Process for chemical conditioning of a heat transfer fluid in a primary circuit of a nuclear power plant.

Abstract

The invention relates to a method for chemically conditioning a heat transfer fluid in a primary circuit of a nuclear power plant, the circuit comprising a nuclear reactor core (NRC) and a steam generator (SG) each made of a metal alloy comprising Nickel, the method comprising the steps: A) injecting reducing and acid-base species into the heat transfer fluid to define, at a given temperature, a reference hydrogen potential - redox potential pair, B) injecting a reducing species at the inlet (MP1) of the steam generator to reduce the redox potential of the heat transfer fluid, this reducing the solubility of the different phases of Nickel in the heat transfer fluid, C) injecting, at the inlet (MP2) of the core, an oxidizing species to compensate for the effects of the reducing species injected in step B), this increasing the redox potential of the heat transfer fluid and the solubility of the different phases of Nickel in the heat transfer fluid.Figure for abstract: Figure 5.

Description

Title of the invention: Method for chemical conditioning of a heat transfer fluid in a primary circuit of a nuclear power plant. Technical field of the invention

[0001] The invention relates to the field of chemical conditioning of a heat transfer fluid in a primary circuit of a nuclear power plant. Technical background

[0002] [Fig.l] shows a diagram of a primary circuit of a nuclear power plant.

[0003] This primary circuit comprises different components, namely a hot source, here the core of the CRN nuclear reactor, a cold source, here a steam generator GV, a CAI pipe connecting the CRN core to the steam generator GV and another CA2 pipe connecting the steam generator GV to the CRN core and integrating a PMP pump to drive the heat transfer fluid. The heat transfer fluid is in liquid form throughout the primary circuit. The heat transfer fluid is generally water.

[0004] In this type of circuit, we encounter phenomena of thinning of the walls in certain places, in particular at the level of the steam generator, and phenomena of deposition of material in other places, in particular at the level of the nuclear fuels (core).

[0005] The phenomena of thinning of the walls of a liquid single-phase circuit are known as dissolution. In the portions of the circuit carrying a heat transfer fluid in liquid single-phase form, corrosion also contributes to the thinning of the walls and to the generation of species in the aqueous phase. The term dissolution-precipitation refers to a chemical action which modifies the passive film likely to be present on the wall by reducing its thickness by dissolution (which activates corrosion) or by increasing it by precipitation (which slows down corrosion).

[0006] The phenomena of material deposition on the walls of a circuit are known as fouling or clogging. Fouling refers to the precipitation of oxides or metals on all of the walls, for example on the fuels of the core, from species present in the aqueous phase, while clogging refers to the precipitation of aqueous species and / or the deposition of particles localized on thermohydraulic singularities.

[0007] To limit these phenomena, which are harmful in the long term to the proper functioning of the nuclear power plant, chemical conditioning of the heat transfer fluid is carried out. Thus, as visible in [Fig.l], the primary circuit comprises a means MCC chemical conditioning, which is generally arranged to take heat transfer fluid at the outlet of the steam generator (before the pump) and inject species for chemical conditioning at the inlet of the core (after or before the pump), forming a circuit parallel to the pipe containing the pump.

[0008] This chemical conditioning means is used to inject reducing and acid-base species into the pipe connecting the steam generator to the nuclear reactor core in order to define for the entire circuit (concentrations of reducing agent, acid and base constant in the circuit) a reference solubility of the different phases of Nickel in the heat transfer fluid. Nickel is in fact an element present in the different types of alloy constituting the different components of the primary circuit.

[0009] The injection of reducing and acid-base species makes it possible to adjust a hydrogen potential - redox potential (pH-Eh) pair to a given reference value. It should be remembered that the hydrogen potential (pH) measures, by the logarithm of the activity of the solvated proton, the acid-base character of the water, and the redox potential (Eh), measures the oxide-reducing character of the water with respect to the species present in the water or the materials in contact with it (oxidation reaction of a metal towards an oxidized form, change in the degree of oxidation of a solvated chemical species, etc.).

