Method for helping to maintain a stable state of a nuclear reactor

Abstract

This disclosure relates to a method (200) for assisting in maintaining a stable state in a nuclear reactor, the method being implemented by computer, the method comprising successive steps of: receiving (201) a previously measured operating point of the nuclear reactor, generating (202) output data comprising: a graph indicating whether the operating point of the nuclear reactor is in a stable operating zone of the nuclear reactor or in an unstable operating zone of the nuclear reactor, in which the unstable operating zone is partitioned into different regions of instability, and different messages respectively associated with the different regions of instability. (Shortcut figure: Fig. 5)

Description

Title of the invention: Method for helping to maintain a stable state of a nuclear reactor technical field

[0001] This application relates to a method for assisting in the control of a nuclear reactor. It aims to facilitate the maintenance of a stable state of the nuclear reactor. STATE OF THE ART

[0002] A nuclear unit is an electricity production unit comprising a nuclear reactor, a current-generating turbine, circuits supplying the current-generating turbine, and a control system enabling an operator to control electricity production.

[0003] As part of the fight against CO2 emissions, renewable energies are increasingly present on the electricity grid. By their very nature, they are linked to climatic conditions, and are therefore intermittent and fluctuating. The electricity grid is vast and interconnected, and the electrical frequency is controlled to provide a high-quality current. It therefore cannot absorb overly disparate and unpredictable sources of production. Nuclear power, which can be controlled, is thus used to complement renewable energies to ensure overall stability in electricity production, in line with consumption.Historically conceived as a means of baseload production, the control of nuclear power units is therefore heavily impacted, as the operation of said nuclear units must adapt to shorter time windows than in previous practice, which are increasingly numerous, in order to cope with the growing number of demands for power variations.

[0004] The control of a nuclear reactor requires knowledge of the present state of the reactor in order to be able to reach a predetermined or desired operating point using control means available to the operating operator.

[0005] However, the physical phenomena occurring within a nuclear reactor are highly complex, requiring the application of multiple branches of physics, including nuclear physics, neutronics (e.g., the laws governing the neutron population), thermo-hydraulics (e.g., fluid mechanics and heat transfer), thermodynamics, and thermomechanics (the effect of stresses on materials subjected to heat); these sciences are applied here to the core of nuclear reactors. As can be seen, the phenomena that occur are diverse in nature; moreover, they interact with one another.

[0006] Thus, for example, fission (nuclear physics) occurs through the interaction of a heavy nucleus with neutrons (neutronics). This fission produces heat that propagates through matter (thermal), which in turn transfers its heat to water, which carries it away (thermohydraulics). Here, neutronics is the central branch because it governs the generation of the aforementioned phenomena. It allows for the characterization of the neutron population, distributed spatially and temporally according to an energy spectrum that depends on the interaction with matter. These interactions are absorption (fissile, fertile, and sterile), scattering, reflection, and neutron leakage. A kinetic component must also be considered, related to the actual production of neutrons by fission, distributed into prompt and delayed neutrons.These latter neutrons, which are in the minority and appear several seconds after prompt neutrons, are essential for enabling the control of a nuclear reactor.

[0007] Thus, it is clear that neutron behavior cannot be fully understood by a nuclear reactor operator who wishes to make a decision based on easily interpretable indicators. The operator has very little information available to assess the stability of a reactor. This information does not allow them, based on the reactor's current state, to predict the operating point trajectory that will subsequently be closest to reality. Moreover, if their assessment of the reactor's initial state is flawed, the anticipated operating point trajectory will also be flawed.

[0008] Flow maps are activity measurements taken in the reactor with the aim of: • verify certain associated safety criteria; • recalibrate the external chambers; • calibrate linear power chain (also called PLIN) and critical heat flux ratio (or DNBR for English departure from nucleate boiling ratio); • calculate quantities of interest necessary for the core protection system and reactor control.

[0009] There are two types of flow maps: partial or complete.

[0010] During a reactor restart, and in accordance with the reactor operating procedures, during the initial power increase after core refueling, numerous complete and partial flux maps must be generated. The purpose of these flux maps is to recalibrate the response of the external chambers (the reactor instrumentation that measures neutron fluxes) and the protection systems so that it remains representative of core activity, using a calibration code specific to the nuclear reactor unit under consideration. They also allow for the detection of any potential refueling errors.

[0011] A periodic test is conducted approximately once a month by the operating teams, the test teams, and the automation engineers. It consists of stabilizing the reactor to allow the test technicians to create a complete flux map. This flux map aims to obtain a three-dimensional measurement of the neutron flux distribution in the reactor, which makes it possible to: • verify compliance with safety studies (in terms of power distribution, hot spot factors, etc.); • monitor fuel depletion and the evolution of power distribution that took place during reactor operation.

[0012] Establishing an accurate flow map requires maintaining the nuclear reactor in a stable state for at least six hours. Therefore, once a stable state is reached, an operator must know what actions to take to maintain the reactor in stable conditions. EXPOSED

[0013] One goal is therefore to facilitate the action of an operator aimed at maintaining a nuclear reactor in a state of stability throughout the duration of the realization of a reactor flow map.

