KR20000074521A - Emergency core cooling system for pressurized water reactor - Google Patents

Abstract

PURPOSE: A safe injection light water in an atomic furnace is provided to maximize the amount of emergency coolant arriving at a core by minimizing the amount of the emergency coolant effusing from a low temperature tube. CONSTITUTION: An atomic furnace comprises an emergency core cooling system which consists of a direct container injection nozzle(6) and a safe injection duct(10). The direct container injection nozzle(6) is installed higher than an atomic furnace input nozzle(7) so as to buffer a pressed heating impact at injecting an emergency coolant through a safe injection tube(5) and to minimize a coolant discharge amount at damage of the safe injection tube(5). The safe injection duct(10) is connected to the direct container injection nozzle(6). The safe injection duct(10) makes the emergency coolant, discharged from the direct container injection nozzle(6), become detour so that an emergency coolant is prevented from being discharged with a coolant into a low temperature tube(4) at damage of the low temperature tube(4).

Description

Emergency core cooling system for pressurized water reactor {EMERGENCY CORE COOLING SYSTEM FOR PRESSURIZED WATER REACTOR}

According to the present invention, in a pressurized water reactor (hereinafter referred to as a pressurized water reactor), an emergency coolant is introduced into the reactor in order to prevent a sudden rise in core temperature due to the lack of coolant during a loss of coolant accelerator (LOCA). It relates to the emergency core cooling system, in particular, in the event of breakage of the cold pipe which is the coolant inlet pipe, it is possible to minimize the amount of emergency coolant that flows out into the cold pipe, and the safety injection pipe when the safety injection pipe of the emergency coolant injection pipe is broken. It relates to an emergency core cooling system for pressurized water reactors that can maintain the coolant level of the core to a minimum height by blocking the siphon effect that allows the coolant to continuously flow through.

Among nuclear reactors that produce energy using nuclear fuel, in particular, light water is used as a reactor coolant, which is a heat energy transport medium generated by nuclear reaction of nuclear fuel, and the coolant is a reactor vessel. A reactor that keeps the pressure inside the reactor above a certain size so as not to boil inside is called a Pressurized Water Reactor.

In general, the system of a pressurized water reactor consists of a reactor, a steam generator, a reactor coolant pump, and a pressurizer as shown in FIG. 1, and includes a cold leg and a hot tube, which are pipes connecting the above components. Legs form one coolant circulation circuit (primary system).

According to this configuration, the hard water as the coolant enters into the vessel 1 of the reactor as shown in FIG. 1 by the coolant pump and is caused by the heat energy generated by the nuclear reaction of the reactor core 2 as the nuclear bundle in the reactor. After being heated to high temperature, the steam generator (not shown) is circulated through the hot tube 3 to heat the secondary system water of the steam generator (for driving the steam turbine) to become steam and then cooled, and then the cold tube Return to reactor vessel 1 through (4). In this process, the water circulated in the secondary system becomes a steam in the steam generator to drive a steam turbine, and the steam turbine drives a generator, and finally electric energy is generated by the generator. From the energy point of view, the thermal energy generated by the nuclear reaction in the reactor is absorbed by the coolant of the primary system and transferred to the coolant of the secondary system through the steam generator, and then converted into electrical energy through the kinetic energy by the steam turbine and generator. Means that.

In the process of designing and constructing such a nuclear power plant, virtual accidents, which are difficult to occur in practice, are considered. Therefore, after analyzing the circumstances and effects of the virtual accidents, the reactor coolant system is abnormal. By adding a system that can minimize the impact on the core in the event of a state accident, the safety of the power plant is ensured and maintained.

Among the accidents considered in the design of nuclear power plants, accidents in which coolant flows out of the system due to damaged boundary of the reactor coolant system are called loss of coolant.

