KR100827567B1 - Core monitoring methodology for nuclear power plant without reactor vessel level indication system - Google Patents

Abstract

A method for monitoring core cooling of a nuclear power plant without an RVLIS(Reactor Vessel Level Indication System) is provided to cope with an event by reducing an event recognition time and improving accuracy of event diagnosis. A method for monitoring core cooling of a nuclear power plant without an RVLIS includes the steps of: setting an event tree according to a temperature of a core outlet thermocouple in case of an event of the nuclear power plant(S100); analyzing the event by using a computer code in analyzing a thermal hydraulic system(S300); classifying event types and determining an event scenario in case of an event after selecting event development obstructing the core cooling with the event tree according to stability by the computer code(S400); setting a standard for monitoring the core cooling by analyzing the event scenario according to the temperature of a core outlet of the event tree based on a super-cooling temperature of the core outlet thermocouple(S500); and evaluating relevancy of a power plant operation strategy based on the standard for monitoring the core cooling(S600).

Description

CORE MONITORING METHODOLOGY FOR NUCLEAR POWER PLANT WITHOUT REACTOR VESSEL LEVEL INDICATION SYSTEM}

1 is a flowchart illustrating a core cooling monitoring method of a nuclear power plant that does not use the reactor level watch holder of the present invention.

The present invention relates to a core cooling monitoring method of a power plant that does not use a reactor level watch holder of a power plant, and in particular, a power plant that does not use a reactor level watch holder of a power plant for developing guidelines for use as a core cooling monitoring method in an emergency accident. Relates to core cooling monitoring method.

In general, the role of the Reactor Vessel Level Indication System (RVLIS) is to monitor coolant inventory through direct core level monitoring and to determine the cooling of the core in conjunction with the core outlet thermocouple temperature. Core level monitoring is also carried out as a result of bubble generation or the presence of non-condensable gases in the core.

At this time, additional installation of the RVLIS for the core cooling state monitoring is a technical difficulty and expensive.

This problem complements this vulnerability by establishing core monitoring criteria that do not use the reactor water gauges and by developing operational response guidelines.

Although not shown in the drawing as a conventional method for monitoring the core temperature pressure, a facility called a core level watch tube must be added, and it is not an emergency procedure guideline focused on the convenience of operator core monitoring. In consideration of the operator's convenience and economy, there has been an urgent problem in the development of a core cooling monitoring method that does not use the core level gauge.

The present invention has been made to solve the above problems, the object of which is to develop a method for monitoring the reactor reactor core cooling in case of emergency in a nuclear power plant, and improve the stability / operability of the power plant through this core monitoring method In addition, the present invention provides a core cooling monitoring method of a nuclear power plant that does not use a reactor level watch holder that enables a smooth operation without installing a power plant reactor level watch holder.

In order to achieve the above object, the present invention sets the core cooling monitoring based on the core outlet thermocouple temperature and the monitoring criteria of the core cooling functional recovery procedure based on the optimum system behavior analysis results as the criteria used in the emergency operation procedure. .

In the process of developing the core cooling monitoring method of a nuclear power plant that does not use the reactor level watch tube, the development of the accident that causes the improper core cooling was selected based on the event tree of the probabilistic safety analysis. The optimal system behavior according to the accident development is characterized by adopting the computer code MARS (RELAP 5) code.

In addition, the monitoring criteria applicable to the core cooling condition tree and related procedures based on the core outlet thermocouple temperature in the emergency operation procedure were established. The core cooling state tree and related functional recovery procedures by the core outlet thermocouple temperature in the emergency operation procedure are the monitoring criteria that can replace the core cooling monitoring and measures by RVLIS, and are developed through the analysis of the adequacy of the operation strategy. .

Hereinafter, with reference to the accompanying drawings, a core cooling monitoring method for a nuclear power plant that does not use the reactor level watch holder of the present invention will be described with reference to the accompanying drawings.

In the core cooling monitoring method of a nuclear power plant that does not use a nuclear reactor level watch according to a preferred embodiment of the present invention, when the guideline is developed for the core cooling monitoring criteria in case of emergency of the power plant as shown in FIG. A step of selecting a state tree by monitoring the core cooling of the nuclear power plant in the event of an accident, including contents related to the core cooling monitoring standard and the operating procedure of the power plant not using the indicator system (S100); Power plant system modeling step (S200); Optimal code analysis step (S300); Incident scenario determination step (S400); Core cooling monitoring criteria determination step (S500); Evaluating the adequacy of the driving strategy when applying the core cooling monitoring standard (S600); And it consists of a power plant application step (S700) .

The state tree selection step (S100) by monitoring the core cooling of the nuclear power plant in the event of an accident (S100) is a step of selecting a state tree for monitoring the core cooling of the nuclear power plant in the emergency operation manual in case of an accident. Monitoring shall determine whether mandatory safety functions are secured, and if recovery of essential safety functions is deteriorated, functional recovery measures should be taken to restore core essential safety functions.

