Method for assisting in the preparation of a reactivity balance of a nuclear reactor

Abstract

This disclosure relates to a method for assisting in the calculation of a nuclear reactor reactivity balance. The method is computer-implemented and comprises the following steps: receiving (101) input data; from the input data, generating (102) simulated data comprising: a simulated evolution of a first parameter representative of xenon reactivity in the nuclear reactor, a simulated evolution of a second parameter of the nuclear reactor, the generation of the simulated data being carried out under a plurality of assumptions; generating (104) a graph superimposing the simulated evolution of the first parameter and the simulated evolution of the second parameter. (Shorthand figure: Fig. 2)

Description

Title of the invention: Method for assisting in the calculation of a reactivity balance of a nuclear reactor FIELD OF INVENTION

[0001] This application relates to a method for assisting in the operation of a nuclear reactor. More specifically, it aims to facilitate the establishment of a reactivity balance of such a 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] It is known that a reactivity balance must be performed for a nuclear reactor. The reactivity balance is a regulatory activity that an operator performs regularly and which must be carried out each time the reactor is shut down.

[0008] To define reactivity, it is necessary to introduce the concept of criticality. A nuclear core is said to be "critical" when the chain reaction is stabilized: one fission generates, on average, another fission—no more, no less. If the number of fissions generated is less than 1, the core is said to be subcritical. Conversely, a core that generates more fissions is said to be "supercritical."

[0009] In normal operation, the reactor must be critical: for each fission, among the neutrons emitted, at least and at most one neutron will generate another fission. The other neutrons produced will then be either absorbed or deflected out of the core. The criticality is therefore equal to 1.

[0010] The reactivity of a core is a quantity that characterizes the deviation of the core's state from criticality. If the core is critical, the reactivity will be 0 (and the criticality 1). In the case of a subcritical core (criticality < 1), the reactivity will be negative, and vice versa. Various factors will impact criticality and thus decrease reactivity, notably the presence in the core of certain species (boron or xenon in particular), or conversely, increase reactivity (for example, a decrease in the moderator temperature in pressurized water reactors). One advantage of reactivity is that it is a quantity that can be applied, on the same scale, to different neutrophil species that may be present in the reactor, and thus allows for comparison of the influence of each of these species on the deviation from criticality.

[0011] Performing a reactivity balance involves calculating the value, at a given moment, of each of the physical quantities that sustain or suppress the chain reaction within the reactor. In particular, it is essential to calculate the amount of xenon in the reactor, as this species is a powerful neutron absorber. Xenon is generated potentially during fission of uranium atoms, but especially through the natural decay of iodine-135. Iodine-135 is itself generated during fission of uranium atoms.

[0012] Under normal operating conditions, the generated xenon quickly absorbs a neutron and thus ceases to be a neutron absorber. An equilibrium is rapidly reached between the generated xenon and the xenon that disappears. However, when the reactor shuts down, the number of neutrons available to produce fission decreases rapidly. Therefore, xenon is no longer "consumed" and its concentration increases. The proportion of xenon in the reactivity balance will thus increase significantly. However, the xenon isotope that strongly absorbs neutrons (135Xe) is not stable and will naturally decay. Half of the 135Xenon will have disappeared after approximately 9 hours. Furthermore, it is considered that approximately 96 hours after core shutdown, all the xenon present in the reactor has disappeared. The reactivity balance will therefore change significantly over the course of the reactor shutdown.

[0013] On the contrary, samarium, which is also a neutrophilic species, is stable over time. Therefore, when the core shuts down, the samarium present in the reactor does not decay, and an additional amount of samarium is generated by the decay of other species present in the core. Thus, the amount of samarium increases after the nuclear core shuts down.

[0014] In order to restart a reactor, it is necessary to achieve a criticality slightly greater than 1: indeed, the number of fission reactions occurring must first be increased. However, the presence of neutron-absorbing species in the core prevents reaching the necessary criticality. Therefore, it is not possible to restart the core for a certain period of time following shutdown. The reactivity balance must allow an operator to estimate the point at which it becomes possible to restart the core, that is, the point at which the quantities of various neutron-absorbing species are sufficiently low so as not to prevent reaching the criticality necessary for restarting.