When a coolant loss occurs in a reactor, the pressure in the reactor coolant system drops below the coolant boiling point, causing nuclear boiling on the fuel rod surface, which rapidly reduces the heat transfer rate between the fuel and the coolant, resulting in a sharp rise in the core surface temperature. . Therefore, in case of a coolant loss accident that rapidly increases the core surface temperature, the reactor is equipped with a safety device that can inject emergency injection water into the reactor vessel at high pressure from the outside when the coolant in the reactor is lost. This is commonly referred to as the Emergency Core Cooling System (ECCS) of the safety injection system.

The emergency core cooling system generally includes a pre-injection tank for storing the emergency coolant to be injected into the core under pressure, a safety injection pump for injecting the emergency coolant stored in the pre-injection tank into the reactor coolant system under reduced pressure, and And a safety injection pipe forming an emergency coolant flow path from the tank and the pump to the reactor coolant system. In such an emergency core cooling system, safety injection pipes connected to the reactor vessel and directly discharging emergency coolant are usually installed in the same number as the number of low temperature pipes. For reference, in a typical PWR reactor, three to four low temperature pipes are usually installed.

In the emergency core cooling system, a part of discharging the emergency coolant from the end of the safety injection pipe into the reactor is called a safety injection nozzle. In the related art, these safety injection nozzles are connected to each low temperature pipe to inject the emergency coolant through the low temperature pipe. Emergency core cooling systems have been used.

However, as in the conventional emergency core cooling system, when the safety injection nozzle is connected to the low temperature pipe, the emergency coolant (safe injection water) to be discharged to the reactor core side through the safety injection nozzle in the event of breakage of the low temperature pipe flows back toward the low temperature pipe. There is a problem of not reaching the core. That is, the emergency coolant flowing through the safety injection nozzle is mixed with the primary system coolant flowing back to the low temperature pipe by the pressure difference, and accompanied by the primary system coolant along with the primary system coolant through the breakage of the low temperature pipe to the external containment building of the reactor coolant system. Because of the spill, the emergency coolant does not reach the core.

In recent years, such a problem has been taken into consideration, and as shown in FIG. 1, the safety injection pipe 5 is directly connected to the reactor vessel 1 so that the emergency coolant is directly discharged into the reactor vessel 1, thereby allowing the low temperature tube 4 to be discharged. Even if) is broken, the safety injection water does not flow out of the system through the broken low temperature pipe (4), thus, a method of cooling the core by allowing the emergency coolant to directly reach the core (2) side has been proposed. The method of directly connecting the safety injection nozzle 6 of the safety injection pipe 5 to the reactor vessel 1 so that the emergency coolant is directly injected into the reactor vessel 1 is called a direct container injection method. The outlet portion of the safety injection pipe (5) directly attached to the container (1) is called a direct vessel injection nozzle (6).

However, even in such a direct container injection method, there is a possibility that the safety injection water flows out. That is, when the direct container injection nozzle 6 is installed above the reactor inlet nozzle 7, which is the primary system coolant inlet, when the low temperature pipe 4, which is the coolant inlet pipe, is broken, the direct container injection is performed. The cold coolant 4 injected through the nozzle 6 moves along the downcomer 9 between the inner wall of the reactor vessel 1 and the core support barrel 8 and is broken. Along with the coolant flowing back to the reactor through the reactor inlet nozzle (7) there is a possibility that a significant amount of outflow. In this case, even if the direct container injection nozzle 6 is directly attached to the reactor vessel 1, the emergency core cooling effect resulting from the safety injection is relatively low. Therefore, in the direct container injection method, attaching the direct container injection nozzle 6 to the lower part of the cold tube 4 can obtain an effect of emergency coolant injection which is much improved than attaching to the upper part of the cold tube 4.

However, in the emergency core cooling system of the direct container injection method, when the direct container injection nozzle 6 is attached to the bottom of the reactor inlet nozzle 7, the low-temperature emergency coolant through the direct container injection nozzle 6 has a high temperature. The rapid inflow into the high-pressure reactor vessel (1) to generate a pressurized thermal shock not only causes a health problem of the reactor vessel (1), but in particular when the safety injection pipe (5) breaks the safety injection pipe (5) There is a problem that the amount of coolant flowing through is much higher than when the direct container injection nozzle (6) is attached on top of the reactor inlet nozzle (7).

