To minimize the chances of such hydrogen accumulation, the NRC has suggested U.S. reactors strengthen vents to ensure that they could be used to relieve such a dangerous buildup of pressure. After all, the accretion of hydrogen could also raise pressures above the design limits of the safety systems. Fukushima Daiichi actually had such hardened vents, which either failed to operate or were not used soon enough to prevent the explosions. One problem may have been that the vents require electricity to operate—and at that point the stricken nuclear power plant had none.
"The NRC is implementing a [recommendation] to enhance the vents by making them 'reliable' under adverse conditions," such as a loss of electricity, says NRC spokesman Scott Burnell. And the agency concludes that such voluntary improvements provide "appropriate protection" of public health and safety. Beyond that, new rules are expected to address any lessons learned from the Fukushima nuclear crisis.
Regardless of the ability of the vents to function appropriately, one clear difference exists between the operation of such boiling-water reactors in the U.S. versus those in Japan—in the U.S., reactor operators have the authority to vent radioactive steam or hydrogen gas as conditions warrant. The employees of the Tokyo Electric Power Co. (TEPCO), which ran Fukushima Daiichi, appear to have required or at least sought government authorization to do so. "They were concerned venting might allow a flow of radioactive materials into the air, and they had not yet fully evacuated the area," explains mechanical engineer Vijay Nilekani of the Nuclear Energy Institute (NEI), an industry group. By the time evacuations and authorizations had taken place, "they had damaged the core and were venting hydrogen that caused explosions," Nilekani explains. "If you don't damage your core, you do not produce the large amounts of hydrogen that resulted in detonation."
In addition, it remains unclear how the hydrogen got from the sealed area containing the reactor vessel into the surrounding building—and then built up in sufficient quantities to explode. One suggestion is that the enormous pressures generated by the boiling steam opened gaps around bolts that allowed the hydrogen to escape or that the vents themselves leaked.
Such problems have long been an issue with the Mark I, the "safety disadvantages" of which were highlighted in an internal memo at the U.S. Atomic Energy Commission —the precursor to the NRC—as far back as 1972. The NRC nonetheless permits its use because "the Mark I can survive long enough to allow for actions that keep the public safe in the event of a radioactive release," Burnell says. In other words, there would be time to evacuate or take other safety precautions.
Modeling the worst
To make that judgment, the NRC relies on computer modeling, the most recent of which is known as State-of-the-Art Reactor Consequence Analyses. That modeling took two representative nuclear power plants in the U.S.—a pressurized-water reactor from the Surry Power Station in Virginia and a boiling-water reactor from Peach Bottom Atomic Power Station in Pennsylvania—and attempted to assess what would happen in a severe accident, such as the loss of all electric power as a result of an earthquake, among other scenarios.