[0010] The interest in adjusting the value of the couple (pH-Eh) can be understood with the support of [Fig.2],

[0011] [Fig.2] gives the Pourbaix diagram (Eh as a function of pH) of Nickel in water at 150 °C and at atmospheric pressure. The two lines (a) and (b) represent the thermodynamic stability domain of water (outside these lines (a) and (b), water decomposes by oxidation-reduction to give dioxygen or dihydrogen as the case may be). Depending on the values ​​taken by the hydrogen potential (pH) and the redox potential (Eh) of the solution, different oxides and aqueous species will appear. For example, at a temperature of 150 °C, Nickel(II) oxide (NiO) can only form for pH values ​​between 6.5 and 13.5 and Eh values ​​between -0.3 V and - 1 V, i.e. in (rather) basic and reducing environments. We thus define, for the given temperature, a stability domain for Nickel(II) oxide (possible form or degree of oxidation for Nickel in water).

[0012] Generally speaking, a Pourbaix diagram shows three domains for a metal: a corrosion domain where the metal oxidizes quickly, an immunity domain where the metal does not corrode (or very little) and a passivity domain where corrosion is prevented by the formation of an oxide layer. The objective of chemical conditioning is to place the metal-water system in the passivity or immunity domain of the metal.

[0013] In the context of the chemical conditioning carried out on the heat transfer fluid of a primary circuit of a nuclear power plant, we therefore seek to define an Eh-pH couple in the passivity domain of a given oxide (NiO in our example) among all the possible oxides of the metal concerned (Ni) to protect against corrosion by the formation of a deposit, or even more favorably, in the immunity domain of the metal where corrosion is almost non-existent. The stability domain of the oxide or the metal is however relatively wide and leaves a wide range of possible values ​​for the pH - Eh couple. This is all the more true since, in a primary circuit of a nuclear power plant, the pressure is high which makes it possible to extend the stability domain of the water (in particular, curve (a) is shifted downwards compared to the representation in [Fig.2]).

[0014] To set the most suitable pH-Eh value, we seek in parallel to reduce the solubility of the oxide thus formed or that of the metal if the immunity domain is reached. In fact, the pH-Eh pair is not chosen by considering only the behavior of a single metallic element but by making compromises for all the alloying elements present in the circuit. In the circuit of nuclear power plants, nickel is however the majority metallic element, particularly in the alloy of the steam generator tubes. Controlling its physicochemical behavior by setting the pH-Eh pair is particularly important to limit contamination in the circuit.

[0015] [Fig.3] shows the solubility diagram of Nickel at 150°C as a function of pH. This diagram shows that a minimum solubility in water exists and is obtained for a pH between 8 and 12.

[0016] This range of pH values ​​reported on the Pourbaix diagram in [Fig.2] makes it possible to define a more restricted domain for the redox potential Eh, between approximately -600mV and -900mV (at 150°C).

[0017] It is therefore this double condition on the stability domain of a given oxide or metal ([Fig.2]) and the minimum solubility ([Fig.3]) which makes it possible to define an optimum pH-Eh couple. The value of this couple evolves with the temperature therefore when the heat transfer fluid moves within the circuit. It should be noted that a classical conditioning is typically taken, at 25°C, such that (pH, Eh) = (9 to 10), (-540 to -600 mV / ENH) which is equivalent at 300°C approximately to the couple (pH, Eh) = (7 to 8), (-800, -900 mV / ENH).

[0018] This limits the phenomena of dissolution-precipitation and erosion-corrosion of the walls of the components of the primary circuit.

[0019] In practice, and as previously specified, the adjustment of the pH-Eh pair is carried out by injecting reducing and acid-base species into the liquid phase (single-phase).

[0020] In a primary circuit of a nuclear power plant (which operates in single-phase), we can choose lithium hydroxide (LiOH, solid), potassium hydroxide (KOH, solid) and boric acid (H3BO3, solid) to adjust the pH hydrogen potential and dihydrogen (H2, gaseous) to adjust the Eh redox potential. All these species are soluble in water.

