More on nuclear risks April 1, 2011Posted by Cameron Shelley in : STV202 , trackback
Yesterday brought more commentary from Science and Nature regarding what can be learned from the disaster at Fukushima. Let me continue the discussion from this post by noting some points relevant to risk assessment.
(Image courtesy of César via Wikimedia Commons.)
This article in Science notes that the possibility of a large earthquake in the region had already been raised in the scientific literature. Japanese researchers excavated sediments in the region found evidence for a major earthquake that resulted in a large tsunami, one that had been recorded by Japanese historians in 869 AD. Their work also prompted them to estimate the hazard of another such quake occurring:
They estimated the Jogan earthquake’s magnitude at 8.3 and concluded that it could recur at 1000-year intervals. “The possibility of a large tsunami striking the Sendai Plain is high,” they wrote in a 2001 article in the Journal of Natural Disaster Science.
In spite of this article, the possibility of such a large quake and tsunami were not considered in risk assessments of the safety of the Fukushima plant. Yukinobu Okamura, the lead scientist in studies that confirmed the initial work, states that an expert panel did not heed his concerns during a review of the safety of the Fukushima plant in 2008. The reasons for not attending to this concern remain unclear.
There is also uncertainty about the cause of the explosion in the spent storage pool for reactor 4. The purpose of this pool is to cool the fuel for reactor 4 when it is not in use, and to shield workers from the radiation it gives off. There was an explosion in the pool on March 15, four days after the initial disaster. Calculations had suggested that such a problem should take several weeks to develop:
During normal operation, 7 meters of roughly 40°C water sit between the top of the fuel rods and the surface of the 1425-ton pool. The water is constantly circulated and replenished. There’s little doubt that temperatures in the pool would have risen steadily after power was lost. But several scientists have independently calculated that it would take much longer than 4 days—perhaps as much as 3 weeks—for the heat of the fresh fuel in the #4 pool to evaporate or boil off the water.
The upshot is that there is some failure mode for this pool that its designers and operators do not yet understand.
Finally, this article in Nature outlines some of the lessons from the Chernobyl disaster that might be applied to Fukushima. One of those lessons concerns the effect of general disinterest in shutting down the Chernobyl reactors once the dust has settled. Funding from international bodies is needed to study the continuing effects of radiation on the people and environment affected by the disaster there, as well as for the construction of new containment structures to prevent any further problems from arising.
But the international Chernobyl Shelter Fund that supports the US$1.4-billion effort still lacks about half of that cash, and the completion date has slipped by almost ten years since the shelter plan was agreed in principle in 2001.
The disaster at Fukushima will likely mobilize the international community to pony up the dough to get this work accomplished. Hopefully it will not take another disaster in future for the consequences of the Fukushima disaster to be probably understood and dealt with.
Among other things, these points serve to remind us that the likelihood of some events, and the hazards that they pose, have been subject to uncertainty and disagreement. So, one of the unfortunate lessons of the Fukishima disaster is that we must avoid overconfidence in assessing the risks posed by new technologies.