[0015] In currently known reactivity balance procedures, two states are defined: - a first state corresponding to the state of the heart before it stops, and - a second state of the core at the time of the scheduled restart.

[0016] The reactivity assessment is therefore carried out in two stages, in order to characterize each of these two states.

[0017] Furthermore, the reactivity balance can be used to indicate the reactor's state during shutdown, or to simulate a shutdown and thus determine the potential restart time. Therefore, several types of parameter settings are possible for the operator. Depending on the current state of the core, at least two states must be parameterized for each reactivity balance calculation. This parameterization requires the operator to specify several values ​​for each state, such as temperature or the position of control groups. Consequently, preparing a reactivity balance by the operator is time-consuming, and the conditions for performing a reactivity balance differ depending on the core's current state, i.e., whether it is shut down or not. EXPOSED

[0018] One goal is therefore to facilitate the task of an operator who is trying to estimate a time at which it is possible to restart a nuclear reactor core, regardless of the current state of the core at the time the operator wishes to estimate this time.

[0019] To this end, a method is proposed to assist in carrying out a reactivity balance of a nuclear reactor, the method being implemented by computer and comprising the following steps: - receiving input data including: • a target moment, • a temperature parameter representative of the nuclear reactor temperature at the target time, • a position of the control rods for temperature control in the nuclear reactor at the target time, - from the input data, generation of simulated data, the simulated data including: • a simulated evolution of a first parameter representative of the reactivity of xenon in the nuclear reactor over a period beginning at an initial time and including the target time, and • a simulated evolution of a second parameter of the nuclear reactor over the period,

[0020] the generation of the simulated data being carried out under the following assumptions: • The thermal power generated in the nuclear reactor is zero at the initial instant, • a simulation parameter is fixed during the period, in which: • The simulation parameter is a position of the control rods for the thermal power generated in the nuclear reactor, and the second parameter is a boron concentration in the nuclear reactor, or • The simulation parameter is the boron concentration in the nuclear reactor, and the second parameter is the position of the thermal power control rods. • The period includes a moment in which the nuclear reactor is in a critical state. • generation of a superimposed graph: • the simulated evolution of the first parameter, and • the simulated evolution of the second parameter.

[0021] Performing a simulation under the assumption that the simulation parameter is fixed and that the nominal power is zero allows for simulated behaviors with the same minimum preparation requirements for the operator and the same output data to be provided to the operator, regardless of the actual state of the nuclear core when the operator launches the simulation. Consequently, the operator can perform a reactivity assessment under familiar conditions and more easily and quickly estimate the time at which a core restart could occur.

[0022] According to one embodiment, the simulated data includes a simulated evolution of a samarium parameter representative of samarium antireactivity in the nuclear reactor.

[0023] According to one embodiment, the method further comprises a determination of a potential restart time of the reactor, the potential restart time being a time in the period in which the simulated evolution of the first parameter goes from a value preventing restart to a value allowing restart, the graph comprising an indicator of the potential restart time.

[0024] According to one embodiment, the graph includes a target time indicator.

[0025] According to one embodiment, the graph further shows: - a measured or calculated change in the first parameter during a second period prior to the period, and - a measured or calculated evolution of the second parameter during the second period.

[0026] According to one embodiment, the method further comprises: - a generation of calculated or measured values ​​of the first parameter associated with an update time, the update time being the initial time or a time later than the initial time, and - updating the graph so as to replace a value of the simulated evolution of the first parameter associated with the time of update with the calculated or measured value.

[0027] According to one embodiment, the method further comprises simultaneously displaying, on a display screen, the graph and a numerical value indicating at least one of: - the position of the thermal power control rods in the nuclear reactor at the target time, - the average temperature of the nuclear reactor at the target time, and - the boron concentration in the nuclear reactor at the target time.

[0028] This disclosure further relates to a computer program product comprising program code instructions for performing the steps of the process as defined above, when this program is executed by a computer.

[0029] This disclosure further relates to a computer-readable memory storing computer-executable instructions for the execution of the steps of the process as defined above. DESCRIPTION OF THE FIGURES

[0030] [Fig.1] schematically represents a nuclear unit.

[0031] [Fig. 2] schematically represents a method for assisting in the performance of a reactivity balance of the nuclear unit.

[0032] [Fig.3] represents a graph generated by a human-machine interface of the slice nuclear within the framework of the process represented in [Fig.2], according to a first embodiment.

