The state-affiliated agency, which recently came under fire for failing to inspect thousands of devices at Monju, is investigating the cause of the latest mishap and devising measures to prevent a recurrence.

According to sources familiar with the matter, the device in question controls the temperature of sodium coolant for the prototype reactor, which is situated in Tsuruga, Fukui Prefecture.

The JAEA was slated to complete the inspection by the end of March, after moving up the previous deadline, which was set at the end of May

Around the end of February, the maintenance management system installed at Monju started to display a warning signal. But it was not until May 27 that agency officials noticed it, the sources said.

JAEA reported the problems to the Nuclear Regulation Authority and completed the inspection of the temperature-controlling device on May 31, the sources said.

A massive sodium leak and coverup bid in December 1995 caused Monju’s operations to be suspended. It was eventually brought back online in May 2010, but halted again by a different problem in August the same year.

In November 2012, it was found that JAEA failed to carry out maintenance checks on more than 10,000 devices at Monju. After that, the NRA effectively banned the JAEA from operating the reactor.

In November last year, the NRA recommended to science and technology minister Hiroshi Hase that JAEA be replaced as Monju’s operator. The ministry is studying the matter.