Chemical conditioning process of a heat transfer fluid in a primary circuit of a nuclear power plant.

Abstract

The invention relates to a chemical conditioning process for 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 metallic alloy containing nickel, the process 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 decrease the redox potential of the heat transfer fluid, thereby decreasing 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), thereby 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 the abbreviation: Figure 5.

Description

Title of the invention: Chemical conditioning process for 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. 1 schematically represents a primary circuit of a nuclear power plant.

[0003] This primary circuit comprises various components, namely a hot source, here the core of the nuclear reactor CRN, a cold source, here a steam generator (SG), a CAI pipeline connecting the CRN core to the SG, and another CA2 pipeline connecting the SG to the CRN core and incorporating a PMP pump to circulate 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, wall thinning phenomena occur in certain places, particularly at the steam generator, and material deposition phenomena occur in other places, particularly at the nuclear fuel (core).

[0005] The thinning of the walls of a single-phase liquid circuit is known as dissolution. In sections of the circuit carrying a heat transfer fluid in single-phase liquid form, corrosion also contributes to wall thinning and the generation of species in the aqueous phase. The term dissolution-precipitation refers to a chemical action that modifies the passive film that may be present on the wall by decreasing its thickness through dissolution (which activates corrosion) or increasing it through 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 the walls, for example on the core fuels, 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 detrimental 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 can be seen in [Fig. 1], the primary circuit includes a means of chemical conditioning MCC, which is generally arranged to take heat transfer fluid from the outlet of the steam generator (before the pump) and inject species for chemical conditioning into the inlet of the core (after or before the pump), forming a circuit parallel to the pipeline carrying the pump.

[0008] This chemical conditioning method is used to inject reducing and acid-base species into the pipeline connecting the steam generator to the nuclear reactor core in order to define, for the entire circuit (with constant concentrations of reducing agent, acid, and base throughout 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 various types of alloys 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 recalled 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 to an oxidized form, change in the degree of oxidation of a solvated chemical species, etc.).

[0010] The advantage of adjusting the torque value (pH-Eh) can be understood from [Fig.2],

[0011] Figure 2 shows the Pourbaix diagram (Eh as a function of pH) of nickel in water at 150 °C and atmospheric pressure. Lines (a) and (b) represent the thermodynamic stability range of water (outside these lines (a) and (b), water decomposes by redox reactions to give dioxygen or dihydrogen, as the case may be). Depending on the values ​​of 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, that is, in (rather) basic and reducing media. Thus, for the given temperature, we define a stability domain for Nickel(II) oxide (the possible form or degree of oxidation of Nickel in water).

[0012] Generally, a Pourbaix diagram highlights three regions for a metal: a corrosion region where the metal oxidizes rapidly, an immunity region where the metal does not corrode (or corrodes very little), and a passivity region 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 metal's passivity or immunity region.

[0013] As part of the chemical conditioning performed on the heat transfer fluid of a In the primary circuit of a nuclear power plant, the goal is to define an Eh-pH pair within the passivity range of a given oxide (NiO in our example) among all possible oxides of the metal in question (Ni), in order to protect against corrosion by deposit formation, or even more favorably, within the metal's immunity range where corrosion is virtually nonexistent. However, the stability range of the oxide or metal is relatively broad, allowing for a wide range of possible values ​​for the pH-Eh pair. This is especially true in a nuclear power plant's primary circuit, where the pressure is high, thus extending the stability range of the water (in particular, curve (a) is shifted downwards compared to the representation in [Fig. 2]).

[0014] To determine the most suitable pH-Eh pair value, the solubility of the oxide thus formed, or that of the metal if the immunity range is reached, is simultaneously reduced. 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 nuclear power plant circuit, however, nickel is the predominant 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] Figure 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 ​​shown on the Pourbaix diagram in [Fig.2] allows us to define a more restricted range for the redox potential Eh, between approximately -600mV and -900mV (at 150°C).

[0017] It is therefore this dual condition on the stability range of a given oxide or metal ([Fig. 2]) and the minimum solubility ([Fig. 3]) that allows us to define an optimum pH-Eh pair. The value of this pair varies with temperature, therefore as the heat transfer fluid moves within the circuit. It should be noted that a typical standard condition is assumed, at 25°C, such that (pH, Eh) = (9 to 10), (-540 to -600 mV / ENH), which is approximately equivalent at 300°C to the pair (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 primary circuit components.

