WO2023105165A1

WO2023105165A1 - Method for maintaining a steam generator involving a model

Info

Publication number: WO2023105165A1
Application number: PCT/FR2022/052276
Authority: WO
Inventors: Enrico DERI; Jérémie NEBES
Original assignee: Electricite De France
Priority date: 2021-12-08
Filing date: 2022-12-07
Publication date: 2023-06-15
Also published as: FR3130013B1; FR3130013A1

Abstract

A method for maintaining a steam generator carrying out heat exchange between a first circuit (10) and a second circuit (20), the method comprising, over the course of a simulation comprising a plurality of simulation times, a simulation of the operation of the steam generator (22) using a model establishing fluid characteristic fields, in which, following a finding of an observed value of a parameter of the steam generator (22), an internal variable is adjusted so as to match a simulation value of the parameter to the observed value, which variable may be the clogging rate of the pipe segments, the pressure drop in the flow of the secondary fluid at the segment and/or the steam pressure, the clogging resistance subsequently being adjusted and the generator scheduled to be cleaned according to the clogging rate and/or the clogging resistance.

Description

Method of maintaining a steam generator involving a model

Technical area

The invention belongs to the field of the maintenance of a steam generator, and more specifically deals with a maintenance of a steam generator operating a heat exchange between a primary circuit and a secondary circuit, for example in a power plant nuclear.

Technology background

A steam generator is generally made up of a bundle of tubes in which the hot fluid circulates, and around which the fluid to be heated circulates. For example, in the case of a steam generator of a Pressurized Water Reactor (PWR) nuclear power plant, the steam generators are heat exchangers that use the energy from the primary circuit resulting from the nuclear reaction to transform the water in the secondary circuit into steam which will power the turbine and thus produce electricity.

Referring to Figure 1 which shows a very simplified sectional view of the structure of a nuclear power plant, the primary circuit 10 circulates a primary heat transfer fluid, typically pressurized water, by means of a pump 11. The primary fluid passes through a reactor vessel 12 to enter the core 13 of the reactor in which the fuel assemblies are located and where nuclear fission occurs, forming the heat source of the primary circuit 10. The primary fluid is brought to high temperature, around 330°C. At the outlet of the tank 12, a pressurizer 14 makes it possible to establish a pressure of approximately 155 bars, so that the primary fluid remains in the liquid state. The primary fluid then enters a steam generator 22 effecting a heat exchange between the primary fluid and a secondary fluid of a secondary circuit 20, typically water. The primary fluid is then returned by pump 11 to reactor vessel 12.

The steam generator 22 brings the secondary fluid from a liquid water state to the vapor state just at the saturation limit, using the heat of the primary fluid. This circulates in tubes around which the secondary water circulates. The outlet of the steam generator 22 is the highest point in temperature and pressure of the secondary circuit 20. The secondary circuit 20 recovers the steam produced by the steam generator 22 to actuate a set of turbines 24, in order to produce electricity via an alternator 26. The secondary fluid returns to the liquid state through a condenser 28 which exchanges its calories with a cold source SF, before returning, via a pump 29, to the steam generator 22.

A nuclear power plant may comprise several steam generators depending on the calories to be evacuated, each steam generator being connected to the reactor vessel and to the turbines according to the simplified representation of FIG. 1. For example and not limited to, a nuclear power plant of the type REP 900 MW generally includes three steam generators, a nuclear power plant of the EPR 1650 MW type will include four steam generators.

The exchange surface, physically separating the primary circuit and the secondary circuit, is thus made up of a tube bundle, made up of 3,000 to 15,000 tubes, depending on the model, in which circulates the primary water heated to high temperature (330° C) and high pressure (155 bar). These tubes of the steam generator are held by spacer plates arranged generally perpendicular to the tubes which pass through them.

In order to allow the fluid which vaporizes to pass, the passages of these spacer plates are foliated, that is to say their shape has lobes around the tubes. As the secondary fluid passes from the liquid state to the vapor state, it deposits on the spacer plates, with each passage, a certain quantity of materials which it transports, forming deposits accumulating and fixing over time , eventually clogging the foliated passages. The deposits are often in the form of an iron oxide called magnetite resulting from the corrosion of the pipe elements, or other iron oxides or even an oxide of another metal.

FIG. 2 schematically illustrates a foliated passage in a spacer plate 30, through which passes a tube 31. The lobe-shaped passages 32 allow water to pass through the spacer plate 30 along the tube 31, thus allowing the water in the steam generator. The number of lobes 32 can vary: there are, for example, steam generator spacer plates comprising foliated passages with three lobes 32 instead of four, which are just as likely to clog. A deposit 33 is visible at the level of a lobe of the passage 32, clogging said lobe. This deposit can be located on the side of the tube 31 and/or on the side of the spacer plate 30. The clogging leads to modifications of the flow of the secondary fluid in the steam generator 22, and thus to promote the excessive tubes 31, as well as induce significant mechanical forces on the internal structures of the steam generators 22 or modify the stability of the steam generator during power transients. This degradation has therefore effects both on the safety and on the performance of the facilities, hence the need for regular maintenance, typically during periodic shutdowns of the nuclear unit concerned.

