The structure of venting pipes may have led to the hydrogen explosions that rocked the Fukushima No. 1 nuclear power plant during the early phases of the accident, a nuclear expert said.
The structure of venting pipes may have led to the hydrogen explosions that rocked the Fukushima No. 1 nuclear power plant during the early phases of the accident, a nuclear expert said.
Water vapor was vented from the containment vessels of the No. 1, No. 2 and No. 3 reactors to reduce the risk of damage caused by high internal pressure withing the vessels.
But the venting pipes were structured so that hydrogen in the vented gas could flow into the reactor buildings.
"Under that structure, venting could turn into a self-bomb," an expert said.
The Nuclear and Industrial Safety Agency (NISA) said it cannot deny the argument that the structure of the venting pipes may have caused the hydrogen explosions at the No. 1, No. 3 and No. 4 reactor buildings. NISA is now discussing safety measures regarding those pipes.
Tokyo Electric Power Co., the operator of the Fukushima No. 1 plant, has so far maintained that the explosion at the No. 3 reactor building was caused by hydrogen leaking from the reactor to the reactor building interior through openings on the lid of the containment vessel and elsewhere.
When TEPCO on Dec. 22 inspected the pipes of the "emergency gas treatment system," which extends from the venting pipes of the No. 3 reactor into the reactor building, it found traces of gas inflow.
At the No. 3 reactor, gas in the reactor containment vessel was vented on multiple occasions starting on March 13. Hydrogen likely flowed into emergency gas treatment system pipes on those occasions.
Those pipes are equipped with valves and a back flow prevention apparatus. But the valves are designed to open automatically when power supply is lost during an accident. It was also learned that the back flow prevention apparatus was not completely airtight, raising the possibility that hydrogen leaked through and entered the reactor building interior.
The venting pipes at the No. 3 reactor are also connected to the No. 4 reactor building. But there is no back flow prevention apparatus on the No. 4 reactor side.
TEPCO said in an investigation report in August that the explosion in the No. 4 reactor building was caused by the inflow of hydrogen gas vented from the No. 3 reactor.
The venting and emergency gas treatment system pipes are structured in a similar manner at: the No. 1 reactor, where another hydrogen explosion occurred; TEPCO's other nuclear reactors; and the boiling water reactors operated by Tohoku Electric Power Co., Chubu Electric Power Co., Hokuriku Electric Power Co., Chugoku Electric Power Co., and Japan Atomic Power Co.
"The interconnection of pipes has enhanced the danger," said Tadashi Narabayashi, a professor of nuclear reactor engineering at Hokkaido University.
NISA on Dec. 27 said subjects for discussions on safety measures include the independence of the venting pipes and installing hydrogen back flow prevention valves along the emergency gas treatment system.
The nuclear watchdog also said an analysis showed that large amounts of hydrogen leaked through openings in the containment vessel.