[0019] In practice, and as previously stated, the pH-Eh couple is adjusted by injecting reducing and acid-base species into the (single-phase) liquid phase.

[0020] In a primary circuit of a nuclear power plant (which operates in single-phase), one Lithium (LiOH, solid), potassium (KOH, solid) and boric acid (H3BO3, solid) can be chosen to regulate the hydrogen potential pH and dihydrogen (H2, gaseous) to regulate the redox potential Eh. 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 (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 (middle) and the resulting thickness of material deposit or thinning (dissolution-precipitation of oxides, corrosion) on the walls of the primary circuit (bottom).

[0022] Thus, on the upper 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, it can be seen that the solubility CM,eq of element M decreases with temperature (the composition of reducing and acid-base species, determined by the quantity of these species injected by the chemical conditioning method of the primary circuit, remains constant, but the pH and Eh vary with temperature). In this case, it can be seen on the middle curve that the solubility CM of 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 towards a wall in the core (hot source). It is therefore precisely here that the primary circuit is likely to encounter precipitation phenomena leading to fouling and / or clogging. Conversely, a flow of material from element M is directed from a wall towards the heat transfer fluid in the steam generator (cold source). It is therefore precisely here that the primary circuit is likely to undergo dissolution phenomena, also activating corrosion of these walls containing element M (typically Ni).

[0025] The lower curve also shows the location of the deposition (hot source, nuclear core) and the location of corrosion (cold source, steam generator). It should be noted that nickel deposition under neutron flux has serious radiological consequences during the operation of the facilities, particularly during maintenance operations.

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

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

[0028] One 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 metallic alloys present in such a circuit, particularly at the level of the steam generator.

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

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

[0032] To achieve 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 metallic alloy containing 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 process includes the following additional steps: B) inject a reducing species at the inlet of the steam generator to decrease the redox potential of the heat transfer fluid, thereby decreasing 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 process according to the invention may include 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 circuit area from the inlet of the steam generator at the inlet of the nuclear reactor core is less 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 coolant 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), potassium hydroxide (KOH) and boric acid (H3BO3) or a mixture of these;

[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 features of the invention will become clearer in the following description, made with reference to the accompanying figures, in which:

[0042] The [Fig.5] is a representative diagram of a primary circuit of a nuclear power plant as intended to implement the process according to the invention;

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

[0044] Fig. 7 shows the evolution of the redox potential (Eh, on the ordinates) as a function of the concentration of dihydrogen (on the abscissas, 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 shown in Fig. 5 as a function of temperature, this evolution 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] The [Fig.5] is a representative diagram of a primary circuit of a power generation plant as provided for implementing the process according to the invention.

[0048] This primary circuit comprises various components, namely a hot source, here the core of the nuclear reactor CRN, a cold source, here a steam generator (SG), a CAI pipeline connecting the CRN core to the SG, and another CA2 pipeline connecting the SG to the CRN core and incorporating a PMP pump to circulate 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 includes a chemical conditioning (CCM) means conforming to prior art principles. This chemical conditioning (CCM) means enables the implementation, in accordance with current prior art practices, of 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 CCM means is generally located at the outlet of the steam generator (cold source), to which it can then be considered equivalent. This step establishes a reducing agent and acid-base species composition for the entire circuit.

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

[0051] The following steps can then be implemented.

[0052] A step B) consisting of injecting a reducing species into the inlet of the generator Steam is used to decrease the redox potential (-AEh) of the heat transfer fluid, thereby 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 device located at the inlet of the steam generator, an inlet to which the said MPI device can be considered equivalent.

[0053] Step C) consists of injecting an oxidizing species at the inlet of the nuclear reactor core 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 device located at the inlet of the nuclear reactor core, an inlet to which said MP2 device can be considered equivalent.

[0054] Implementing steps B) and C) reduces fouling or sealing the walls of the primary circuit.

[0055] Furthermore, advantageously, during step B) a quantity of reducing agent can be injected such that the solubility of the different phases of nickel throughout the primary circuit, from the steam generator inlet to the nuclear reactor core inlet, is lower than the solubility of the different phases of nickel at the outlet of the nuclear reactor core. Indeed, by decreasing the solubility of the nickel phases through the addition of a reducing agent, this blocks the dissolution of the steam generator surfaces and can even lead to the precipitation of nickel in aqueous form exiting the reactor core.