Here, the degraded core cooling state occurs in the process of overheating of the core in the case of an accident exceeding the design standard accident such as a component failure or multiple accident, and the core state is inadequate if no appropriate operator action is taken in this state. Deteriorates with core cooling. Inadequate core cooling is just before serious damage to the core, which can lead to serious damage to the core if immediate operator action is not taken.

The power plant system modeling step (S200) is a step of modeling three primary loops, such as loop 1, loop 2, and loop 3, to simulate the asymmetry of the reactor coolant loop of the primary system. Although not shown, it is composed of a high temperature tube, a heat generator tube of a steam generator, a pump suction tube, a reactor coolant pump, and a low temperature tube. The pressurizer and the jungle tube are connected to the loop 2 and include a reactor vessel, a steam generator, a loop, a pressurizer, and a jungle tube.

The optimal code analysis step (S300) is a step using a computer code, and evaluated using a MARS (RELAP 5) code, which is a code developed for optimal thermal hydraulic system analysis of a pressurized water reactor, and a reactor vessel level indication system (RVLIS). The accident analysis for the development of the alternative plan used the one-dimensional module for the accident analysis because the main system variables measured in the core level change, the core exit temperature, and other reactor systems were the main focus of the analysis.

The accident scenario determination step (S400) selects an accident deployment that inhibits core cooling based on the accident tree produced through the probabilistic safety analysis by the optimal code in the optimal code analysis step (S300), and the reactor system at the time of accident It is a step to determine the accident scenario by classifying the types of accidents according to the behavior characteristics.
Here, the degradation of the core cooling function is caused by the decrease of the core water level due to the loss of the reactor coolant inventory amount and the loss of the reactor inventory amount recovery function due to the pipe breakage of the reactor system or the fixing opening of the pressure relief valve.

Accidents that lead to such degradation of the core cooling function are those that exceed the design criteria, and whether or not the core cooling is lowered depends on the development of the accident, such as various initial thoughts, operator measures, and the operation of the system.

Therefore, based on the event tree produced through the probabilistic safety analysis, the accident development that inhibited core cooling was selected, and the accident types were classified according to the characteristics of the reactor system. Of the 14 internal accidents that caused damage to the core, 14 accidents were selected as the target accidents. Among the internal accidents, the development of accidents that resulted in more than 0.5% core melt frequency were selected as the target accidents, which covered more than 95% of the internal accidents that caused severe core damage.

In this case, the loss of the reactor shutdown function was excluded from the accident because it was first monitored and recovered by the essential safety function 'core criticality', and the external accident was excluded because it is outside the scope of emergency operation procedures.

Core cooling begins to decline as reactor coolant inventories are reduced to less than 30% of initial inventories and the core level decreases. Most of the reactor coolant is in the form of vertical laminar flow in the core. The core level decreases with loss of coolant inventory through the break, and the core is cooled by Boil-off Cooling until the core is fully exposed. In this state, the pressure in the reactor system is determined by the core decay heat, breakage and mass and heat removal through the steam generator. In particular, the secondary generator inventory is linked to the core cooling recovery procedure. Thus, the thermohydraulic conditions of the reactor system in a degraded core cooling state are characterized by the system pressure and the steam generator secondary heat removal capability.

Accordingly, the system pressure conditions were divided into high, medium, and low stages based on the pressurizer pressure relief valve pressure and the low pressure safety injection pressure, and the secondary side inventory conditions were divided into normal and low stages based on the 30% wide water level. According to the reactor system conditions in the reduced core cooling condition, the selected accident development was classified into four types.

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The core cooling monitoring standard setting step (S500) is a matter of the core cooling monitoring standard, and the core monitoring standard using the state tree in the existing emergency operation procedure based on the core exit temperature of the state tree is set through the optimal analysis. And the optimal behavior of the reactor system was analyzed with respect to the incident expansion described in the adequacy evaluation step (S600) of monitoring the core cooling operation strategy based upon application. In the analysis of the behavior of the reactor system, the inventories of the reactor coolant according to the development of initial thoughts and accidents were used as the dominant variable.

After the exposure of the upper plenum, the core level began to decrease, indicating that the core's exposure would occur when the reactor system inventory reached about 30-40%. That is, since the accidents that impair core cooling proceed very slowly, the behavior of the reactor system is dominated by the drainage characteristics of the reactor system due to the reduction of the reactor coolant inventory regardless of the initial thought and the development of the accident.

It can be seen that the decay heat of the reactor core is reduced to about 30-40% of the reactor coolant inventory according to the drainage characteristics of the reactor system, and the core cooling is maintained by heat removal through the break and steam generator until the core is exposed. have. In the core cooling, after the steam generator secondary cooling capacity is maintained, the two-phase flow natural convection cooling, counter-condensation cooling and high temperature pipe through the steam generator according to the drainage characteristics of the reactor system as well as energy removal through the breakage part are completely exposed. Is maintained by core boiling cooling.

