The available data coming out of Japan indicates that there is still only a “low risk” of a Chernobyl-scale disaster in Fukushima, according to a nuclear expert.
All the explosions to have occurred there have been external to the central nuclear core and, as yet, none of the nuclear reactors’ cores has been breached, said Kamran Nikbin, professor of structural integrity at Imperial College, London.
The Fukushima Daiichi site on Japan’s north-east coast has been hit hardest of all the country’s nuclear power plants. Some 200 kilometres north of the capital Tokyo, the plant has six reactors. Reactor number 3, which has had the highest radiation leak levels is the only one with plutonium in its core.
Authorities are pumping sea water into the reactors to prevent them overheating. TEPCO, the company running the plant, has confirmed the spent fuel pool may have heated up, but the change was not critical.
“Substantial thicknesses of stainless steel are used to protect the nuclear core. It is most likely that these have been over-designed in order to take into account the level of seismic events that would be possible in this region,” Mr. Nikbin said. “Pressure build-up is envisaged in the design of these vessels and substantial pressure would have to build up for the vessel to fail.”
Although the plant was designed almost 40 years ago, it did withstand the tremors of the strong earthquake, which is one of the strongest measured in the last century, he added.
Up to now, plant workers have had access to the reactors and have been able to implement cooling procedures, he said. They have allowed intermittent pressure release of the steam build-up to reduce the risk of a catastrophic failure.
Nuclear plants are designed with very conservative safeguard measures especially in earthquake-prone regions, with a failure probability of 1 in 100,000 years for the nuclear material related components. Core safety is therefore taken very seriously since the worst possible earthquakes are taken into consideration in seismic regions, Nikbin said.
As a matter of standard safety, a 20-kilometre zone around Fukushima Daiichi has been evacuated, according to the International Atomic Energy Agency, citing Japanese authorities. The IAEA publishes updates on the situation on this website.
The maximum radiation recorded at the plant was 400 millisieverts (mSv) per hour. Those Chernobyl workers who died within a month of the disaster in the Ukrainian nuclear plant were recorded to have dosages of about 6,000 mSv, according to the Guardian. For a single dose to be fatal in weeks, radiation levels need to be as high as 10,000 mSv, it said. A dental x-ray exposes us to a radiation of 0.01 mSv, while a heart CT scan contains 16 mSv, the newspaper reported.
Reactors are surrounded by four to eight inches of steel, called the primary containment vessel, according to this Reuters analysis. If that first line of defence is breached, a containment building made of steel and concrete is the final defence against radiation leak into the atmosphere.
By Ali Sheikholeslami