[0014] To this end, a method is proposed to assist in maintaining a stable state in a nuclear reactor, the method being implemented by computer, the method comprising successive steps of: • Receipt of a previously measured operating point of the nuclear reactor, the operating point of the nuclear reactor being defined by: • a value of a power parameter indicative of an axial imbalance of thermal power generated in the nuclear reactor, and • a value of a temperature parameter indicative of a temperature in the nuclear reactor, • generation of output data including: • a graph indicating whether the operating point of the nuclear reactor is located in a stable operating zone of the nuclear reactor or in an unstable operating zone of the nuclear reactor, in which the unstable operating zone is partitioned into different regions of instability, • different messages respectively associated with the different instability regions, each message indicating a modification of a parameter of the nuclear reactor to be controlled to cause a shift in the operating point of the nuclear reactor from the instability region to the stability zone, in which the parameter is a quantity of boric acid to be used to cool the nuclear reactor or a position of the nuclear reactor's neutralization control rods.

[0015] The associations between the indicative messages of a corrective action to be applied and the regions of instability allow an operator to quickly understand the actions to be taken to maintain the reactor in its stable state. Indeed, the operator simply has to identify the reactor's operating point and then read the message associated with the instability zone containing that point. This is applicable to any position of the nuclear reactor's operating point and within a suitable timeframe. Thus, it is possible to create a flow map without the risk that unstable reactor conditions will distort the results obtained from this flow map.

[0016] According to one embodiment, each region of instability contains the message associated with it.

[0017] According to one embodiment, the various messages include • a message indicating an increase in the amount of boric acid, and • a message indicating a decrease in the amount of boric acid.

[0018] According to one embodiment, the various messages include: • a message indicating the insertion of the neutralization control rods into the nuclear reactor, and • a message indicating the extraction of the neutrophil control rods from the nuclear reactor.

[0019] According to one embodiment, the unstable operating zone is partitioned into four regions.

[0020] According to one embodiment, the unstable operating zone surrounds the stable operating zone.

[0021] According to one embodiment, the stable operating zone is a rectangular zone defined by a range of values ​​for the power parameter and a range of values ​​for the temperature parameter.

[0022] A method for controlling a nuclear reactor is also proposed, comprising successively implementing the method for assisting in maintaining the state of stability as defined above and applying a corrective action or not depending on the output data.

[0023] A computer program product is further proposed comprising program code instructions for executing the steps of the method for assisting in maintaining a state of stability as defined above, when this program is executed by a computer.

[0024] It is further proposed a computer-readable memory storing instructions executable by the computer for the execution of the steps of the process of maintaining a stable state as defined above. DESCRIPTION OF THE FIGURES

[0025] Other features, purposes and advantages will become apparent from the following description, which is purely illustrative and not limiting, and which should be read in conjunction with the accompanying drawings on which:

[0026] Fig. 1 schematically illustrates a nuclear unit according to one embodiment.

[0027] [Fig.2] schematically illustrates a process implemented to achieve a state of stability of the nuclear unit shown in [Fig.1].

[0028] Fig. 3 illustrates a graph produced as part of the nuclear unit stabilization process illustrated in Fig. 2.

[0029] Fig. 4 schematically illustrates a process implemented to maintain the nuclear unit in its stable state.

[0030] Fig. 5 illustrates a graph produced within the framework of the process for maintaining the stability of the nuclear unit illustrated in Fig. 4.

[0031] Throughout the figures, similar elements bear identical reference symbols. DETAILED DESCRIPTION

[0032] With reference to [Fig.1], a nuclear power plant comprises one or more nuclear units 1.

[0033] Each nuclear unit 1 is an electricity production unit comprising a nuclear reactor 2, a current generating turbine 4 connected to the nuclear reactor 2, and a control system 6 enabling an operator to control the electricity production by the nuclear unit 1.

[0034] Nuclear reactor 2 is the site of a fission chain reaction, primarily of uranium-235 nuclei by neutrons. The neutrons emitted during fission are too energetic to produce further fissions; therefore, it is necessary to slow them down to reach lower energy levels, where the probability of producing fissions is higher. To this end, nuclear reactor 2 includes a circuit for circulating water through the nuclear core to slow down the neutrons. The water acts as a moderator and also as a heat transfer fluid, carrying away the heat produced during fission.

[0035] Nuclear reactor 2 is known from the prior art.

[0036] In a manner known per se, the nuclear reactor 2 comprises rods adapted for neutron absorption bars, or neutrophil bars. Neutrophil bars are Movable devices relative to the reactor frame in an axial direction are used to absorb neutrons and reduce the chain reaction if necessary. The amount of neutrons absorbed by the neutron absorption rods depends on their position relative to the frame. The axial direction is typically vertical.

[0037] Nuclear reactor 2 includes, for example, two groups of neutrophil control rods: • A first group of control bars, called group R, intended for controlling the core temperature, • A power compensation group, called GCP group: this group is itself made up of several subgroups of different absorption bars, and allows the power produced in the core to be modulated.

