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by Cecil Adams

I recently moved to southern California from the Chicago area. I know about the Three Mile Island and Chernobyl nuclear accidents, but now I'm also hearing rumors of a reactor meltdown in China Flats, in the hills between Simi Valley and Thousand Oaks, northwest of Los Angeles. What gives? —Steve Johnson, Thousand Oaks, California

You heard right, cowboy. There was a meltdown at the Santa Susana Field Laboratory in 1959, one of several accidents at that site over its checkered history. Though nobody died, at least right away, as many as 1,800 cases of cancer may have resulted from the '59 incident alone. Fact is, we know of at least a half-dozen nuclear meltdowns over the years, though none recently. I don't say this to scare you off nukes—given the inexorable depletion of other energy sources, I might as well beef about the perils of the sun—but rather to make the point that, with nuclear power as with any emerging technology involving vast forces, you have these little oopsies in which things melt, stuff explodes, and sometimes people get hurt.

What's a meltdown? Most nuclear reactors generate steam power using heat from clusters of radioactive fuel rods housed in the reactor core. If the rods get too hot you can have a partial meltdown, damaging the reactor and releasing radiation; if things really go off the rails you can have a full-scale meltdown, where the core melts and breaches the containment vessel, typically causing fires, steam explosions, and a massive release of radiation. In the worst case, the molten reactor mass burns through the bottom of the containment structure and sinks into the earth, melting the rocks underneath—a scenario vividly if hyperbolically conveyed by the term "China syndrome," which envisions the stuff uncontrollably tunneling through to the other side of the planet. Not to worry: the core would likely only reach the water table, at which point a steam explosion could blast radioactive material all over the place.

The first known partial meltdown occurred on December 12, 1952, at the NRX research reactor in Chalk River, Ontario. The result of mechanical failure and operator error, it was a relatively low-impact event, notable in part for the involvement of U.S. Navy lieutenant Jimmy Carter in the cleanup. The U.S. has had four similar accidents: a minor 1955 fuel rod meltdown at an experimental breeder reactor in Idaho; the aforementioned incident at Santa Susana, where a third of the reactor's fuel melted due to blocked cooling channels; and a 1966 meltdown stemming from a blocked coolant nozzle at the experimental Enrico Fermi breeder reactor near Detroit. ("China syndrome" first appeared in print soon after this incident.)

The fourth and most famous U.S. nuclear accident occurred in March 1979 with the partial meltdown of Unit 2 at the Three Mile Island plant near Harrisburg, Pennsylvania. (Oddly, the fictional meltdown movie The China Syndrome had come out just 12 days earlier.) As with prior partial meltdowns, a combination of mechanical problems and human error was blamed—in one case, a warning light had been obscured by a yellow maintenance tag. About half the uranium fuel melted, permanently disabling the reactor. Plenty of radiation was released but no workers were killed or injured, nor have any subsequent deaths been conclusively linked to the disaster.

The mother of nuclear mishaps, of course, was at the Chernobyl power plant in what was then the Soviet Union in April 1986, where a jaw-dropping cavalcade of screwups led to a massive explosion and meltdown of Reactor 4. Although many pin ultimate blame for the accident on lousy plant design, the immediate cause was a test of the reactor's safety systems that went horribly wrong, in part because so many of said systems were disabled for the test. A steam explosion in the overheated reactor blew off the containment lid and flung chunks of fuel into the surrounding area, after which a graphite-fueled fire sent a plume of radioactive debris across much of Europe. Thirty-one people died, and the long-term health effects are warmly debated. A 2005 report by the International Atomic Energy Agency estimates a potential 4 percent increase in eventual cancer deaths (4,000 cases) among the 600,000 people most heavily exposed. Increased thyroid cancer has been found among those who were children at the time, but despite initial fears there's no clear evidence of a spike in leukemia or birth defects.

New reactor designs supposedly are much safer, with triple-redundant cooling systems and greater reliance on passive safety features such as gravity feeds to circulate coolant rather than failure-prone pumps. Still, it'd be foolish to think no one else will ever die in a nuclear accident, and one can offer only the brittle consolation that in China, extraction of coal—the principal competing energy source capable of being ramped up significantly—kills 5,000 or more miners a year.

Comments, questions? Take it up with Cecil on the Straight Dope Message Board, www.straightdope.com, or write him at the Chicago Reader, 11 E. Illinois, Chicago 60611. Cecil's most recent compendium of knowledge, Triumph of the Straight Dope, is available at bookstores everywhere.

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