A water injection cooling system for a reactor melt debris bed

Abstract

The present invention belongs to the technical field of nuclear power plant reactor safety systems, and specifically relates to a water injection cooling system for a reactor molten material debris bed, comprising a water collecting bottom plate (4) arranged in a pile pit at the bottom end of a containment vessel (10), wherein cooling water (8) can flow upward from the inside of the water collecting bottom plate (4) to the upper surface of the water collecting bottom plate (4). The present invention can quickly and effectively extract the decay heat in the molten material debris bed when a serious accident occurs in a nuclear power plant, and maintain the coolability of the molten material debris bed in a long period of time, thereby maintaining the integrity of the containment structure and mitigating the consequences of the accident.

Description

Technical Field

[0001] The invention belongs to the technical field of nuclear power plant reactor safety systems, and in particular relates to a water injection cooling system for a reactor molten material fragment bed. Background Art

[0002] When a serious accident occurs in a light water reactor (LWR) nuclear power plant, the reactor core heats up rapidly due to the loss of cooling capacity, causing the fuel assemblies to melt and the internal components to collapse. The above-mentioned core melt will then migrate and accumulate in the lower chamber of the reactor pressure vessel. If the wall of the reactor pressure vessel ruptures due to thermal shock, these core melts will be ejected from the rupture into the reactor pit. If the reactor pit has been flooded with cooling water at this time, the ejected high-temperature molten material will further undergo a strong physical and chemical reaction with the cooling water, breaking into fine particles and depositing at the bottom of the pit, thus forming a porous molten debris bed. After this, the molten debris bed needs to be continuously cooled and the decay heat carried by the debris particles is continuously discharged in order to successfully terminate the accident process; otherwise, the debris particles will heat up again and melt to form a molten pool, threatening the integrity of the concrete structure of the containment floor, and may cause a large amount of radioactive material to be released into the external environment.

[0003] There is no special mitigation measure for cooling the molten debris bed outside the reactor in the design of traditional second-generation nuclear power plants. The mitigation strategies for the molten debris bed outside the reactor in third-generation advanced nuclear power plants can be roughly divided into two categories: one is the molten material retention in the reactor pressure vessel (IVR) strategy represented by the US AP1000, which is to cool the outer wall of the reactor pressure vessel by flooding the reactor pit under accident conditions, thereby maintaining the integrity of the reactor pressure vessel. However, this mitigation measure only targets the melt inside the reactor pressure vessel. Once the reactor pressure vessel fails, it will be impossible to further cool the melt outside the reactor pressure vessel; in addition, due to the limitation of external cooling capacity, the IVR strategy cannot be applied to reactors with higher power. The other is the French EPR (see Figure 1 ) and Russian VVER (see Figure 2) is a core catcher design represented by the core catcher. Its basic principle is to first mix the sprayed high-temperature molten material with sacrificial concrete, then spread it on a steel plate pre-coated with a high-temperature inert material, and then cool it by injecting cooling water from the top of the molten material to submerge the molten material. The core catcher strategy also has its own limitations. On the one hand, the cooling water is submerged from the top of the molten fragment bed from top to bottom, while the large amount of steam generated during the cooling process flows from bottom to top. Since the two fluids flow in opposite directions, the cooling water may not be able to effectively penetrate downward and flow into the molten fragments due to the obstruction of the upward steam flow (called the CCFL phenomenon). In particular, the area near the bottom is likely to be difficult to be effectively cooled. On the other hand, existing experimental studies and program calculation results also show that: under the condition of cooling water submerging from the top, the coolability of the molten fragment bed depends largely on the height of the formed molten fragment bed. Therefore, if the molten fragment bed cannot be effectively thinned and flattened during the formation process, it may result in the molten fragment bed not being effectively cooled in the end. Summary of the invention

[0004] The purpose of the present invention is to provide an efficient passive cooling system for the molten debris bed formed in the reactor pit under severe accidents in nuclear power plants, which is used to quickly and effectively remove the decay heat in the molten debris bed and maintain the coolability of the molten debris bed in the long term, thereby maintaining the integrity of the containment structure and mitigating the consequences of the accident.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is a water injection cooling system for a reactor melt debris bed, which includes a water collecting pan arranged in a pile pit at the bottom end of the containment vessel, and cooling water can flow upward from the inside of the water collecting pan to the upper surface of the water collecting pan.