[0038] Nuclear reactor 2 can also use other absorbents, introduced intentionally or produced by reactions in the core, to capture neutrons so as to prevent them from producing fissions. These may include: • boron (introduced in a dilute form into the moderator, in order to limit the chain reaction). The operator can adjust the concentration of boron in the moderator; • Xenon-135: a highly efficient neutron absorber produced during core operation. Variations in reactor power involve complex physics that cause spatial and temporal oscillations in xenon concentration. An operator cannot directly control the xenon, but must implement methods to control it when operating the reactor; • Samarium-149: also a neutron absorber, but less effective than xenon. Samarium accumulates (unlike xenon, which decays with a half-life of approximately 9 hours).

[0039] There are also absorbents that are fission products generated during the operation of the heart, but which have less significant effects than xenon or samarium. An operator cannot act on these elements.

[0040] The nuclear reactor 2 comprises two neutron reflectors having different respective positions in the axial direction. Thus, when this axial direction is vertical, the nuclear reactor 2 comprises an upper neutron reflector located above the reactor and at least one lower neutron reflector located below the reactor and therefore consequently below the upper neutron reflector.

[0041] Each neutron reflector is capable of reflecting a portion of the neutron flux back towards the reactor. This reflected flux has a direct impact on the half-height of the reactor at which the reflector is positioned. The upper reflector therefore impacts the part The upper core and vice versa for the lower reflector and the lower part of the reactor. The thermal power from these two upper and lower regions is the result of the neutron flux and the effects induced by this flux. These combined effects determine an axial power imbalance in nuclear reactor 2. The axial power imbalance (axial offset), generally denoted AO, is a known difference in power between two regions of nuclear reactor 2 (the upper and lower parts).

[0042] More specifically, we have: [°°431 100

[0044] where: • PH denotes thermal power generated in the upper region of nuclear reactor 2, • PB denotes thermal power generated in the lower region of nuclear reactor 2.

[0045] When the absorber rods are inserted from the top of nuclear reactor 2, the flux tends to be greater at the bottom of the core. Fuel wear therefore occurs gradually from the bottom to the top of the core. If the flux were to become greater at the top than at the bottom of the core, there would be, on the one hand, uneven wear of the fuel core and, on the other hand, a risk of boiling in the upper part of the core. Indeed, since the water is hotter at the top of the core, it is more likely that the water will reach saturation conditions there.

[0046] By analogy, an axial imbalance of iodine [AO] and an axial imbalance of xenon [AOXe] can be defined, corresponding respectively to a difference in the quantity of these two species between the top and the bottom of the nuclear reactor:

[0047] f AO 100 {AUXe~ MHKXe+MBXe X

[0048] where: - MH] designates a mass of iodine present in the upper region of nuclear reactor 2, - MB] refers to a mass of iodine present in the upper region of nuclear reactor 2, - MHXe designates a mass of xenon present in the upper region of nuclear reactor 2, - MBXe refers to a mass of xenon present in the upper region of nuclear reactor 2.

[0049] Other parameters are also indicative of the axial imbalance of neutron flux in the reactor, such as the parameter Dpax.

[0050] In particular, we have:

[0051] Dpax = AO.Prel

[0052] or Prel is the relative power, expressed as a percentage of the rated power. It should therefore be noted that, when the power is the rated power, Dpax is identical to the parameter AO. The quantity of xenon generated increases when the relative power decreases, because, at high power, xenon decays by neutron absorption.

[0053] The control system 6 has the function of controlling the operation of the nuclear reactor 2, in order to control the reactions which occur inside, and to be able to modulate the electrical power delivered by the nuclear unit 1.

[0054] The control system 6 includes a communication interface 8, a human-machine interface 10, a processing unit 12 and a memory 14.

[0055] The communication interface 8 is adapted for communicating with components of the nuclear reactor 2 to be controlled or monitored. In particular, the control system 6 is capable of controlling the movement of the control rods in order to monitor their position, by sending them appropriate commands via the communication interface 8.

[0056] The communication interface 8 is also adapted to receive measurements acquired by sensors of the nuclear reactor 2, in order to assess its operating status. The reactor's Dpax at a given time is one of these measurements. Electrical power delivered by the nuclear unit at a given time is another.

[0057] The communication interface 8 is of any type, for example wired or wireless radio type.

[0058] The human-machine interface 10 is adapted to provide an operator with data generated by the processing unit 12 or measurements provided by the sensors of the nuclear reactor 2.

[0059] The human-machine interface 10 includes a display screen enabling this data to be presented graphically to the operator.

[0060] The human-machine interface 10 also includes an input device enabling the operator to provide data that can be processed by the processing unit 12, and to initiate commands to the nuclear reactor 2.

[0061] The processing unit 12 is adapted to perform processing based on the received measurements. This processing will be described later. The processing unit 12 is also adapted to generate commands that can then be transmitted to the nuclear reactor 2 via the communication interface 8.