If the deposits of material are made in the lobes of the passages 32, they reduce the free passage: this is the clogging, which is therefore the progressive sealing, by deposits, of the holes intended for the passage of the liquid/vapor mixture. If the deposits of material are made along the tubes, this is fouling which limits the heat transfer between the primary circuit and the secondary circuit, causing a reduction in the operating pressure of the steam generator 22 for the same power. and therefore a drop in the overall efficiency of the plant and of electricity production.

In order to reduce, or even eliminate, these deposits, it is possible to clean the tubes and the spacer plates by chemical processes. These processes are implemented during periodic shutdowns of the nuclear power plant and consist of injecting chemical reagents into the secondary circuit of the steam generators in order to destructure and dissolve these magnetite deposits.

These processes are cumbersome, the industrial capacity of the market is limited, and the preparation is done years in advance. In addition, a steam generator 22 can only undergo a limited number of cleanings in order to avoid any damage induced by the chemicals used. The following two situations must therefore be avoided:

1) carry out a cleaning of a steam generator 22 which does not yet need it;

2) discovering during a periodic shutdown that a steam generator 22 has too much deposit, and not having the authorization to restart before carrying out an emergency cleaning.

The second case notably involves a shutdown that lasts for several months, in order to prepare for the clean-up operation.

In order to fight against clogging and fouling and thus bring the steam generator 22 back to a state compatible with safe and profitable operation, it is then necessary to have a tool which is capable of carrying out estimates and predictions of the evolution over time of clogging and fouling for a steam generator 22 in particular in order to limit the hazards and to optimize the cleaning schedule: we want to avoid cleaning the steam generators too early but not too late to stay below the prescribed technical limits.

Conventionally, the level of clogging or fouling is estimated by extrapolating measurements taken during inspection. Currently, the only non-destructive examination system that is capable of accessing all of the tube/spacer plate intersections of the steam generators 22 is the axial eddy current probe. Eddy currents appear in a conductive material when the magnetic flux nearby is varied. A multi-frequency eddy current probe is thus circulated in a tube of said exchanger and a measurement signal is measured with it, depending on the environment in which the probe is located, from which information can be extracted as to anomalies in the heat exchanger. A variation of the magnetic induction, typically by a coil in which an alternating current circulates, generates eddy currents, the induced variation of the magnetic field of which is detected. Typically, the voltage difference generated by the impedance variation of the coil is measured.

However, such inspections are occasional, and the evolution of the clogging rate is then based on assumptions such as a linear progression of the latter over time, which can only constitute a rough approximation of reality. As a result, the rate of clogging or fouling is not known with precision, and cannot be predicted, even though the stakes in terms of safety or electrical production are high.

It is also well known the practice of Television Examinations which allow the direct observation of the presence of deposit 33 inside the passages 32 of the spacer plates 30. However, these examinations are on the one hand too spaced out in time, and on the other hand they only allow a small portion of the spacer plate 30 to be examined and only a few spacer plates of the same steam generator 22 are accessible.

Presentation of the invention

The invention therefore aims to allow the maintenance of a steam generator operating a heat exchange between a primary circuit and a secondary circuit, by setting up a digital twin of said steam generator, which by appropriately exploiting certain data, allows to reproduce the evolution of clogging and fouling in the steam generator as well as possible, and therefore to plan the implementation of a steam generator cleaning operation as well as possible.

A method for maintaining a steam generator operating a heat exchange between a primary circuit and a secondary circuit is proposed, the steam generator comprising a plurality of tubes passing through an internal medium, a primary fluid of the primary circuit circulating in said tubes while a secondary fluid of the secondary circuit circulates in the internal medium, the steam generator further comprising a plurality of plates through which said tubes pass and in which passages are provided along the tubes allowing the circulation of the secondary fluid, the method comprising, during a period of simulation comprising a plurality of simulation instants, a simulation of the operation of the steam generator using a model of said steam generator comprising geometric parameters of the steam generator and taking into account imposed operating data, the model establishing characteristics related to the flow of the primary fluid and the secondary fluid and to the heat exchange between the primary fluid and the secondary fluid at each instant of simulation, from the geometric parameters, the imposed operating data, and internal variables, and in the method, following an observation of an observed value of a parameter of the steam generator on said steam generator at a simulation instant following previous simulation instants, an internal variable is adjusted so as to match a simulation value of said parameter at said simulation instant with the observed value of the parameter, the simulation being continued during the following simulation instants from this adjusted internal variable, and the observed value of the parameter of the steam generator is a rate of clogging of passages observed during an inspection of said steam generator, the adjusted internal variable then being a pressure drop in the flow of the secondary fluid at the level of said passage, and/or the observed value of the parameter of the steam generator is a steam pressure measured in the internal medium, the adjusted internal variable then being a fouling resistance affecting the heat transfer, and at the end of the simulation time, the implementation of a steam generator cleaning operation is planned based on clogging rate and/or fouling resistance.

