NR Digital

Overreaction

by William Tucker
A call for perspective on the Japanese nuclear crisis

Japan’s nuclear agony has proved one of two things: Either humanity was never meant to tap the power of the atom, or the long, slow climb toward a nuclear economy will be marked by the perils and setbacks that have always dogged human endeavors of similar scope and complexity. In the long run, it is likely to be the latter. If we are wise, we will learn from our mistakes, not abandon generations worth of technological progress.

“Prepare for the worst” has been the motto of nuclear engineers for almost three decades, but the earthquake and tsunami that struck the island nation on March 11 exceeded even worst-case expectations: The quake was a 9.0 on the Richter scale, the worst in Japan’s experience and tied for fourth-strongest in recorded history. Japan’s 53 nuclear reactors were built to withstand the shock of an 8.1 earthquake; this disaster surpassed that threshold by a factor of nine — but still the walls of the concrete containment structures held, a testament to the genius of Japanese engineering.

Unfortunately, a 9.0 earthquake was not the end of it.

Nuclear reactors are programmed to automatically shut down in emergency circumstances. All eleven of the reactors impacted by the Japanese earthquake did just that. The control rods were inserted into the giant fuel assemblies, and the reactors stopped reacting. But once that is achieved, there remains “decay heat” from the breakdown of unstable isotopes created by the fission process. This decay heat will remain at very high levels for about a week. During that time, the fuel core must be continually bathed in cooling waters to circulate heat away from it.

In a Generation II reactor — a category that covers nearly all reactors currently in use — this cooling relies on electrical pumps.  At Fukushima, the cooling pumps lost connection to the electrical grid, but backup diesel generators were on hand — until the tsunami hit a few hours later and wiped out the generators for Unit 2. Batteries were quickly substituted, but those last only a day. When they expired, a new set was located, but they had the wrong sort of connectors and could not be used. As a consequence, cooling ceased, and the fuel rods began to overheat.

When a reactor overheats, two things happen. First, some of the cooling water begins to evaporate into steam. This builds up, and the pressure eventually must be relieved by venting. The released steam will contain low levels of radiation. Ordinarily it can be contained in various reservoirs, but these, too, can eventually become overloaded. (At Three Mile Island, the major steam release came when the seal of a storage tank failed.) These steam releases will carry a radioactive charge of only about 10 millirems, meaning about the level of the radiation exposure associated with a single chest X-ray. That radiation will quickly dissipate.

The critical problem comes if the water level drops below the fuel assembly and the rods become exposed. Rising temperatures will start splitting water molecules into free hydrogen and oxygen. This is an explosive mix: Some of the oxygen binds to the fuel rods, leaving the hydrogen free. When this free hydrogen is released, it reacts immediately with the oxygen in the atmosphere and causes an explosion. Several such explosions took place in the days after the earthquake, blowing the tops off two of the steel sheath buildings that cover the reactors and their concrete containment structures.

Even at this point, nothing truly catastrophic had happened. Fuel rods themselves are housed in a steel pressure vessel that is cast from a single ingot and is nearly invulnerable to cracks and fissures. During the 1970s, anti-nuclear activists dreamed up the “China syndrome,” an implausible scenario in which the molten fuel would burn right through the steel pressure vessel and then through the concrete containment structure, “all the way to China.” On the way, the theory went, it would hit groundwater and cause a steam explosion that would wipe out half of Los Angeles. In the real world, at Three Mile Island as at Fukushima, the molten fuel didn’t even melt the chromium lining of the pressure vessel, much less the vessel itself.

Nonetheless, by March 14, pumping had flagged at several of the reactors, and the cores were exposed in at least three of the six units. Sea water was pumped into Units 1 and 3 as a last-ditch measure. This is “emergency core cooling,” which abandons all effort to cool the pressure vessel and simply inundates the containment structure, surrounding the pressure vessel with enough water to absorb the heat. Salt water is very corrosive and will probably ruin the reactor, but at such a point no other choice remains. But flooding the reactors does not solve all of the problems: Evaporation and hydrogen formation continue, meaning that there must be regular venting of radioactive steam, and that hydrogen explosions remain a possibility. All this occurred at Fukushima during the ensuing days.

