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

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

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

Because the Fukushima reactor has been flooded and is not on fire, there is currently no threat of it rising to the level of the Chernobyl disaster.

FORBES: Reports Claim Meltdown At Japanese Reactor