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[0071] The processing unit 12 typically includes a processor configured to execute the code instructions of a computer program, thereby causing the implementation of these processes. The processor can be of any type (it may have one or more cores). In one embodiment, the processing unit 12 includes a calibration module and a nuclear reactor physics code 2. The calibration module is a module configured to calibrate quantities that do not take into account two types of elements: • The piloting history, which obviously cannot be known to the code developer; • phenomena not taken into account, because they are too complex, or too computationally intensive (the code must be fast enough to have industrial applications), or related to the imperfection of experimental measurements (bias, noise, uncertainties). Because of these two factors, some values ​​processed by the processing unit 12 may not accurately reflect reality at any given moment. The registration module compensates for this. For example, the registration module is suitable for registering the fast scattering coefficients, denoted Dl, of axial neutron reflectors. Registration can be performed as follows: = (2-ô)Dfp (5 — Ôq + ~ Pnom) In which: • J)™1* denotes the axial scattering coefficient of the neutron reflector superior, * denotes the axial scattering coefficient of the neutron reflector lower, • 50 is the parameter used to recalibrate the historical effects, • aP is the coefficient used to improve the predictive accuracy of simulations during power variations. It is a function of fuel wear, • P denotes current power • Pnom denotes the nominal power. This mathematical model will be all the more effective if it is not burdened with physical effects modeled elsewhere. In particular, to be effective, it is preferable to disable the recalibration to a phase of xenon oscillation.

[0072] The nuclear physics code is a module adapted to simulate the behavior of the nuclear core. It integrates implementations of the nuclear physics equations and numerical solvers to solve them. The code preferably uses a 3D model of the reactor to perform the calculations. This 3D aspect can correspond either to an explicit three-dimensional model of the core in the calculation code, or to a two-dimensional model (generally, a core reduced to a radial plane) followed by a deployment of the results in the axial direction (a 2D+1D approach).

[0073] Before describing in more detail a process implemented by the control system 6, and in order to understand the need for powerful computing resources to achieve the aforementioned objectives of assisting in the control of nuclear reactor 2, it is useful to recall what is encompassed by the concept of a calculation code for the physics of nuclear reactor cores implementing its various specialized modules. The physical parameters introduced are calculated by a neutronics calculation code, which is more precisely a nuclear reactor physics code, the main principles of which are now explained.

[0074] The term "nuclear reactor physics code" applies to software capable of calculating the three-dimensional power distribution (in Watts) in a nuclear reactor core 2 from structural data (geometry, chemical composition, heavy nucleus composition, etc.). To do this, the software can, for example, in 3D geometry: • Calculate a temperature distribution of the coolant in the reactor core. The coolant is the fluid that removes the heat produced by nuclear fission. This calculation is performed by a module of the code called the "thermohydraulic module"; • Calculate a temperature distribution of the nuclear fuel. This calculation is performed by a module of the code called "thermal module" or "thermomechanical module" if mechanical aspects are also treated (example a pellet-cladding interaction); • Calculate a neutron flux distribution from which the power is derived, using a "neutron modulus";

[0075] The coupled interaction of these three modules makes it possible to calculate a power in the core of the reactor in three dimensions.

[0076] Neutron physics modifies the coolant temperature, and the coolant temperature modifies the fuel temperature. The moderator and fuel temperatures modify neutron physics. Indeed, fission (nuclear physics) is caused by an interaction of a heavy nucleus with neutrons (managed by the neutron module), said fission producing heat which propagates in the material (managed by the thermal module), which transmits its calories to the water, the heat transfer fluid, which transports them by raising its temperature (managed by the thermohydraulic module) and consequently modifies the temperature of the fuel.

[0077] Counter-reaction phenomena are then observed. Since the coolant water is also the moderator by which neutrons are slowed down to promote fission, its temperature variation will modify its density and therefore the neutron slowing down, which in turn impacts the future generation of fission reactions. At the same time, a change in fuel temperature increases neutron absorption reactions, particularly in Uranium-238, which modifies the neutronics. Neutronics is the central branch of nuclear reactor physics because it governs the generation of the aforementioned phenomena. It allows the characterization of the neutron population, distributed spatially and temporally according to an energy spectrum that depends on the interaction with matter. These interactions are absorption (fissile, fertile, and sterile), scattering, reflection, and neutron leakage.A kinetic component must be added, linked to the actual production of neutrons by fission, divided into prompt and delayed neutrons. The latter, being less numerous and appearing several seconds after the prompt neutrons, are essential for controlling a nuclear reactor. Other phenomena, such as xenon-135 poisoning, which generates axial power oscillations impacting the axial offset, a product of fission, will also modify neutronics.

[0078] The reactor physics code can produce results necessary for the operation and safety of the reactor, by providing considerable assistance in controlling the core of nuclear reactor 2: • Location and values ​​of hotspots (Fxy); • “Intelligent” power distribution; • Calculated responses from various heart instrumentations; • Reactor control and stability over time; • Irradiation of nuclear fuels (burn-up assessment); • 3D distribution of xenon 135; • Etc.

