The invention relates to the field of reactor coolant pumps of pressurized water nuclear reactors (PWRs).

[0002] More specifically, the invention relates to ice cream, also called active faces of the sealing gasket of the gasket 1 of a sealing primary pump unit shaft system of a nuclear reactor.

STATE OF THE ART

[0003] In the water nuclear reactors under pressure, the primary pump unit, also more simply called primary pump ensures the circulation of water in the primary circuit of the reactor. A shaft dynamic sealing system provides a seal between the primary circuit and the atmosphere. This primary pump unit shaft seal system is a controlled leak system. It has three seals arranged in series. Each joint has two windows that provide the primary seal. One of the lenses, called rotating mirror, is mounted in a fixed rotating shaft assembly, the other ice, said floating ice, is mounted in an assembly does not rotate but is free to move axially to follow the axial displacements potential of the tree.

[0004] The gasket 1 provides most of the pressure drop between the primary circuit and the atmosphere. It allows you to switch a pressure of 1 55 bar to a pressure of 2 bars. This gasket 1 is a hydrostatic seal, film of water with a thickness of the order of 1 0 μηι. The particular geometry of the faces of the ice ensures the main and enables sealing to the judgment as to rotate, an automatic adjustment of their spacing.

[0005] The gasket 1 works with a controlled leakage rate, of the order of 600 liters per hour in nominal operation, with the machined profile of its active faces. The hot primary fluid is confined in the primary circuit through an upstream cold water injection of the gasket 1 at a slightly greater than that of the primary circuit pressure. Part of the cold water passes through the primary circuit and a part passes through the seal 1 so as to cool it to maintain a still lower temperature 1 00 ° C.

[0006] Historically, the ice seal No. 1 were alumina, but are now more often made of silicon nitride more abrasion resistant.

[0007] In an accident situation, type SBO (for Black Out Station in English), corresponding to a total loss of electrical power to the nuclear plant, the cooling circuits of the group of shaft sealing system become primary pump inoperative and cause the loss of cold water injected at high pressure upstream of the seal 1 and the cooling of the thermal barrier of the pump. Therefore, the hot water of the primary circuit goes back to the joints of the shaft sealing system.

[0008] The applicant has identified in the study of these scenarios that the hot water passage between the ice silicon nitride of the joint 1 causes degradation. Indeed, in an environment of superheated water at a temperature above 200 ° C and under pressure (pressure greater than or equal to the saturation vapor pressure of water), which corresponds to an accident situation type SBO, ice silicon nitride undergo degradation and decomposition. Indeed, under the conditions of SBO, the silicon nitride is converted into ammonia and silica. This results in dissolution and erosion of the ice lost from the surface in the ice profile changes accordingly and therefore causes a sharp increase in leakage rate of the gasket 1 can no longer ensure its function.

[0009] This is problematic because it can quickly lead to the uncovering of the heart if the necessary remedial measures are not taken in time by the operator.

DISCLOSURE OF INVENTION

[0010] In this context the invention aims to overcome the disadvantages of the prior art by providing an efficient, easy to implement, avoiding degradation ice silicon gasket 1 nitride system sealing shaft of primary pump of a nuclear reactor group, including accident conditions of loss of all cooling sources of the shaft seal system (typical situation SBO).

[0011] To do this, the invention proposes to cover the ice surface of the gasket 1 of a particular protective layer conferring protection hydrothermal silicon nitride thereby preventing its dissolution in normal operation and in an accident situation of Type SBO.

[0012] More specifically, the invention provides a crystal of silicon nitride for group shaft sealing system primary pump of a nuclear reactor for sealing between the primary circuit and the atmosphere, said ice having a surface covered by a protective layer made of a non-porous material and chemically inert to superheated water at a temperature greater than or equal to 200 ° C and under pressure (pressure greater than or equal to the saturation vapor pressure of water) .

[0013] The invention therefore consists in using an inert protective layer and non-degradable in an aqueous medium and in SBO type accident conditions (temperature at the upper wipers 200 ° C), and capable of preventing degradation and erosion of the ice surface of silicon nitride which is transformed into silica under conditions of temperatures higher than 200 ° C. The protective layer according to the invention resists erosion both in normal operating conditions in SBO type conditions.

[0014] With the addition of a protective layer of the invention on ice in silicon nitride, they now have a hydrothermal resistance SBO type conditions and degrade more.

[0015] Therefore, this protection against degradation nitride ice responds to a specific problem different from the problem of fouling of ice known per se.

