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How bad was the impact of sea water on Homaoka reactor No.5?

July 25, 2012



Seawater inflow accident could lead to decommissioning of Hamaoka nuke reactor



An expert panel to the government will start considering whether to decommission a reactor at the Hamaoka Nuclear Power Plant in Shizuoka Prefecture after assessing possible damage from a seawater inflow accident last year, it has been learned.

The move comes after the nuclear plant operated by Chubu Electric Power Co. was suspended at the government's request on May 14 last year out of consideration for potential quake damage. Shortly after the suspension, an estimated five metric tons of seawater gushed into the plant's No. 5 reactor due to a malfunction.

"There are no such precedents in the world in which a large volume of seawater made its way into the reactor core," said an official with the government's Nuclear and Industrial Safety Agency (NISA). "There are possibilities that nuclear fuel rods have eroded, and we need to carefully look into the repercussions (of the seawater inflow)."

The expert meeting will be convened on July 25 to determine whether the nuclear complex can be reactivated after such a large amount of seawater covered an extensive area including the No. 5 reactor. Because nuclear reactors are mainly made of metal prone to corrosion by saline matter, the No. 5 reactor may likely face decommission in the worst-case scenario.

In the wake of the nuclear disaster at the Fukushima No. 1 Nuclear Power Plant in Fukushima Prefecture in March last year, a large volume of seawater was injected to cool down its No. 1 through No. 3 reactors after freshwater ran short, leading plant operator Tokyo Electric Power Co. to later decide to decommission the reactors.

Chubu Electric, however, is reluctant to decommission the Hamaoka nuclear plant's No. 5 reactor. "The seawater that reached the reactor had low saline concentrations, and decommission is not necessary," said a Chubu Electric official, suggesting that the utility will only repair and replace any necessary equipment.

In the Hamaoka plant's accident in May last year, a pipe holding coolant seawater ruptured inside a condenser, prompting the leaked seawater to circulate in the No. 5 reactor. Chubu Electric estimates that some 400 tons of seawater leaked in the facility, of which about five tons reached the reactor.

The utility has been investigating the impact of the seawater leakage on all affected equipment. In March this year, several holes believed to have been made due to corrosion were found inside a tank holding emergency coolant water for a reactor. Furthermore, screws of devices to maneuver control rods were found to be covered with rust the following month, prompting the utility to file a report with NISA. The utility will wrap up its investigation on the facility by the end of this year before drawing up a final report.

During the expert meeting on July 25, attendants will carefully review the current conditions of the No. 5 reactor based on Chubu Electric's report to NISA. The discussion will be taken over by a nuclear regulatory commission to be launched in September, where the validity of the final report to be complied by Chubu Electric will be determined.

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