RCP 기동 시 과압방지를 위한 2차-1차 냉각재 온도편차 연구

Abstract

가압경수로형의 한빛1,2호기는 기동과 냉각시 발생할 수 있는 저온과압사고가 원자로 압력용기의 취성파괴를 유발할 위험이 있다. 저온과압사고를 유발할 수 있는 유형은 질량주입과 에너지주입이 있다. 이 연구보고서는 발전소에서 발생 가능 빈도가 비교적 높은 에너지 주입사고에 대해 분석하였다. 발전소는 저온과압을 방지하기 위해 운영기술지침서의 압력-온도 곡선을 토대로 운전온도에 따른 압력상승을 제한하고 있으며, 저온과압방지보호 설비로는 가압기 압력방출밸브(PORV)나 잔열제거계통의 안전밸브(PSV)를 갖추고 있고, 설비 운영에 필요한 운영기술지침서와 발전소 운영절차서가 있다. 저온과압방지를 위한 다음과 같은 분석을 수행하여 저온과압보호계통의 적절성을 확인하였고, 저온과압방지를 위한 운영절차서 개정안을 제시하였다. 첫째, 원자로냉각재계통의 가압기 만수위 운전 상태에서 증기발생기 2차측 냉각재 온도가 원자로냉각재계통 냉각재온도보다 50℉ 높은 상태에서 원자로냉각재펌프 기동으로 원자로냉각재계통으로 에너지가 주입된 경우 원자로냉각재계통 압력이 원자로 압력용기 압력-온도 제한 값 이하에서 안정됨을 확인하였다. 둘째, 열 유입시 압력 과도상태가 발생되지 않는 즉, 저온과압보호설비[잔열제거계통의 안전밸브(PSV) 또는 가압기 압력방출밸브(PORV)] 동작 설정값 이하에서 원자로냉각재계통 압력이 안정될 수 있는 증기발생기 2차측 냉각재와 1차측 원자로냉각재간 온도 차이를 과도상태분석과 현장 경험 값 분석을 통하여 산출하고 현장에 반영하여 발전소 안전운전에 도모하고자 한다.|The Unit 1 and 2 the Hanbit nuclear power plant (NPP) are of the pressurized water reactor (PWR) type. A low-temperature over-pressurization accident, which can occur during the startup or the cooling-down, carries a risk of inducing a brittle fracture within pressure vessels of these reactors. A low-temperature over-pressurization accident can be caused either by mass inflow or by energy inflow. This paper analyzed energy-inflow-induced accidents at a NPP, as its likelihood is relatively higher than the other one. When the pressure climbs along with the rise of the operating temperature, the Hanbit NPP limits the pressure increase to prevent a possible low-temperature over-pressurization, based on the pressure-temperature curve specified in its technical specification. In addition, as a protective component, pilot-operated relief valves (PORVs) and pressure safety valves (PSVs) were installed in the pressurizer and in the residual heat removal (RHR) system respectively. Furthermore, technical specifications and operating procedures were established for the safe operation of the relevant facilities. With an aim to curb a low-temperature over-pressurization, the following analyses were performed to verify the adequacy of the protection system, and accordingly the revision of the operating procedures was proposed. First, it was confirmed that during the water-solid operation of the pressurizer in the reactor coolant system (RCS), when the coolant temperature in the secondary side of the steam generator is 50 degrees Fahrenheit higher than the coolant temperature in the RCS, even if energy is injected into the RCS followed by the startup of the reactor coolant pump (RCP), the pressure within the RCS remains stable below the upper pressure-temperature limit of the pressure vessel. Second, the historical operational data of the NPP were analyzed to understand what is required to create a favorable condition for the stabilization of the RCS pressure in terms of the difference between the coolant temperature in the secondary side of the steam generator and the coolant temperature in the RCS, meaning that the transient pressure condition is not created despite an energy inflow or the actual values of the protective equipment - the PSVs in the RHR system or the PORVs in the pressurizers – are below their operation setpoint. The outcome of the calculation and the analysis shall be reflected in the operating procedures for safer operation of the NPP.