© Reuters/Tepco
"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) 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 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 "drywell" and the "wetwell" are both considered part of the primary containment system.
© General Electric
Theofanous found that as long as there was a nominal 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 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 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 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 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 a large portion of the nuclear fuel was released into the drywell in the early days of the emergency, it's still possible that it will melt through the concrete floor of the drywell.
If melted fuel escapes the reactor vessel and collects on the floor of the drywell 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 wetwells, which are connected to the drywells by a series of pipes, have likely been damaged by the explosions and appear 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 number 2 and 3 that were badly damaged, most of the fuel is out and on the drywell floor. If it is there, it may keep on eating through the concrete."
But the drywell'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 drywells, he explains, but due to 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.
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.
Nuking FUTS!