IEEE Spectrum has an article on the possibility of fuel escaping from the damaged nuclear reactors at Fukushima - Nuclear Risk Expert: Fukushima's Fuel Could Still Seep Out .
It's Theo Theofanous's job to worry about worst-case scenarios. As director of the Center for Risk Studies and Safety at UC Santa Barbara, he tries to quantify the unthinkable and calculate the likelihood of utter disaster. He has studied everything from chemical weapons to natural gas pipelines--but for a 15-year stretch in the 1980s and 1990s, he focused on nuclear reactors.
"It was the post-Three-Mile-Island, post-Chernobyl period," Theofanous says. "There was a lot of interest in hardening our reactors to prevent failures."
His findings on reactors' vulnerabilities have given him insight into the emergency that continues to grip Japan. Four of the six reactors at the damaged Fukushima Dai-1 nuclear plant are boiling water reactors that use a "Mark 1" containment system designed by General Electric in the 1960s. Theofanous studied what would happen in a Mark 1 reactor if the cooling systems failed and the nuclear fuel overheated and melted, as some experts think may have happened in at least one of Fukushima Dai-1's reactors.
"We wanted to assess whether this particular design would be operable in the case of a failure, or whether the fuel could possibly violate the containment," says Theofanous. The "containment" he's referring to is a protective concrete and steel structure that surrounds the reactor vessel, where the nuclear fission reaction takes place. In the diagram above, the "drywell" and the "wetwell" are both considered part of the primary containment system.
For his study of the Mark 1 design, Theofanous, a professor of chemical and mechanical engineering, modeled how the materials in a reactor would interact during a partial meltdown. He assumed the following situation: The nuclear fuel overheats, begins to melt, drips down onto the floor of the reactor vessel, and eats though the steel floor of that vessel. The melted, highly radioactive fuel then leaks down onto the concrete floor of the drywell. Theofanous found that as long as there was a typical amount of water in the drywell--about half a meter--and that water was continuously cycled through to prevent it from heating up and boiling away, the nuclear fuel would not immediately make its way out into the environment. "We showed that if there's a severe accident, you must make sure there's water in the drywell," says Theofanous.
Theofanous's findings on the immediate effects of a release of fuel into the drywell may seem to bode well for Fukushima Dai-1. Although the tsunami that struck on 11 March wiped out the automatic pumping systems meant to cycle cooling water through the reactor, the plant's operators were able to pipe in seawater. "That addresses the short-term containment, the effort to make sure it doesn't fail in the first 24 hours," he says.
But the Fukushima emergency didn't end there: Subsequent explosions in reactor buildings No. 1, 2, and 3 may have damaged critical structures. So far, officials from Japan's Nuclear and Industrial Safety Agency have maintained that these explosions caused no "major breach" to any of the reactor vessels or drywells. But the discovery of radioactive water throughout the power plant's grounds indicates that something, somewhere is leaking. And according to Theofanous's research, if some of the nuclear fuel melted in the first days of the emergency, a major release of radiation is still possible.
If melted fuel escapes the reactor vessel and collects on the floor of the drywell it stills need to be continuously cooled, since it still produces decay heat even after the nuclear fission reaction has stopped. And that cooling procedure can be difficult. "Even if you flood the place with water, there's no guarantee that you can keep it cool," says Theofanous. "The fuel can form a crust, isolate itself from the water, and keep on eating through the concrete floor." The fuel could then eat all the way through the floor and drip into an area called the pedestal room--a chamber below the drywell that's surrounded by the doughnut-shaped wetwell. From there, the radioactive material could more easily reach the environment.
Has this scenario been playing out at Fukushima Dai-1 in the three weeks since the earthquake and tsunami damaged the nuclear plant? Theofanous says that from the current reports, it's impossible to say for sure. "We don't really know where the fuel is," he says. "My guess is that for the bigger 740 megawatt reactors that were badly damaged, reactors 2 and 3, the fuel is out into the drywell. If it is there, it keeps on eating through the concrete."