WASHINGTON, DC (March 7, 2014)—Three years after hydrogen explosions wreaked havoc in the Fukushima nuclear disaster, the U.S. Nuclear Regulatory Commission (NRC) is still not adequately protecting American nuclear reactors from the risk of similar hydrogen blasts in a severe accident, according to a new report by the Natural Resources Defense Council (NRDC).
As the report explains in detail, tons of combustible hydrogen can be produced in severe loss-of-coolant nuclear accidents when uranium-filled fuel rods, made of zirconium, react with steam. In the March, 2011, Fukushima accident, the accumulation and subsequent detonation of hydrogen gas breached the reactors’ containment structures, spewing radioactive contamination into the air, forcing evacuations and intensifying the disaster emergency.
Yet the NRC, relying on outdated computer models that don’t accurately account for the rapid buildup of explosive hydrogen gas, has ordered American utilities to take only token steps to address the problem and left in place safety systems which, the NRDC report argues, are just as likely to trigger a hydrogen detonation as prevent one.
“U.S. reactors remain vulnerable to the threat of runaway hydrogen production and leakage in a severe nuclear accident, with little or no capacity to safely reduce or vent potentially explosive concentrations of this gas before it explodes and contaminates the surrounding region,” said Christopher Paine, senior policy adviser in NRDC’s nuclear program and contributing editor to the report.
Despite the three devastating hydrogen explosions at Fukushima Daiichi, the NRC has relegated severe-accident hydrogen safety issues to the least proactive stage of its post-Fukushima regulatory responses to the accident (termed “Tier 3”). Important steps can be taken to minimize the risk of hydrogen explosions in severe nuclear accidents, and the NRDC report offers six recommendations for agency action:
- The NRC should develop and validate accurate, conservative computer safety models to predict rates of hydrogen generation in severe nuclear accidents;
- The safety of one type of device for hydrogen control in reactors—Passive Autocatalytic Recombiners (PARs)—should be reassessed, with their use potentially discontinued until technical improvements are developed and certified. There is evidence that they are as likely to cause an explosion as prevent one;
- Oxygen and hydrogen monitoring instruments at nuclear plants should be significantly improved;
- Reactor core diagnostic capabilities should be upgraded to provide plant operators with a better signal for when to transition from emergency operating procedures (designed to protect the reactor core) to severe accident management guidelines (intended to contain and limit the damage from an accident);
- The NRC should require that periodic reactor containment leak rate test data be used to help predict the rate of severe accident hydrogen leaks from the primary containments of the 31 U.S. Mark I and Mark II boiling water reactors (BWR) under a range of accident scenarios; and
- The NRC should require all nuclear power plants to develop and install systems to control the total quantity of hydrogen that could be generated in a severe accident.
“The primary containments of the Mark I BWRs, the oldest design still operating, have failed a number of leak rate tests; such tests are used to determine how much radiation would leak in an accident. New Jersey’s Oyster Creek, similar to Fukushima Daiichi Unit 1, has failed at least five tests. In one case it leaked air at a rate 18 times greater than allowed,” explained Mark Leyse, a technical consultant to NRDC and lead author of the report. “In the U.S. there are over 20 BWR Mark I reactors in operation; any of them could leak a large amount of hydrogen in a severe accident—the gas would most likely detonate, as happened at Fukushima. The NRC needs to address this problem.”
As the NRDC report documents, in the aftermath of the Fukushima Daiichi accident the NRC has simply declared that severe nuclear accidents are vanishingly rare events that can either be prevented or sharply limited in scope, thereby avoiding any significant buildup of hydrogen and the attendant explosion risk. However the risk of hydrogen explosions remains an unresolved safety problem for the U.S. nuclear industry.
“It is clear from this report that it will be costly to maintain the safety of the aging reactors in the current U.S. nuclear fleet, which will increasingly operate beyond their 40-year initial license terms,” said Paine. “Nuclear power faces significant competition from lower-cost electricity sources, creating an unsettling tradeoff between economic viability and public safety.”
For more information, see Chris Paine’s blog: http://switchboard.nrdc.org/blogs/cpaine/us_nuclear_safety_regulators_c.html