WO2021002552A1

WO2021002552A1 - Diverse molten core cooling method

Info

Publication number: WO2021002552A1
Application number: PCT/KR2020/000192
Authority: WO
Inventors: 배병환
Original assignee: 한국수력원자력 주식회사
Priority date: 2019-07-03
Filing date: 2020-01-06
Publication date: 2021-01-07
Also published as: KR102066813B1

Abstract

The present invention relates to a diverse molten core cooling method comprising the steps of: determining whether or not a severe accident stage has been entered; if it is determined that a severe accident stage has been entered, determining whether or not in-core cooling is possible; if it is determined that in-core cooling is possible, performing in-core cooling; determining whether or not a nuclear reactor vessel is damaged after the in-core cooling; and, if the nuclear reactor vessel is determined to have been damaged, performing out-core cooling.

Description

A variety of core melt cooling methods

The present invention relates to a method for cooling a core melt having a variety.

Core melt refers to a high-temperature melted material in which enriched uranium, which is the nuclear fuel of the reactor core installed inside the reactor core installed in the reactor core, and zirconium used as a covering material, and a number of substances inside the pressure container are mixed when a serious accident occurs.

[0003] A conventional core-melt external cooling device called a core catcher is complicated because it consists of a pre-catcher for collecting the core melt, a core melt transfer channel, and a space for spreading and cooling the core melt. In addition, the flow path cross-sectional area is small and the flow path resistance is relatively large, and thus the time for cooling the high-temperature core melt is delayed, making it difficult to respond quickly.

As another method, in the event of a major accident in which the core material melts, there is a furnace cooling method for cooling the core melt by confining the core melt in the reactor vessel by performing cooling on the outer wall of the reactor vessel to ensure the integrity of the reactor vessel. However, in this method, there is a risk that the furnace cooling will fail when all the core melt is melted and relocated to the lower head of the reactor.

Accordingly, an object of the present invention is to provide a method for cooling a core melt having a variety of improved cooling success probability.

It is an object of the present invention to provide a method for cooling a core melt having a variety, including the steps of determining whether to enter a serious accident; Determining whether furnace cooling is possible when it is determined that a major accident is entered; If it is determined that the furnace cooling is possible, performing the furnace cooling; Determining whether the reactor vessel is damaged after cooling in the furnace; If it is determined that the reactor vessel is damaged, it is achieved by including the step of performing external cooling.

The determination of the entry into the serious accident may include determining whether the core exit temperature is equal to or higher than the first temperature.

The determination of whether the reactor is damaged may include determining whether the core exit temperature is equal to or higher than the second temperature, the duration is maintained within a first hour, and the radiation level of the containment building is equal to or higher than the first level.

If it is determined that cooling in the furnace is not possible, cooling outside the furnace may be performed.

The first temperature and the second temperature may be, independently of each other, 600°C to 700°C.

The first time may be 4 to 8 hours.

The first level may be 800,000mR/hr to 1,200,000mR/hr.

According to the present invention, there is provided a method for cooling a core melt having a variety of improved cooling success probability.

1 is a graph showing the possibility of core damage according to the radiation level above the containment building,

2 is a flow chart showing a method of cooling a core melt according to an embodiment of the present invention,

Figure 3 shows the furnace cooling in an embodiment of the present invention,

Figure 4 shows the outside cooling in an embodiment of the present invention.

The present invention includes external cooling in which the core melt is cooled in the reactor cavity by prioritizing the internal cooling by determining whether to relocate the core melt to the lower hemisphere of the reactor vessel at the entry stage of a serious accident. We propose a method of cooling the core melt in various ways to ensure the integrity of the nuclear reactor building and minimize the amount of radiation emitted outside the reactor building.

In the present invention, as a countermeasure for a serious accident, when the core exit temperature reaches a certain temperature, it is determined that it is a step of entering a serious accident, and if cooling inside the furnace is possible by determining whether cooling inside the furnace is possible, the reactor cavity cooling water injection is performed to cool the outer wall of the reactor vessel (inner cooling method) A static cooling system and a chemical and volume control system are used to implement the strategy.

If the reactor joint coolant for cooling in the furnace cannot be injected, or if the core outlet temperature continues for a certain period of time at a certain temperature or higher, and the radiation level in the containment building rapidly rises above a certain level, all core melts are relocated to the lower head of the reactor. As a result, it is regarded as a case where the reactor vessel is damaged, and it is judged as an external cooling method step, and the reactor cavity immersion system (CFS) is used for the reactor cavity immersion. In addition, cooling water can be injected into the reactor cavity using a water source outside the reactor building using the emergency reactor building sprinkling system, etc., providing a method of cooling the core melt in various ways for all serious accidents.