[0033] [Fig.4] represents a graph generated by a human-machine interface of the slice nuclear within the framework of the process represented in [Fig.2], according to a second embodiment.

[0034] In what follows, identical reference signs denote the same or similar elements. DETAILED DESCRIPTION OF THE INVENTION

[0035] With reference to [Fig.1], a nuclear power plant comprises one or more nuclear units 1.

[0036] 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.

[0037] 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 in order to slow down neutrons. Water acts as a moderator, and also acts as a heat transfer medium by transporting the heat produced during fission.

[0038] Nuclear reactor 2 is known from the prior art.

[0039] In a manner known per se, the nuclear reactor 2 comprises rods adapted for Neutron absorbers, or neutron-absorbing rods. Neutron-absorbing rods are devices that move relative to the reactor frame in an axial direction to absorb neutrons and slow the chain reaction as needed. The amount of neutrons absorbed by the neutron-absorbing rods depends on their position relative to the frame. The axial direction is typically vertical.

[0040] Nuclear reactor 2 includes, for example, two groups of neutralizing 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.

[0041] 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).

[0042] 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.

[0043] 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.

[0044] The control system 6 includes a communication interface 8, a human-machine interface 10, a processing unit 12 and a memory 14.

[0045] 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.

[0046] 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. Electrical power delivered by the nuclear unit 1 is one of these measurements.

[0047] The communication interface 8 is of any type, for example wired or wireless radio type.

[0048] 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.

[0049] The human-machine interface 10 includes a display screen enabling this data to be presented graphically to the operator.

[0050] 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.

[0051] The processing unit 12 is adapted to implement 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.

[0052] The processing unit 12 typically includes a processor configured to execute the code instructions of a computer program, so as to cause the implementation of these processes. The processor is of any type (it may include one or more cores).

[0053] In one embodiment, the processing unit 12 includes a calibration module and a nuclear reactor physics code 2.

[0054] The recalibration module is a module configured to recalibrate 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 indeed be fast enough to

[0055]

[0056]

[0057]

[0058]

[0059]

[0060]

[0061]

[0062]

[0063] 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 fast scattering coefficients, denoted Δl, of axial neutron reflectors. Registration can be performed as follows: ^registered = ^freeregistered = ΔI^nf 5 = ô0 + aP(P-Pni^ In which: • D™p denotes the axial scattering coefficient of the upper neutron reflector, • p'W denotes the axial scattering coefficient of the lower neutron reflector, • 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. The nuclear physics code is a module designed to simulate the behavior of a nuclear reactor core. It integrates implementations of nuclear physics equations and numerical solvers to solve them. The code preferably uses a 3D model of the reactor to perform calculations. This 3D aspect can correspond either to an explicit three-dimensional model of the core within 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). Before describing in more detail a process implemented by 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 the concept of a calculation code for physics encompasses. nuclear reactor cores utilizing 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.

[0064] 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";

[0065] The coupled interaction of these three modules makes it possible to calculate a power in the core of the reactor in three dimensions.

[0066] Neutron physics modifies the temperatures of the coolant, and the temperature of the coolant modifies the temperatures of the fuel. The temperatures of the moderator and the fuel modify neutron physics. Indeed, fission (nuclear physics) is caused by an interaction of a heavy nucleus with neutrons (managed by the neutron module). This fission produces heat that propagates through the matter (managed by the thermal module). This matter then transfers its heat to the water, the coolant, which transports it by raising its temperature (managed by the thermohydraulic module) and consequently modifies the temperature of the fuel.

[0067] 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 fissions. 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 will depend on the interaction with matter. These interactions include 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, 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 fission product), will also modify neutronics.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] Method for assisting in the preparation of the reactivity balance

[0075] A proposed method for controlling nuclear unit 1, shown in [Fig.2], comprises the following steps.

[0076] An operator wishes to establish a reactivity balance of the nuclear reactor, either during an actual reactor shutdown, in order to validate or invalidate a target time tc at which the reactor can be restarted, or as part of a training exercise aimed at improving the operator's ability to react appropriately to changing conditions in the reactor when it is shut down. The reactivity balance indicates, over a given period, the reactor's criticality state, that is, whether the number of fission reactions occurring in the reactor is increasing, decreasing, or remaining stable. The reactivity balance also makes it possible to determine the contribution of various factors to this criticality state.