In order to solve this problem, US Patent Publication No. 5135708, "Method of Injection To or Near Core Inlet", is directed from a direct vessel injection nozzle (6) at the top of a cold tube inside a reactor vessel for the purpose of maximizing the effect of safety injection. It discloses a method of installing a cylindrical or tubular safety injection passage to the bottom of the core support (8).

However, in this case, the emergency coolant can be directly supplied to the lower end of the core 2 in the case of the low temperature pipe 4 breakage accident, but if the safety injection pipe 4 outside the reactor vessel 1 breaks, Because of the siphon effect, the coolant level in the reactor vessel continuously exits until the core level is completely exposed by the siphon effect, thereby preventing the core from melting in the reactor vessel (1). The result is a serious shortage of sheep.

To solve this problem, US Patent Publication No. 5377242, "Method and System for Emergency Core Cooling", surrounds the core support container (8) and is discharged from the direct container injection nozzle (6) in the event of breakage of the low temperature pipe (4). Inducing the emergency coolant to reach the bottom of the core support (8) while revealing the emergency core cooling system including a cylinder having a hole in the upper portion to block the siphon effect in the event of breakage accident.

However, the two prior arts mentioned above (US Published Patent Nos. 5377242 and 5135708) greatly increase the weight of the core holder 8, so that the reactor vessel 1 containing the core holder 8 is included. The reactor vessel support structures supporting the reactor had to be designed to withstand large loads.

The present invention is an emergency core cooling system capable of eliminating the problems of the conventional emergency core cooling systems as described above, and reaches the core by minimizing the amount of emergency coolant flowing into the cold tube in the event of a cold tube breakage that is a coolant inlet tube. The amount of emergency coolant can be maximized, and in case of breakage of the safety injection pipe, which is an emergency coolant injection pipe, the level of coolant level in the reactor can be reduced by blocking the siphon effect that causes the coolant in the reactor to continuously flow through the safety injection pipe. Its purpose is to provide an emergency core cooling system for pressurized water reactors that can be maintained at the minimum height that the core is submerged and, in addition, hardly increases the load design of the reactor support structure by hardly increasing the weight of the core bearings. have.

1 is a vertical cross-sectional view of a pressurized water reactor in which an emergency core cooling system for a conventional pressurized water reactor is installed.

Figure 2 is a schematic view of the reactor longitudinal section of the pressurized water reactor is installed emergency core cooling system for the pressurized water reactor according to an embodiment of the present invention.

3 is an enlarged view of FIG. 2;

4 is a cross-sectional view taken along the line IV-IV of FIG. 2.

<Description of Symbols for Main Parts of Drawings>

1: reactor vessel 2: core

4: low temperature pipe 5: safety injection pipe

6: direct container injection nozzle 7: reactor entrance nozzle

8: core support 10: safety injection duct

11 protrusion 12 flange

14: niche

The present invention provides a pre-injection tank for storing the emergency coolant under pressure; A safety injection pipe connecting the blood injection tank to a direct container injection nozzle of a reactor vessel; In the emergency core cooling system comprising a safety injection pump for pumping the emergency coolant stored in the pre-injection tank to the direct container injection nozzle of the reactor vessel in case of loss of the reactor coolant between the pre-injection tank and the safety injection pipe, The direct container injection nozzle is installed at a position relatively higher than the reactor inlet nozzle, the inlet is coupled to the direct container injection nozzle and the inlet is combined with the emergency coolant discharged from the direct container injection nozzle bypasses the reactor inlet nozzle, It further comprises a safety injection duct (Safty Injection Duct) for inducing to discharge to the outlet of the position lower than the reactor inlet nozzle.

In the configuration of the emergency core cooling system for the pressurized water reactor as described above, the safety injection duct preferably has an outlet at a position higher than the upper end of the core.