[0056] This also helps to limit the dissolution-precipitation and erosion-corrosion of the metallic alloys present in the primary circuit.

[0057] A situation which makes it possible to reduce or even eliminate deposits fouling or clogging the walls of the primary circuit while limiting the erosion-corrosion of the metallic alloys present in this circuit is explained in 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 in motion, in terms of temperature (top), solubility of an element M (Nickel in this case) (middle) and the resulting thickness of material deposit or thinning (dissolution-precipitation, erosion-corrosion) on the walls of the primary circuit (bottom).

[0059] Thus, on the upper curve of [Fig. 6], it can be seen that the heat transfer fluid alternately increases in temperature in the nuclear reactor core (hot source) and then decreases in the steam generator (cold source), remaining relatively constant in the pipes. This upper curve of [Fig. 6] is identical to the corresponding curve in [Fig. 4].

[0060] On the right-hand curve of [Fig. 6], it is noted that there are two operating points PFht, PFbt, unlike what occurs for the corresponding curve of [Fig. 4]. The transition from one operating point to the other is achieved through the implementation of steps B) and C). For each operating point, the solubility CMjeq of element M decreases with temperature.

[0061] On the mid-curve of [Fig.6], we note, just before the inlet 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 decreasing the solubility of 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 until the outlet of the steam generator.

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

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

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

[0067] This solubility remains essentially constant in the pipeline leading to the inlet of the steam generator (up to point D).

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

[0069] On the lower curve of [Fig. 6], and unlike the corresponding curve in [Fig. 4], it can be seen that there is 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 solubilities between the two states PFHT and PFbt (contrary to what is shown in [Fig. 6]), for example because the quantity of reducing species introduced at the inlet of the steam generator is insufficient, the efficiency is not maximized. Nevertheless, this is still advantageous for the primary circuit because it reduces the amount of matter transported between the steam generator (dissolution, erosion) and the nuclear reactor core (precipitation, but also deposition, which corresponds to the sedimentation of solid particles on the walls) compared to conventional single-point operating conditions.

[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 A method according to the invention for changing the redox potential as desired.

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

[0074] In this example, the conventional chemical conditioning (step A)) is carried out with lithium. The addition of reducing agent at the inlet of the steam generator allows, in this example, the dihydrogen concentration to be increased from 17 cm3 / kg to 70 cmV / kg at a temperature of 320 °C, which decreases 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 concentration of dihydrogen from 70 cm3 / kg to 17 cm3 / kg at a temperature of 280°C, which in turn 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.Under these conditions, no precipitation of nickel oxides will occur in the nuclear reactor core since the solubility of the oxides remains constantly greater than the concentration of nickel present in the coolant at the inlet of the nuclear reactor core.

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

[0076] Figure 8 shows more clearly what happens with this conditioning example. Figure 8 represents the solubility of nickel in the heat transfer fluid as a function of temperature when following the heat transfer fluid through a cycle in the primary circuit shown in Figure 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] (in the 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 inlet of the steam generator.

[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 decrease 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 observe 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 its outlet, we arrive at point D. The temperature remains substantially constant 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, it is possible to work with lithium under other conditions. It is also possible to work with other conventional conditioning agents, namely with potash (KOH, solid) and / or boric acid (H3BO3, solid) or mixtures of acids and bases to regulate the hydrogen potential pH and dihydrogen (H2, gaseous) to regulate the redox potential Eh.

Claims

Demands

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 (NRC), - a steam generator (SG), - a first pipe (CAI) connecting the outlet of the nuclear reactor core (NRC) to the inlet of the steam generator (SG), - a second pipe (CA2) connecting the outlet of the steam generator (SG) to the inlet of the core (NRC), each of said components being made of a metallic alloy containing 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 process comprises the following additional steps: B) injecting a reducing species at the inlet (MPI) of the steam generator (SG) to decrease the redox potential (Eh) of the heat transfer fluid, thereby decreasing 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), thereby 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. A 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 less than the solubility of the different phases of Nickel at the outlet of the nuclear reactor core.

3. A process according to any 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. A method according to any 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. A method according to any 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. A method according to any one of the preceding claims, characterized in that the acid-base species injected during step A) is selected from lithium (LiOH), potash (KOH) and boric acid (H3BO3) or a mixture thereof.

7. A process according to any 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. A method according to any 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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