[0077] The proposed method for assisting in the reactivity balance calculation of 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 reactivity balance calculation assistance process. For example, the operator clicks on a start button provided by a graphical interface of the human-machine interface 10, or specifies a future start time to the human-machine interface 10.

[0078] The method includes a first input data reception step 101, in which the processing unit 12 receives input data representative of a target time U corresponding to a time at which the operator wishes to restart the nuclear reactor 2. In the case of a reactivity balance performed not following a core shutdown, but rather for operator training purposes, the target time corresponds to a hypothetical core restart time. The input data can be entered by the operator via the human-machine interface 10.

[0079] The processing unit 12 receives at least input data representative of a temperature of the nuclear reactor core at the target time U and a position of the group R at the target time tc, as well as the target time tc itself. The target time tc is a time of re-divergence (the divergence of a reactor is the return to criticality from a reactor that has no or no longer has a chain fission reaction) desired by the operator, corresponding to a moment in the simulated period at which the conditions in the nuclear reactor - namely, the quantity of different neutrophil species and the position of the control groups - are such as to allow a restart of the reactor, that is to say that they do not prevent the attainment of a criticality sufficient to restart nuclear reactor 2.

[0080] The processing unit 12 can also receive, during step 101, information indicating the operator's choice of one of two simulation modes. The first simulation mode corresponds to a fixed position of the GCP group, while the second simulation mode corresponds to a constant boron concentration in the nuclear reactor. Alternatively, the operator can specify, prior to implementing the proposed process, which simulation mode will be used. Depending on the simulation mode chosen, the process is implemented assuming that the position of the GCP group or the boron concentration in the nuclear reactor is maintained fixed or constant throughout the simulated period.If the position of the GCP group is kept constant, the boron concentration evolves to allow nuclear reactor 2 to reach a criticality state for restarting, taking into account the constant position of the GCP group. If the boron concentration is kept fixed, the position of the GCP group evolves to allow nuclear reactor 2 to reach a criticality state for restarting, taking into account the fixed boron concentration in reactor 2. Of course, it is possible that at the given time, no value of the evolving parameter will allow the reactor to restart, in which case no value of this parameter is simulated for that time.

[0081] The implementation of the process simulation is further based on the assumption that the core is at rest at an initial time t0 represented in the simulation. The assumption that the core is at rest corresponds to a nominal power of zero at the initial time t0. The assumption that the core is at rest is used regardless of the actual state of the core: • If the heart is in a critical state, or possibly slightly hypercritical, then we assume that the heart has just "converged," meaning that the chain reaction begins to gradually stop at the initial instant t0. This allows us to perform a reactivity assessment without the heart actually stopping, particularly to train an operator in performing the reactivity assessment. • If the heart is in a subcritical state, then the hypothesis of cardiac arrest is representative of reality – cardiac arrest occurred before the initial time to-

[0082] The input data can be retrieved by the processing unit 12 before the start of the simulation step detailed below. Alternatively, the input data can be stored in memory 14, so that it can be retrieved by the processing unit 12 when the simulation is started.

[0083] The preparation required by the operator for implementing the process is therefore simple and quick: the operator only needs to specify three data points (temperature, position of group R, and position of group GCP or boron concentration if this data is not already available for the processing unit 12), a target time for re-divergence (real or fictitious) desired by the operator, and optionally a binary choice of simulation mode. In comparison, for currently known processes for assisting in the performance of a stability balance, two distinct types of balances are performed (depending on whether the core is stopped or not when the balance is initiated), so the operator must also indicate a core convergence date (core shutdown date).

[0084] In a second step 102, the processing unit simulates the evolution of several parameters over the simulated period, which begins at the initial time to and includes the target time tc specified by the operator. The period can extend over 96 hours, which is the time after which all the xenon present in the core at the initial time to can be considered to have completely disappeared.

[0085] The simulation takes into account the simulation mode indicated to the processing unit at the setpoint data reception step 101: if the fixed position operating mode of the GCP group has been chosen, the simulation is generated on the basis of information relating to the constant position of the GCP group, while if the constant boron concentration operating mode has been chosen, the simulation is generated on the basis of information relating to the constant concentration of boron in the reactor.