More preferably, the safety injection duct is attached to the outer wall of the core support in the reactor, and a flange around the inlet is projected by a direct vessel lower intrusion around the direct container injection nozzle. Are spaced apart and combined.

Hereinafter, the functions and operations of the above components will be described in more detail with reference to the accompanying drawings.

In describing the present embodiment, the same reference numerals are used for the configuration of the present invention that is the same as the conventional technology described at the outset.

First, a brief description of the accompanying drawings, FIG. 2 is a schematic cross-sectional view of a reactor in a pressurized water reactor equipped with an emergency core cooling system for a pressurized water reactor according to an embodiment of the present invention, and FIGS. 3 and 4 are enlarged views of FIG. 2, respectively. In addition, it can be seen that the cross-sectional view of the IV-IV line of FIG.

Generally, a pressurized water reactor refers to a reactor that uses hard water as a coolant as a medium for transporting thermal energy generated as a result of nuclear reaction of nuclear fuel and pressurizes the inside of the coolant system to a predetermined pressure with a pressurizer to prevent boiling of the coolant.

The system of pressurized water reactor consists of a reactor, steam generator, reactor coolant pump, pressurizer, and low temperature tube and high temperature tube. A circuit is formed.

In this system, a nuclear reactor is a nuclear reactor that generates thermal energy by nuclear reaction of nuclear fuel in an enclosed space. As shown in FIG. 2, a reactor vessel (1) forming a confined space in which a nuclear reaction occurs, and the vessel (1) And a core 2 formed by the nuclear bundles regularly arranged therein, and a core support cylinder 8 supporting the core 2 with respect to the container. The reactor 1 of this reactor is pumped by a reactor coolant pump (not shown), and a reactor, a high temperature tube 3, a steam generator (not shown), a reactor coolant pump (not shown), a cold tube 4, and again The coolant circulated in the reactor vessel (1) is filled. In the coolant circulation process, the coolant is heated by absorbing thermal energy generated through the nuclear reaction of the fuel bundle in the core (2), and then, the steam is a primary system heat exchanger. Transferred to the secondary system coolant circulated in the secondary system as it passes through the steam generator's tubing and then passes through the low temperature pipe (4) by the coolant pump and into the reactor vessel (1) through the reactor inlet nozzle (7). Regress Subsequently, the reactor vessel 1 flows down the reactor vessel 1 through a downward passage 9 between the inner wall of the reactor vessel 1 and the core support 8 to the lower vessel 1a of the reactor vessel 1 and then pivots upward. Flows back into 2). As described above, the coolant starting from the core 2 and returning back to the core 2 is referred to as a reactor coolant or a primary system coolant as distinguished from a secondary coolant circulating in the steam generator and the steam turbine.

In the system of such a pressurized water reactor, the emergency core cooling system for a pressurized water reactor according to the present invention composed of components such as a pre-injection tank, a safety injection pipe, a safety injection pump, and the like, in particular, is illustrated in FIGS. 2 and 3. Similarly, a direct container injection nozzle 6, which is a discharge port for discharging emergency coolant into the reactor vessel, and a safety injection duct 10 for guiding the emergency coolant discharged from the direct container injection nozzle 6 to the core 2 are included. Is done.

In the configuration according to the present invention, the direct container injection nozzle (6), while reducing the emergency thermal shock when the emergency coolant input through the safety injection pipe (5) while minimizing the amount of cooling water outflow when breaking the safety injection pipe (5) It is installed at a position higher than the reactor inlet nozzle (7), which is the primary system coolant inlet.

This direct container injection nozzle 6 is located on a different azimuth angle from the reactor inlet nozzle 7 when viewed in cross section as shown in FIG. 4, but is shown in the same section for the sake of convenience in the longitudinal cross-sectional view of FIG. 3.