The present invention proposes a method of effectively cooling the core melt even when the core melt, which is a case of a serious accident with a very low probability of occurrence, is ejected out of the reactor vessel at a time too quickly, or when the core melt slowly melts.

By overcoming the technical limitations of the ability to cope with serious accidents in conventional nuclear power plants, and by proposing a method for cooling the core melt in various ways, the core melt is safely confined and cooled in the reactor building, preventing damage to the reactor building, and allowing the public to It dramatically improves the safety of nuclear power plants by minimizing radiation exposure.

In the prior art, the core melt cooling employs only one of an in-furnace cooling method and an out-of-furnace cooling method, whereas the present invention allows a cooling method to be selected according to the state of the core melt, thereby effectively coping with a serious accident in various ways.

The phenomenon of serious accidents can be broadly divided into before and after reactor vessel damage.

From the perspective of overall accident management, it is possible to subdivide before and after the breakdown of the reactor vessel, and before and after the entry into the management of a serious accident, and before entering into the management of a serious accident can be divided again according to the exposure of the core in terms of the environmental conditions of the reactor coolant system (RCS). Therefore, it can be classified into a total of four time periods: from the occurrence of the accident to the exposure of the core, from the exposure of the core to the entry of a serious accident, from the entry of a serious accident to the destruction of the reactor vessel, and from the destruction of the reactor vessel to the destruction of the reactor building.

That is, the definition of the time zone according to the progress of a serious accident is as follows.

○ Time zone 0: Accident start ∼ Core exposure

○ Time zone 1 (Severe accident entry stage): Core exposure ∼ Rapid hydrogen oxidation (core exit temperature exceeding 1,200 ℉)

○ Time Zone 2 (Severe Accident Progress Stage): Rapid hydrogen oxidation (core exit temperature exceeding 1,200 ℉) ∼ Stable stationary state or reactor vessel damage

For the reactor cavity coolant injection, a stationary cooling system and a chemical and volume control system can be used to perform the strategy for cooling the outer wall of the reactor vessel (inner cooling method), and the reactor cavity immersion system can be used for the reactor cavity submersion as an external cooling method. In addition, cooling water can be injected into the reactor cavity by using a water source outside the reactor building using the emergency reactor building sprinkling system.

○ Time Zone 3 (Severe Accident End Stage): Reactor vessel damage ∼ Power plant controlled and stable safety shutdown

In order to end a serious accident, it can be known by the following variables that the power plant is in a controlled and stable state.

① As the core temperature is stable, it is expected that there will be no further movement of the core.

② The release of fission products is controlled and there are no fission products that are seriously released.

③ As the reactor building pressure is controlled, it is expected that there will be no significant leakage of fission products, and there is sufficient margin to the threat of the reactor building.

1 is an example, to provide a convenient means for determining whether to start and end serious accident management by evaluating whether a reactor vessel is damaged through radiation level information in a containment building measured by a containment radiation monitoring device.

If a serious accident occurs that damages the core during full-power operation, the occurrence of the serious accident is recognized through the core exit temperature, and the management of the serious accident starts.

If the maximum core temperature is, for example, 2200°F or higher, it is defined as a core damage. In this case, the core cannot be sufficiently cooled and the core outlet temperature rises to 1200°F (649°C).

If the core exceeds about 2,200 ℉, it is regarded as damage to the core, and the nuclear fuel cladding is damaged, and fission products collected in the gap between the nuclear fuel element and the cladding are discharged to the RCS. Fission products are released to the top of the containment building. The radiation caused by fission products emitted to the upper part of the containment can be measured through a radioactivity monitor on the upper part of the containment building, and is suitable as a monitoring variable to predict the failure of the nuclear reactor according to the measured radiation level. For this reason, the radiation level of the containment building can be used as a criterion for determining whether the reactor is damaged.

In FIG. 1, as an example, when 100% of the covering material of the effective core is damaged, it is determined whether or not a nuclear reactor has been damaged based on a radiation level value at the top of a containment building by the amount of fission products emitted.

In the stage of a major accident, it is an important step to determine how to cool the melted core, and there are two methods: the furnace cooling method and the outside cooling method.

