It was the first rupture of an emergency core cooling system pipe in Japan. As the 840MWe unit 2 is similar in design to the 540MWe unit 1, plant owners Chubu Electric have suspended operation of both units 1 and 2 while the cause of the incident is investigated. The Nuclear and Industrial Safety Agency of the Ministry of Economy, Trade and Industry said the rupture in the 1cm-thick, 15cm-diameter carbon steel pipe was found in an elbowed part of the pipe, which carries 290 degrees C steam at a pressure of 70 atmospheres.

The incident happened during start up tests of the unit’s emergency core cooling system’s high-pressure coolant injection system on 7 November. Radioactive steam leaked into the reactor building but there was no outside release of radiation. The radioactive concentration of the leaked steam was estimated to be 400Bq/m3. Chubu said it checks the high-pressure core injection system on a monthly basis and conducts annual overall inspections to look for leaks, but had found no problems.

Investigators from Chubu and Japan’s agency for nuclear and industrial safety determined that the rupture was probably instantaneous and caused by tremendous pressure immediately after the start of the test. The piping was last changed in 1993-1994.

Meanwhile, an investigation has begun into a water leak at another part of unit 1, which was discovered three days later. Radioactive water was found dripping inside the reactor vessel near the lower part of the control rod drive mechanism at the Hamaoka 1 reactor on 9 November. Chubu and agency officials said that the water was dripping at a rate of 60ml/hr with a radioactive concentration of 223Bq/cm3. Chubu has said that the two incidents were not related.

Chubu said it would remove all fuel from the reactor to investigate the leak, an unusual step that may keep the reactor off line for a long time. The company said operation data indicates water could have started leaking from the BWR around July or August.

A third reactor at Hamaoka, with a capacity of 1100MWe, has been closed since the beginning of September for maintenance checks, leaving only one 1137MWe reactor operating at the facility.