Bottom closure of reactor pressure vessel and preparation method therefor

Abstract

The present invention provides a bottom closure of a reactor pressure vessel and a preparation method therefor. The bottom closure of a reactor pressure vessel comprises a bottom closure body and a po

Description

The present disclosure relates to the technical field of reactor pressure vessels, and more particularly to a reactor pressure vessel lower head and a preparation method therefor. BACKGROUND A reactor pressure vessel (RPV) is an important device in the first circuit system of a pressurized water reactor nuclear power plant, and it is a sealed pressure-bearing boundary. When a severe accident occurs in a pressurized water reactor nuclear powder plant, the meltdown of the core may further cause damage to the pressure vessel, which seriously threatens the integrity of the containment. In the case of a core melting accident, water is injected into the reactor cavity to cool the outer wall of the reactor pressure vessel, so that the heat of the melting pool can be exported to maintain the integrity of the pressure vessel, thereby retaining radioactive materials in the reactor pressure vessel, preventing the radioactive materials from spreading and avoiding severe nuclear pollution. Therefore, the In-Vessel Retention (IVR) technology is one of important emergency safety measures in severe accidents. When the heat flux on the heating surface reaches a certain value, bubbles on the outer surface of the reactor pressure vessel wall increase and form a gas film covering the heating surface. Since the gas film may hinder the heat transfer, the wall temperature will rise rapidly and get out of control. This critical point is referred to as the Critical Heat Flux (CHF) or boiling crisis. A criterion for the success of the IVR is that, the heat flux does not exceed the Critical Heat Flux (CHF). The ratio of the pressure vessel wall surface heat flux to the local critical heat flux is one of key indicators for evaluating the effectiveness of the IVR, and the smaller the ratio indicates the greater the margin of IVR measures. At present, measures to enhance the boiling heat transfer performance of the outer wall surface of the reactor pressure vessel lower head are mainly to machine the surface to form surface channels that are conducive to the flow7 of vapor and liquid. However, machining will change the mechanical properties of the pressure vessel wall body and reduce the security of the pressure vessel. CN204066752U discloses a pressure vessel with dense ring-shaped groove structures on outer surfaces. SUMMARY The present disclosure provides a reactor pressure vessel lower head and a preparation method therefor, which can increase the critical heat flux of the outer wall surface of the reactor pressure vessel and facilitate the implementation of the In-Vessel Retention technology for in-vessel melt. The reactor pressure vessel lower head of the present disclosure includes a lower head body and a porous coating formed on an outer wall surface of the lower head body, and an end of the lower head body is connectable to a barrel section of the reactor pressure vessel. A primer layer is provided between the outer w'all surface of the lower head body and the porous coating so as to improve a bonding force between the porous coating and the outer wall surface of the lower head body. The porous coating has a porosity in a range of 35%-55%. The porous coaling has a large number of micron-scale interconnected pores. Preferably, the porous coating is formed of stainless steel powder by means of spraying. Preferably, the spraying is cold spraying or thermal spraying. Preferably, the porous coating has a thickness in a range of 200-500 microns. Preferably, the end of the lower head body is connected to the barrel section of the reactor pressure vessel through a full penetration weld. Preferably, the porous coating is provided on all or part of the outer wall surface of the lower head body. The preparation method for the reactor pressure vessel lower head of the present disclosure includes the following steps: pretreating the outer wall surface of the lower head body so as to increase the surface roughness of the outer wail surface of the lower head body; preheating the outer wall surface of the lower head body; and forming the porous coating on the outer wall surface of the lower head body. Preferably, before the step of forming the porous coating on the outer wall surface of the lower head body, the preparation method further includes a step of spaying a primer layer, and the porous coating is formed on the primer layer. According to the reactor pressure vessel lower head and the preparation method therefor of the present disclosure, the surface of the lower head is formed with a porous coating, which does not damage the safety of the pressure vessel . The surface of the lower head with the porous coating has a large number of micron-scale interconnected pores, so that bubbles wall preferentially nucl eate in these pore structures, thereby greatly increasing the number of vapori zati on cores and accelerating the nucleation speed of bubbles. At the same time, the pore structure of the porous coating expands the surface area of the lower head, thereby significantly increasing the heat exchange area. Furthermore, the bubbles nucleate in the pores of the porous coating and grow out of the interconnected pores, thereby hindering the merging of the bubbles, stirring the local fluid, and enhancing surface convection, so that the heat exchange capability between the fluid and the outer wall surface of the reactor pressure vessel is enhanced. Compared with a smooth outer surface, the surface of the lower head with the porous coating can increase the critical heat flux of the fluid at the corresponding position on the outer wall surface of the reactor pressure vessel by more than 50%, which is conducive to the implementation of the In-Vessel Retention technology for in-vessel melt, and can improve the safety of nuclear power plants in serious accidents. BRIEF DESCRIPTION OF THE DRAWINGS In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the accompanying drawings that need to be used in the embodiments of the present application. Obviously, the accompanying drawings described below are only some embodiments of the present application, and those of ordinary skill in the art can also obtain other drawings based on the accompanying drawings without creative efforts. FIG. 1 is a schematic diagram of the connection of the reactor pressure vessel lower head to the barrel section according to an embodiment of the present disclosure; FIG. 2 is a partially enlarged view of the part A in FIG. 1; FIG. 3 is a schematic diagram of the reactor pressure vessel lower head implemented with the InVessel Retention technology for in-vessel melt in severe accidents according to an embodiment of the present disclosure; FIG. 4 is a flowchart of a preparation method of the reactor pressure vessel lower head according to