Editor's Note: This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency. For more details on how Fukushima Dai-1's nuclear reactors work and what has gone wrong so far, see our explainer and our timeline.
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 and learning how to manage severe accidents."
His findings on reactors' vulnerabilities under extreme conditions have given him insight into the emergency that continues to grip Japan. All of the six reactors at the damaged Fukushima Dai-1 nuclear plant are boiling water reactors, and five of those (including three that are damaged) 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 manageable in the case of a large-scale core melt accident, 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 dry well and the wet well 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 bottom of the reactor vessel, and eats though the steel of the vessel. The melted, highly radioactive fuel then leaks down onto the concrete floor of the dry well.
Theofanous found that as long as there was a nominal amount of water in the dry well--about half a meter--and as long as 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 dry well prior to a vessel melt-through," says Theofanous. This requirement became part of the emergency procedures for such reactors internationally.
Theofanous's findings on the immediate effects of a release of fuel into the dry well 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 pressure build-ups and hydrogen explosions in reactor buildings No. 1, 2, and 3 are very likely to 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 dry wells. 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 a large portion of the nuclear fuel was released into the dry well in the early days of the emergency, it's still possible that it will melt through the concrete floor of the dry well.
If melted fuel escapes the reactor vessel and collects on the floor of the dry well it still needs 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. First, the melted fuel and debris (which could be up to 2 meters in depth) must be covered in water. That may not be possible in the Fukushima reactor buildings, because the wet wells, which are connected to the dry wells by a series of pipes, appear to have been damaged by the explosions and to be leaking. "Even if you could flood the place with water, there's no guarantee that you can keep it cool throughout," says Theofanous. "The fuel can form a crust, isolate itself from the water, and keep on eating through the concrete floor." If the fuel eats all the way through the floor, the chances of a major release of radiation into the environment go up.
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, "and the communications from the plant have been very limited and often erratic. My guess is that for the 740-megawatt reactors No. 2 and 3 that were badly damaged, most of the fuel is out and on the dry well floor. If it is there, it may keep on eating through the concrete."
But the dry well's concrete floor is probably 5 to 10 meters thick, so Theofanous says there's not an immediate risk of a release of radioactive materials via this route. "A lot of melting has to take place before you get through 5 meters of concrete," he says.
The acute concern, he says, is the radioactive water that workers have found pooled in turbine buildings and tunnels near the reactors. "It's a real Catch-22 here," he says. To keep the fuel cool workers need to keep spraying water into the reactor vessels and dry wells, he explains, but because of some still-mysterious leaks that water is being heavily contaminated with radiation, and is making its way outside. This dangerously radioactive water has "filled up all the empty spaces around, and it's ready to flow out to the ocean," says Theofanous. If the cooling process only required a modest amount of water, he says, the plant operators might be better able to cobble together a closed loop system where the radioactive elements were filtered out of the waste water to allow it to be used again. "But there's a huge amount of highly radioactive water. I still don't know how they're going to get out of this."
Theofanous says he encouraged the U.S. Nuclear Regulatory Commission to do further studies on whether reactors could fail disastrously days or even weeks after an initial accident. "I highlighted the importance of knowing what happens in the long-term," he says. "This doesn't exist as a problem just in Fukushima, but also all plants around the world. But they didn't want to deal with it. Moreover, they declared 'sucess' and abandoned the attempt to understand severe accidents altogether. Then I got disgusted and left the nuclear business," he says.
IMAGE: General Electric