Meanwhile, a safety injection duct 10 is connected to the direct container injection nozzle 6, and for this connection, as illustrated in FIGS. 3 and 4, an inner wall of the reactor vessel 1 around the direct container injection nozzle 6 is provided. The protrusion 11 is formed, and a flange 12 corresponding to the protrusion 11 is formed around the inlet 16 of the safety injection duct 10 which is coupled to the protrusion 11. Thus, the emergency coolant flowing directly into the container injection nozzle (6) flows down through the protrusion (11) and the flange 12 along the safety injection duct (10) and mixed with the reactor coolant flowing into the reactor inlet nozzle (7). After that, it flows down the downflow path 9 and pivots in the lower space 1a of the reactor vessel 1 and flows back into the core 2 to cool the core 2. At this time, the safety injection duct 10 directs the emergency coolant discharged from the direct container injection nozzle 6 downwardly by bypassing the reactor inlet nozzle 6, so that the emergency coolant at the time of breaking the cold pipe 4 is lowered. The pipe (4) serves to prevent the outflow with the coolant by the steam flow.

On the other hand, the flange 12 of the inlet 16 of the safety injection duct 10 which is opposed to the protrusion 11 of the direct container injection nozzle 6 has a core support cylinder 8 to which the safety injection duct 10 is fixed. When fitted into the reactor vessel 1, the reactor vessel 13 supporting the core support 8 does not interfere with the vessel jaw 13 of the upper inner wall of the reactor vessel 1. It is extruded to a height not exceeding. The flange 12 of the safety injection duct 10 is abutted against the protrusion 11 of the direct container injection nozzle 6 to form a gap 14 between the flange 12 and the protrusion 11. The gap between the core support cylinder 8 and the reactor vessel jaw 13 can prevent the excessive coolant flowing into the gap 14 from the emergency coolant flowing into the direct container injection nozzle 6. It is limited in size to allow for lifting without interference.

However, if this gap 14 is not present, in the event of an accident that the safety injection pipe 5 breaks, the pressure in the reactor vessel 1 is equal to the pressure outside the vessel 1 by the siphon effect. Since the coolant in the reactor vessel 1 flows back to the broken safety injection pipe 5 and flows out, the safety injection duct 10 may cause adverse effects such that the amount of coolant required for core cooling is greatly reduced. Therefore, the gap 14 is the vapor in the reactor vessel 1 into the safety injection pipe 5 when the coolant level of the reactor vessel 1 is lowered below the height of the gap due to breakage of the safety injection pipe 4. By allowing air to flow, it acts as a siphon effect blocker to stop the continuous outflow of coolant due to the siphon effect.

On the other hand, the safety injection duct 10 is attached to the outer wall of the core support cylinder (8) serves to guide the emergency coolant, the cross-sectional area of the direct coolant injection nozzle (6) by changing the flow path of the emergency coolant by changing the cross-sectional area Do not be large.

In addition, the length of the safety injection duct 10 is designed such that the outlet 15 at the lower end thereof is lower than the bottom of the reactor inlet nozzle 7 and higher than the upper surface of the core 2.

Lowering the height of the safety injection duct (10) outlet (15) below the bottom of the reactor inlet nozzle (7) is accompanied by the coolant through the low temperature pipe (4) due to the steam flow in case of the low temperature pipe (4) failure Since there is a possibility of leakage, the emergency coolant is prevented from flowing out by blocking the contact with the steam flow rate, and the emergency coolant discharged from the direct container injection nozzle 6 bypasses the reactor inlet nozzle 7 and the reactor vessel ( 1) To be injected into the reactor coolant accumulated in the lower part. In addition, the height of the safety injection duct 10 and the exit 15 higher than the upper surface of the core 2 is such that a gap between the flange 12 and the protrusion 11 when the safety injection pipe 5 breakage occurs outside the reactor. This is in case 14 is blocked by debris that has been deposited in the reactor vessel (1) to prevent the siphon effect blocker. That is, even if the gap 14 is blocked by debris and prevents the function (siphon effect blocking function), the coolant outflow due to the siphon effect is higher than the upper surface of the core 2 (safe injection duct outlet 15 height). By not proceeding further at, the coolant level in the reactor is maintained at least above the top surface of the core 2.

