Reactor still OK after 58 years!!

July 28, 2012

Nuclear agency deems aging No. 1 reactor at Genkai plant safe until 2033

http://mainichi.jp/english/english/newsselect/news/20120728p2a00m0na012000c.html

The Nuclear and Industrial Safety Agency (NISA) has deemed that the idled No. 1 reactor of the Genkai Nuclear Power Plant in Saga Prefecture, whose pressure vessel is feared to have deteriorated more than expected, will remain safe until 2033 -- 58 years after the start of its operations, agency officials have announced.

NISA's view is inconsistent with the government's general policy of decommissioning nuclear reactors 40 years after the start of their operations for safety reasons. The assessment is expected to influence a review of the government's policy by a new nuclear power regulatory body that the government will launch in September.

Pressure vessels of nuclear reactors become fragile after being constantly exposed to neutrons generated through nuclear fission. In order to check the extent of deterioration, nuclear plant operators insert test specimens made of the same materials into such vessels, and take them out and examine them during regular inspections to ascertain the so-called "fracture appearance transition temperature." An extremely high temperature raises the risk of the vessels being damaged if water is injected in the event of an accident.

Kyushu Electric Power Co., the operator of the Genkai nuclear plant, checked the test specimens in the its No. 1 reactor's pressure vessel in 2009 and confirmed that the fracture appearance transition temperature was 98 degrees Celsius, about 14 degrees higher than the estimated level, and the highest figure observed in Japan -- raising questions about the safety of the vessel.

In response, NISA's panel of experts began to examine the safety of the vessel in November last year. Experts precisely examined the specimens using an electronic microscope, and concluded that the vessel is still safe.

The inner wall of the reactor pressure vessel is farther from the reactor core than the spot where the specimens are placed, and is therefore exposed to fewer neutrons. Because of this, it deteriorates at a slower pace than that of the specimens, according to NISA officials.

The experts deemed that the No. 1 reactor's pressure vessel would not deteriorate to the extent of its specimens being damaged by water injection until around 2033 -- 58 years after it began operations in 1975.