Three years after the tsunami that caused the meltdown at Fukushima Daichii, Japanese academics shared concern over the continued use of “absolute safety” measures in the country. The methods used by the Japanese Nuclear Regulatory Authority (NRA) rely upon the concept, where historical earthquake data is used to establish a standard which all nuclear plants are required to meet. However, these methods are considered to be excessively conservative, and may put several nuclear plants at risk of shutting down if they prevail.
Professor Koji Okumura of Hiroshima University explained that the NRA seismic activity standards do not allow for the use of probabilistic analysis methods, even following the accident at Fukushima Daichii. This differs from the United States, where probabilistic analysis methods have been used by the U.S. Nuclear Regulatory Commission (NRC) for characterizing seismic behavior since 1997.
The benefit of probabilistic analysis methods is that they allow for a technical judgment to be made on the effect of an uncommon hypothetical situation, such as an earthquake, on nuclear plant operation. Characterization of these uncertainties allow for regulators and plant operators to distinguish between real plant safety issues and issues which just appear to be unsafe. They may also allow for more economic operation of plants, as safety features can be applied where they are needed most.
Much of the panel discussion, which took place at the 2014 American Nuclear Society (ANS) Annual Meeting, focused on the technical aspects of characterizing the uncertainty of seismic behavior in regions which lack detectable faults, also called diffuse or background seismicity regions. Experts from the NRC encouraged Japanese regulators to embrace probabilistic analysis methods in order to prevent getting blindsided by more likely but smaller scale incidents that may occur.
They also recommended Japanese regulators to pursue establishing a committee of global experts. These experts could then assist in interpreting seismic analysis results and their effect on the design and operation of nuclear plants. This committee would follow the example of the Senior Seismic Hazard Analysis Committee, established by the NRC in the 1980’s.