Scandal-plagued Tokyo Electric Power Co. may have tried to cover up an unstable air-sealing system at the No. 1 reactor container at its Fukushima No. 1 nuclear plant before a routine government inspection in 1991, sources at Hitachi Ltd. said Thursday.

The air-tight seal of a reactor container is the last line of defense in preventing radiation leaks from becoming serious accidents, meaning sealing tests are a priority in government safety inspections.

According to the sources, internal company documents suggest Tepco tried to manipulate data right up until the government inspection of the sealing system began. Hitachi conducted prep tests for the Tepco inspection.

They said Hitachi uncovered the documents during an internal search that was carried out after similar documents were found in September regarding a routine inspection in 1992.

However, it has not been confirmed whether Tepco actually took any manipulative measures in 1991 or 1992, they added.

The preparatory test results for the 1991 case show that the reactor’s sealing system was insufficient, and Tepco asked Hitachi workers to recommend ways to fix the problem, they said.

According to the sources, there is documentary evidence showing Hitachi workers calculated the amount of oxygen to inject into the reactor’s container pipes.

Tepco and Hitachi are looking into both cases, they added.

Upon hearing of the the 1992 case in September, the Nuclear and Industrial Safety Agency, affiliated with the Ministry of Economy, Trade and Industry, told the two firms to check their documents.

The possible manipulation of air-sealing data comes in the wake of a series of earlier allegations that Tepco covered up defects in reactor core shrouds, cooling water pipes and other parts at its nuclear plants.

Tepco has owned up to some of the coverups. The utility’s chairman stepped down Monday as a gesture of accepting blame for the scandal, and its president will follow suit later this month.