[0021] [Fig.4] schematically represents what happens in the primary circuit when the heat transfer fluid is in motion, in terms of temperature (at the top), the solubility of an element M (typically Nickel contained in the alloys of the different components of the primary circuit) in the chemically conditioned heat transfer fluid (in the middle) and the thickness of material deposit or thinning (dissolution-precipitation of oxides, corrosion) which results on the walls of the primary circuit (at the bottom).

[0022] Thus, on the top curve, we note that the heat transfer fluid alternately sees its temperature increase in the core of the nuclear reactor (hot source) then decrease in the steam generator (cold source) and remain relatively constant in the pipes.

[0023] On the right-hand curve in the middle, we note that the solubility CM,eq of the element M decreases with the temperature (the composition in reducing and acid-base species determined by the quantity injected of these species by the chemical conditioning means of the primary circuit remain constant, but the pH and the Eh vary with the temperature). In this case, we note on the middle curve that the solubility CM of the element M in the heat transfer fluid decreases in the core (hot source) then increases in the steam generator (cold source) and remains relatively constant in the pipes.

[0024] This reflects the fact that the flow of material from element M is directed from the heat transfer fluid to a wall in the core (hot source). This is therefore where the primary circuit is likely to encounter precipitation phenomena involving fouling and / or clogging. Conversely, a flow of material from element M is directed from a wall to the heat transfer fluid in the steam generator (cold source). This is therefore where the primary circuit is likely to experience dissolution phenomena, also activating the corrosion of these walls containing element M (as a reminder, typically Ni).

[0025] On the bottom curve, we also note the location of the deposit (hot source, nuclear core) and the location of corrosion (cold source, steam generator). It should be noted that the deposition of Nickel under neutron flux has serious radiological consequences during the operation of the installations, in particular during maintenance operations.

[0026] The chemical conditioning currently in operation makes it possible to greatly limit the phenomena of dissolution-precipitation (fouling, clogging) and also corrosion.

[0027] However, these phenomena exist and remain observable on long time scales but nevertheless shorter than the operating times of electricity production plants.

[0028] An objective of the invention is therefore to reduce or even eliminate deposits fouling or clogging the walls of a primary circuit of a nuclear power plant, in particular on the nuclear fuels of the core.

[0029] Another objective of the invention is to be able to limit the corrosion of the metal alloys present in such a circuit, in particular at the level of the steam generator.

[0030] Another objective of the invention is furthermore to limit or even eliminate the oxidizing species produced by the radiolysis of water.

[0031] Another objective is to limit radioactive contamination of circuits by reducing the precipitation of metallic species, particularly nickel. Summary of the invention

[0032] To solve at least one of the objectives, the invention proposes a method for chemically conditioning a heat transfer fluid in a primary circuit of a nuclear power plant, the circuit comprising the following components: a nuclear reactor core, a steam generator, a first pipe connecting the outlet of the nuclear reactor core to the inlet of the steam generator, a second pipe connecting the outlet of the steam generator to the inlet of the core, each of said components being made of a metal alloy comprising Nickel, the method comprising a step consisting of: A) inject reducing and acid-base species into the second pipe to define, at a given temperature, a reference hydrogen potential - redox potential pair in the heat transfer fluid, characterized in that the method comprises the following additional steps: B) inject a reducing species at the inlet of the steam generator to reduce the redox potential of the heat transfer fluid, this reducing the solubility of the different phases of Nickel in the heat transfer fluid, and C) inject, at the inlet of the nuclear reactor, an oxidizing species to compensate for the effects of the reducing species injected in step B), this increasing the redox potential of the heat transfer fluid and the solubility of the different phases of Nickel in the heat transfer fluid.

