Too much heat during welding, poor water quality and inferior materials are being examined as possible causes of a radioactive water leak at a nuclear reactor in Hamaoka, Shizuoka Prefecture, Chubu Electric Power Co. said Tuesday.

The Nagoya-based utility said the mistakes may have caused a welded part at the bottom of a pressure vessel in the 540,000-kw plant's No. 1 reactor to crack, leading to a leak in early July that averaged 80 liters of radioactive water each day.

The leak was not discovered until November.

The company reported the findings Tuesday to the Nuclear and Industrial Safety Agency, which is a branch of the Ministry of Economy, Trade and Industry.

Chubu Electric said it will send samples from the welded part to a facility linked to Toshiba Corp. for further investigation.

The welded part and others surrounding it will be replaced, the officials said. The weld links one of the reactor's 89 control rod driving units to the bottom of a pressure vessel.

Officials said the task will likely be difficult, since it has never been done before in Japan.

Fumio Kawaguchi, president of Chubu Electric Power, visited the town of Hamaoka on Tuesday afternoon and apologized to Mayor Yoshiaki Honma for the accident.

"I once again want to apologize for causing great worries to town residents and others," Kawaguchi said.

Radioactive steam was discovered leaking from a pressure-injection system at the Hamaoka plant's No. 1 reactor on Nov. 7.