Patent Application
20200161009
The invention relates to the field of primary reactor coolant pumps of pressurized water nuclear reactors (PWR).
More specifically, the invention relates to face plates, also called active faces, of the seal package of seal no. 1 of a shaft sealing system of a primary reactor coolant pump of a nuclear reactor.
In pressurized water reactors, the primary reactor coolant pump, also more simply called primary pump, sees to the circulation of water in the primary circuit of the reactor. A dynamic sealing system of the shaft ensures the sealing between the primary circuit and the atmosphere. This shaft sealing system of the primary reactor coolant pump is a controlled leak system. It includes three seals arranged in series. Each seal includes two face plates that ensure the main sealing. One of the face plates, called rotary face plate, is mounted in a rotating assembly secured to the shaft; the other face plate, called floating face plate, is mounted in an assembly that does not rotate, but that is free to move axially to follow any axial movements of the shaft.
Seal no. 1 ensures the majority of the pressure drop between the primary circuit and the atmosphere. It makes it possible to go from a pressure of 155 bars to a pressure of about 2 bars. This seal no. 1 is a seal of the hydrostatic type, with water film having a thickness of about 10 μm. The particular geometry of the faces of the face plates ensures the main sealing and allows, both stopped and rotating, an automatic adjustment of their separation.
Seal no. 1 works with a controlled leak rate, in the order of 600 liters per hour in nominal operation, owing to the specific profile machined on its active faces. The hot primary fluid is confined in the primary circuit owing to an injection of cold water upstream from seal no. 1 at a pressure slightly higher than that of the primary circuit. Part of this cold water passes in the primary circuit and part passes in seal no. 1 so as to cool it to maintain a temperature still below 100° C.
Historically, the face plates of seal no. 1 were made from alumina, but they are now most often made from silicon nitride, which better withstands friction.
In an accidental situation, of the SBO (Station Black Out) type, corresponding to a total loss of electrical power to the nuclear power plant, the cooling circuits of the shaft sealing system of the primary reactor coolant pump become inoperative and cause the loss of the high-pressure injection of cold water upstream from seal no. 1 and the cooling of the thermal barrier of the pump. As a result, the hot water from the primary circuit rises to the seals of the shaft sealing system.
During the study of these scenarios, the applicant identified that the passage of hot water between the silicon nitride face plates of seal no. 1 causes damage to them. Indeed, in an environment with water superheated to a temperature exceeding 200° C. and under pressure (pressure greater than or equal to the saturated vapor pressure of the water), which corresponds to an accidental situation of the SBO type, the silicon nitride face plates experience damage and decomposition. Indeed, under SBO its conditions, the silicon nitride turns into ammonia and silica. This results in a dissolution and erosion of the face plates, which lose surface material, the profile of the face plates therefore evolving and therefore causing a strong increase in the leak flow rate of seal no. 1, which no longer performs its function.
This situation is problematic, since it can quickly lead to the exposure of the core if the necessary palliative measures are not taken in time by the operator.
In this context, an effective solution is provided that is easy to implement, preventing the degradation of the silicon nitride face plates of seal no. 1 of the shaft sealing system of the primary reactor coolant pump of a nuclear reactor, in particular under accidental conditions involving loss of all of the cooling sources of the shaft seal system (SBO-type situation).
To that end, the surface of the face plates of seal no. 1 are covered with a specific protective layer imparting a hydrothermal protection to the silicon nitride, thus preventing it from dissolving under normal operating conditions and in an accidental situation of the SBO type.
More specifically, a face plate is provided, made of silicon nitride, for a sealing system of a shaft of a primary reactor coolant pump in a nuclear reactor intended to provide the sealing between the primary circuit and the atmosphere, said face plate having a surface covered by a protective layer made from a nonporous material that is 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 saturated vapor pressure of the water).
An inert and non-degradable protective layer is used in an aqueous medium and under accidental conditions of the SBO type (temperature at the face plates greater than 200° C.), and capable of preventing the degradation and erosion of the surface of the silicon nitride face plates, which turns into silica under temperature conditions higher than 200° C. The protective layer withstands erosion under both normal operating conditions and conditions of the SBO type.
Owing to the addition of the protective layer on the silicon nitride face plates, the latter henceforth have a hydrothermal resistance to conditions of the SBO type and no longer suffer degradation.
Therefore, this protection against degradation of the nitride face plates responds to a specific issue, different from the issues of dirtying of the face plates known elsewhere.
The face plate according to the invention can also have one or several of the features below considered individually or according to all technically possible combinations:
A seal package is also provided including at least one of the face plates.
A shaft sealing system of a primary reactor coolant pump in a nuclear reactor is also provided including at least one of the seal packages.
A primary reactor coolant pump is also provided including the shaft sealing system.
A pressurized water reactor is also provided including the primary reactor coolant pump.
Other features and advantages of the invention will better emerge from reading the following description, in reference to the appended figures.
In all of the figures, like elements bear like references unless otherwise specified.
Seal no. 1 is shown more specifically in
The face plates 10, 11 of seal no. 1 are shown more specifically in
This protective layer 13 is made from a nonporous material that is chemically inert in an aqueous medium and at a temperature greater than or equal to 200° C. This protective layer 13 makes it possible to prevent the degradation and erosion of the surface of the silicon nitride face plates under conditions of the SBO type and does not disrupt the normal operation of seal no. 1.
The protective layer 13 also has properties of chemical resistance to corrosion and in particular boric acid, lithium hydroxide and potassium hydroxide, and withstands erosion. More generally, the protective layer 13 withstands all of the conditions that seal no. 1 may experience under normal operating conditions and under accidental conditions, and in particular under conditions of the SBO type for several hours, or even several days.
The protective layer 13 preferably has a thickness e of between 0.1 and 30 μm. The thickness of the protective layer e is preferably between 0.2 and 10 micrometers. Preferably, the protective layer 13 has a thickness e of between 0.2 and 2 micrometers.
The protective layer 13 is deposited uniformly with ad hoc means, that is to say, with a constant and homogeneous thickness while respecting the shape of the support.
The protective layer 13 has a great hardness and is suitable for withstanding scratches and the incidental friction that may occur between the two active faces of the face plates.
The protective layer 13 withstands substantial thermal shocks, such as the transition from a temperature of 15° C.-95° C. to a temperature greater than 200° C. in several seconds.
The protective layer 13 can be made from nano- or microcrystalline diamond, or from zirconium oxide.
As a comparison,
More specifically,
One can then easily see by comparison that the silicon nitride face plate with the protective layer 13 in
Thus, one can easily see the gain obtained with a protective layer 13. Indeed, in the graph of
Naturally, the invention is not limited to the embodiments described in reference to the figures, and variants may be considered without going beyond the scope of the invention. One may in particular use materials other than those cited in the detailed description as long as these materials are nonporous, inert and stable under conditions of the SBO type.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1755117 | Jun 2017 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/065116 | 6/8/2018 | WO | 00 |