[0033] The method according to the invention may comprise at least one of the following additional steps, taken alone or in combination:

[0034] - during step B), a quantity of reducing species is injected ensuring that the solubility of the different phases of Nickel over the entire area of ​​the circuit going from the inlet from the steam generator to the inlet of the nuclear reactor core is lower than the solubility of the different phases of Nickel at the outlet of the nuclear reactor core;

[0035] - the reducing species injected at the inlet of the steam generator in step B) is the di- hydrogen, in gaseous form or dissolved in the heat transfer fluid;

[0036] - the oxidant injected into the heat transfer fluid at the inlet of the nuclear reactor core in step C) is dioxygen, in gaseous form or dissolved in the heat transfer fluid;

[0037] - step A) consists of imposing a pH between 7 and 8, at 300°C, and a potential redox between -800 mV / ENH and -900 mV / ENH, at 300 °C;

[0038] - the acid-base species injected during step A) is chosen from lithium (LiOH), potash (KOH) and boric acid (H3BO3) or a mixture thereof;

[0039] - the reducing species injected during step A) is dihydrogen, in the form gaseous or dissolved in the heat transfer fluid;

[0040] - step A) is carried out at the outlet of the steam generator. Brief description of the figures

[0041] Other objects and characteristics of the invention will appear more clearly in the description which follows, made with reference to the appended figures, in which:

[0042] [Fig. 5] is a representative diagram of a primary circuit of a nuclear power plant as provided for implementing the method according to the invention;

[0043] [Fig.6] represents what happens in the primary circuit shown in [Fig.5] when the heat transfer fluid is moving there, in terms of temperature (at the top), the solubility of an element M, Nickel in this case (in the middle), and the thickness of the material deposit or thinning which results on the walls of the primary circuit (at the bottom)

[0044] [Fig.7] provides the evolution of the redox potential (Eh, on the ordinate) as a function of the dihydrogen concentration (on the abscissa, logarithmic scale) for several values ​​of the dihydrogen pressure (curves mainly oriented vertically, lowest pressure on the left and highest on the right) and for several values ​​of the temperature (curves mainly oriented horizontally, lowest temperature at the top and highest at the bottom);

[0045] [Fig.8] represents the evolution of the solubility of Nickel in the heat transfer fluid (water) of the primary circuit of the installation represented in [Fig.5] as a function of the temperature, this evolution therefore being based, in accordance with the framework of the invention, on two operating points: a low temperature operating point PFbt and a high temperature operating point PFHT. Detailed description of the invention

[0046] The following description is made in support of the attached figures 5 to 8.

[0047] [Fig. 5] is a representative diagram of a primary circuit of an electricity production plant as provided for implementing the method according to the invention.

[0048] This primary circuit comprises different components, namely a hot source, here the core of the CRN nuclear reactor, a cold source, here a steam generator GV, a CAI pipe connecting the CRN core to the steam generator GV and another CA2 pipe connecting the steam generator GV to the CRN core and integrating a PMP pump to drive the heat transfer fluid. The heat transfer fluid is in liquid form throughout the primary circuit. The heat transfer fluid is generally water.

[0049] This primary circuit also comprises a chemical conditioning MCC means in accordance with that of the prior art. This chemical conditioning MCC means makes it possible to implement, in accordance with what is currently carried out in the state of the art, a chemical conditioning step A) consisting of injecting reducing and acid-base species into the second pipe to define, at a given temperature, a reference hydrogen potential - redox potential (pH-Eh) pair in the heat transfer fluid. This MCC means is generally located at the outlet of the steam generator (cold source) to which it can then be assimilated. This step sets a reducing agent composition in acid-base species for the entire circuit.

[0050] However, the primary circuit further comprises a means MPI for introducing a product into the primary circuit, this means MPI being located at the inlet of the steam generator GV (cold source). Furthermore, the primary circuit further comprises a means MP2 for introducing or extracting a product from the primary circuit, this means MP2 being located at the inlet of the nuclear reactor core (hot source).

[0051] The following steps can thus be implemented.

