Fukushima: the danger of going ‘critical’

The critical question

So far efforts at the stricken Fukushima plant in Japan have focused on keeping three working nuclear reactors cool. However, evidence is now emerging of a far greater, but so far unforeseen, threat.

There is a suggestion that fuel rods, stored in a pool at reactor 4, somehow went “critical” during Wednesday. It’s a technical term for nuclear fuel that is undergoing nuclear fission – the highly energetic process normally contained within a nuclear reactor. Without the shielding a reactor provides however, fuel going critical releases a short flash of extremely intense radiation.

“A criticality event outside a containment vessel is very serious,” said consultant nuclear engineer John Large. “Not only does it liberate an enormous amount of energy, it generates neutrons that con penetrate heavy shielding structures,”

Channel 4 News special report – Japan: tsunami to nuclear crisis

Because the containment structure around reactor number 4 may have been damaged, the risk to human health is much greater. “The explosion would have fragmented the fuel and ejected it out of the building. Now you have a nuclear site which could have fragments of intensely radioactive material lying around,” said Large.

Seriously concerned

That presents a new problem: preventing workers from getting near the reactors and cooling ponds that need constant supervision to stop them getting further out of control. In a press conference today, engineers from Tokyo Electric Power Company admitted that they were seriously concerned. “The possibility of re-criticality is not zero,” the utility said.

The fact a “criticality” incident may already have occurred may account for a spike in radiation levels measured at the plant on Wednesday.

The possibility of a repeat incident may be behind a significant announcement by the US Nuclear Regulatory Commission. “There is no water in the spent fuel pool and we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures,” its head Gregory Jaczko, told the House energy and commerce subcommittee this evening.

Advice from the US embassy in Tokyo has also changed significantly. “We are recommending, as a precaution, that American citizens who live within 50 miles (80 km) of the Fukushima Nuclear Power Plant evacuate the area or to take shelter indoors if safe evacuation is not practical,” it said.