[0086] Throughout the simulated period, the processing unit 12 simulates at regular intervals the quantity of xenon as well as the position of the GCP group (when the chosen simulation mode is at constant boron concentration) or the concentration of boron in the reactor (when the chosen simulation mode is at constant position of the GCP group). The chosen simulation interval can, for example, be one hour.

[0087] The amount of xenon simulated by the processing unit 12 can also take the form of a xenon reactivity such as would be with a zero-power critical reactor 2.

[0088] The processing unit 12 can also simulate other data, including, for a given moment, one or more of the following: - the position of group R, - the position of group GCP, - the thermal power generated in the nuclear reactor, - the average temperature in the nuclear reactor, - the quantity or reactivity of samarium in the nuclear reactor, - the slope of variation (i.e., the derivative) of the reactivity of xenon, - the critical concentration for restarting neutrophilic bar boron fixed. This data can only be simulated in simulation mode with a constant GCP group position, and corresponds to the boron concentration that would need to be reached, at a given time in the simulation, to allow a reactor restart. Alternatively, this data can be expressed as a quantity of boron to be introduced or removed from the reactor, at the given time, in order to achieve sufficient criticality for reactor restart, taking into account the state of the reactor at that simulation point (quantity of neutrophilic species and position of the control groups). - data relating to the effect of the position of a control group (R group or GCP) on responsiveness, - data relating to the effect of temperature (and therefore power) on reactivity, - a maximum divergence rating of the GCP group, - an irradiation rate (typically expressed in megawatt-days per tonne or MWj / t).

[0089] According to one embodiment, the processing unit simulates a potential restart time h of the reactor. The potential restart time h corresponds to a point in the simulated period at which the simulated evolution changes from a value preventing restart to a value allowing the nuclear reactor to restart. At a minimum, the simulation of the potential restart time tr takes into account the reactivity of xenon in the reactor: in simulation mode with a fixed boron concentration, the restart time h is not reached as long as the quantity of xenon in the reactor prevents reaching criticality, regardless of the position of the GCP group (within the limits of the positions authorized for the simulation, particularly according to regulations). As soon as the quantity of xenon becomes sufficiently low to allow a reactor restart, subject to maximum extraction of the group GCP - within the limits of the allowed positions - the potential restart time h is reached.

[0090] Correspondingly, in fixed-position simulation mode of the GCP group, the restart time tr is not reached as long as the amount of xenon in the reactor prevents criticality from being reached, regardless of the boron concentration in the reactor (within the limits of concentrations authorized for the simulation, particularly according to regulations). As soon as the amount of xenon becomes sufficiently low to allow a reactor restart, provided the boron concentration remains within the permitted limits, the potential restart time h is reached.

[0091] Naturally, a more complex model can be used to simulate the potential restart time tr, taking into account not only the reactivity of xenon, but also the reactivity of other neutrophilic species in the reactor and / or the position of the neutrophilic control bars.

[0092] According to one embodiment, the processing unit 12 identifies, among the simulated data, output data associated with the target time tc. The processing unit 12 can advantageously indicate the output data to the operator, so as to indicate to the operator the reactivity conditions of the nuclear reactor 2 at the target time tc. The output data may include one or more of the data mentioned in the preceding paragraph, at the target time tc.

[0093] During a step 103, the processing unit can, in particular, generate visual indicators for the operator, showing the output data. These visual indicators advantageously take the form of a digital display of output data (for example, a percentage value for the insertion of the GCP group or a boron concentration value). The output data can, for example, be displayed as a list or a table, projected onto the display screen of the human-machine interface 10.

[0094] The proposed method includes a graph generation step 104, the graph being displayed on the display screen of the human-machine interface 10. The graph is shown in [Fig. 3], for the constant position simulation mode of the GCP group. Step 104 can generate the graph during simulation step 102, with each simulated point being represented on the graph immediately after being simulated by the processing unit 12. Alternatively, graph generation step 104 is subsequent to simulation step 102, i.e., the simulation points are represented on the graph only after the simulation is complete.

[0095] The graph includes a curve Ci of the simulated evolution of the quantity of xenon in the reactor, or preferably of the reactivity of xenon (or more precisely, of antireactivity, that is to say the opposite of reactivity, the reactivity of xenon being negative). The graph also includes one of a C2 curve of simulated evolution of the boron concentration in nuclear reactor 2 (for the simulation mode with fixed position of the GCP group) and a curve of evolution of the position of the GCP group (not shown, for the simulation mode with constant boron concentration in the reactor).