This method is advantageously supplemented by the following characteristics, taken alone or in any of their technically possible combination:

the observed value of the parameter of the steam generator is a temperature related to the pressure in the internal medium, said temperature being measured instead of the pressure, the adjusted internal variable still being a fouling resistance affecting the heat transfer;

- the steam generator operates for a time interval between a first simulation instant and a second simulation instant, and operating data imposed on the model during this second simulation instant come from the operation of the steam generator during this time interval; - the operating data imposed include at least one of a thermal power corresponding to the power exchanged between the primary circuit and the secondary circuit, a temperature difference in the primary circuit between the inlet of the steam generator and the outlet of the steam generator, and a steam flow in the secondary circuit at the outlet of the steam generator;

- at at least one instant of simulation, the observed value of the parameter of the steam generator is a rate of clogging of passages or a fouling resistance following cleaning of the steam generator, the adjusted internal variable then being said rate of clogging or said fouling resistor, respectively;

- at at least one instant of simulation, the observed value of the parameter of the steam generator describes a blockage of a tube, and a modifier reflecting said blockage is applied to the geometric parameters of the model of the steam generator;

- the simulation, at each moment of simulation, implements: a) a determination of characteristic fields linked to the flow of the primary fluid and the secondary fluid and to the heat exchange between the primary fluid and the secondary fluid, b) a determination of chemical quantities of the secondary fluid from the thermo-hydraulic fields, c) a determination of a relation of transport and deposition of matter, the rate of clogging and/or fouling being determined from said transport relation and material deposition;

- at least one simulation instant, the clogging rate is determined from internal variables using an estimate of a deposit accumulation rate involving a particle deposit term and a precipitation term of soluble species, from which is subtracted a term representing a tearing flux, and in which at least one instant of simulation, the fouling is determined from the sum of two fluxes of materials: a flux induced by the parietal precipitation of the soluble species, and a flux induced by parietal boiling;

- the method comprises the implementation of a steam generator cleaning operation following the determination of the clogging rate and/or the fouling resistance;

- the cleaning is a chemical cleaning of the steam generator requiring the shutdown of the steam generator. Presentation of figures

Other characteristics, objects and advantages of the invention will emerge from the description which follows, which is purely illustrative and not limiting, and which must be read in conjunction with the appended drawings in which:

- Figure 1, already discussed, shows a very simplified sectional view of the primary circuit and the secondary circuit of a nuclear power station unit;

- Figure 2, already discussed, schematically illustrates a foliated passage in a spacer plate 30;

- Figure 3 shows an example of a steam generator.

detailed description

Steam generator

By way of non-limiting, the present description is given within the framework of a steam generator 22 using water as the heat transfer fluid both for the primary circuit and for the secondary circuit. Generally, a steam generator 22 measures approximately 20 meters in height, 3 meters in diameter, and weighs up to 300 tons.

Referring to Figure 3, a steam generator 22 comprises a set of tubes 31 in which the primary fluid circulates, spacer plates 30 holding said tubes, 31. The tubes 31, for example in the shape of an inverted U, and form a tube bundle. The tubes 31 are relatively thin since their diameter measures about 2 cm, and their number varies between 3000 and 15000, tubes depending on the type of steam generator 22. The tube bundle has a large exchange surface, between 4700 m2 and 10000 m2, in order to allow maximum heat transfer between the fluid of the primary circuit circulating inside the tubes and the fluid of the secondary circuit circulating outside the tubes. Anti-vibration bars 34 enclosing the bundle of tubes 31 prevent vibrations and maintain the tubes 31 at the level of the curved part at the top of the bundle of tubes 31.

The steam generator 22 comprises plates for holding the bundle of tubes: a tube plate 35 on which the tubes 31 are fixed at the bottom of the steam generator, is pierced with passage holes for the tubes 31, the spacer plates 30 are positioned horizontally along the steam generator 22. The number of spacer plates depends on the type of steam generator 22 and the bearing of the nuclear power plant, the bearing being defined among other things by the power of the nuclear power plant; we will talk about 900 MW, 1300 MW, 1650 MW etc. As mentioned previously, the spacer plates 30 are provided with foliated passages through which the tubes 31 pass. Typically, these foliated passages are distributed uniformly over each spacer plate 30, and there are as many per spacer plate 30 as there are tubes 31.