[0079] In particular, the COCCINELLE calculation code, the official code for the calculation chain currently in operation for the French fleet of generating reactors, falls into the class of "reactor physics codes". It includes: • an axial 1D thermo-hydraulic module in the channel containing the water heat transfer fluid; • a radial 1D thermal module in the cylindrical fuel bar; • a 3D neutronics module in neutron scattering theory.

[0080] Neutron scattering is a theoretical model widely used worldwide to address a simplified form of the Boltzmann equation that governs the behavior of neutrons in matter. Descriptions of the theoretical models used in COCCINELLE can be found in the French-language reference "La physique des nucléaires nucléaires, 3ème édition" (author Serge Marguet, ISBN 978-2-7430-1105-5, Lavoisier edition) and the English-language reference "The physics of nuclear reactors" (author Serge Marguet, ISBN 978-3-319-59558-7, Springer edition). Other, older reference works present the fundamental principles of nuclear reactor physics, such as the "Traité de Neutronique" by Jean Bussac and Paul Reuss, published by HERMANN, ISBN 2-705-6011-9 - second edition, 1985.

[0081] The memory 14 is adapted to store data received by the human-machine interface 10, by the communication interface 8, or calculated by the processing unit 12.

[0082] Memory 14 further stores the calibration module and the nuclear reactor physics code 2 in the form of a computer program executable by the processor of the processing unit 12.

[0083] Furthermore, data representing an operating range of nuclear reactor 2 is stored in memory 14. The predefined operating range consists of a set of operating points. These points represent physical quantities associated with the normal operation of nuclear reactor 2. One objective of a nuclear reactor 2 controller is to maintain the state of nuclear unit 1 within said operating range. Stabilization aid method

[0084] A proposed method for controlling nuclear unit 1 comprises the following steps. The method is carried out at a constant average temperature of reactor 2, and for a given position of the neutralization rod groups (group R and group GCP).

[0085] An operator wishes to know, for a specific position of the neutralizing rod groups, the evolution of the stability of the nuclear reactor over a period of time D. Before the implementation of the stabilization aid process, the operator places the nuclear unit 1 at maximum net available power (PMD).

[0086] The proposed method for stabilizing the nuclear reactor 2 may include the operator receiving a start-up instruction. The operator indicates, via the human-machine interface 10, that they wish to initiate the stabilization process. For example, the operator clicks a start button provided by a graphical interface of the human-machine interface 10, or indicates a future start time to the human-machine interface 10. Advantageously, and in order to ensure that a stable state of reactor 2 can be reached sufficiently early, The operator starts the implementation of the process approximately 24 hours before the time at which he plans to create a flow map of reactor 2.

[0087] In an input data reception step 101, the processing unit receives input data representative of a state of the nuclear reactor 2 at the time the process implementation is initiated. The input data includes indicative data on the position of the neutralization rods. The input data can be retrieved by the processing unit 12 before the process implementation is initiated. Alternatively, the input data can be stored in memory 14, so that it can be retrieved by the processing unit 12 when the process implementation is initiated.

[0088] In a simulation step 102, the processing unit 12 generates, as a function of the position of the neutrophil control bars, a simulated evolution of representative parameters, respectively: - the axial imbalance of the thermal power generated in the nuclear reactor, - the axial imbalance of the amount of iodine generated in the nuclear reactor, and - the axial imbalance of the amount of xenon generated in the nuclear reactor.

[0089] In what follows, these parameters will respectively be referred to as power parameter, iodine parameter and xenon parameter.

[0090] The power parameter can be directly the axial power imbalance in the nuclear reactor 2, denoted AO. Alternatively, it can be Dpax, or any other parameter related to AO.

[0091] The iodine parameter is preferably parameter AOb II, however it can be any parameter related to AOb

[0092] The xenon parameter can be the AOXe- parameter

[0093] Xenon is obtained, within nuclear reactor 2, primarily by decay radioactive iodine. Therefore, the quantity of xenon present in reactor 2 at a given time ti is actually representative of a state of the reactor at an earlier time to, at which a quantity of iodine present in reactor 2 begins its disintegration which will form the xenon present in the reactor at time tb deducted from the quantity of Xenon which will have captured a neutron between times to and ti if the reactor is at power.

[0094] Thus, according to an embodiment aimed at allowing better comparability of the xenon parameter with the iodine and power parameters, the method includes a step of generating a simulated evolution of the xenon parameter, as well as a This step involves applying a transformation to the xenon parameter AOXe to obtain a corrected xenon parameter AOxe,c, taking into account the time At = ti - to required for the iodine to decay into xenon. Advantageously, for a given time t, the corrected xenon parameter AOxe,c represents the same reactor state as the power parameter AO and the iodine parameter AOb

[0095] The transformation applied to the xenon parameter AOXe to obtain the corrected xenon parameter AOXe,c is an affine transformation:

[0096] AOXec = Ax AOXe +B

[0097] The affine coefficients A and B can be obtained by two static calculations with the group R positioned at two positions on either side of its maneuvering band, so as to be at two distinct stability points of the nuclear reactor 2.

