Could the March 2011 disaster at the Fukushima nuclear power plant have been prevented? And have the lessons learned from the event been adequately assimilated by the US nuclear industry? Those questions were pondered at a 6 March symposium hosted by the Carnegie Endowment for International Peace.
One year after the accident, the US Nuclear Regulatory Commission is close to issuing several orders to US plant operators in response to lessons learned from Fukushima. One of the orders will require NRC licensees to implement “mitigating strategies” for accidents that exceed the worst case accident scenarios considered at the time the plants were built. Such “beyond design basis” events include the “station blackout” situation—the loss of all power at the plant, including that provided by emergency on-site generators—that faced the operators at Fukushima in the wake of the tsunami.
The US nuclear industry has proposed a mitigation strategy called FLEX to place portable emergency equipment—generators, battery packs, pumps, air compressors, and battery chargers—at multiple locations around the plants. A second layer of protection will be provided by placing additional emergency equipment in secure off-site locations, according to the industry plan. In the event of a station blackout or damage to the pumps that supply coolant to the reactor core, the plant would have the equipment on hand to effect emergency repairs, said Marvin Fertel, president of the Nuclear Energy Institute, the US trade group for nuclear utilities.
A similar approach was taken by US nuclear operators to mitigate the impacts of a potential terrorist following the 11 September attacks, said Richard Meserve, who chaired the NRC at the time. But little information about those measures was made public, and the possibility of flooding wasn’t considered to be a threat at that time, noted Fertel.
Arguing for a different approach, Christopher Paine of the Natural Resources Defense Council said US operators should be required to retrofit the plants with “robust in-plant systems” capable of supplying emergency power and cooling for days following a station blackout. The FLEX approach would require accident responders to lay and connect pipes “under highly stressful conditions,” he argued, and “involves a level of improvisation.”
The approach advocated by Paine is more in keeping with that taken by European regulators, who generally have required nuclear power plants to undergo expensive engineering modifications to enhance safety, noted a white paper authored by James Acton and Mark Hibbs of the Carnegie Endowment for International Peace. German reactors, for example, are required to have at least one standby grid connection and at least two diesel generators that are protected against external impacts. Many European plants also have gas-fired turbines on site to supply emergency power.
Fertel countered that nuclear plants can’t be engineered to meet every possible accident scenario and that trained and well-equipped operators should be capable of responding to all adverse events.
As for the threat of earthquakes, Paine asserted that 47 of the 104 US reactors do not meet their current seismic threat assessments. Fertel noted that the North Anna, Virginia, nuclear plant maintained safe shutdown following the August 2011 earthquake, which exceeded its seismic design requirement. But Paine noted that the plant was disconnected from the grid during the temblor and that three of the plant’s four emergency generators were leaking and one had to be shut down.
Both Fertel and Paine agreed that concerns about the spent fuel pools at the stricken Fukushima plant proved to be unfounded. Fertel said that the lack of information available about the pools led to speculation that some of the spent fuel may have become uncovered and was contributing to the radiation released. One of the new NRC orders will require operators to install instruments for monitoring the water level in the pools. That will help prevent the pools from becoming a distraction for operators who are coping with an event in the reactor, he noted. Paine said that the amount of spent fuel kept in the pools should be minimized, in view of its vulnerability to a terrorist attack or a tornado. Once sufficiently cooled, the spent fuel should be placed into dry casks for storage.
Acton and Hibbs argued that the Fukushima Daiichi disaster could have been avoided had the operator implemented best practices adopted in other countries to guard against the threats of severe floods, station blackout, and the loss of the ultimate heat sink (seawater, in the case of Fukushima). The Tokyo Electric Power Co (TEPCO), which operated Fukushima, and the Nuclear and Industrial Safety Agency (NISA), its regulator, should have been prepared for a tsunami of the magnitude generated on 11 March 2011, because tsunamis of that size occur about once every 1000 years. The white paper noted that computer simulations performed in 2008 suggested the tsunami risk to the Fukushima power plant had been seriously underestimated, but those findings weren’t reported to NISA until 7 March 2011. Fukushima was not designed to withstand a tsunami even half the size of the estimated 14- to 15-meter wave that struck the plant four days later, their report stated. Best practices promulgated by the International Atomic Energy Agency call for plants to be protected against extreme seismic events that may occur only once every 10 000 years.
NRC Commissioner George Apostolakis agreed that the Japanese accident “was not unthinkable and not unforeseen.” Risk calculations based on a once-in-a-thousand-years occurrence are “intolerable” from the NRC’s perspective, he said, and the commission would not have allowed Fukushima to operate had it been the regulator.
Acton and Hibbs asserted that TEPCO and NISA were aware of European practices such as protecting diesel generators and batteries by moving them to higher ground or placing them in watertight bunkers, establishing watertight connections between emergency power supplies and key safety systems, and improving protection for seawater pumps that are used to transfer heat from the reactors and to cool diesel generators. The seawater pumps at Fukushima were flooded and rendered inoperable by the tsunami.
Takuya Hattori, president of the Japan Atomic Industrial Forum, said the public’s trust in the nuclear industry has been lost since Fukushima, with 70% of Japanese citizens surveyed in opinion polls saying they do not support nuclear power. Currently, just two of Japan’s 54 reactors are operating, because local government officials refuse to grant permission to restart reactors until further safety checks are made. Prior to the 2011 disaster, Japan’s policy was to increase its reliance on nuclear energy from the current capacity of 30% of electric demand to 50%. Now, the government wants to decrease reliance on nuclear, but to what level has yet to be determined, Hattori said.
