assemblies are gradually withdrawn during the cycle. At the end of their life, the assemblies are almost completely removed from the core.
[0120] Fig. 8 illustrates in the form of a curve the evolution of the 2D and 3D power peaks of the heart during the cycle.
[0121] The inventor also carried out calculations of the residual power.
[0122] At the end of the cycle, the residual power of each assembly is calculated over a cooling period of 6 months.
[0123] Figure 9 shows that the residual power of the assemblies in the optimal core configuration obtained according to the process of the invention is much lower than that of typical PWR generator assembly configurations according to the state of the art. This can be explained by:
[0124] - the low discharge combustion rate, which is approximately ~22 GWj / tM according to the invention compared to approximately 50 to 60 GWj / tM for the state of the art,
[0125] - the low volumetric power according to the invention despite an irradiation cycle of 10 years versus approximately 2 years for PWR generator reactor configurations according to the state of the art.
[0126] An enlargement of the beginning of the curves in [Fig. 9], illustrated in [Fig. 10], shows that the residual power of the optimal configuration according to the invention drops to less than 4 kW after 6 days of cooling. This is compatible with unloading the assemblies directly into the transport towers after a few days of reactor shutdown. A cooling pool is therefore unnecessary, and the reactor building can thus be eliminated.
[0127] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.
[0128] Other variants and embodiments may be envisaged without departing from the scope of the invention.
[0129] Thus, if the criteria chosen in substep i5 / are the 2D power peak and the core reactivity at the beginning of the cycle, other criteria can be considered such as minimizing the 3D power peak, maximizing the combustion rate at the end of the cycle, or others. List of cited references
[0130] [1]: P. Mosca et al. "APOLLO3®: OverView of the New Code Capabilities for Reactor Physics Analysis” published May 20, 2024, Nuclear Science and Engineering.
[0131] [2]: R. Sanchez et al. “APOLLO2 YEAR 2010” NUCLEAR ENGINEERING AND TECHNOLOGY, VOL.42 NO.5 OCTOBER 2010.
Claims
Demands
1. A method for constructing a light water nuclear reactor core, in particular a pressurized water reactor (PWR), comprising the following steps: i / defining a loading plan for the fuel assemblies constituting the core, according to four zones arranged in a three-dimensional pattern symmetrical with respect to two orthogonal planes, comprising the following sub-steps: i1 / choosing a total number of assemblies, i2 / defining, as a function of the total number chosen in sub-step i1 / , the position of the assemblies, which gives a number n of zoning types, i3 / defining for each of the n zoning types and for each of the four zones, the nature of the assemblies defined by a percentage of neutron poison, the number of fuel rods containing this percentage of neutron poison and an enrichment percentage, considering that: each of the four zones is made up of assemblies having the same initial isotopic composition,and that the fuel assemblies each comprise uranium oxide (UOX) fuel rods enriched to less than 5% in uranium-235 (235U), fuel rods containing the defined percentage of neutron poison, and guide tubes each adapted to accommodate by insertion one of the number of neutron absorber fuel rods grouped in a cluster, i4 / determine the possible combinations between the number of assemblies, the number n of zoning types and the nature of the assemblies, i5 / select from the possible combinations those for which the effective multiplication coefficient (keff) is greater than one, which corresponds to a state where the core reactivity is not subcritical at the beginning of life, then those resulting in a predetermined lifetime T, ii / once the loading plan is defined according to step i / , define a core control scheme which consists of: i1 / considering at least three groups of neutron absorber fuel rod clusters (PI, P2,P3) whose number and / or material, neutron absorption is different from one group of clusters compared to another, ii2 / first insert one of the groups into the areas of the core exhibiting the greatest reactivity in DDV then modify the positions of the clusters of each group, the number of clusters in a group, the total number of groups, their order of insertion in the core, their overlap in the core, and the nature of their neutron absorber until a value is obtained that respects a power limit (Fxy) to avoid boiling of the water, over the entire duration of the reactor cycle, and this while keeping the effective multiplication coefficient of the core (keff) equal to 1 throughout the cycle.
2. Method according to claim 1, the assemblies according to step i3 / comprising pencils containing a percentage of neutron poison.
3. The method according to claim 2, step i3 / consisting of defining four types of assemblies having different enrichments and pencils containing a percentage of neutron poison as follows: - assemblies enriched with 1.5% in 235U and eight gadolinium pencils, - assemblies enriched with 2% in 235U and eight gadolinium pencils, - assemblies enriched with 3% in 235U and twenty-four gadolinium pencils, - assemblies enriched with 4% in 235U and sixteen gadolinium pencils,
4. Method according to claim 1 to 3, the at least two criteria of step i5 / being at least two criteria of peak power in evolution, and / or combustion rate and / or reactivity at the beginning or end of cycle.
5. The method according to claim 4, the at least two criteria of step i5 / being respectively the maximum two-dimensional core power peak reached during evolution (FXY) and the effective core multiplication coefficient (keff) at the beginning of life (DDV) calculated by considering all clusters inserted into the core to be minimal.
6. A method according to any one of the preceding claims, fuel assemblies each comprising a number equal to 17x17 uranium oxide (UOX) fuel rods enriched to less than 5% in uranium 235 (235U) and having a fissile column height less than or equal to 1.60 m.
7. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to implement step i / of the method according to claim 1.
8. Light water nuclear reactor core, in particular pressurized water (PWR), of batch type, obtained by implementing the process as defined in any one of claims 1 to 6.
9. Reactor core according to claim 8, operating at a thermal power of less than or equal to 300 MWth, typically equal to 50 MWth,
10. Use of the reactor core according to claim 8 or 9 to provide heat at a temperature below 200°C, typically equal to 150°C.