Thank you for your letter addressing public fears about the safety of nuclear power plants in the US after the catastrophic failure of three nuclear reactors at Fukushima 1 (Daiichi) nuclear power station in Japan in the wake of the Tohoku-Oki Earthquake Mar. 11, 2011.
We wish to respond to two issues raised in your letter that remain of fundamental concern:
1) “All U.S. nuclear power plants are built to withstand external hazards, including earthquakes, flooding, and tsunamis, as appropriate.”
How can we be certain that the measures implemented in the US are appropriate? We have not even fully uncovered the causes for the reactor accidents at Fukushima Daiichi nuclear power station yet.
The loss of off-site alternating current (AC) power and the early loss of emergency diesel generators may represent major causes underlying the accident. However, the reactor protection systems of unit 1, as well as of units 2 and 3, are supposedly designed such that the reactor can be cooled and its core covered with water for eight hours, using direct current battery power and decay heat alone. Both were still available for several hours at Fukushima after the loss of all AC power. Despite, lack of coolant caused core melt-down at unit 1 within four hours. Therefore, the extended loss of AC power and subsequent attempts of restoration seem to have contributed significantly to the rapid unravelling of reactor protection system controls. How can the NRC claim that reactors of similar design are safe in the US, considering that we do not fully understand the causes instrumental to the catastrophic reactor failures at Fukushima?
2) “ The NRC believes that it is highly unlikely that a similar combination of events such as those which occurred in Japan could occur in the United States…”
Since a different combination of similar events cannot be ruled out, this belief seems unwarranted, particularly because the probabilistic risk assessment (PRA) methods commonly used in nuclear reactor safety analyses may fail to realistically estimate the risks associated with the concomitant occurrence of at times independent events with non-linear dynamics.
For further explanation, we cite from the criticism of probabilistic risk assessment published on wikipedia:
“Theoretically, the probabilistic risk assessment method suffers from several problems:
Nancy Leveson of MIT and her collaborators have argued PDF that the chain-of-event conception of accidents typically used for such risk assessments cannot account for the indirect, non-linear, and feedback relationships that characterize many accidents in complex systems. These risk assessments do a poor job of modeling human actions and their impact on known, let alone unknown, failure modes. Also, as a 1978 Risk Assessment Review Group Report to the NRC pointed out, it is “conceptually impossible to be complete in a mathematical sense in the construction of event-trees and fault-trees … This inherent limitation means that any calculation using this methodology is always subject to revision and to doubt as to its completeness.“
In the case of many accidents, probabilistic risk assessment models do not account for unexpected failure modes:
At Japan’s Kashiwazaki Kariwa reactors, for example, after the 2007 Chuetsu earthquake some radioactive materials escaped into the sea when ground subsidence pulled underground electric cables downward and created an opening in the reactor’s basement wall. As a Tokyo Electric Power Company official remarked then, “It was beyond our imagination that a space could be made in the hole on the outer wall for the electric cables.”
When it comes to future safety, nuclear designers and operators often assume that they know what is likely to happen, which is what allows them to assert that they have planned for all possible contingencies. Yet there is one weakness of the probabilistic risk assessment method that has been emphatically demonstrated with the Fukushima I nuclear accidents — the difficulty of modeling common-cause or common-mode failures:
From most reports it seems clear that a single event, the tsunami, resulted in a number of failures that set the stage for the accidents. These failures included the loss of offsite electrical power to the reactor complex, the loss of oil tanks and replacement fuel for diesel generators, the flooding of the electrical switchyard, and perhaps damage to the inlets that brought in cooling water from the ocean. As a result, even though there were multiple ways of removing heat from the core, all of them failed.
- M. V. Ramana (19 April 2011). “Beyond our imagination: Fukushima and the problem of assessing risk”. Bulletin of the Atomic Scientists.”
Taken the above concerns into account, we recommend that the NRC initiate re-analyses of PRAs of all plants with reactors similar to those at Fukushima Daiichi nuclear power station to include the common-cause, common-mode and special-cause variations which led to the accidents at Fukushima Daiichi. In addition, the utilities should demonstrate to the NRC that the results of the PRAs based on the revised conditions do not raise risks to the safe operation of the plants or unacceptable increases in radiation dose to the public. Should the results of the analyses require improvements, modifications or additional redundancies, each utility should submit the list with financial costs to the NRC which will complete a detailed review and recommend mandatory compliance to complete. Costs should not override the necessity for completing all of the recommended changes and those mandatory upgrades should be presented to congressional oversight to ensure their implementation.
If any of the PRAs indicate increased risks, probabilities and frequencies that approach safety limits, a review will determine actions up to and including operation at a reduced total reactor power until the required remedies are carried out and approved by the NRC. A reduced full power should be initiated until a clear understanding of the causes for the three catastrophic reactor failures is reached, most notably because each failure was anticipated to occur only once in ten-thousand years or less.
Thank you very much for your consideration.
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