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

In late March, Tepco said readings of airborne radiation inside the containment vessel at the No. 2 reactor had surged to a deadly 79 sieverts, the highest level since the crisis began March 11 last year.

WSJ: New Leak at Japan Reactor Threatens Ocean

's nuclear watchdog, the International Atomic Energy Agency, said in late December that Tehran had amassed 232 kilograms of uranium enriched to the 20% level but that almost 100 kilograms of that amount is being converted into fuel plates to power Tehran's research reactor.

WSJ: Iran Cools Nuclear Work as Vote Looms