In February 2002, during a routine inspection at Ohio’s Davis-Besse nuclear power station, inspectors found three cracks in the lid of the reactor’s pressure vessel, the mighty steel cylinder that encloses the radioactive core. One crack was in the housing of a mechanism that drives control rods into the reactor core to manage the nuclear reaction. The flaws needed to be repaired, but there was no sense of urgency—that is, until workers began fixing the crack in the control rod mechanism and they felt a wiggle. A wiggle that was all wrong.
The control-rod housing moved slightly, which should have been impossible, as it was supposed to be surrounded on all sides by the 15-centimeter-thick steel of the reactor vessel. When workers investigated, they found a cavity roughly the size of an American football in the steel next to the housing. This void left less than one centimeter of metal protecting the pressurized interior of the reactor vessel, with its radioactive core. If the vessel had ruptured while the reactor was in operation and at pressure, the water that cooled the core would have gushed out through the hole. Such a serious “loss of coolant” accident might have led to serious core damage. To fix the vessel, the plant’s owners installed a new lid at an estimated cost of US $600 million.
Investigations by the U.S. Nuclear Regulatory Commission (NRC) and the plant’s owner determined that a tiny fissure probably appeared in the control rod mechanism as early as 1990. By around 1995, acidic water from inside the reactor was leaking through the crack and eroding the surrounding steel of the pressure vessel; it ate away at the steel for seven years before workers discovered the metal loss. Nuclear researchers are acutely aware that this kind of slow, steady degradation becomes more likely as nuclear power stations age. Every day of operation, the rugged steel and concrete that make up a reactor’s containment structures are bombarded with radiation and stressed by both high temperature and high pressure. Given enough time, these forces can potentially weaken even the toughest materials.
In the aftermath of Japan’s Fukushima Dai-ichi nuclear accident, governments all over the world are reevaluating the safety of their nuclear power plants. In the United States, where nuclear power supplies 20 percent of the country’s electricity, attention is focused on the aging of the country’s 104 active nuclear reactors, which are 32 years old on average. (Four Westinghouse AP-1000 reactors now being built in the United States are the country’s first new nuclear construction projects in decades.) When these reactors went online, regulators granted them licenses to operate for 40 years, a conservative estimate of their life span. Now these plants are being awarded license extensions; 73 reactors have already received approval to operate until they’re 60 years old, and 10 of those reactors have already entered this new era of extended operation.
But that’s not the end of the story. Operators are performing major “midlife” refurbishments that can cost $1 billion per plant. Meanwhile, regulators and nuclear researchers are studying these aging plants to find an answer to one of the most important questions now facing the industry: Would it be safe and economically sound to keep these plants running until they reach 80 or more years of age?