In the reactor vessel outer wall cooling condition, which is a method of cooling the inside of the core melt, the damage of the reactor vessel is comprehensively examined in terms of rearrangement of the core melt and cooling at the outer wall of the reactor vessel.

When the temperature of the core increases, the core and nuclear fuel structure composed of a low melting point iron (Fe) or zirconium (Zr) alloy are melted, and then the nuclear fuel pellets composed of uranium oxide with a high melting point are expected to melt. This mixture of molten nuclear fuel and structures is called a core melt (Molten Core or Corium). Since the core melt has a higher density than cooling water or steam, it moves to the lower hemisphere of the reactor vessel and is relocated in a semicircular shape in the lower hemisphere of the reactor vessel.

The main components of the core melt disposed in the lower hemisphere of the reactor vessel are uranium (U), iron (Fe), and zirconium (Zr), and these metal atoms exist in a metallic state or in the form of an oxide bonded with oxygen. Uranium was originally uranium oxide (UO ₂ ), and iron and zirconium reacted with steam at a high temperature to be oxidized to become iron oxide (Fe2O3) or zirconium oxide (ZrO ₂ ). At this time, since the oxidation degree of zirconium is higher than that of iron, the core melt is composed of uranium oxide (UO ₂ ), zirconium oxide (ZrO ₂ ), zirconium (Zr), and iron (Fe). At this time, since the density of the oxide and the metal are different, it is highly likely to be separated into two layers. This is called rearrangement of a two-layer model reactor, and the upper metal layer contains zirconium (Zr) and iron (Fe), and the lower molten oxide layer contains uranium oxide (UO ₂ ) and zirconium oxide (ZrO ₂ ).

Recently, it has been confirmed that there is a heavy metal layer formed inside the oxide layer, which is in the form of a metal due to precipitation of uranium from the oxide under high temperature conditions with a high content of the molten metal. Uranium is heavier than an oxide melt composed of uranium oxide and zirconium oxide, so it is highly likely to form under the oxide melt layer. This configuration of three layers of the upper metal layer, the oxide melt layer, and the heavy uranium melt is called a three-layer model reactor relocation. However, when uranium oxide (UO ₂ ) is reduced to uranium (U), the remaining oxygen (O ₂ ) oxidizes zirconium, and the amount of zirconium oxide contained in the oxide melt layer increases, and the weight of zirconium and the upper metal layer The thickness is further reduced.

Considering that the elapsed time after the reactor shutdown is a decisive factor in the determination of the outer wall heat flux of a nuclear reactor, this analysis is to estimate the time after the reactor shutdown, which can have a sufficient chance of cooling the outer wall. This is to evaluate the probability of success and to derive the time after the minimum reactor shutdown, which is judged to have a sufficient probability of success for cooling the outer wall (success probability is 95% or more). For example, after the reactor is stopped, the time is set to 10 hours, and the probability of the maximum heat flux ratio (maximum heat flux/critical heat flux) not exceeding 1.0 is estimated by performing an analysis of the absence of study for this, and if the probability is 95% or more. In this case, it is possible to judge that cooling of the outer wall can be successful if 10 hours have passed since the reactor was stopped.

According to the results of the thermal load analysis according to the stopping time of each reactor, there are four variables: the heat flux ratio to the reactor vessel at the top of the oxide charge and the heat flux ratio to the reactor vessel at the top of the metal layer. . It is shown that the heat flux ratio in the oxide layer has sufficient margin for the elapsed time after all reactor shutdowns. On the other hand, when the heat flux ratio in the metal layer is based on the 95 percentile maximum, the heat flux ratio exceeds 1.0 in the case of 3 hours and 4 hours after the reactor is stopped. This means that the reliability that the maximum heat flux does not exceed the critical heat flux when the melt is completely rearranged when the elapsed time after the reactor is stopped is less than 4 hours is less than 95%. On the other hand, when the elapsed time after the reactor is stopped is 4 hours, the confidence that the outer wall cooling can succeed can be judged to be at the level of 90%. In the case of 5 hours elapsed time after stopping the reactor, the reliability of cooling the outer wall is about 95%, and in the case of 6 hours, the reliability is estimated to be about 99%. The reliability of the success of cooling the outer wall 3 hours after the reactor was shut down was evaluated at about 75%. The shorter the elapsed time after the reactor is stopped, the higher the level of decay heat of the oxide layer. Therefore, the elapsed time after the reactor shutdown and the reliability of the outer wall cooling success have an inverse relationship.

