This is a phenomenon engineers call the bathtub curve: In the early days of a piece of machinery's operation, as the kinks are being worked out, problems are numerous, but then it declines into a period of smooth operation. The question is: How long will that period be in the case of nuclear reactors? Because on the far side of the bathtub curve is a climb in the number of problems.
According to the NRC's analysis of so-called "accident sequence precursors," or events that could lead to a nuclear accident, U.S. nuclear reactors averaged seven such events per year between 1993 and 2004—three per year at boiling-water reactors and 11 at pressurized water reactors, most of which could be attributed to the loss of auxiliary, nonreactor-generated power required to run the safety equipment—an increase in such precursors that was statistically significant even when such loss-of-power events were not counted.
It may not be problems with the nuclear reactors themselves, however, that pose the greatest safety concern. Of all the failures, human error among the 50 to 55 reactor operators who staff a nuclear power plant control room in shifts looms largest, particularly in the case of the most notorious nuclear accident in U.S. history: Three Mile Island (TMI).
On March 28, 1979, Unit 2 at the TMI nuclear power plant near Middletown, Pa., suffered a partial meltdown—an overheating of the nuclear fuel rods caused by radioactive decay in the fission by-products. If unchecked, a meltdown can send superheated fuel through the steel and concrete that surrounds it, damaging or destroying the reactor and releasing extreme levels of radiation into the environment.
At about 4 A.M. local time, the main pumps feeding cooling water into Unit 2 failed and, due to confusion amidst the klaxon of alarms and flashing warning lights, the men operating the reactor made the situation worse when they mistakenly thought there was too much water in the core and shut off emergency pumps, thereby reducing further the amount of coolant reaching the reactor. Within three hours the nuclear fuel had boiled itself dry and then burst through its zirconium cladding before beginning to melt inside the reactor. "If they kept their hands in their pockets, everything goes much better," notes Gary Callaway, a former reactor operator at the Palo Verde and Indian Point nuclear plants in Arizona and New York State, respectively, and now an NRC trainer.
Before the problem was fixed, one half of the core had melted, though it never breached the walls of steel and concrete designed to contain it and only a small amount of vented radioactive steam, other gases and water was released—enough to give a dose of an average of eight millirems to people living within 10 miles (16 kilometers) of the plant, or the same as the dose from a chest x-ray, according to the American Nuclear Society (ANS), an industry group.
Nevertheless, the TMI accident essentially brought to a halt the first round of nuclear power plant construction in the U.S. And such operator error has been behind many of the worst disasters in the global nuclear power industry, including Chernobyl, in Ukraine (then part of the U.S.S.R.), which involved a reactor that on April 26, 1986, exploded through its containment after a series of mistakes by Soviet operators during an experiment. "You cannot regulate against stupidity," says John Ricci, manager of specialized technical training at the NRC. "As far as I know we've never killed anyone with a nuclear power plant."