[0098] The affine transformation is applied to the xenon parameter AOXe by the processing unit 12, so as to obtain the corrected xenon parameter AOXe,c.

[0099] In a step 103, the processing unit 12 generates a graph representing the respective simulated evolutions of the power parameter, the iodine parameter and the xenon parameter as a function of time, during the period studied D.

[0100] The graph generation step 103 can be carried out simultaneously with the simulation, i.e. the graph displays each new operating point of the nuclear reactor 2 (including a time data point and a data point for each of the three parameters studied) as soon as this point is simulated by the processing unit 12. The graph then represents operating points located in the future.

[0101] Alternatively, the processing unit 12 performs the simulation 102 over the entire period studied D before proceeding to the graph generation step 103.

[0102] According to another embodiment, the simulation is performed in real time, and the graph also represents the operating points in real time. Each simulated operating point is then representative of the conditions in the reactor at the time the operating point is simulated. The graph displays the operating points as the simulation progresses, and thus indicates past operating points and the current operating point.

[0103] The simulation can combine a real-time simulation allowing the indication of past operating points and the present operating point, as well as a simulation of future operating points, so as to give the operator an overview of the state of the reactor over the entirety of a studied period D extending into the past and into the future.

[0104] The generated graph, shown in [Fig. 3], superimposes the simulated evolutions of the three parameters considered. Thus, the operator can easily compare the values ​​of these three parameters in order to know the moment when a stable state of the reactor is reached. In particular, when the three parameters considered are AOb, AO, and AOxe, a convergence of these three parameters corresponds to a point of reactor stability. The generated graph thus allows the operator to know the time required to reach a stable state of reactor 2. Therefore, the operator knows when a flow map can soon be produced under satisfactory conditions.

[0105] When the simulation is performed in real time, the operator can detect, through the convergence of the curves representing the three parameters AOi, AO, and AOXe,c, that a stable state of reactor 2 has been reached. The operator can adjust the position of the neutralization control rods in real time to react to the curves representing the three parameters under study and achieve convergence of these curves more quickly. When the simulation covers future operating points, the graph helps the operator to proactively manage the reactor, enabling them to prepare their control strategy to achieve reactor stability.

[0106] The graph includes a curve representing the power parameter, in particular the AO parameter (curve I), a curve representing the iodine parameter, in particular the AO parameter (curve II), a curve representing the xenon parameter, in particular the AOXe,c parameter (curve III), and optionally a curve representing the position of the neutralization control rods (curve IV). This last curve is particularly useful when the operator needs to interact in real time with the displayed data by changing the position of the neutralization control rods. The graph may also include a straight line indicating a reference value AOUef of the iodine parameter, representative of the stability state of reactor 2 when group R is positioned in the middle of its operating band. This reference value AOIjrf can be obtained in a manner described below.This reference value AOi>ref then becomes part of the input data for the stabilization aid process, and is communicated to the processing unit, either by the operator via the human-machine interface 10, or by memory 14 when the reference value AOUef is stored in memory 14. Alternatively, the reference value AOi>ref is itself calculated by the processing unit 12 prior to the implementation of the stabilization aid process, as will be seen later.

[0107] In conventional processes, the operator identifies a stable state by the absence of change in Dpax for a certain period of time. However, due to the appearance of xenon through iodine decay, Dpax exhibits an oscillatory temporal evolution. In order to ensure that the stable state is not a pseudo-stability resulting from this oscillatory nature of the temporal evolution of Dpax, the operator must wait, once they have identified an absence of change in Dpax, for a period of time that varies depending on the evolution of the quantity of xenon. in reactor 2 - typically, several hours. He then knows that a true state of stability is reached if the Dpax does not vary during this period.

[0108] On the contrary, for the proposed method, the convergence of the parameters AO, AO, and AOXe,c allows the operator to know when a true state of stability will be reached, without the risk of being misled by a state of pseudo-stability. The simulation therefore not only makes it possible to know in advance when the state of stability will be reached, but also to avoid, once this moment has arrived, the need to wait several hours before generating a flow map. Each flow map can thus be generated more quickly, and the shutdown of nuclear unit 1 can therefore be ended sooner.

[0109] The stability achieved is of high quality, linked to a reinforced equilibrium state of the reactor. The data obtained for different flux maps will therefore be comparable, and will reduce the biases between the calculated quantities and the quantities measured during the flux mapping.

[0110] The proposed stabilization assistance method provides a uniform approach for operator teams, thereby standardizing control practices, and also contributes to improving operator team training. The proposed method also enables a better response rate to demands on the electrical grid and improved calculation accuracy, preventing the exclusion of transient power variations due to the long timescale and conditions required to restore the nuclear unit to a stable state.

[0111] To create a flow map, it is desirable to have a convergence of the three parameters AO, AO, and A0xe,c to + / -1%.