The steam generator 22 comprises an enclosure 36 confining the secondary water, both in liquid form at the bottom of the steam generator and in steam form. At the bottom, a water box 37 onto which the tubes 31 open is part of the primary circuit and allows the primary water to join the tubes 31. The water box 37 is typically in two compartments, one forming the inlet of the steam generator 22 for the primary water, and the other forming the outlet of the steam generator 22 for the primary water after having passed through the tubes 31. The water box 37 generally allows access to the tubes 31, by example to plug them with plugs.

The water from the primary circuit 10 enters the tube bundle at the bottom of the steam generator 22 on the Hot Leg (BC) side. At the steam generator inlet 22, the water in the primary circuit 10 is at a temperature of between 320° C. and 330° C., and at a pressure of approximately 155 bars. During its flow in the tubes 31, the primary water cools by transferring its thermal energy by conduction and convection through the wall of the tubes 31 to the secondary fluid circulating outside the tubes 31. After its ascent, the water primary begins a descent phase in the tubes 31 on the Eroide Branch (BE) side where it continues to transfer its thermal energy to the secondary fluid. The primary water then leaves the steam generator 22 then is reinjected by a pump 11 into the core of the reactor to recover thermal energy from the nuclear reaction. At the outlet of the steam generator 22, the temperature of the water in the primary circuit is between 284° C. and 293° C. The area located between the hot leg and the cold leg in the uncased part at the center of the steam generator 22 is called the water street.

The secondary water, also called feed water for the steam generator 22, is injected into the upper part of the steam generator 22 via a secondary inlet 38 where it is distributed over the entire periphery of the steam generator 22 via a feed torus 40. This feed water, on leaving the torus 40, mixes with the drainage water from the steam separation and drying equipment then descends in the water return to the bottom of the steam generator 22. Feed water then flows from bottom to top along the tubes 31, and in the water channel of the steam generator 22, either on the hot branch side or on the cold branch side. In contact with the tubes, the water in the secondary circuit 20 absorbs the thermal energy supplied by the primary circuit 10, heats up and then vaporizes. The steam generated on the secondary side rises along the bundle of tubes 31 where it reaches a pressure of between 55 bars and 74 bars, depending on the type of steam generator 22 associated with a given stage. The temperature of the vapor is the saturation temperature associated with the pressure, namely of the order of 270°C to 290°C.

The steam continues its ascent through the equipment for separating and drying the water-steam mixture, the function of which is to separate the liquid phase and the gaseous phase. This equipment comprises separators of the cyclone type 42 and dryer frames 44 located above the separators 42 delivering dry steam with a titer preferably greater than 99%, or even 99.5%, in order to be dry enough to avoid any deterioration of the fins of the turbine 24. The steam then escapes through the steam exhaust 46 located on the upper bottom at the top of the steam generator 22, and is brought through the secondary circuit 20 to the turbine 24.

The primary circuit 10 and the secondary circuit 20 are equipped with sensors, making it possible to measure certain physical quantities, from which the operating parameters of the steam generator 22 can be deduced. The primary circuit 10 is equipped with temperature sensors, typically thermometers such as than thermocouples, making it possible to determine the temperature at the inlet and at the outlet of the heat exchanger 22. The secondary circuit 20 is equipped with at least one sensor making it possible to obtain the flow rate of steam at the outlet of the steam generator 22 ( a steam flow meter), and at least one sensor for measuring the pressure at the top of the steam generator 22, downstream of the steam exhaust 46.

It is also possible to have a temperature sensor measuring the temperature of the feed water, at the inlet of the steam generator 22, and a steam humidity rate sensor, for example at the steam exhaust. 46. Multiple other sensors may be provided, in particular to ensure the operational safety of the steam generator 22. It should be noted however that due to the numerous constraints of certain parts of the steam generator 22 (temperature , pressure, radioactivity, etc.), it is not possible to have sensors that can measure any physical quantity at any location. The conduct of the operation of the steam generator 22 is generally done via the imposition of a thermal power, which corresponds to the power exchanged between the primary circuit 10 and the secondary circuit 20. Indeed, it is this thermal power which conditions the power supplied to the turbine 24 and therefore to the alternator 26. This thermal power is generally deduced from the temperature differential in the primary circuit 10 between the inlet of the steam generator 22 and the outlet of the steam generator 22, and/ or the steam flow in the secondary circuit 22 at the outlet of the steam generator 22.

Simulation of steam generator operation

A computer simulation of the steam generator 22 simulates the operation of the steam generator 22 during a simulation period comprising a plurality of successive simulation instants. The simulation is implemented by a computer comprising a processor and a memory, and communication interfaces. The simulation of the operation of the steam generator 22 using a model of the steam generator 22 comprising geometric parameters of the steam generator 22 and taking into account imposed operating data. The model establishes fields of characteristics linked to the flow of the primary fluid and of the secondary fluid and to the heat exchange between primary fluid and secondary fluid at each instant of simulation. To do this, the model uses geometric parameters, imposed operating data, and internal variables.

