Nuclear reactors become less safe as they age

Nuclear power in the future: risks of a lifetime DAVID LOCHBAUM, 26 Feb 16,  A nuclear safety engineer,Lochbaum is one of the nation’s top independent experts on nuclear power.

Following the March 1979 reactor core meltdown at the Three Mile Island nuclear plant in Pennsylvania, the US Nuclear Regulatory Commission (NRC) established a safety policy that sought to limit the chance of another meltdown to no more than once every 10,000 years of reactor operation—reasonably remote odds for a reactor licensed to operate for 40 years. But since that safety goal was established, the NRC has extended the operating licenses of more than three quarters of the US fleet of 100 reactors by 20 years and is contemplating extending the licenses for an additional 20 years. The new license process, called Subsequent License Renewal, would extend operations from 60 years to 80 years. Although some reactors in unregulated markets have retired early because they can’t compete economically with cheap natural gas, reactors in regulated markets face a very different set of economic circumstances and may be kept in service well past their originally planned retirement dates.

The chance of one reactor experiencing a meltdown among a fleet of 100 reactors operating within the NRC’s safety goal for 40 years is nearly one in three (32.97 percent), or slightly higher than the risk from taking two turns on a six-chamber revolver during Russian roulette. The chance of a meltdown from that fleet operating for 60 years rises to 45.12 percent, or slightly higher than taking three Russian roulette turns. And the meltdown risk from the fleet operating for 80 years is 55.07 percent, or roughly the risk from taking four and one-half Russian roulette turns.

Time is a risk factor being ignored by the NRC. The agency’s safety goal put the risk of meltdown at one-in-three for the 100 reactors licensed for 40 years. When the NRC began renewing licenses for 20 and perhaps now 40 additional years, the agency did not revisit its safety goal and seems tolerant of the meltdown risk rising to one-in-two or greater. This is a failure to recognize that aging takes a significant safety toll on nuclear reactors—not just because parts wear out over time, but also because refurbishment and replacement sometimes have unanticipated consequences.

The bathtub curve. The NRC’s safety goal is a constant number for all reactors at every point during their operation. In reality, the risk over a reactor’s lifetime varies by what is called the bathtub curve due to its shape.

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