[0052] A step B) consisting of injecting a reducing species at the input of the generator of steam, to reduce the redox potential (-AEh) of the heat transfer fluid, this reducing the solubility of the different phases of Nickel in the heat transfer fluid. It is understood that this step B) is implemented using the MPI means located at the inlet of the steam generator, inlet to which said MPI means can be assimilated.

[0053] Step C) consists of injecting, at the inlet of the nuclear reactor core, an oxidizing species to compensate for the effects of the reducing species injected in step B). This increases the redox potential and the solubility of the different phases of Nickel in the heat transfer fluid. In all cases, it is understood that this step C) is implemented using the MP2 means located at the inlet of the nuclear reactor core, an inlet to which said MP2 means can be assimilated.

[0054] The implementation of steps B) and C) makes it possible to reduce fouling deposits or clogging the walls of the primary circuit.

[0055] Furthermore, advantageously, during step B) it is possible to inject a quantity of reducing species so that the solubility of the different phases of Nickel over the entire zone of the primary circuit going from the inlet of the steam generator GV to the inlet of the nuclear reactor core CRN is lower than the solubility of the different phases of Nickel at the outlet of the nuclear reactor core. Indeed, by reducing the solubility of the phases of Nickel by the addition of a reducing agent, this blocks the dissolution of the surfaces of the steam generator and can even lead to the precipitation of the nickel in aqueous form leaving the reactor core.

[0056] This also makes it possible to limit the dissolution-precipitation and erosion-corrosion of the metal alloys present in the primary circuit.

[0057] A situation making it possible to both reduce or even eliminate deposits fouling or clogging the walls of the primary circuit while limiting the erosion-corrosion of the metal alloys present in this circuit is explained with the support of [Fig.6].

[0058] [Fig.6] schematically represents what happens in the primary circuit shown in [Fig.5] when the heat transfer fluid is moving there, in terms of temperature (at the top), the solubility of an element M (Nickel in this case) (in the middle) and the thickness of material deposit or thinning (dissolution-precipitation, erosion-corrosion) which results on the walls of the primary circuit (at the bottom).

[0059] Thus, on the top curve of [Fig.6], we note that the heat transfer fluid alternately sees its temperature increase in the nuclear reactor core (hot source) then decrease in the steam generator (cold source) and remain relatively constant in the pipes. This top curve of [Fig.6] is identical to the corresponding one in [Fig.4].

[0060] On the right-hand curve of [Fig.6], we note that there are two operating points PFht, PFbt, unlike what happens for the corresponding curve of [Fig.4]. The transition from one operating point to the other is carried out thanks to the implementation of step B) and step C). For each operating point, the solubility CMjeq of the element M decreases with the temperature.

[0061] On the middle curve of [Fig.6], we note, just before the entry of the steam generator (point D), that the solubility CM of the element M in the heat transfer fluid is that provided by the chemical conditioning of step A). ​​We are then in a completely classic operating point.

[0062] Step B) is then implemented at the inlet of the steam generator (cold source), which has the effect of immediately reducing the solubility of the element M in the heat transfer fluid (point A). We then move from a first operating point PFH T, conventional, to a second operating point PFBT.

[0063] The solubility of element M then increases in the steam generator (cold source) with the decrease in temperature, but with values ​​linked to this second operating point up to the outlet of the steam generator.

[0064] The solubility remains substantially constant in the pipe leading to the inlet of the nuclear reactor core (point B).

[0065] At the inlet of the nuclear reactor core, step C) of the method according to the invention is implemented. The solubility of element M in the heat transfer fluid then increases immediately (point C). We then move from the PFbt operating point to the conventional PFHT operating point. Dioxygen can be used in gaseous form or dissolved in the heat transfer fluid as an oxidant.

[0066] Then, the solubility of the element M in the heat transfer fluid decreases in the nuclear reactor core until its exit with the increase in temperature.

[0067] This solubility ultimately remains substantially constant in the pipe going to the inlet of the steam generator (up to point D therefore).

