Fukushima lesson: be ready for unanticipated nuclear accidents

that puts you in a situation that you didn’t anticipate,” said Ewing, Edward H. Kraus Distinguished University Professor. “So the research focus should be on the situations you don’t expect to deal with. Right now, that kind of knowledge is fragmentary, at best.”

Ewing’s co-authors on the Science article are Peter Burns of the University of Notre Dame and Alexandra Navrotsky of the University of California, Davis. In the article, the authors note that the “studies outlined here are both difficult and expensive, but are essential to reduce the risk associated with an increasing reliance on nuclear energy.”

Burns, Ewing, and Navrotsky lead a Department of Energy-funded Energy Frontier Research Center that focuses on the materials science of actinides, the heaviest elements in the periodic table and critical elements in nuclear fuel. The research center includes scientists and engineers from six U.S. universities and three national laboratories.

On 11 March 2011 the three operational Fukushima reactors shut down promptly after the earthquake. Most of the fuel in the reactors was uranium dioxide.

When the tsunami inundated the site about forty minutes after the earthquake, electrical power was lost, followed by the loss of onsite backup power, resulting in a station blackout and the loss of reactor coolant. A partial core-melt event ensued in units 1, 2, and 3.

The Japanese operator, TEPCO, has surmised that there was a nearly immediate loss of core cooling in unit 1, and almost all of its fuel assemblies melted and accumulated in the bottom of the pressure vessel. Partial melting of the cores in units 2 and 3, damaging one-third of the fuel assemblies in each, occurred over the following days.

The release notes that reaction of the zirconium alloy fuel cladding with water at high temperatures generated hydrogen gas that accumulated and exploded in four of the reactor units. The release of radioactivity, other than gaseous and volatile fission products, was dominated by the many tons of seawater used to cool the cores and storage pools.

Ewing said that — despite all the uncertainties and unknowns about the short- and long-term effects — using seawater to cool the Fukushima reactors was probably the right call.

“You have a crisis, you have to cool the cores, and you can’t afford to wait around,” he said. “Using the seawater sounds like the right thing to do.”

During the nuclear crisis that followed last year’s earthquake and tsunami, 80,000 or more people were evacuated from the area nearest the Fukushima plant; a year later, they remain displaced. The Japanese government is awarding an initial $13 billion in contracts to begin decontamination and rehabilitation of the more than 8,000-square-mile region most exposed to radioactive fallout.

All but two of Japan’s 54 nuclear reactors remain shut down a year later, in a country where nuclear power once supplied nearly 30 percent of the electricity.

— Read more in Peter C. Burns1, Rodney C. Ewing, and Alexandra Navrotsky, “Nuclear Fuel in a Reactor Accident,” Science 335, no. 6073 (9 March 2012): 1184-88 (DOI: 10.1126/science.1211285)