The geometric parameters of the steam generator 22 reflect the spatial structure of the steam generator 22, in particular in order to model the volumes in which the primary fluid and the secondary fluid flow. The geometric parameters of the steam generator 22 model, for example, the tubes and the passages along the tubes passing through the plates, but include many other data allowing a three-dimensional modeling of the steam generator 22. Typically, the model of the steam generator 22 presents a resolution of the structure of a few centimeters, allowing the modeling of the fields of characteristics related to the flow of the primary fluid and the secondary fluid with a resolution of the order of the liter.

If most of the geometric parameters of the steam generator 22 are fixed once and for all since they depend on the nature of the steam generator 22, it is however possible to complete them by means of modifiers. For example, during the life of a steam generator 22, it is common for certain tubes 31 to be voluntarily blocked during a maintenance operation. A tube 31 is blocked by means of a stopper, for example when a structural anomaly of this tube 31 has been observed threatening the separation between the primary circuit 10 and the secondary circuit 20. When a tube 31 is blocked, it there is no longer any primary fluid circulation in said tube 31, although the tube 31 is still geometrically present in the steam generator 22.

The operating data imposed include at least the thermal power at each simulation instant, with at least the temperature difference of the primary fluid between the inlet in the steam generator 22 and the outlet of the steam generator 22. Others imposed operating data can be used, such as for example the steam flow at the outlet of the steam generator 22. These imposed operating data are typically those used by the operator of the steam generator 22 to control the operation of the latter. this. Other operating data form boundary conditions, such as feedwater temperatures, for example.

The internal variables are variables involved in the model of the steam generator 22, which result from the operation of the steam generator 22 and are necessary to model the relationships between the different physical quantities involved in the operation of the steam generator 22. These internal variables are very numerous, and include for example the temperature of the secondary fluid at different points of the secondary circuit, the evolution of the temperature of the primary fluid along the tubes 31, and many others. Significantly, certain internal variables can account for the evolution of the steam generator 22 over a long period, extending over several weeks, months, or even years.

In particular, certain internal variables relate to the physicochemical characteristics of the secondary fluid, within the framework of a thermo-hydraulic approach as described in the thesis of T. Prusek. “Modelling and numerical simulation of clogging at the scale of the sub-channel in steam generators”, Doctoral thesis of the University of Aix-Marseille, 2012, which the person skilled in the art will consult advantageously to obtain many details on the modeling of a steam generator 22.

The internal variables make it possible in particular to define the fields of characteristics linked to the flow of the primary fluid and of the secondary fluid, also called thermo-hydraulic fields, comprising for example a temperature field for the primary fluid, and for the secondary fluid a field of temperature, a field of vector velocity, a density field, a void fraction, and a pressure or temperature field (the two being linked for the secondary fluid).

The simulation carried out at a simulation time determines the internal variables at this simulation time, in particular those intervening in the thermal-hydraulic fields, from the internal variables of the previous simulation time, the geometric parameters, and the imposed operating data. , including boundary conditions. The determination of the thermo-hydraulic fields therefore makes it possible to have, at each instant of simulation, and for a plurality of points spatially distributed in the steam generator 22, at least for points of the secondary fluid the pressure P, a vacuum rate ∈ (representative of the quantity of bubbles - on a given section, the ratio between the surface occupied by the vapor and the total surface), a density p and a velocity U of secondary fluid (vector), as well as a temperature for the primary fluid.

It is then possible to determine the chemical quantities of the secondary fluid depending on the location: oxidation-reduction potential Eh, the solubility Si of the i species in solution in the secondary water (metal oxides in particular, iron and/or copper) , distribution Ci and particle size of the i species in suspension, as well as the distribution Cj of the j dissolved species.

Some of these variables are used directly to, by means of transport equations, determine the clogging of passages of plates 30 or fouling of tubes 31, while other internal variables can be used essentially to allow updating operating conditions of the steam generator 22.

Clogging and fouling

The deposit accumulation rate rh on a spacer plate 30 is represented by the following equation:

This material accumulation rate is therefore written as the sum of a term for the deposition of particles Φ _p and a term for the precipitation of soluble species Φ _s , from which we subtract a term Φ _r which represents the flow of tearing, all within the framework of the secondary fluid.

with T _p the particle mass fraction in the flow, K _p a particle deposition rate at the wall, p a density of the secondary fluid, and Ci the concentration in solution in the secondary water of species i (typically iron oxide).

The flux of precipitated soluble species Φ _s forming the fouling along a tube 31 is determined from the sum of two fluxes of matter: a flux Φ _s ^th induced by the parietal precipitation of the soluble species, and a flux Φ _s ^eb induced by parietal boiling:

The flux Φ _s ^th induced by the parietal precipitation of soluble species involves two main mechanisms, a mechanism for transporting soluble species to the wall, characterized by a speed Kt, _s , and a crystal growth mechanism characterized by a speed Kcrist:

with r _s the mass fraction of soluble species in the flow, and Γ _s ^max the saturation mass fraction of soluble species.