[0006] Furthermore, it also includes a water tank arranged inside the containment shell for storing the cooling water. The position of the water tank is higher than the position of the water collecting pan. The water tank is connected to the water collecting pan through a downcomer. The cooling water can enter the water collecting pan from the water tank through the downcomer through gravity and flow upward to the upper surface of the water collecting pan.

[0007] Furthermore, a plurality of injection nozzles are provided on the upper surface of the water collecting bottom pan, and the cooling water inside the water collecting bottom pan flows out to the upper surface of the water collecting bottom pan through the injection nozzles.

[0008] Furthermore, it also includes a sacrificial concrete material layer covering the water collecting bottom plate and the injection nozzle, and the composition of the sacrificial concrete material layer is Fe2O3.

[0009] Furthermore, one end of the downpipe is connected to the bottom of the water tank, and the other end of the downpipe is connected to the side wall of the water collecting bottom pan.

[0010] Furthermore, the descending pipeline is provided with a valve which can be remotely controlled.

[0011] Furthermore, there are a plurality of downcomers which are evenly distributed around the circumference of the water tank.

[0012] Furthermore, the water tank is an annular structure and is disposed close to the inner wall of the containment vessel.

[0013] Furthermore, the injection nozzle is made of high temperature resistant ceramics.

[0014] The beneficial effects of the present invention are:

[0015] 1. The accident consequences after the failure of the melt inside the reactor pressure vessel (IVR) strategy were considered, and corresponding mitigation measures were proposed, which further improved the ability and safety of nuclear power plants to cope with severe accidents.

[0016] 2. Under accident conditions, the molten debris bed is cooled by passive means, which does not rely on safety-grade power supplies and active pumps. This simplifies the system and equipment, improves the inherent safety of the nuclear power plant, and ensures its economy.

[0017] 3. The use of water injection from the bottom instead of the traditional top flooding method avoids the injected cooling water being hindered by the reverse flow of steam, overcomes the limitation of the CCFL phenomenon, and effectively improves the penetration capacity and cooling efficiency of the cooling water.

[0018] 4. The cooling capacity mainly depends on the flow rate of cooling water injected at the bottom, rather than the height of the chip bed playing a decisive role in the coolability as in the top flooding scheme, thereby reducing the requirements for flattening during the melt formation process and improving coolability. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the core catcher system of a third-generation nuclear power plant (EPR, France) in the background technology part of the present invention;

[0020] Figure 2 It is a schematic diagram of the core catcher system of a third-generation nuclear power plant (Russian VVER) in the background technology section of the present invention;

[0021] Figure 3 It is a schematic diagram of a water injection cooling system for a reactor melt debris bed described in the specific implementation mode of the present invention (including the positional relationship between the containment vessel 10 and the core 11);

[0022] Figure 4It is a three-dimensional schematic diagram of a water injection cooling system for a reactor melt debris bed described in the specific implementation method of the present invention;

[0023] Figure 5 It is a schematic diagram of the working principle of a water injection cooling system for a reactor melt debris bed described in the specific implementation mode of the present invention;

[0024] In the figure: 1-water tank, 2-down pipe, 3-valve, 4-water collecting bottom plate, 5-injection nozzle, 6-sacrificial concrete material layer, 7-molten material fragment bed, 8-cooling water, 9-steam, 10-containment, 11-reactor pressure vessel. DETAILED DESCRIPTION

[0025] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0026] like Figure 3 , Figure 4 As shown, the present invention provides a water injection cooling system for a reactor melt debris bed, which includes a water collecting pan 4 arranged in a pile pit at the bottom end of the containment vessel 10, and cooling water 8 can flow upward from the inside of the water collecting pan 4 to the upper surface of the water collecting pan 4.

[0027] It also includes a water tank 1 for storing cooling water, which is arranged inside the containment shell 10. The position of the water tank 1 is higher than that of the water collecting pan 4. The water tank 1 is connected to the water collecting pan 4 through a downcomer 2. The cooling water can enter the water collecting pan 4 from the water tank 1 through the downcomer 2 through gravity and flow upward to the upper surface of the water collecting pan 4.

[0028] A plurality of injection nozzles 5 are provided on the upper surface of the water collecting bottom plate 4 , and the cooling water 8 inside the water collecting bottom plate 4 flows out to the upper surface of the water collecting bottom plate 4 through the injection nozzles 5 .

[0029] It also includes a sacrificial concrete material layer 6 covering the water collecting bottom plate 4 and the injection nozzle 5. The sacrificial concrete material layer 6 is composed of oxides such as Fe2O3 and is used to mix with the sprayed high-temperature molten material, thereby reducing its temperature and power density and protecting the integrity of the mechanical structure of the injection nozzle 5.

