The ruptured pipe that caused four deaths at a Japanese nuclear power plant Monday had never been inspected in 28 years, the Mainichi Shimbun said Tuesday.

The accident, in the secondary system of the Mihama Nuclear Power Plant's No.3 reactor, killed four people and left seven badly burned when high-pressure steam began gushing out of the pipe.

Inspections after the accident revealed that the thickness of the pipe at its thinnest section had worn down to just 0.06 inch from 0.4 inches, less than half that required for minimum safety.

The plant's operator, the Kansai Electric Power Company, was alerted last November about the section but did not implement safety measures, the paper said.

The company's guidelines listed the ruptured section among parts to be included in main inspections. But ultrasonic inspections to determine the thickness of the pipe had not been carried out even once since operations began in December 1976, the paper added.