Considering the 95% reliability, which is commonly used from a statistical point of view, as a criterion for the success of outer wall cooling, it is judged that the outer wall cooling can be successful if the point at which all core melts are relocated to the lower head of the reactor is 6 hours after the reactor is stopped. do. When the outer wall cooling is not performed, the time point at which the core melt is relocated to the lower head of the reactor and the reactor is damaged is shorter than when the outer wall cooling is performed.

A method of cooling a core melt according to an embodiment of the present invention will be described with reference to FIGS. 2 to 4.

Figure 2 is a flow chart showing a method of cooling the melted core according to an embodiment of the present invention, Figure 3 is a furnace cooling in an embodiment of the present invention, Figure 4 is an outside furnace in an embodiment of the present invention It shows cooling.

It is determined whether the core exit temperature is greater than or equal to the first temperature in step S100 before entering the serious accident (S200). Here, the first temperature may be 600°C to 700°C, and specifically 649°C.

If the core exit temperature is greater than or equal to the first temperature, it is determined as a serious accident entry step (S300). If it is determined that the furnace cooling is possible (S400) and it is determined that it is possible, the furnace cooling (IVR filling water, S500) is performed as shown in FIG. 3. In furnace cooling, a static cooling system and a chemical and volume control system can be used. If it is determined that the furnace cooling is not possible, the furnace cooling (S700) is performed as shown in FIG. 4.

Whether the reactor vessel is damaged or not is determined whether the core exit temperature is greater than or equal to the second temperature after the furnace cooling (S500), the duration is maintained within the first hour, and the radiation level of the containment building is greater than or equal to the first level (S600). The second temperature may be different from or equal to the first temperature. The first hour is 4 to 8 hours, and specifically, may be 6 hours. The first level may be 800,000mR/hr to 1,200,000mR/hr, and specifically 1,000,000mR/hr.

As a result of the determination (S600), if the requirements are met, it is regarded as the reactor vessel damage, and external cooling (S700) is performed. According to the requirement, the point at which all the core melts are relocated to the lower head of the reactor is regarded as a case where the core melt melts in the furnace and breaks the reactor in a short time after the reactor is stopped, and proceeds to the outside cooling method step.

In the outside cooling (S700), CFS filling or filling of an external water source may be performed. After the outside cooling (S700), the core melt is cooled and solidified, and when the stabilization stage is achieved, the serious accident is terminated (S800).

The present invention relates to a method for coping with a serious accident using a diverse core melt cooling device, and it is possible to effectively cope with a serious accident by minimizing radiation exposure to the public by maintaining the integrity of a nuclear reactor building.

The present invention can effectively cope with all serious accidents in which nuclear fuel is melted, and thus has a significant economic benefit. Even in the event of a major accident in which nuclear fuel is melted, the core melt, which is a material in which nuclear fuel is melted, is trapped inside the reactor building and cooled to prevent damage to the containment building so that there is no radiation exposure to the public.

The above-described embodiments are examples for explaining the present invention, and the present invention is not limited thereto. Since those of ordinary skill in the art to which the present invention pertains will be able to implement the present invention by various modifications therefrom, the technical protection scope of the present invention should be determined by the appended claims.

Claims

In the method of cooling a core melt with a variety,

Determining whether to enter a serious accident;

Determining whether furnace cooling is possible when it is determined that a major accident is entered;

If it is determined that the furnace cooling is possible, performing the furnace cooling;

Determining whether the reactor vessel is damaged after cooling in the furnace;

A cooling method comprising the step of performing external cooling when it is determined that the reactor vessel is damaged.
The method of claim 1,

The decision to enter the serious accident,

A cooling method comprising the step of determining whether the core outlet temperature is greater than or equal to the first temperature.
The method of claim 1,

The determination of whether the reactor is damaged,

And determining whether the core exit temperature is greater than or equal to the second temperature, the duration lasts within a first hour, and the radiation level of the containment building is greater than or equal to the first level.
The method of claim 2,

A cooling method that performs external cooling when it is determined that cooling in the furnace is not possible.
The method of claim 1,

The first temperature and the second temperature are, independently of each other, 600 ℃ to 700 ℃ cooling method.
The method of claim 3,

The first time is a cooling method that is 4 to 8 hours.
The method of claim 3,

The first level is 800,000mR/hr to 1,200,000mR/hr cooling method.