an embodiment of the present disclosure. In the drawings, the drawings are not drawn to scale. Reference numbers: 10 - barrel section; 20 - lower head body; 21 - porous coating; 100 - reactor cavity; 101 - thermal insulation layer of barrel section; 200 - cooling water channel; 201 - thermal insulation layer of lower head; 202 - cooling water inlet; 300 - in-vessel melt. DETAILED DESCRIPTION The implementations of the present application will be further described in detail below with reference to the drawings and embodiments. The detailed description and drawings of the following embodiments are used to schematically illustrate the principles of the present application, but not to limit the scope of the present application, that is, the present application is not limited to the described embodiments. In the description of the present application, it should be noted that, unless otherwise specified, the meaning of "a plurality of means two or more; the orientation or positional relationship indicated by the terms “upper", "lower", "left", “right", "inner", "external" and so on are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that, the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as restrictions to the present application. The orientation words appearing in the following description are the directions shown in the drawings, and do not limit the specific structure of the present application. In the description of the present application, it should also be noted that, unless otherwise specified and limited, the terms "connected", "interconnected", and "attached" should be understood in a broad sense, for example, it can be a fixed connection, a removable connection or an integral connection; it can be directly connected or indirectly connected through an intermediary. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to specific situations. FIG. 1 is a schematic diagram of the connection of the reactor pressure vessel lower head to the barrel section according to an embodiment of the present disclosure, and FIG. 2 is a partial enlarged view of the part A in FIG. 1. As shown in FIG. 1 and FIG. 2, the reactor pressure vessel lower head of the present disclosure includes a lower head body 20 and a porous coating 21 formed on the outer wall surface of the lower head body 20. The lower head body 20 is bowl-shaped and has a rotary shell structure, and an end of the lower head body 20 is connected with a barrel section 10 of the reactor pressure vessel . The end of the lower head body 20 here refers to the end of the lower head body 20 on its opening side. The lower head body 20 may be a complete forging, or may be formed by butt welding of several forgings through full-penetration welds. The outer wall surface of the lower head body 20 formed with the porous coating 21 has a large number of micron-scale interconnected pores, and bubbles will preferentially nucleate in these pore structures, thereby greatly increasing the number of vaporization cores and accelerating the nucleation speed of bubbles. At the same time, the pore structure of the porous coating 21 expands the surface area of the outer wall surface of the lower head body 20, thereby significantly increasing the heat exchange area. Furthermore, the bubbles nucleate in the pores of the porous coating 21 and grow out of the interconnected pores, thereby hindering the merging of the bubbles, stirring the local fluid, and enhancing surface convection, so that the heat exchange capability between the fluid and the outer wall surface of the reactor pressure vessel is enhanced. Compared with a smooth outer surface, the critical heat flux of the fluid at the corresponding position on the outer wall surface of the reactor pressure vessel can be increased by more than 50%, which is conducive to the implementation of the In-Vessel Retention technology for in-vessel melt, and can improve the safety margin of nuclear power plants in serious accidents. In one embodiment of the present disclosure, the porous coating 21 is formed by spraying stainless steel powder. By controlling spraying process parameters, the melting degree and stacking state of the stainless steel powder are adjusted to obtain the porous coating 21 with different porosities and structures. In one embodiment of the present disclosure, the spraying is cold spraying or thermal spraying. In a cold spraying process, compressed air is used to accelerate metal particles to reach the critical speed (supersonic speed), so that the metal particles collide with a surface of a substrate, become flattened and adhere to the surface firmly. The particles do not melt during the cold spraying process, and the process can be controlled to create gaps between sprayed particles so as to produce the porous coating 21. However, since the porous coating 21 produced by the cold spraying process generally has a relatively low porosity, it is necessary' to perform corrosion and other treatments after spraying to obtain the porous coating 21 with a higher porosity. While in a thermal spraying process, the melting degree of powder particles is controlled by adjusting the spraying process to obtain semi-melted powder particles with melted surface layer and rigid inner core. These semi-melted powder particles are deposited and stacked on the substrate, wherein the rigid inner core constitutes a pore skeleton and melt of surface layer is filled between the powder particles and between the powder particles and the substrate, thereby ensuring the bonding strength of the coating, and the porous structure 21 is formed due to gaps between the powder particles. In one embodiment of the present disclosure, the porous coating 21 has a porosity in a range of 35%-55%. If the porosity is too low, the coating would be denser, and the strengthening effect would be poor or even the heat transfer would be deteriorated; and if the porosity is too high, the outer wail surface of the lower head body 20 would be exposed. In one embodiment of the present disclosure, the porous coating 21 has a thickness in a range of 200-500 microns. If the thickness of the porous coating 21 is too large, the strengthening effect of the coating would be poor or even the heat transfer would be deteriorated; and if the thickness of the porous coating 21 is too small, the bonding between the porous coating 21 and the outer wall surface of the lower head body 20 would be unstable. In one embodiment of the present disclosure, the end of the lower head body 20 is connected to the barrel section 10 of the reactor pressure vessel through a full penetration weld. Fullpenetration welds are adopted to ensure the connection strength between the lower head body 20 and the barrel section 10 and ensure the safety of the reactor pressure vessel. In one embodiment of the present disclosure, the porous coating 21 is disposed on all or part, of the outer wail surface of the lower head body 20. By coating all of the outer wall surface, the overall CHF can be improved; and by merely coating parts other than the bottom of the outer