Table 1 below shows the safety and safety of the conventional emergency core cooling system in which the emergency coolant is injected only by the direct container injection nozzle (6) in the event of breakage of the low temperature pipe (4), the most serious virtual accident among the reactor coolant loss accidents. When the emergency core cooling system according to the present invention which injects emergency coolant into the reactor inlet nozzle 7 as the injection duct 10 is applied for thermal-hydraulic transient analysis. The results of the example calculations using the RELAP5 / MOD3 / K computer code are compared.

As can be seen in Table 1, after the breakdown of the cold tube 4 breakage and the loss of coolant, the core is filled with emergency coolant and the fuel is prepared for the current allowable reference temperature (1204 ℃). The temperature ratio of the cladding was only about 0.7 when the emergency core cooling system according to the present invention was applied, and increased to about 1.0 when the conventional emergency core cooling system was applied. As described above, the emergency core cooling system according to the present invention exhibits a significantly improved cooling effect on the core while showing a difference in temperature ratio of about 0.3 from the conventional emergency core cooling system in view of the temperature ratio to the allowable reference temperature in the re-watering stage. Appeared.

Major emergency core cooling endpoints When the direct container injection nozzle of the conventionally proposed method is located at about 2 M above the reactor vessel inlet nozzle, When the safety injection duct of the invention is added to the direct container injection nozzle Nuclear Fuel Cladding Temperature Ratio to Acceptance Criteria Temperature after Emergency Coolant Injection 1.0 0.7 Evaluation results In the re-watering step, the effect of the safety injection duct of the present invention is reflected, so that the cladding temperature is relatively low.

Table 1 Temperature Ratio of Nuclear Fuel Cladding in Re-Irrigation Phase

According to the emergency core cooling system for the pressurized water reactor according to the present invention as described in detail above, in the case of a cold tube breakage, which is a coolant inlet pipe, the amount of emergency coolant reaching the core is maximized by minimizing the amount of emergency coolant flowing into the cold tube. In addition, in case of breakdown of the safety injection pipe, which is an emergency coolant injection pipe, the siphon effect that causes the coolant to continuously flow through the safety injection pipe is blocked so that the level of the coolant in the reactor to a minimum height above a certain level where the core is submerged in the coolant. By maintaining it, the safety of the pressurized water reactor can be greatly increased.

In addition, the emergency core cooling system according to the present invention greatly reduces the construction cost by facilitating the load design and construction of the reactor support structure by hardly increasing the weight of the core support cylinder compared to the conventional emergency core cooling system. do.

Claims (4)

A pre-injection tank for storing the emergency coolant under pressure; A safety injection pipe connecting the blood injection tank to a direct container injection nozzle of a reactor vessel; Emergency core cooling system for pressurized water reactor comprising a safety injection pump for pumping the emergency coolant stored in the pre-injection tank to the direct container injection nozzle of the reactor vessel in case of loss of the reactor coolant between the pre-injection tank and the safety injection pipe. To The direct container injection nozzle is installed at a position relatively higher than the reactor inlet nozzle, Safety injection for discharging the emergency coolant discharged from the direct container injection nozzle, bypassing the reactor inlet nozzle to discharge the coolant discharged from the direct container injection nozzle through the outlet lower than the reactor inlet nozzle Emergency core cooling system for pressurized water reactor characterized in that it further comprises a duct. The method of claim 1, The safety injection duct is an emergency core cooling system for a pressurized water reactor, characterized in that the flange of the inlet circumference is coupled to the protrusion projecting from the circumference of the direct container injection nozzle. The method according to claim 1 or 2, The safety injection duct, the emergency core cooling system for pressurized water, characterized in that having an outlet at a position higher than the upper end of the core. The method according to claim 1 or 2, The safety injection duct is an emergency core cooling system for a pressurized water reactor, characterized in that attached to the outer wall of the core support in the reactor.