[0112] The stabilization aid procedure can be implemented within the framework of a stabilization procedure for nuclear reactor 2, also illustrated in [Fig. 2]. Following the display step of a graph 103, the operator knows that their operating strategy is validated if they estimate that a stable state is reached sufficiently early. Conversely, if they estimate that it should be possible to reach a stable state for reactor 2 more quickly, the operator modifies their operating strategy for reactor 2. In particular, the operator can change the position of the neutralization control rods. They can then initiate a new implementation of the stabilization aid procedure, so as to identify a time when the next stable state will be reached after modifying the operating strategy, and to determine whether this occurs sufficiently early or whether it is necessary to modify the operating strategy again, specifically the position of the neutralization control rods.The operator can initiate the stabilization aid process and perform corrective actions as many times as necessary to reach a stable state for reactor 2 as quickly as possible. This allows the operator to prepare reactor 2 for flow mapping under sustained stable conditions. When simulations take place in real time, Data from several successive implementations of the process can be displayed on the same graph, as illustrated in [Fig.3], with the operator acting or not on the position of the control bars after the display of one or more operating points following a given implementation of the proposed process.

[0113] Calculation of the reference value of the iodine parameter

[0114] The reactor 2 stabilization process may include, prior to the implementation of the stabilization aid process 100, a calculation of the reference value of the iodine parameter AO1>ref. This calculation is performed by computer. It includes a first step of receiving setpoint data by the processing unit 12, this setpoint data being transmitted by the operator via the input device of the human-machine interface 10. The setpoint data includes a PCS setpoint power, notably expressed as a relative power to the nominal power. The setpoint data may also include the position of the neutralizing rods in reactor 2 (group R and group GCP). The setpoint data may also include a temperature deviation, which corresponds to the difference between the reactor temperature and a reference temperature value.

[0115] The steps for calculating the reference value AOi>ref can be implemented by a processing unit separate from the processing unit 12. Advantageously, these steps start approximately 48 hours before the time when a flow map is planned to be produced - that is, approximately 24 hours before the start of the implementation of the proposed stabilization aid process.

[0116] After calculating the reference value of the iodine parameter AO1>ref, if the operator considers that the iodine parameter AOi is too far from the reference value AOIjrf, he slightly modifies the position of group R so as to bring the iodine parameter AO closer to its reference value AOUef- Once the iodine parameter AOi is sufficiently close to its reference value AOi>ref, the operator starts the implementation of the stabilization aid process 100.

[0117] Method for maintaining a stable state of the nuclear reactor

[0118] This disclosure relates, in another aspect, to a maintenance method 200 of a stable state in nuclear reactor 2. The method 200 for maintaining the stable state is shown in [Fig. 4]. The stable state is achieved, in particular, by means of the stabilization aid method 100 described above. The method 200 for maintaining the stable state is implemented by computer.

[0119] In order to produce a flow map, it is desirable to maintain the stability of nuclear reactor 2 for a period of at least six hours. The process The proposed approach aims to enable an operator to maintain the reactor in its stable state for this duration.

[0120] In a first step 201, a processing unit 12 receives at least one previously measured operating point of the nuclear reactor 2, this operating point corresponding to a given instant and being defined by two values, namely: - a value of a power parameter, indicative of an axial imbalance of thermal power generated in reactor 2. In one embodiment, the power parameter is Dpax. Alternatively, it may be the axial offset AO, or any parameter related to Dpax or axial offset AO; - a value of a temperature parameter, indicative of a temperature in the nuclear reactor. This temperature parameter can be the average temperature in the reactor, or the temperature deviation defined by the difference between the average temperature in the reactor and a reference temperature value.

[0121] According to one embodiment, the treatment unit 12 receives several previously measured operating points of reactor 2, for two distinct instants.

[0122] From the received operating points, the processing unit 12 generates output data (step 202). The output data includes a graph, displayed on the display screen of the human-machine interface 10, an example of which is shown in [Fig. 5]. The graph can display the operating points in real time.

[0123] The graph indicates to the operator whether the operating point(s) is / are located within a stable operating zone of nuclear reactor 2. Conversely, the graph indicates to the operator whether the operating point(s) is / are located within one of a plurality of unstable operating zones of reactor 2. Together, the unstable operating zones comprise all possible operating points of reactor 2 for which the operating point is not located within the stable operating zone. In other words, the stable operating zone and the unstable operating zones together encompass all possible operating points of nuclear reactor 2.

[0124] The graph may, in particular, include a stability region 11, for which any point represented in the stability region 11 corresponds to an operating point in the stable operating zone. The graph may also include a plurality of instability regions, for which any point represented in an instability region of the graph corresponds to an operating point in a zone of corresponding unstable operation. Together, the stability region 11 and the instability regions cover the entire graph.

[0125] For each unstable operating zone, the output data includes an indication to the operator of an action to be taken to prevent the operating point from deviating further from the stable state. These indications advantageously take the form of messages 13 associated with the unstable regions, each message 13 indicating a modification of a parameter of the nuclear reactor 2 to be controlled, aimed at moving the operating point towards the stable zone.

[0126] Thus, as soon as the operator notices that the operating point is outside the stability area on the graph, and is in a given instability area, the operator can become aware of the message associated with this instability area in order to know what action to take - more precisely, what reactor parameter the operator must modify - in order to move the operating point towards the stability region.