In the case where the steam generator's heat removal capacity is reduced, most of the core decay heat is removed by the breakout energy discharge through the pressurizer. However, as the reactor coolant inventory decreases to about 30-40% or less as the coolant spills through the break and the loss of coolant recovery, the core starts to heat up and the core exit temperature rises. Can be.

From the above analysis results, it was proved that the core outlet temperature is the main variable indicating the state of the core cooling, which can be provided as a technical validity and basis for the core cooling state tree of the existing emergency operation procedure based on the core outlet thermocouple temperature.

A step of the adequacy evaluation of driving strategy when applied to the core cooling monitoring criteria applied during adequacy evaluation step (S600) is a core cooling monitor criteria of the operation strategy, after the emergency operating instruction is monitoring the cooling conditions of the reactor core Core Cooling If deteriorated, the operator should perform a core cooling function recovery procedure to restore the core cooling function. The application of the monitoring criteria for cooling monitoring of plants without RVLIS in the emergency operation procedure determines the appropriateness of the operation strategy .

When the core cooling monitoring criteria are applied, the analysis of the adequacy of the operation tree based on the validity of the state tree by the core exit temperature discussed in the step S600 of evaluating the adequacy of the operation strategy is performed. For this purpose, the optimal analysis of the core cooling function recovery procedure in the inappropriate core cooling state, which is the most restrictive state of the degraded core cooling state, was analyzed. Inadequate core cooling condition monitoring criteria apply when the core outlet temperature is overheated above 649 ° C, with the operator performing the procedure given in the relevant functional recovery procedure.

In order to verify the validity of these operator measures, an accident analysis scenario for driver measures was constructed for the accident development representing each type of accident. The accident analysis scenario was established according to the pressure conditions of the reactor system according to the development of the accident and the operability of the related system, and the optimal analysis of the accident analysis scenario was carried out. If overheated to be started.

As a result, the restart of the safety injection system was evaluated as the most effective core cooling function recovery measure. The discharge and low pressure safety injection measures on the secondary side of the steam generator were evaluated as the most appropriate measures when the secondary side had inventory. Reactivation of the reactor coolant pump is desirable in that it can recover the core cooling condition in the short term, thus ensuring additional operator time.However, if no additional measures are taken to recover the reactor coolant inventory, the core will be damaged. It can be seen that recovery measures of injection are required. Reactor system depressurization and safety injection by opening the pressurizer pressure relief valve are very effective in case of the development of the core cooling through the pressurizer pressure relief valve, such as the loss of heat removal capacity of the steam generator secondary side. In this case, additional reactor coolant loss through the pressurizer pressure relief valve can be seen as an ineffective measure to recover the core cooling.

Furthermore, if the result of the core cooling monitoring criteria is satisfied in the result of the operation strategy adequacy evaluation step (S600), if it is not satisfied, go to the power plant application step (S700) to be described later, if not satisfied, back to the optimal code analysis step (S300) and computerized You will analyze and determine the code.

The power plant application step (S700) is a step of applying the power plant core cooling monitoring standards , As described above, the adequacy of various operator measures for recovering the core cooling function in the improper core cooling state indicated by the core exit temperature was analyzed through the optimal system behavior analysis. The core cooling state tree and associated functional recovery procedures by the core outlet thermocouple supercooling degree in the emergency operation procedure are determined to be suitable as core cooling monitoring criteria in power plants without RVLIS and applied to the power plant.

The core cooling monitoring method of a nuclear power plant that does not use the reactor level watch container of the present invention sets a monitoring standard that monitors the core cooling smoothly without installing the reactor reactor level watch box at the time of an emergency at the nuclear power plant. Providing a procedure for developing core cooling monitoring and response methods for power plants that do not have an indication system, it has the effect of improving plant safety / operability.

In addition, the core cooling monitoring method of the nuclear power plant that does not use the reactor level watch tube of the present invention was developed in consideration of the operator's convenience, and shortens the accident recognition time, which is an early stage of the accident, and improves the accuracy of the accident diagnosis. In addition to specifying the initial response system of the Guidelines for effectively coping with the initial response to the response, there is an effect that can be organized.

Claims (2)

1) establishing an event tree according to the core outlet thermocouple temperature in case of an accident of the power plant; 2) analyzing the accident using a computer code when analyzing the thermal hydraulic system after setting the state tree; 3) determining an accident scenario by selecting an accident development that inhibits core cooling by an accident tree according to safety by the computer code, and classifying an accident type in an accident; 4) setting the core cooling monitoring criteria by analyzing the accident scenario according to the core outlet temperature of the state tree based on the supercooling degree of the core outlet thermocouple; And 5) evaluating the adequacy of the plant operation strategy according to the core cooling monitoring criteria; Core cooling monitoring method of a nuclear power plant that does not use a reactor water level clock, characterized in that comprises a. The method of claim 1, wherein before performing step 2), In order to simulate the asymmetry of the reactor coolant loop of the primary system, the reactor system modeling step of modeling the primary loop into three such as loopo 1, loop 2, and loop 3 is further included. Monitoring method of core cooling of nuclear power plant.

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