In one of the examples cited in the report, the Nuclear Regulatory Commission (NRC) has twice raised the acceptable level of radiation damage reactor vessels can sustain, after several plants violated or came close to violating the original standard.

FORBES: The Biggest Threat to Nuclear Power: 'Pencil Engineering'

Tepco had been pumping water to raise the level in the No. 2 reactor.

WSJ: Japan's Nuclear Crisis Escalates

Meanwhile, engineers are continuing to pump some 11, 500 tonnes of low-level radioactive seawater into the sea so the more highly contaminated water from reactor No 2 can be stored in waste buildings.

BBC: Japan quake: Nitrogen pumped into nuclear reactor