The admission by Kansai Electric Power on Tuesday is likely to further dent public confidence in Japan's nuclear policy, raising questions about the condition of some of Japan's ageing plants and management's apparent laxity on safety matters.
"The pipe was to have been checked at an upcoming regular inspection," said a company official.
Four workers were killed in Japan's deadliest nuclear accident on Monday when super-heated steam escaped from a ruptured pipe in a building housing turbines for a reactor at the Mihama nuclear power plant, 320km west of Tokyo.
There was no radiation leak, but the accident raised further concerns about Japan's nuclear-safety record.
Ageing pipes may now have to be
replaced at all N-power plants
Kansai said the pipe had not been checked since 1976 because it was not on an inspection list - something the company was notified of in November by a maintenance sub-contractor.
Some independent analysts said the accident could force the government to shut down its nuclear reactors for inspections.
"If the accident proves to have originated in a critical system, the implications of the 9 August non-radioactive steam leak will prove deep and immediate, forcing the government to order another round of safety inspections," said Strategic Forecasting Inc, a US-based consulting group.
"Early indications are that the bursting pipe that released the steam was already through 28 years of its 30-year life span, raising the possibility that similar pipes on all plants might have to be replaced," it said in a report.
"The pipe was to
have been checked
at an upcoming regular inspection"
Kansai Electric Power
The government ordered utilities to check records to ensure they had not missed checking the pipes that Kansai had not.
Kouji Yamashita, a nuclear plant safety inspector at the Trade Ministry, said there were 22 other nuclear-power generators in Japan of the same design as the Mihama reactor, 10 run by Kansai Electric, the rest operated by four other firms.
Police were investigating whether the company neglected safety standards by letting more than 200 workers prepare for an annual inspection while the reactor, which was in a separate building, was still running.
A police spokesman said investigations were continuing.