[0016] The ice cream according to the invention may also have one or more of the following characteristics taken individually or in any combinations technically possible:

the protective layer has properties of adhesion with the silicon of said ice nitride;

the protective layer has chemical resistance properties to boric acid and / or lithium hydroxide and / or potassium hydroxide;

the protective layer has a uniform thickness; the protective layer is uniform in thickness and complies with the shape of the support;

the protective layer has a hardness to withstand the friction (in particular between the two active faces of the ices) and scratch resistance;

the protective layer has a roughness equivalent to the active surface of said mirror;

the protective layer has properties of resistance to thermal shock;

the protective layer is diamond micro or nanocrystalline or zirconium oxide;

said protective layer has a thickness of a few micrometers; for example between 0.1 and 30 microns and advantageously between 0.2 and 10 microns; and preferably between 0.2 and 2 μηι;

the active surface of said lens intended to be in contact with a water film is covered entirely by a protective layer;

the protective layer is resistant to erosion caused by water under normal conditions and SBO type accident conditions;

the protective layer does not disturb the normal operation of the ice;

the protective layer is resistant to all conditions that can undergo the joint No. 1 in normal operation or under accident conditions;

said crystal is a floating ice or a rotating mirror;

[0017] The invention also relates to a gasket comprising at least one blade according to the invention.

[0018] The invention also relates to a primary motor-driven pump shaft seal system of a nuclear reactor comprising at least one seal according to the invention.

[0019] The invention also relates to a primary motor-driven pump of a nuclear reactor having a shaft sealing system according to the invention.

[0020] The invention also relates to a pressurized water nuclear reactor comprising a primary pump unit according to the invention.

BREVE DESCRIPTION DES FIGURES

[0021] Other features and advantages of the invention will emerge on reading the description which follows, with reference to the accompanying figures.

[0022] Figure 1 is a sectional view of a sealing system for a primary motor-driven pump shaft according to one embodiment of the invention.

[0023] Figure 2 is a schematic sectional view of a seal 1 in accordance with one embodiment of Figure 1.

[0024] Figure 3 is a schematic sectional view of the ice of the seal 1 in accordance with one embodiment of the invention.

[0025] Figure 4a is an image formed by electron microscopy showing the surface condition of a sample of ice of a joint 1 having a protective layer according to the invention after exposure to an aqueous medium 290 ° C and under pressure.

[0026] Figure 4b is an image formed by electron microscopy showing the surface condition of a sample of ice of a gasket 1 without the protective layer after exposure to an aqueous medium to 290 ° C and pressure.

[0027] Figure 5a is a graph illustrating the change of the leak rate over time of a joint 1 having ice with a protective layer according to the invention, during an increase of the temperature at the gasket 1 due to a rise in the primary fluid SBO type conditions.

[0028] Figure 5b is a graph illustrating the change of the leak rate over time of a joint 1 having ice without protective layer, at a temperature increase at the joint 1 following a rise in the primary fluid SBO type conditions.

[0029] In all the figures, common elements bear the same references unless indicated otherwise.

DETAILED DESCRIPTION OF AN EMBODIMENT

[0030] Figure 1 shows a sealing mechanical seals system 4 shaft 7 of a primary pump unit of a pressurized water nuclear reactor. This shaft seal system comprises a seal # 1 referenced 1 in Figure 1, a seal 2 referenced 2 in Figure 1 and a seal # 3 referenced 3 in Figure 1. Each joint No. 1, 2, 3 consists of a rotating mirror integral with the shaft 7 and a floating ice may follow the axial displacements of the shaft 7 but not rotating.

[0031] The seal 1 is shown more specifically in Figure 2. The seal 1 provides most of the pressure drop between the primary circuit 8 and the atmosphere 9. The gasket 1 is hydrostatic type, water film with a thickness of about 10 μιτι. The seal 1 comprises a rotating mirror 10 secured to the shaft 7 and a floating crystal 1 1 can follow the axial displacements of the shaft 7. The seal 1 leakage rate is determined by the dual slope floating glass 1 1 or even by the respective slopes of ice

floating and rotating according to an alternative embodiment of the ice seal No. 1 (not shown). Ice 1 0, 1 1 are made of silicon nitride.

[0032] The wipers 1 0, 1 1 of the gasket 1 in accordance with the invention are shown more specifically in Figure 3. The surface 1 2 of at least one of the lenses 1 0, 1 1 is covered with a layer protective 13. Preferably, the two mirrors 10 to 1 1 are covered by a protective layer-1 3 at their active surface.

[0033] This protective layer-1 3 is made of a nonporous material and chemically inert in aqueous medium and at a temperature above or equal to 200 ° C. This protective layer 3 allows one to prevent the degradation and erosion of the ice surface silicon nitride provided SBO type and does not disturb the normal operation of the seal 1.

[0034] The protective layer-1 3 also shows the properties of chemical resistance to corrosion and particularly boric acid, with lithium hydroxide and potassium hydroxide and resists erosion. More generally, the protective layer 13 is resistant to all conditions that can undergo the joint No. 1 in normal operating conditions and under accident conditions and including SBO like conditions for several hours or even days.

[0035] The protective layer-1 3 has preferably a thickness e between 0.1 and 30 micrometers. The thickness of the protective layer e is preferably between 0.2 and 1 0 micrometers. Preferably, the protective layer 13 has a thickness e between 0.2 and 2 micrometers.

[0036] The protective layer-1 3 is uniformly deposited with ad-hoc means, that is to say with a constant and uniform thickness in accordance with the shape of the support.