[0030] One end of the downpipe 2 is connected to the bottom of the water tank 1 , and the other end of the downpipe 2 is connected to the side wall of the water collecting bottom pan 4 .

[0031] A remotely controlled valve 3 is provided on the downcomer 2, which is in a closed state during normal operation and is opened upon receiving a signal indicating damage to the reactor pressure vessel or an instruction from an operator after an accident occurs.

[0032] There are several downpipes 2, evenly distributed around the circumference of the water tank 1 (in the vicinity of Figure 4There are 4 independent series of downcomers 2 shown in the figure, which is for example only).

[0033] The water tank 1 is an annular structure and is disposed close to the inner wall of the containment vessel 10 .

[0034] The injection nozzle 5 is made of high temperature resistant ceramics and is used to inject cooling water into the molten material debris bed in the pit.

[0035] Finally, the practical application of a water injection cooling system for a reactor melt fragment bed provided by the present invention is introduced:

[0036] Combination Figure 5 When a serious accident of core melting occurs in a nuclear power plant and the wall of the reactor pressure vessel 11 is damaged, a large amount of high-temperature core melt is sprayed into the reactor pit at the bottom and reacts strongly with the cooling water. On the one hand, the melt fragment bed 7 formed by the crushing of the high-temperature melt is deposited in the reactor pit and contacts and mixes with the sacrificial concrete material layer 6, and the temperature and energy density of the melt are reduced through mutual physical and chemical reactions. On the other hand, when the reactor pressure vessel is damaged or the operator's instruction is received, the control valves 3 on all the downpipes 2 of the system are opened at the same time, and then the cooling water 8 stored in the water tank 1 flows into the water collecting bottom plate 4 through the downpipe 2 under the action of gravity, and then penetrates upward through the injection nozzle 5 above and is injected into the pores in the mixed sacrificial concrete material layer 6 and the melt fragment bed 7. The cooling water 8 is heated by the fragment particles to generate steam 9, which flows upward, thereby taking away the heat in the melt fragment bed 7, lowering its temperature, and finally reaching and maintaining a coolable state. In particular, since the cooling water 8 and the steam 9 flow in the same direction, the upward permeation flow of the cooling water 8 will not be hindered by the steam 9, which is more conducive to the cooling of the molten material debris bed 7. The water injection cooling system for the reactor molten material debris bed can be used to deal with the serious accident process after the failure of the reactor pressure vessel 11 and alleviate the consequences of the accident.

[0037] The device described in the present invention is not limited to the embodiments described in the specific implementation manner. Those skilled in the art may derive other implementation manners based on the technical solution of the present invention, which also fall within the technical innovation scope of the present invention.

Claims

1. A water injection cooling system for a reactor melt debris bed, characterized in that: The invention comprises a water collecting pan (4) arranged in a pile pit at the bottom end of a containment vessel (10), wherein cooling water (8) can flow upward from the inside of the water collecting pan (4) to the upper surface of the water collecting pan (4); and further comprises a water tank (1) arranged inside the containment vessel (10) for storing the cooling water, wherein the position of the water tank (1) is higher than the position of the water collecting pan (4), and the water tank (1) is connected to the water collecting pan (4) via a downcomer (2), wherein the cooling water can flow from the water tank (1) through the downcomer (2) into the inside of the water collecting pan (4) by gravity and flow upward to the upper surface of the water collecting pan (4); and a valve (3) capable of remote control is provided on the downcomer (2); The water collecting bottom plate (4) is a disc-shaped structure; The upper surface of the water collecting bottom plate (4) is provided with a plurality of injection nozzles (5), and the cooling water (8) inside the water collecting bottom plate (4) flows out to the upper surface of the water collecting bottom plate (4) through the injection nozzles (5); It also includes a sacrificial concrete material layer (6) covering the water collecting bottom plate (4) and the injection nozzle (5), wherein the component of the sacrificial concrete material layer (6) is Fe2O3 oxide; One end of the downpipe (2) is connected to the bottom of the water tank (1), and the other end of the downpipe (2) is connected to the side wall of the water collecting bottom pan (4); The downcomer pipelines (2) are multiple and evenly distributed around the circumference of the water tank (1); The water tank (1) is an annular structure and is arranged close to the inner wall of the containment vessel (10); The injection nozzle (5) is made of high temperature resistant ceramics.

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