WH1000형 원전의 보조급수계통 유효성 연구

Abstract

Nuclear power plant is a facility which produces electricity by heat from reactor, steam generator and turbine. The steam generator uses a high-pressure water system to produce steam from the reactor coolant system to operate the turbines, and forms the boundary between the reactor coolant system and the secondary system. The high temperature of the high pressure reactor flows into the lower head of the steam generator and flows through the inside of U-tube, heating the water on the outside of the secondary water. The auxiliary feedwater system removes heat from the reactor core by supplying feedwater to prevent core damage when feedwater system is inoperable. For this reason, the auxiliary feedwater system contributes to the safety of the reactor and ESF (Engineered Safety Facility). The function of the auxiliary feedwater system is as follows.

First, supplying feedwater to steam generator during abnormal accident Second, supplying feedwater during the feedwater system inoperable due to the loss of offsite power. Third, supplying feedwater during the maintaining hot stand-by condition. Fourth, supplying feedwater to steam generator in a condition required hot shutdown condition.

Compared to the OPR1000 type plants, the WH1000 type plants have relatively small steam generators volume and tube number. Therefore, the falling rate of Tavg in the WH1000 type plants are faster and the fluctuation of steam generator level is variable. This means that the steam generation of the WH1000 type plants relatively reach to full range level faster than the OPR1000 type plants. When it happens to the auxiliary feedwater activation signal, the auxiliary feedwater flow control valves are fully opened and the rate of flow is maximum. Steam generator level can be maintained from 7 to 50 % by operator`s controlling. However, the steam flow rate of the steam generator by level of steam generator in the OPR1000 type can be maintained automatically to 50% level without the intervention of operator. Overall, whether the steam generator level is full or not is dependent on operator`s level control capability

The negative effects of the steam generator full level are as follows. First, if water flows into a steam pipe, vapor condenses and the water hammer phenomenon can be triggered. This causes damage to piping hangers, supports structures, valves, and operators, and loses the integrity of the steam pipe. Second, when significant amount of water is poured into the steam pipe, the design criteria for the spring support structure of steam pipe is exceeded stress limitation by the extra loads. it causes severe deformation of steam pipe. Third, valves are likely to be opened and not to reopened in a steam generator full level condition because they are not designed to fit into this environment. Fourth, if the water flows into the turbine driven auxiliary feedwater pump through the steam pipe the turbine of auxiliary feedwater pump may not be seriously damaged, causing the pressure boundary to be compromised. Fifth, the acceleration of a liquid that is deposited inside a pipe can cause breakage of piping or equipment.

This study is to verify the problems of the WH1000 type`s facility and improvement plan about Tavg falling, plan of prevention of steam generator full level based on tests and analysis by simulator`s test.