The worst problem occurred at Unit 2, at which either the earthquake or a subsequent hydrogen explosion damaged the “torus,” a shallow pool of water at the bottom of the containment structure designed to catch the molten core if it should ever make its way through the bottom of the steel vessel. This structural damage apparently opened the containment structure to the outside world. And while workers were frantically trying to cool the three reactors, they failed to notice that the spent-fuel pool at Unit 4 was also losing cooling water. This is extremely dangerous because, if exposed to the atmosphere, the spent fuel will burn, causing a true Chernobyl-style release of radioactive debris, a genuine catastrophe. The spent fuel did catch fire briefly on the morning of March 15, before emergency workers put it out.

The International Atomic Energy Agency (IAEA) has a scale to measure the seriousness of nuclear accidents, running from 1 to 7. Chernobyl was a 7, while Three Mile Island was a 5. After the damage to the torus and the spent-fuel fire, Fukushima was raised to a 6, making it the second-worst accident in history.

What are the health effects of this? Workers on the site have suffered radiation exposure that carries serious consequences. A typical American experiences background radiation levels between 300 and 600 millirems, a millirem being 1/1000th of a rem. No health consequences have ever been observed at exposure below 10 rems, but at 50 rems people begin to show signs of radiation sickness. For a few moments on March 15, the level at the Fukushima plant hit 80 rems. Tokyo Electric pulled out everyone except a skeleton crew, who heroically soldiered on.

The IAEA’s ranking of the Fukushima accident seems perfectly accurate. Public exposure was nowhere near the level of Chernobyl, a unique event at a facility for which the Soviets had not ever bothered to build a containment structure. (All the world’s reactors, including those in Russia, now have them.) But Fukushima surpassed Three Mile Island, where neither the public nor plant workers ever experienced seriously dangerous levels of radiation exposure.

Where do we go from here? Reaction around the world has been mixed: Germany, which had just extended the life of its seven reactors beyond a self-imposed deadline of 2021, immediately closed them down. Switzerland called off plans for three new reactors for the time being. France, on the other hand, said it had no intention of revamping its energy infrastructure, which relies upon nuclear sources for 75 percent of its power. Russia, Korea, and China, which are rapidly becoming world leaders in nuclear-power technology, all plan to forge ahead. Along with Japan, those three countries are the principal actors in the world nuclear renaissance; there are 65 reactors under construction worldwide, almost all of those four countries’ design. They are unlikely to give up their leading role.

Where does that leave the United States, once the world’s leader in nuclear engineering, now lagging far behind? Before the earthquake, there appeared to be creeping progress toward a nuclear revival, with many former opponents on board and the support of President Obama. The president reiterated his support for nuclear power in the immediate aftermath of the earthquake, and it seems unlikely he will succumb to the hysteria of nuclear opponents to close our reactors down.

But those signs of progress may have been deceptive even before Fukushima. The regulatory process in the United States moves so slowly that it will be years before any new reactors are built. Take, for example, the Westinghouse AP1000, a Generation III reactor designed to overcome precisely the design flaw behind the troubles at Fukushima: the need for cooling pumps. Engineers realized in the 1990s that the need for electrical power in the face of a cataclysmic event was a critical vulnerability, so they designed new reactors with passive circulatory systems that rely on natural convection to keep water moving. General Electric’s Economic Simplified Boiling Water Reactor has the same design. But after seven years of review, it has not yet received design approval from the Nuclear Regulatory Commission, much less an operational go-ahead, even though four are already under construction in China.

Another promising innovation is small modular reactors, which are approximately one-tenth the size of the current behemoths. These gazebo-sized units can be buried underground and power a town of 20,000 people. They would be much safer, operating at lower temperatures and without the large concentration of fuel that characterizes their bigger cousins.

These technologies stand waiting in the wings, but the glacial pace of regulatory review puts realizing their potential at least ten years down the road. Perversely, this probably makes us less safe: We are left with a fleet of 30- and 40-year-old reactors that cannot be retired, because there is nothing to replace them. We are not going to return to the Age of Coal, nor should we be foolish enough to base our economy upon expensive and uncertain supplies of natural gas. The way toward greater nuclear safety to allow new innovations to replace outmoded technology — not to lock ourselves into an aging nuclear infrastructure.

– Mr. Tucker is author of Terrestrial Energy: How Nuclear Power Will Lead the Green Revolution and End America’s Energy Odyssey.

Send a letter to the editor.