Among the worst case scenarios at the Fukushima plant is that the spent nuclear fuel, which sits in essentially open cooling pools near the six nuclear reactors, could catch fire for a prolonged period and spew tons of radioactive dust in a radioactive plume. It appears some of the spent fuel has been on fire at reactor #4; fire occurs if the rods get hot enough to burn their cladding. Reports say that high levels of radioactivity have made it difficult to fight the fire, which appears to have continued to burn late Wednesday. Today, U.S. Nuclear Regulatory Commission (NRC) head Gregory Jaczko said that the pool at reactor #4 had run dry; Japanese authorities denied this.
Industry and academic scientists have long been at odds over the risk of such an event. U.S. regulators have mostly sided with industry, which before today said such a fire was impossible. Today, they appear to have changed that view.
On the other hand, the National Academies' National Research Council examined the issue in a 2006 study that covered the risk of a zirconium fire caused by terrorism, earthquake, or another major breach of the pools by accident. That study concluded that the risk was real, but that up until that point, federal regulators had not taken steps to deal with it:
The Nuclear Regulatory Commission and its contractors have periodically reanalyzed the safety of spent nuclear fuel storage (see Benjamin et al., 1979; BNL, 1987, 1997; USNRC, 1983, 2001a, 2003b). All of these studies suggest that a loss-of-pool-coolant event could trigger a zirconium cladding fire in the exposed spent fuel. The Nuclear Regulatory Commission considered such an accident to be so unlikely that no specific action was warranted.
NRC's position, the Research Council's report noted, came despite "more densely packed spent fuel pools" in U.S. reactors as spent nuclear fuel piled up. In response to the classified version of the report, released in 2004, the NRC sent a letter to lawmakers explaining that it had given U.S. nuclear plants "additional guidance" on protecting nuclear fuel. But the letter didn't specify what guidance. The NRC has not released the results of a reactor-by-reactor study of vulnerabilities to spent fuel pools, said Kevin Crowley, a nuclear energy expert who ran the two Research Council studies. That study was "very contentious," he said.
As of 4 p.m. today, when fire had been seen in the pool at reactor #4, a fact sheet dated yesterday from the Nuclear Energy Institute (NEI), which represents the nuclear industry, commented that:
There has been some speculation that, if the used fuel pool were completely drained, the zirconium cladding might ignite and a "zirconium fire" might occur. Studies performed by the Department of Energy indicate that it is virtually impossible to ignite zirconium tubing.
Since zirconium is presumably what was burning in the pools, ScienceInsider asked NEI for a reference to those studies. Spokesperson Steve Kerekes wrote in an e-mail response:
Just today we were made aware of additional analysis that leads us to amend that view. Without getting into the details of how or when, we acknowledge the potential in certain circumstances and should by now have changed the fact sheet you reference.
Apologies for not properly reflecting the latest scientific research in this area.
The new fact sheet, "updated 3/16/11," says, "There has been some speculation that, if the used fuel pool were completely drained, the zirconium cladding might ignite and a 'zirconium fire' might occur." But it omits the contention that such an event would be "virtually impossible."
Kerekes would not elaborate on the earlier studies or the "additional analysis" NEI had received, suggesting that Insider ask the Nuclear Regulatory Commission.