[0068] Thus, between point C (inlet of the nuclear reactor core) and point D (inlet of the steam generator), we are in a first operating point PFHt, classic, which is the one shown in [Fig.4]. On the contrary, between point A (inlet of the steam generator) and point B (inlet of the nuclear reactor core), we are in another operating point PFBt, radically different from the previous operating point due to the introduction of a reducing species in the primary circuit at the inlet of the steam generator.

[0069] On the bottom curve of [Fig.6], and unlike the corresponding curve of [Fig.4], we note that there is then neither deposition of material (no precipitation) at the level of the nuclear reactor core (hot source) nor thinning (no dissolution) at the level of the steam generator (cold source). This means that the precipitation of element M in the nuclear reactor core becomes impossible and that the dissolution of element M in the steam generator also becomes impossible.

[0070] In the case where there is a partial overlap of the solubilities between the two states PF HT and PFbt (contrary to what is represented in [Fig.6]) for example because the quantity of reducing species introduced at the inlet of the steam generator is not sufficient, the efficiency is then not maximum. Nevertheless, this is still of interest for the primary circuit because there remains a reduction in the quantity of material displaced between the steam generator (dissolution, erosion) and the nuclear reactor core (precipitation, but also deposition which corresponds to a sedimentation of solid particles on the walls) compared to a conventional conditioning at a single operating point.

[0071] From a practical point of view, it is important to determine the quantity of reducing species to be introduced at the inlet of the steam generator during step B) of the method according to the invention to change the redox potential as desired.

[0072] By definition, the change in the redox potential depends directly on the change in the concentration of dihydrogen in the heat transfer fluid (chemical species electrochemically equivalent to all reducing species). For example, we can refer to [Fig.7] which shows this link for different values ​​of temperature (substantially horizontal lines) ranging from 25°C to 350°C and for different values ​​of dihydrogen pressure (substantially vertical lines, lowest pressure on the left and pressure increasing towards the right) ranging from 103 bar to 10 bar. By definition also, the concentration of dihydrogen will depend on the additional mass flow rate of dihydrogen added or removed from the heat transfer fluid, taking into account the mass flow rate of heat transfer fluid circulating in the primary circuit.The mass flow rate of heat transfer fluid present in the primary circuit depends on the installation concerned, but it is known. Dihydrogen can be in gaseous form or dissolved in the heat transfer fluid.

[0073] We will first present an example for a given installation and therefore a given (known) heat transfer fluid mass flow rate. Examples

[0074] In this example, the conventional chemical conditioning (step A) is carried out with lithium hydroxide. The addition of reducing agent at the inlet of the steam generator makes it possible, in this example, to increase the dihydrogen concentration from 17 cm3 / kg to 70 cmVkg at a temperature of 320°C, which reduces the redox potential from -850 mV / ENH to -890 mV / ENH and the solubility of nickel from 3.109 mol / kg to 109 mol / kg. At the inlet of the nuclear reactor core, an oxidant is added which, in this example, reduces the hydrogen concentration from 70 cm3 / kg to 17 cm3 / kg at a temperature of 280°C, which this time increases the redox potential from -800 mV / ENH to -750 mV / ENH and the solubility of Nickel from 3.5 109 mol / kg to 14.109 mol / kg.With these conditions, no precipitation of nickel oxides will occur in the nuclear reactor core since the solubility of the oxides remains constantly higher than the concentration of nickel present in the heat transfer fluid entering the nuclear reactor core.

[0075] Dihydrogen (gaseous) can be chosen as the reducing agent added at the steam generator inlet. Dioxygen (gaseous) can be chosen as the oxidant at the nuclear reactor core inlet.

[0076] [Fig.8] shows more clearly what happens with this conditioning example. [Fig.8] represents the solubility of Nickel in the heat transfer fluid as a function of temperature when following the heat transfer fluid in a cycle in the primary circuit shown in [Fig.5]. Two curves PFHT and PFBT are shown for a high temperature operating point and a low temperature operating point, respectively. Points A, B, C and D are the same as those shown in [Fig.5] (middle).

