A steam pipe at Chubu Electric Power Co.'s Hamaoka nuclear plant in Shizuoka Prefecture ruptured instantly under enormous pressure last month, causing a radioactive steam leak, the plant's operator and the government's nuclear body said Thursday.

The utility firm and the Nuclear Safety and Industrial Agency, which have been looking into the cause of the Nov. 7 accident, said a "ductile fraction" caused the rupture, as characteristic dimples were detected in a cross section of the carbon steel pipe.

A ductile fraction -- in which the pipe is stretched to breaking point -- has never occurred before in Japanese nuclear plants, according to the company.

Masatoshi Sakaguchi, deputy head of the Hamaoka plant, said the phenomenon was "never expected."

Chubu officials said they suspect the pressure was due to explosive hydrogen combustion inside the pipe.

The pipe may also have ruptured as a result of shock caused by the flow of water within, they said.

The utility released the results of its analysis of the cross section, which is 15 cm in diameter 1.1 cm thick.

The officials said a microscopic examination was conducted on 17 pipe fragments at a facility affiliated with Toshiba Corp. and the Japan Atomic Energy Research Institute.

It did not show signs of metal fatigue or corrosion, they added.

Hydrogen could have been generated in the pipe as radiation split water molecules in the reactor, the agency said. Agency officials also said it is difficult to pinpoint the cause of the rupture due to the lack of direct evidence.

Steam containing a small amount of radioactive material leaked from a pressure injection system at the plant's 540,000-kw No. 1 reactor on Nov. 7.

Three days later, radioactive water was found to be leaking inside the reactor at the rate of about 60 milliliters per hour.