After a second explosion today at Japan’s tsunami-ravaged Fukushima Daiichi nuclear plant, is the world faced with another Chernobyl? No. Another Three Mile Island? Probably. The upside of this would be that it might help people to remember exactly what did happen at Three Mile Island — and what didn’t. No one was hurt, for one thing. It was America’s worst nuclear accident, but the main damage it did was to the nation’s psyche.
Nuclear power has long lived under the shadows of Chernobyl and Three Mile Island. The current events in Japan may change this. Three of the reactors at Fukushima Daiichi proved immune to attempts to cool them with backup power, and so were flooded with seawater — the last-ditch alternative for overheating power plants — and now face meltdown. It’s a horrible scene, fraught with unpredictable circumstances. But so far, all the predictions have held true: Japan’s 53 reactors have withstood the largest earthquake in Japanese history with only three of them succumbing to damage from the ensuing tsunami.
Right now, it looks like the country’s biggest problem will be the loss of electricity, which it will need desperately in the coming weeks as it struggles to recover. The dreaded nuclear catastrophe that topped the headlines during the first days after the disaster, and which always are predicted by those prophets of doom who oppose nuclear power, did not materialize. You can feel the public sobering up a bit. Maybe this nuclear stuff isn’t so apocalyptically dangerous after all.
First, what happened? When the earthquake hit, all eleven reactors affected by the impact immediately shut down, as they are designed to do. There is never any danger of a runaway reactor. Whenever such an incident occurs, the control rods are automatically inserted, and the reactor stops reacting. A nuclear reactor cannot explode into a mushroom cloud, contrary to popular lore. The proportion of fissionable uranium present in a reactor is 3 percent, while the proportion required for a bomb is 90 percent. They are two different technologies.
What you do have in a shutdown reactor is “decay heat,” the result of the continued disintegration of radioactive isotopes formed during the fission process. This decay heat is intense at first, but in a week or so it will cool down to below the boiling point of water. Meanwhile, the fuel rods must be kept in a constant bath of water to carry away the heat.
In the Generation II reactors currently in use, this circulation system requires pumping. This design shortcoming was recognized in the 1990s, and the new Generation III reactors use passive circulation systems that rely on convection currents, without active pumping. If we ever get any of these new reactors built, they will be safer than the ones we have now. There are already four under construction in China.
When the earthquake hit, the four reactors at Fukushima Daiichi lost their connection to the electrical grid and required backup power to keep their cooling systems operating. This was supplied by diesel generators on site. But the ensuing tsunami washed away the generators, so batteries had to be substituted. The first round worked, but they last only about a day. When the second set arrived, it was discovered their plugs didn’t fit the connections.
At this point, the situation became critical. If the water does not circulate, some of it starts to evaporate. This builds within the pressure vessel, so that eventually some steam must be released. This was done at Three Mile Island and has been done at three of the four reactors at Fukushima Daiichi. The consequences to the outside world are negligible. At the point of release, the steam produces about the same radiation exposure as a cross-country airplane flight. This slight amount quickly dissipates into the surrounding oceans of air and water.
The real difficulty comes as the temperature in the core rises sufficiently high to begin splitting water molecules into hydrogen and oxygen, which form an explosive mixture. A hydrogen bubble formed in the cooling system at Three Mile Island and caused much anxiety until it was finally bled off successfully. At Fukushima Daiichi, attempts to vent the steam have caused hydrogen explosions in adjoining buildings. These explosions took place outside the nuclear containment structure, however, and did not affect the reactor core. Still, it poses a problem that the engineers have not yet been able to resolve.
Recent reports indicate that fuel rods in three reactors are now uncovered, and meltdown has begun. The uranium pellets are encased in a ceramic material that has a very high melting point; the zirconium alloy that makes up the fuel rods will melt first. In the worst-case scenario, the entire fuel assembly will sink into a puddle at the bottom of the pressure vessel, where it will be funneled into a “core catcher” designed to spread out its heat so that it will not penetrate the six-inch steel enclosure. In the early 1970s, one imaginative scientist at Oak Ridge National Laboratory speculated that, in the worst case, the melted fuel might act as a welding torch, cutting through the pressure vessel and perhaps even the ten-foot floor of the concrete containment structure. Elaboration of this scenario went on until the whole core was hitting groundwater like a white-hot basketball being dropped into a bathtub, causing a steam explosion that would wipe out half of Los Angeles. This fanciful scenario was adopted into script of the China Syndrome, which opened ten days before Three Mile Island, which is mostly why we haven’t licensed any nuclear plants since.
In fact, Three Mile Island proved the China Syndrome wouldn’t happen. More than half the core melted at that plant, and all it did was collect at the bottom of the pressure vessel. It wasn’t even hot enough to penetrate the chromium liner, let alone the steel. But all this was forgotten in the ensuing years.
Chernobyl, of course, was something entirely different. The Soviets had come up with the novel idea of dispensing with the concrete containment structure, leaving the reactor in the open air. Soviet science, they were confident, would compensate for this shortcoming. Unfortunately, Soviet science did not compensate for the fact that two rival teams ended up wrestling over the Chernobyl reactor, with one trying to operate it while the other was shutting it down to test how long the turbines would run on their momentum. The conflicting actions sent a surge of power through the reactor, which overheated and blew its lid off. To top things off, the Soviets had used graphite as the “moderator” (a structure that slows the neutrons, an essential function) instead of water, as in all commercial reactors. The graphite caught fire and burned for four days, scattering radioactive debris all over the northern hemisphere.
There won’t be “another Chernobyl” in Japan, or anywhere else. Every reactor in the world — including the old Russian ones — now has a complete containment structure and has abandoned the graphite design.
So if we can’t have another Chernobyl and Three Mile Island didn’t injure anyone, what are the prospects for Fukushima Daiichi?
If nuclear power were treated like any other technology, we would have become reconciled to it long ago. Other technologies have similar dangers: Natural gas is pumped right into your house, and explosions occur all the time, sometimes leveling whole blocks. In earthquakes and hurricanes, the worst disasters usually come at hydroelectric dams. A series of dam failures after Typhoon Nina killed 170,000 people in China in 1975, yet nobody talks about abandoning hydroelectricity.
In that respect, the current crisis in Japan may turn out to be helpful in that may make people realize that a “nuclear meltdown” is basically an industrial accident in which the public is only minimally endangered. The only real casualty is the reactor itself. That doesn’t mean we should abandon all those evacuation plans or become overconfident in the operation of nuclear reactors. But on the other side of the ledger comes the realization that every technology has its risks, and other countries are finding them acceptable. There are now 54 reactors under construction around the world, none of them in the United States. We’re not going to remain an industrial giant very long if we don’t adopt what will undoubtedly be the principal source of energy in the 21st century.
The earthquake has done such untold damage that it may be weeks or months before anything resembling normal life can return, and years before reconstruction can be completed. In the light of these much larger concerns, the nuclear fright at Fukushima Daiichi may eventually be relegated to its proper place: a footnote.
— William Tucker is author of Terrestrial Energy: How Nuclear Power Will Lead the Green Revolution and End America’s Long Energy Odyssey.