[0037] The protective layer-1 3 has a high hardness and is adapted to withstand incidental scratching and scuffing that may occur between the two active faces of the ice.

[0038] The protective layer-1 3 resist to high thermal shocks such as the passage of a temperature of 1 5. -95 ° C to a temperature above 200 ° C in a few seconds.

[0039] The protective layer-1 3 may be made of nano- or microcrystalline diamond, or zirconium oxide.

[0040] For comparison, Figures 4a and 4b are two photographs taken under the electron microscope showing the status of the ice surface with and without a protective layer-1 3 with exposure to an aqueous medium at a temperature of 290 ° C and pressure of 155 bar.

[0041] More specifically, Figure 4a is a photograph of a wiper seal 1 according to the invention having a protective layer 1 3 of the invention with a thickness of 2 micrometers and Figure 4b a snapshot of a mirror gasket 1 without protective layer.

[0042] It is then found easily by comparing the ice silicon nitride with the protective layer-1 3 of Figure 4a is intact whereas the ice surface without the protective layer in Figure 4b is greatly degraded silica (S1O2) to a thickness of a few tens of micrometers to a few hundred micrometers. Furthermore, the upper layer silica dissolves over time causing degradation and dissolution of a larger amount of ice in silicon nitride, of the order of several hundred micrometers.

[0043] Figure 5a is a graph illustrating the change of the leak rate over time of a joint 1 having ice with a protective layer according to the invention during an increase of the primary fluid temperature at the joint No. 1. Figure 5b is a graph illustrating the evolution as a function of leak flow time of a joint 1 having ice without protective layer during an increase of the primary fluid temperature at the joint No. 1.

[0044] Thus, it is easily noticed the gain obtained with the protective layer-1 3 of the invention. Indeed, in the graph of Figure 5b, the ice according to the state of the protective layer without technical degrade

quickly following a major change in temperature, causing a sharp increase in leakage rate of the gasket 1 within hours. In comparison, the leakage rate of the joint 1 remains constant under the same conditions with ice according to the invention having the protective layer.

[0045] Naturally the invention is not limited to the embodiments described with reference to figures and variants could be envisaged without departing from the scope of the invention. This could include using materials other than those mentioned in the description from the time when these materials are non-porous, inert and stable in SBO-like conditions.

WE CLAIMS

Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) of the primary pump unit of a nuclear reactor designed to ensure sealing between the primary circuit and the atmosphere, characterized in that said ice (10: 1 1) has an active surface covered with a protective layer (13) made of a nonporous material and chemically inert to superheated water at a temperature greater than or equal to 200 ° C and under pressure.

Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) of the primary pump unit of a nuclear reactor according to the preceding claim characterized in that the protective layer (13) has adhesion with said silicon nitride ice properties.

Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) primary pump assembly of a nuclear reactor according to one of the preceding claims characterized in that the protective layer (13) has chemical resistance properties to boric acid and / or lithium and / or potassium hydroxide.

Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) primary pump assembly of a nuclear reactor according to one of the preceding claims characterized in that the protective layer (13) has a uniform thickness.

Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) primary pump assembly of a nuclear reactor according to one of the preceding claims characterized in that the protective layer (13) has a hardness to withstand the friction and scratching.

6. Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) of the primary pump of a nuclear reactor group according to one of claims preceding characterized in that the protective layer (13) has a roughness equivalent to the active surface of said crystal (10: 1 1).

7. Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) of the primary pump of a nuclear reactor group according to one of claims preceding characterized in that the protective layer (13) has thermal shock resistance properties.

8. Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) of the primary pump of a nuclear reactor group according to one of claims preceding characterized in that the protective layer (13) is diamond micro or nanocrystalline or zirconium oxide.

9. Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) of the primary pump of a nuclear reactor group according to one of claims preceding characterized in that said protective layer (13) has a thickness (e) comprised between 0.1 and 30 microns or a thickness (e) comprised between 0.2 and 10 microns or a thickness (e) comprised between 0.2 and 2 μηι.

10. Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) of the primary pump of a nuclear reactor group according to one of claims preceding characterized in that the active surface of said crystal (10: 1 1) intended to be in contact with a film of water is covered entirely by a protective layer (13).

January 1. Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) primary pump assembly of a nuclear reactor according to one of the preceding claims characterized in that the protective layer is resistant to erosion caused by water in normal and accident conditions type SBO.

12. Ice (10, 1 1) silicon nitride seal (1) for sealing system (4) shaft (7) of the primary pump of a nuclear reactor group according to one of claims preceding characterized in that said ice (10: 1 1) is a floating ice or a rotating mirror.

13. A seal (1) comprising at least one blade (10, 1 1) according to one of the preceding claims.

14. A sealing system (4) shaft (7) primary pump assembly of a nuclear reactor comprising at least one seal according to claim 13.

15. primary pump unit of a nuclear reactor having a shaft sealing system according to claim 14.

1 6. pressurized water nuclear reactor comprising a primary pump unit according to claim 15.