[0077] At point D, we are just before the inlet of the steam generator but before the injection of the reducer into the heat transfer fluid.

[0078] At point A, the reducer has been injected and we are just after the steam generator inlet.

[0079] Between points D and A, we therefore observe the effect of adding the reducer according to step B) at the inlet of the steam generator which results in a drastic reduction in the solubility of Nickel in the heat transfer fluid.

[0080] Then, between point A and point B, we move from the inlet of the steam generator to its outlet. We see that the solubility increases with the decrease in temperature.

[0081] The reducer is then removed in accordance with step C) of the process at the inlet of the nuclear reactor core, which results in the passage from point B to point C and therefore in a drastic increase in the solubility of Nickel in the heat transfer fluid.

[0082] The nuclear reactor core increases the temperature and at the outlet of the reactor core, we arrive at point D. The temperature remains substantially the same in the pipe going from the outlet of the nuclear reactor core to the inlet of the steam generator.

[0083] End of example.

[0084] Of course, beyond this example, we can work with lithium hydroxide in other conditions. We can also work with other classic conditioning, namely with potash (KOH, solid) and / or boric acid (H3BO3, solid) or mixtures of acids and bases to adjust the pH hydrogen potential and di-hydrogen (H2, gaseous) to adjust the Eh redox potential.

Claims

Claims

1. A method for chemically conditioning a heat transfer fluid in a primary circuit of a nuclear power plant, the circuit comprising the following components: - a nuclear reactor core (CRN), - a steam generator (GV), - a first pipe (CAI) connecting the outlet of the nuclear reactor core (CRN) to the inlet of the steam generator (GV), - a second pipe (CA2) connecting the outlet of the steam generator (GV) to the inlet of the core (CRN), each of said components being made of a metal alloy comprising Nickel, the method comprising a step consisting of: A) injecting reducing and acid-base species into the second pipe (CA2) to define, at a given temperature, a reference hydrogen potential - redox potential (pH-Eh) pair in the heat transfer fluid,characterized in that the method comprises the following additional steps: B) injecting a reducing species at the inlet (MPI) of the steam generator (GV) to reduce the redox potential (Eh) of the heat transfer fluid, this reducing the solubility of the different phases of Nickel in the heat transfer fluid, and C) injecting, at the inlet (MP2) of the nuclear reactor, an oxidizing species to compensate for the effects of the reducing species injected in step B), this increasing the redox potential (Eh) of the heat transfer fluid and the solubility of the different phases of Nickel in the heat transfer fluid.,

2. Method according to claim 1, characterized in that during step B), a quantity of reducing species is injected ensuring that the solubility of the different phases of Nickel over the entire area of ​​the circuit going from the inlet of the steam generator to the inlet of the nuclear reactor core is lower than the solubility of the different phases of Nickel at the outlet of the nuclear reactor core.

3. Method according to one of the preceding claims, characterized in that the reducing species injected at the inlet of the steam generator in step B) is dihydrogen, in gaseous form or dissolved in the heat transfer fluid.

4. Method according to one of the preceding claims, characterized in that the oxidant injected into the heat transfer fluid at the inlet of the nuclear reactor core in step C) is dioxygen, in gaseous form or dissolved in the heat transfer fluid.

5. Method according to one of the preceding claims, characterized in that step A) consists of imposing a pH between 7 and 8, at 300°C, and a redox potential between -800 mV / ENH and -900 mV / ENH, at 300°C.

6. Method according to one of the preceding claims, characterized in that the acid-base species injected during step A) is chosen from lithium hydroxide (LiOH), potassium hydroxide (KOH) and boric acid (H3BO3) or a mixture thereof.

7. Method according to one of the preceding claims, characterized in that the reducing species injected during step A) is di-hydrogen, in gaseous form or dissolved in the heat transfer fluid.

8. Method according to one of the preceding claims, characterized in that step A) is carried out at the outlet (MCC) of the steam generator (GV).

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