All U.S. reactors are in principle designed to withstand the largest earthquake in the local seismological record. "That earthquake becomes the design basis for engineering at that site," says Scott Burnell, a spokesman for the AEC's successor, the U.S. Nuclear Regulatory Commission (NRC). "The reactor must be able to safely shut down even if there is an earthquake of that magnitude."
Boiling-water reactors, such as those at Fukushima Daiichi or Oyster Creek, directly produce the steam that then turns the turbine to make electricity—adding a sheen of radioactivity to the power-generating equipment. Surrounding this steam-producing core is an upside down lightbulb-shaped steel and concrete containment structure that is connected via large pipes to a donut-shaped pool, which is half-filled with cooling water. In the event of an accident hot steam shoots down those pipes into the ringing pool where it is cooled.
But in a meltdown that ring can be prone to cracking or leakage—as may have occurred at Fukushima Daiichi in multiple reactors. Because of this "generic" flaw built into the original plants, a special vent was added after the Three Mile Island accident to reactors in the U.S. and Japan that allows operators to release radioactive steam before pressure gets too high—steam that can also carry even longer-lived radioactive particles, such as iodine 131 or cesium 137, in the event of a meltdown.
"It was made stronger to accept the higher pressures that would be involved during venting," Johnson explains. But the pressures reported at Fukushima—more than seven kilograms per square centimeter at times—more than double the pressures even the hardened vent was designed to handle.
"The NRC's actions in the 1980s and 1990s regarding Mark I [model boiling-water reactor] containment issues significantly improved the Mark I's ability to deal with accident conditions," Burnell says. "The agency continues to conclude the Mark I containment design provides appropriate protection of public health and safety."
These reactors also face a more insidious threat: age. Concrete, pumps, pipes and wiring face a daily load of some combination of high temperatures and pressures, vibration and—unique to nuclear infrastructure—bombardment with the neutrons thrown off by splitting atoms. Thick steel walls become brittle over time when exposed to a reactor's extreme temperatures, pressures and radiation. In fact, the NRC has shown that the stainless steel surrounding the reactor cores in boiling-water reactors degrades over time. Cracks also form at welds or joints.
Of course, it's not just the old General Electric boiling-water reactors aging in place. Pressurized-water reactors designed by Westinghouse—such those that employ pressurized water and heat exchangers to produce power, unlike the boiling-water variety—face similar challenges. In the 1990s the two pressurized-water reactors at the Salem nuclear facility in New Jersey were shut down for two years due to leaks, faulty reactor controls, poor maintenance and other serious issues.
Plus, parts fail: In the 1970s and 1980s, nuclear power plants endured a rash of steam-filled tubes bursting as a result of a faulty alloy—Inconel 600—used in their construction. Patches held the vital component together but, ultimately, entire steam generators had to be replaced as a result. Leaks have released radioactive hydrogen—tritium—into the environment at reactors from Vermont to Illinois.