wall surface, a better economical efficiency may be achieved, since the parts other than the bottom of the outer wall surface of the lower head body 20 generally have a relatively low IVR margin. FIG. 3 is a schematic diagram of the reactor pressure vessel lower head implemented with the InVessel Retention technology for in-vessel melt in severe accidents according to an embodiment of the present disclosure. As shown in FIG. 3, a thermal insulation layer is provided outside the reactor pressure vessel, which includes a thermal insulation layer 101 of the barrel section and a thermal insulation layer 201of the lower head. A cooling water channel 200 is formed between the thermal insulation layer 101 of the barrel section and the thermal insulation layer 201 of the lower head, and the barrel section 10 of the reactor pressure vessel and the outer surface of the lower head. In the case of a core melting accident, the outer wall of the reactor pressure vessel is cooled by injecting cooling water into the reactor cavity 100. The cooling water Hows along a line indicated by the arrow in FIG. 3: the cooling w'ater enters the cooling water channel 200 from the cooling water inlet 202, and the cooling water can cool the reactor pressure vessel lower head. Since the outer wall surface of the lower head body 20 of the reactor pressure vessel lower head is formed with the porous coating 21, the pore structure of the porous coating 21 expands the surface area of the outer wall surface of the lower head body 20, thereby significantly increasing the heat exchange area, which allows the heat of the in-vessel melt 300 to be exported, so as to hold radioactive materials in the reactor pressure vessel and avoid serious nuclear pollution caused by diffusion. The porous coating 21 increases the critical heat flux at the corresponding position on the outer wall surface of the reactor pressure vessel, which is conductive to the implementation of the In-Vessel Retention technology for the in-vessel melt 300, and can improve the safety of the nuclear power plant in severe accidents. FIG. 4 is a flowchart of a preparation method of the reactor pressure vessel lower head according to an embodiment of the present disclosure. As shown in FIG. 4, the preparation method of reactor pressure vessel lower head of the present disclosure includes steps SI to S3. In step SI, the outer wall surface of the lower head body 20 is pretreated so as to increase the surface roughness of the outer wall surface of the lower head body 20. The pretreating on the outer wall surface of the lower head body 20 may be implemented by a sandblasting process, which may remove oil stains and oxides on the outer wall surface of the lower head body 20 and increase the surface roughness, so as to improve the bonding strength between the porous coating 21 and the outer wall surface of the lower head body 20. In step S2, the outer wall surface of the lower head body 20 is preheated. The preheating on the outer wall surface of the lower head body 20 may be implemented with a spray gun tail flame to reduce the internal thermal stress of the coating, and the preheating temperature is controlled between 100-150°C. In step S3, the outer wall surface of the lower head body 20 is formed with the porous coating 21. In order to form the porous coating 21 with a certain porosity, flame spraying, arc spraying and other methods can be used for a spraying process. During the spraying process, porous coatings with different porosities can be obtained by changing flame power, spraying distance, powder feeding amount and spraying angle. The flame power, the spraying distance, etc., mainly affect the melting degree of particles. The flame power mainly affects the flame temperature, wherein the lower the flame temperature, the less the melting degree of particles. In a case of constant spraying flame power, the spraying distance, the powder feeding amount, etc., will also affect the melting degree of particles. If the spraying distance is reduced, the flight, time of particles in flame would be reduced; or in the case of the same heat input, the larger the powder feeding amount, the less the heat shared by a single particle, in turn the less the melting degree of particles. The spraying angle mainly affects the deposition behavior of the particles. Particles that, are not completely melted have two velocities, in the vertical and horizontal directions respectively, and the velocity in the horizontal direction will bring the molten liquid metal and particles to spread along a certain direction on the substrate, wherein the smaller the spraying angle, the larger the velocity in the horizontal direction. The formation of the final porous coating means stacking of unmolten particles, and the bonding between particles and between particles and the substrate is formed through the molten liquid metal. In one embodiment of the present disclosure, before the outer wail surface of the lower head body 20 is formed with the porous coating 21, a primer layer (not shown) is sprayed first, and then the porous coating layer 21 is formed on the primer layer, so as to improve the bonding force between the coating and the substrate. In the reactor pressure vessel lower head and the preparation method thereof of the present-disclosure, the surface of the lower head is formed with a porous coating, which is not harmful to the safety of the pressure vessel. The surface of the lower head formed with the porous coating has a large number of micron-scale pores which are comunicated with each other, so that bubbles will preferentially nucleate in these pore structures, thereby greatly increasing the number of vaporization cores and accelerating the nucleation speed of bubbles. At the same time, the pore structure of the porous coating expands the surface area of the lower head, thereby significantly increasing the heat exchange area. Furthermore, the bubbles nucleate in the pores of the porous coating and grow out of the connected pores, thereby hindering the merging of the bubbles, stirring the local fluid, and enhancing surface convection, so that the heat exchange capability between the fluid and the outer wall surface of the reactor pressure vessel is enhanced. Compared with a smooth outer surface, the critical heat flux of the fluid at the corresponding position on the outer wall surface of the reactor pressure vessel can be increased by more than 50%, which is conducive to the implementation of the In-Vessel Retention technology for invessel melt, and can improve the safety of nuclear power plants in serious accidents. While the application has been described with reference to preferred embodiments, various modifications may be made and equivalents may be substituted for elements thereof without departing from the scope of the present application. In particular, as long as there is no structural conflict, the technical features mentioned in the various embodiments can be combined in any manner. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of claims.