[0096] The graph may further include a curve C4 indicating the slope of the quantity or reactivity (or antireactivity) of xenon. Such a curve allows the operator to identify, at its inflection point, the instant when the xenon slope stops decreasing and begins to increase. The inflection point can be represented on the graph by a vertical line V3 corresponding to the instant at which the inflection point occurs.

[0097] The boron concentration curve C2 allows the operator to ensure that the boron concentration is above the minimum required by regulations at any time during the simulated period. The operator can also react by injecting or removing boron from the reactor if deemed necessary.

[0098] The graph may include a visual indicator, in particular a vertical line Vj, so as to materialize the target time tc.

[0099] The graph thus generated can be used by the operator to assist in the operation of the nuclear reactor. In particular, it highlights the relevance or not of the choice of the target time tL; the reactivity balance at that time; the earliest possible predicted time of redivergence; the minimum critical boron level to be able to rediverge (i.e. restart nuclear reactor 2) during the simulated period or the minimum redivergence level of the GCP group over the simulated period, depending on the simulation mode chosen by the operator.

[0100] According to one embodiment, when the process includes the generation, by the processing unit, of a potential restart instant h, this instant is materialized by a vertical line V2 on the graph.

[0101] According to one embodiment, the screen of the human-machine interface 10 is configured so that the user can, by moving a cursor or pointer to a given instant during the simulated period, obtain a display of simulated data at that instant, corresponding, for example, to the values ​​of the different curves Ci - C4 for that instant. This helps the operator to identify a precise instant at which the re-divergence of reactor 2 can be initiated. The position of the cursor or pointer can be represented on the graph by a vertical line (not shown).

[0102] With reference to [Fig. 4], the graph may include, in addition to the display of simulated data for the period, a display of measured or calculated data of values corresponding to a past period, prior to the simulated period. The graph then comprises a first zone Zi for which the displayed data are measured, or calculated from measurements obtained in reality, and a second zone z2 for which the displayed data are the simulated data. The operator can thus assess whether the simulated data appear consistent with the measured or calculated data from the past, and can more easily identify an erroneous simulation than in the absence of the first zone zb

[0103] According to one embodiment, which may optionally be combined with the embodiment indicated in the preceding paragraph, step 104 of graph generation is followed by step 105 of graph update. During this step, which can only take place upon reaching or after reaching the initial time t0 in reality, at least one simulated value is replaced by a real value, measured or calculated, corresponding to the same instant as the simulated value.

[0104] Advantageously, at each real-world instant of the simulation, starting from the initial instant t0, the simulated values ​​associated with that instant are replaced by measured or calculated values. Here again, displaying real data superimposed on the simulated data on the same graph facilitates the operator's assessment of the quality of the simulated data. Thus, the graph includes a first time zone, corresponding to the past period extending: - either from the initial instant t0 to the current real instant, or current instant tb - or, when calculated or measured data prior to the initial instant t0 are also represented, as indicated in the preceding paragraph, from an instant prior to the initial instant t0 to the current instant tb

[0105] for which the data shown on the graph are representative of measurements or calculations made on measurements of the actual state of nuclear reactor 2, and a second time zone, corresponding to the future period extending from the current instant ti until the end of the simulated period, for which the data shown on the graph are obtained by the simulation carried out in step 102.

[0106] As time progresses, the first time zone covers an increasingly larger portion of the graph. The operator thus has a clear indication of both the historical reactivity state of the reactor and the estimated future evolution, enabling them, in particular, to assess the relevance of the simulated evolution. If the operator considers the simulated evolution no longer satisfactory given the measured and / or calculated data displayed in the first time zone, they can repeat the process to obtain new simulated data based on updated input data.

[0107] Compared with known prior art methods, the proposed method requires no distinction between cases where the core of nuclear reactor 2 is actually shut down and cases where the core is in a critical or slightly supercritical state, since the simulation is performed under the assumption, in both cases, that at the initial time t0 the core is shut down or has just shut down. The shutdown of the core itself is not simulated because the shutdown is considered immediate and therefore has no impact on reactivity. The kinetics of such a rapid shutdown have no impact on the xenon level during the descent of the neutralization control rods.