The flux Φ _s ^eb induced by parietal boiling assumes that all of the soluble species, initially present in the vaporized liquid phase, precipitates and settles. This flow is characterized by a precipitation rate K _eb,s induced by parietal boiling:

The deposit pull-out flux Φ _r can be taken into account as being proportional to the surface mass m _d previously deposited on the wall:

The term E _r represents a tearing rate which is linked to the turbulent shear stresses in the wall. This tearing rate is calculated from the turbulent friction velocity U _τ at the wall and the kinematic viscosity v of the secondary fluid:

where the parameter a _r represents a modeling constant and adjusts the intensity of the pull-out phenomenon.

This information makes it possible to calculate the quantities which then give the mass accumulation rate m on the spacer plates 30. Then it is possible to link the mass deposited at the reduction of the passage section at the level of the passages 32 of the plates

30 (clogging) via a correlation.

The clogging rate τ _c of a foliated passage 32 quantifies its fluid passage restriction induced by the formation of a deposit. It is defined as being the ratio between the section S _c of the clogging deposit in a foliated passage 32 and the initial total section So of this same “clean” foliated passage, that is to say not having undergone any phenomenon of deposit. A clogging rate value equal to 0 means that the foliated passage is not clogged at all, while a value equal to 1 means that it is completely clogged:

The clogging rate can also be expressed in the form of a clogging percentage p _c of between 0% and 100%.

It is thus possible to determine at any instant of simulation, the rate of clogging and the fouling of the steam generator 22, thanks to the internal variables which make it possible to determine the terms of deposit of particles Φ _p and of precipitation of soluble species Φ _s , and tearing Φ _r .

The simulation duration is defined as the duration during which the operation of the steam generator 22 is simulated. This simulation duration extends over several days, preferably over several months, or even several years. This long simulation time makes it possible to simulate the long-term alterations of the steam generator 22, and not only to determine the operation of the steam generator 22 for the purpose of controlling the latter. However, over such a long simulation time, certain parameters of the steam generator 22 vary (variation of geometry parameter or of internal variables). These variations can result both from the operation of the steam generator 22, such as clogging or fouling, or can be caused by causes external to the steam generator, such as for example the clogging of tubes

31 or chemical cleaning of the steam generator.

Several parameters of the steam generator 22 whose operation is simulated may become available, for example by measurement or by inspection during a shutdown of the steam generator 22. This additional information should be taken into account in order to improve the accuracy. with which the model of the steam generator 22 simulates the operation of said steam generator 22. Thus, following a finding of a value found for a parameter of the steam generator 22 on said steam generator 22 at a simulation instant following previous simulation instants, an internal variable of the model is adjusted so as to matching a simulation value of said parameter to said simulation instant with the observed value of the parameter, the simulation being continued during subsequent simulation instants from this adjusted internal variable.

In particular, the observed value of the parameter of the steam generator 22 may be a rate of clogging of passages 32 observed during an inspection of said steam generator 22. In fact, inspections of the steam generator 22 are periodically carried out. During these inspections, measurements of the clogging rate can be carried out. For example, patent applications FR3028042, FR2999776 or even WO2015097221 deal with such inspection methods making it possible to obtain clogging rates of passages 32 of plates 30. These observed clogging rates, derived from measurements, are estimated to be more accurate than those from the simulation, because from the real steam generator 22. In order to reinforce the accuracy of the simulation on other parameters of the steam generator 22, which in turn will make it possible to reinforce the accuracy of the simulation on the clogging and fouling, the observed clogging rate is taken into account. in the model by adjusting a pressure drop in the flow of the secondary fluid at the level of said passage 32 whose clogging rate has been observed.

The observed value of the parameter of the steam generator 22 can be a steam pressure measured in the environment internal to the steam generator, in the secondary circuit, and typically be a pressure measurement at the top of the steam generator 22, downstream of the steam generator. steam exhaust 46, for example obtained by means of a steam sensor. This pressure may correspond to an internal variable determined during the simulation. If too great a difference happens to exist between this observed pressure and the pressure determined as an internal variable by the simulation in the model of the steam generator 22 at the same point, this means that the heat transfer is not correctly modeled in the model of the steam generator 22. The main internal variable affecting the heat transfer, and liable to change, is the fouling resistance, which reflects the negative effect of the fouling of the tubes 31 on the heat transfer between the primary circuit 10 and the secondary circuit 20. Consequently, the fouling resistance is then modified so that the heat transfer in the simulation approaches that of the operation of the steam generator 22, which is verified by the rapprochement between the observed pressure and the pressure determined by simulation. Typically, if the observed pressure is lower than the determined pressure, the fouling resistance is increased, whereas if the observed pressure is higher than the determined pressure, the fouling resistance is decreased.