Claims

1. A reactor pressure vessel lower head of a reactor pressure vessel, said reactor pressure vessel lower head comprisinga lower head body, anda porous coating formed on an outer wall surface of the lower head body, wherein the porous coating has micron-scale interconnected pores,whereinan end of the lower head body is connectable to a barrel section of the reactor pressure vessel,a primer layer is provided between the outer wall surface of the lower head body and the porous coating so as to improve a bonding force between the porous coating and the outer wall surface of the lower head body,the porous coating has a porosity in a range of 3 5%-5 5%.

2. The reactor pressure vessel lower head according to claim 1, wherein the porous coating is formed of stainless steel powder.

3. The reactor pressure vessel lower head according to claim 1, wherein the porous coating has a thickness in a range of 200-500 microns.

4. The reactor pressure vessel lower head according to claim 1, wherein the porous coating is provided on all of the outer wall surface of the lower head body.

5. The reactor pressure vessel lower head according to claim 1, wherein the porous coating is provided on part of the outer wall surface of the lower head body.

6. A preparation method for the reactor pressure vessel lower head according to any one of claims 1-5, comprising the following steps:pretreating the outer wall surface of the lower head body to increase the surface roughness of the outer wall surface of the lower head body;preheating the outer wall surface of the lower head body; andforming the porous coating on the outer wall surface of the lower head body, wherein the porous coating is formed of stainless steel powder by means of spraying, wherein the porous coating has micron-scale interconnected pores, wherein a primer layer is provided between the outer wall surface of the lower head body and the porous coating so as to improve a bonding force between the porous coating and the outer wall surface of the lower head body, wherein the porous coating has a porosity in a range of 3 5%-5 5%.

7. The preparation method according to claim 6, further comprising the step ofspraying the primer layer on the outer wall surface of the lower head body before forming the porous coating on the outer wall surface of the lower head body,whereinthe porous coating is formed on the primer layer such that the primer layer is between the outer wall surface of the lower head body and the porous coating.

8. The preparation method according to claim 6 or 7, wherein the spraying is cold spraying or thermal spraying.

9. The preparation method according to any of claims 6-8, comprising connecting the end of the lower head body to the barrel section of the reactor pressure vessel through a full penetration weld.

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