Investigation of Fukushima Daiichi Nuclear Power Plant Accident

(3)Vent failure by loss of support system function

Abstract

Many lessons can be learned from the Fukushima Daiichi Nuclear Power Plant (NPP) accident. After the loss of Emergency Core Cooling System (ECCS) and IC core cooling, fuels in the core melted down. Leak of fission product and hydrogen began because of high-temperature damage to the PCV packing. A hydrogen explosion occurred in the upper floor in the reactor building in Units 1, 3 and 4. At Unit 2, reactor core isolation cooling (RCIC) continued to function for about 3 days. Soon after the loss of RCIC water injection, the water level in the RPV declined. Drywell pressure increased from 400 kPa to 750 kPa, and PCV top flange might leak began through silicon rubber O-ring. It was an initiation of severe contamination around the NPS. In the afternoon on March 15, wind blew toward Iidate village. Melted core relocation into lower plenum caused the radiation level increased. The radiation level was measured by containment atmospheric monitoring system (CAMS).