It should be noted that the pressure and the temperature of the secondary fluid are linked since the steam generator 22 operates in steam saturation. The fouling of the tubes 31 therefore affects both the temperature and the pressure of the secondary fluid. It is therefore also possible to observe a temperature in the secondary fluid at the level of the steam generator 22, and if too great a difference happens to exist between this observed temperature and the temperature determined as an internal variable by the simulation in the model of the steam generator 22 at the same point, the fouling resistance can be similarly changed.

The adjustment of the model, by modifying an internal variable, is taken into account during the simulation times following the simulation duration. The adjustment of the internal variables of the model of the steam generator model results in a discontinuity in the evolution of these internal variables compared to their evolution prior to this adjustment. It follows that during a second part of the simulation time following the adjustment of the model of the steam generator 22, the model is no longer a simulated evolution of the initial model.

Thus, by taking into account the observed values of operating parameters of the steam generator 22 in reality, it is possible to readjust the model of the steam generator 22 during the simulation period, so that it reflects the most faithfully possible the actual operation of the steam generator 22, and that the clogging rates and/or the fouling resistance determined at simulation instants are as close as possible to reality. It is then possible to plan the implementation of a steam generator cleaning operation on the basis of the clogging rate and/or the fouling resistance determined at the end of the simulation duration. The cleaning is a chemical cleaning comprising the injection of chemical reagents into the secondary circuit of the steam generators 22 in order to destructure and dissolve the deposits responsible for clogging and fouling.

Further adjustments to the steam generator 22 model may be made during the simulation time. When a chemical cleaning of the steam generator 22 takes place, the event is recorded in the maintenance databases, and the model of the steam generator 22 is adjusted by modifying the values of the clogging rate and fouling resistance. Chemical cleaning can reduce or eliminate both the rate of clogging of the spacer plates 30 and the level of fouling of the tubes 31. Thus, the main effects of cleaning which are to be taken into account on said model are the change in the losses of singular loads of the spacer plates 30 on the one hand, and the modification of the heat transfer between the primary circuit 10 and the secondary circuit 20 taking place through the tubes 31. These two effects require in particular the updating of the thermal-hydraulic calculation and its readjustment on the pressure of the post-cleaning steam generator 22 (this pressure being typically derived from measurements carried out in situ in real time). Then, the calculations of the chemical state of the steam generator 22 then the transport of species are also updated during the following simulation instants.

Similarly, the clogging of the tubes 31 can be informed with the number and position of the tubes 31 clogged. The information on the blockages of tubes 31 can be recorded in the maintenance databases, and be taken into account to adjust the model. The model can be updated with each new capping or once a given number of capped tubes has been reached (depending on the level of detail desired by the operator). Thus, from the moment the tubes have been plugged on the site, this is immediately taken into account in the simulation, in particular for the geometric parameters of the model. As a result, the effects of this clogging will be taken into account by the calculation of new thermo-hydraulic fields, by the re-evaluation of the chemical quantities and by a new calculation of the distributions of suspended and dissolved species, during the following simulation moments.

The simulation of the operation of the steam generator 22 using a model of said steam generator 22 aims to determine the most opportune time to carry out the cleaning, neither too late nor too early, and is part of a maintenance process for the steam generator. steam generator 22. The steam generator 22 actually operates for at least part of the simulation time, the model of the steam generator 22 therefore constitutes a digital twin of the steam generator 22. This assumes that the operating data imposed on the model are the same as those to which the steam generator 22 is subjected. Preferably, the model is interfaced with sensors equipping the steam generator 22, for example via a database accessible both by a system collection of values measured by the sensors in order to inform these measured values, and by the computer operating the simulation so that it can draw therefrom the data measured for the simulation. In particular, the operating data imposed are accessible to the computer operating the simulation, such as for example the thermal power at each instant of simulation, the temperature difference of the primary fluid between the inlet and the outlet of the steam generator 22, the steam flow at the outlet of the generator of steam 22, or food water temperatures. Similarly, the computer operating the simulation can access a maintenance database, in which can be entered events such as the clogging of the tubes 31, the rate of clogging observed, or even the occurrence of a cleaning operation. .

As indicated above, the simulation duration is defined as the duration during which the operation of the steam generator 22 is simulated. Preferably, at least part of the simulation duration is synchronous with the actual operation of the steam generator 22, i.e. the steam generator 22 operates for an interval between a first simulation instant and a second instant. simulation, and operating data imposed on the model during this second simulation instant come from the operation of the steam generator 22 during this interval. When the simulation is implemented on a steam generator 22 already in operation, the simulation duration includes operation prior to this implementation, which the simulation can catch up by means of a history of operating data and data of maintenance. When the simulation is implemented at the same time as the operation of a steam generator 22 begins, there is of course no need for this adjustment. The simulation of the operation of the steam generator 22 in parallel with the steam generator 22 makes it possible to accurately estimate the state of the steam generator 22, and in particular the rate of clogging and the fouling resistance. Of course, it is however possible to make the simulation diverge with respect to the operation of the steam generator 22, in particular by extending the simulation duration beyond the present moment, to estimate the future evolution of the state of the generator steam generator 22. However, this anticipation of the state of the steam generator 22 is made on the basis of a best updated state of the steam generator 22.