[0127] The actions indicated by the messages may include: - a change in the amount of boric acid used to cool the nuclear reactor, and - a position of the neutralization control rods of the nuclear reactor.

[0128] Thus, the graph includes at least two regions of instability, each associated with an indicative message, respectively, of one of these two actions.

[0129] According to one embodiment, the messages 13 are displayed directly on the graph. Each instability region can integrate the corresponding message 13, so as to allow the operator to see the message 13 when looking at the operating point which is located in that instability region.

[0130] The graph may include pairs of instability regions, each pair comprising a first instability region for which the action to be taken, as indicated by message 13, is an increase in a nuclear reactor parameter, and a second instability region for which the action to be taken, as indicated by message 13, is a decrease in said nuclear reactor parameter. In this case, message 13 associated with the first instability region includes an upward arrow, while message 13 associated with the second instability region includes a downward arrow.

[0131] In particular, the graph may include: - a region of instability 3 for which the action to be taken, as indicated by message 13, is a reduction in the amount of boric acid to be used to cool the nuclear reactor, - an instability region 5 for which the action to be taken, as indicated by message 13, is an increase in the amount of boric acid to be used to cool the nuclear reactor, - an instability region 7 for which the action to be taken, as indicated by message 13, is the removal of neutralization control rods from the nuclear reactor (i.e., a reduction in the number of neutralization control rods present in the reactor), and - an instability region 9 for which the action to be taken, as indicated by message 13, is an insertion of neutralization control rods into the nuclear reactor (i.e. an increase in the number of neutralization control rods present in the reactor).

[0132] The graph may include, in addition to the stability region 11, only the instability regions 3 and 5 relating to the quantity of boric acid. The graph may also include, in addition to the stability region 11, only the instability regions 7 and 9 relating to the position of the neutralization control bars. According to another embodiment, the graph may include the stability region 11 and the four instability regions 3, 5, 7, and 9.

[0133] According to one embodiment, the instability regions surround the stability region 11. This makes it possible to define a stable operating zone which excludes extreme values ​​of the power parameter or the temperature parameter, only intermediate values ​​between extreme value intervals of the power or temperature parameters being considered as representing stable operation of the nuclear reactor.

[0134] The stability region 11 can in particular form a rectangle, around which the instability regions are arranged. Thus, the stability region 11 corresponds to intermediate value intervals of the power parameter and the temperature parameter, these value intervals being fixed: the operating point is not considered to correspond to stable operation of the nuclear reactor 2 as soon as the temperature parameter falls outside its reference value interval, and / or as soon as the power parameter falls outside its reference value interval, these two reference value intervals being independent of each other.

[0135] This disclosure also relates to a computer program stored in a computer-readable medium, comprising computer code for implementing the stabilization aid process 100, and / or for implementing the stability maintenance aid process 200.

Claims

Demands

1. A method (200) for assisting in maintaining a stable state in a nuclear reactor, the method being computer-implemented, the method comprising successive steps of: • receiving (201) a previously measured operating point of the nuclear reactor, the operating point of the nuclear reactor being defined by: • a value of a power parameter indicative of an axial imbalance of thermal power generated in the nuclear reactor, and • a value of a temperature parameter indicative of a temperature in the nuclear reactor, • generating (202) output data comprising: • a graph indicating whether the operating point of the nuclear reactor is in a stable operating zone of the nuclear reactor or in an unstable operating zone of the nuclear reactor,in which the unstable operating zone is partitioned into different instability regions, • different messages respectively associated with the different instability regions, each message indicating a change in a nuclear reactor control parameter to cause a shift in the nuclear reactor operating point from the instability region to the stability zone, in which the parameter is a quantity of boric acid to be used to cool the nuclear reactor or a position of the nuclear reactor's neutralization control rods.

2. A method according to the preceding claim, wherein each region of instability contains the message associated with it.

3. A method according to the preceding claim, wherein the different messages include • a message indicating an increase in the amount of boric acid, and • a message indicating a decrease in the amount of boric acid.

4. A method according to any one of the preceding claims, wherein the various messages include: • a message indicating insertion of the neutralization control rods into the nuclear reactor, and • a message indicating extraction of the neutralization control rods from the nuclear reactor.

5. A method according to any one of the preceding claims, wherein the unstable operating zone is partitioned into four regions.

6. A method according to any one of the preceding claims, wherein the unstable operating zone surrounds the stable operating zone.

7. A method according to any one of the preceding claims, wherein the stable operating zone is a rectangular zone defined by a range of values ​​for the power parameter and a range of values ​​for the temperature parameter.

8. Method for controlling a nuclear reactor, comprising successively implementing the method for assisting in maintaining the state of stability according to any one of claims 1 to 7 and applying a corrective action (203) or not depending on the output data.

9. Product computer program comprising program code instructions for carrying out the steps of the process according to any one of claims 1 to 7, when such program is executed by a computer.

10. Computer-readable memory storing computer-executable instructions for carrying out the steps of the process according to any one of claims 1 to 7.

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