Tepco said employees at various levels in the plant and company had felt a "strong sense of responsibility" to keep plants on line and not extend outages. "Those concerns fostered the mistaken idea in all the maintenance sections that they did not have to report problems to the regulator and local governments in the vicinity of the nuclear power stations as long as they did not cause any safety problems."
The internal investigation followed reports that Tepco had hidden the results of core shroud crack tests dating back to the late 1980s - and that the Ministry of Energy, Trade and Industry (METI) had colluded in hiding the problems after it was informed of them in July 2000.
Cracking in the core shroud and other areas of GE-designed reactors were the subject of extensive checks in the late 1980s and early 1990s. Japanese operators agreed voluntary tests with the safety authority, and in Tepco's case General Electric was contracted to carry them out. Some 29 sets of irregularities were found at Tepco's Kashiwazaki and Fukushima plants, but the information was concealed from the safety authority.
Over the 1990s 18 of the components showing indications underwent repair or replacement - remedial actions that have themselves come under suspicion, as there is speculation that the engineering changes may have violated nuclear regulations. Eleven more are being investigated.
Japanese safety authority METI was informed of the cracking by a GE "whistleblower" in 2000, but it was not until 29 August this year that it admitted the concealment and that it had been working with Tepco to reconcile all the faults for the intervening 18 months.
• The revelations have been a disaster for the Japanese nuclear industry. That Tepco, its flagship utility, should be involved in a cover-up was shocking enough: the actions of the safety authority, METI, in effectively concealing Tepco's wrongdoing has extended the crisis of confidence to the entire industry.
Four Tepco executives have tendered their resignation: chairman Hiroshi Araki said he would resign at the end of September; president Nobuya Minami will resign by mid-October; executive vice president Toshiaki Enomoto was due to resign at the end of September, as were counsellors Gaishi Hiraiwa and Shoh Nasu.
Tepco said in its statement it is committed to four measures to prevent a recurrence:
• To improve transparency and the disclosure of public information.
• To create an environment for conducting appropriate business activities.
• To conduct more stringent internal audits and to reform the corporate culture.
• To thoroughly comply with corporate ethics.
So far statements from METI have been inconsistent in assigning blame for the cover up.
The revelations came at a time when Tepco was dealing with new cracking problems in its reactors. Crack indications had been found in core shroud welds at the Fukushima II-3 plant in July 2001 and tie rods installed to strengthen the shroud. But the cracks were in welds made of 316L, a material replacing 304SS and 304L and thought to be less vulnerable to cracking.
This year, crack indications were found in the core shroud welds at Kashiwazaki-Kariwa 3 and the control rod drive mechanism tubes at Fukushima I-3. These are the two lead plants for Tepco's MOX fuel programme. When that programme will be restarted, if ever, is now uncertain on technical grounds. It is also politically uncertain: following the news about the Tepco cover-up, local mayors have cancelled agreements to allow MOX fuel to be burned.
Tepco's plants will be closed on a rolling programme over the next few months to check the crack indications. METI and other organisations can expect to find themselves under scrutiny and it is hard to see what the industry can do in the short term to redeem its standing. International organisations might offer some help; however, so far none of the Japanese organisations has requested it. The IAEA, for example, offered support - perhaps in the form of an ASSET mission - but told NEI its actions had to be "responsive" and it had not been asked for help.