The invention is not limited to the embodiment described and shown in the appended figures. Modifications remain possible, in particular from the point of view of the constitution of the various technical characteristics or by substitution of technical equivalents, without thereby departing from the scope of protection of the invention.

Claims

1. A method for maintaining a steam generator operating a heat exchange between a primary circuit (10) and a secondary circuit (20), the steam generator (22) comprising a plurality of tubes (31) passing through an internal medium , a primary fluid of the primary circuit (10) circulating in said tubes (31) while a secondary fluid of the secondary circuit (20) circulates in the internal medium, the steam generator (22) further comprising a plurality of plates ( 30) through which said tubes (31) pass and in which passages (32) are formed along the tubes allowing the circulation of the secondary fluid, the method comprising, during a simulation period comprising a plurality of simulation instants , a simulation of the operation of the steam generator (22) using a model of said steam generator comprising geometric parameters of the steam generator and taking into account imposed operating data, the model establishing fields of characteristics related to the flow of the primary fluid and of the secondary fluid and to the heat exchange between primary fluid and secondary fluid at each instant of simulation, from the geometric parameters, from the imposed operating data, and from internal variables, characterized in that, following an observation of an observed value of a parameter of the steam generator (22) on said steam generator (22) at a simulation instant following previous simulation instants, an internal variable is adjusted so as to match a simulation value of said parameter at said simulation instant with the observed value of the parameter, the simulation being continued during the following simulation instants from this adjusted internal variable, and in that the observed value of the parameter of the steam generator is a rate of clogging of passages observed during an inspection of said steam generator, the adjusted internal variable then being a pressure drop in the flow of the secondary fluid at the level of said passage, and/or the observed value of the parameter of the generator of vapor is a vapor pressure measured in the internal medium, the adjusted internal variable then being a fouling resistance affecting the heat transfer, and in that at the end of the simulation period, the implementation of a steam generator cleaning operation is planned based on the clogging rate and/or fouling resistance.

2. Method according to claim 1, in which the observed value of the parameter of the steam generator is a temperature linked to the pressure in the internal medium, said temperature being measured instead of the pressure, the adjusted internal variable still being a resistance fouling affecting heat transfer.

3. Method according to any one of the preceding claims, in which the steam generator (22) operates for a time interval between a first simulation instant and a second simulation instant, and operating data imposed on the model during of this second simulation instant are derived from the operation of the steam generator (22) during this time interval.

4. Method according to any one of the preceding claims, in which the imposed operating data comprise at least one of a thermal power corresponding to the power exchanged between the primary circuit (10) and the secondary circuit (20), a temperature difference in the primary circuit (10) between the inlet of the steam generator (22) and the outlet of the steam generator (22), and a flow rate of steam in the secondary circuit (22) at the outlet of the steam (22).

5. Method according to any one of the preceding claims, in which at least one instant of simulation, the observed value of the parameter of the steam generator is a clogging rate of passages or a fouling resistance following cleaning of the generator steam, the adjusted internal variable then being said clogging rate or said fouling resistance, respectively.

6. Method according to any one of the preceding claims, in which at least one instant of simulation, the observed value of the parameter of the steam generator describes a blockage of a tube, and a modifier reflecting said blockage is applied to the geometric parameters of the steam generator model.

7. Method according to any one of the preceding claims, in which the simulation, at each simulation instant, implements: a) a determination of characteristic fields related to the flow of the primary fluid and the secondary fluid and to the heat exchange between primary fluid and secondary fluid, b) a determination of chemical quantities of the secondary fluid from the thermo-hydraulic fields , c) a determination of a transport and material deposition relationship, the rate of clogging and/or fouling being determined from said transport and material deposition relationship.

8. Method according to any one of the preceding claims, in which at least one instant of simulation, the clogging rate is determined from internal variables by using an estimate of a deposit accumulation rate involving a term deposition of particles and a term for the precipitation of soluble species, from which is subtracted a term representing a tearing flux, and in which at least one instant of simulation, the fouling is determined from the sum of two flows of materials: a flow induced by the parietal precipitation of the soluble species, and a flow induced by the parietal boiling.

9. Method according to any one of the preceding claims, comprising the implementation of a steam generator cleaning operation following the determination of the clogging rate and/or the fouling resistance.

10. Method according to any one of the preceding claims, in which the cleaning is a chemical cleaning of the steam generator requiring the stopping of the operation of the steam generator.