[0108] If the core is already stopped, the current xenon level is retrieved from the current reactor monitoring by the processing unit 12. It is also not necessary to define the time when the reactor stopped in the past, since this element is known in the historical monitoring via the current reactor monitoring by the processing unit 12.

[0109] Thus, the two cases are similar in their use with respect to the xenon level.

[0110] The proposed method makes it possible to replace, in a single method, the role of two distinct functions currently used: the reactivity balance of the nuclear reactor 2 when shut down, and the reactivity balance performed in critical condition for operator training purposes. The calculation code of the processing unit 12 simulates the evolution of physical quantities over the simulated period. If the reactor is already shut down and the operator wishes to perform the reactivity balance, then the simulation starts directly from this reactor state, taking advantage of the history already calculated by the processing unit 12 during normal reactor operation.If, on the other hand, the operator wishes to perform reactivity balance training, the simulation assumes that the reactor stopped just before the simulation was launched, and calculates the elements that the operator needs in the event that the reactor stops at the moment he performs his reactivity balance training.

[0111] If an essential maintenance operation requiring a core shutdown is necessary, the reactivity balance allows the operator to determine the xenon level and potentially choose the optimal shutdown time to minimize its duration. For example, if the core has a current xenon level that is too high, the balance will indicate that the re-divergence date is quite far off because a reactor cannot be restarted if the xenon level is too high. If feasible, the operator may wish to wait a short time at full power until this xenon peak is absorbed before shutting down the reactor to perform the maintenance operation, thus minimizing the shutdown time.

Claims

1. Demands Method for assisting in the calculation of a reactivity balance of a nuclear reactor, the method being implemented by computer and comprising the following steps: - reception (101) of input data including: • a target instant (Q, • a temperature parameter representative of the nuclear reactor temperature at the target time (te), • a position of the control rods for temperature control in the nuclear reactor at the target time (Q, - from the input data, generation (102) of simulated data, the simulated data comprising: • a simulated evolution of a first parameter representative of the reactivity of xenon in the nuclear reactor during a period beginning at an initial time (t0) and including the target time (tc), and • a simulated evolution of a second parameter of the nuclear reactor over the period, The generation of simulated data is carried out under the following assumptions: the thermal power generated in the nuclear reactor is zero at the initial instant (t0), a simulation parameter is fixed during the period, in which: • The simulation parameter is a position of the control rods for the thermal power generated in the nuclear reactor, and the second parameter is a boron concentration in the nuclear reactor, or • The simulation parameter is the boron concentration in the nuclear reactor, and the second parameter is the position of the thermal power control rods. • the period includes an instant in which the nuclear reactor (2) is in a critical state, • generation (104) of a graph superimposing: • the simulated evolution of the first parameter, and • the simulated evolution of the second parameter.

2. A method according to claim 1, wherein the simulated data includes a simulated evolution of a samarium parameter representative of samarium antireactivity in the nuclear reactor (2).

3. A method according to any one of claims 1 and 2, further comprising a determination of a potential restart time (tr) of the reactor, the potential restart time being a time in the period in which the simulated evolution of the first parameter goes from a value preventing restart to a value allowing restart, the graph comprising an indicator of the potential restart time (h).

4. Method according to the preceding claim, wherein the graph includes an indicator of the target time (tc).

5. A method according to any one of claims 1 to 4, wherein the graph further shows: - a measured or calculated evolution of the first parameter during a second period prior to the period, and - a measured or calculated evolution of the second parameter during the second period.

6. A method according to any one of claims 1 to 5, comprising: - a generation of calculated or measured values ​​of the first parameter associated with an update time (tm), the update time (tm) being the initial time (to) or a time later than the initial time (t0), and - updating the graph so as to replace a value of the simulated evolution of the first parameter associated with the update time (tm) with the calculated or measured value.

7. A method according to any one of claims 1 to 6, further comprising a simultaneous display, on a display screen, of the graph and a numerical value indicating at least one of: - the position of the thermal power control neutralization bars in the nuclear reactor at the target time, - the average temperature of the nuclear reactor at the target time, and - the boron concentration in the nuclear reactor at the target time.

8. 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.

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

Images