The Nuclear Regulatory Authority has broadened the definition of active geological faults that is used in the review of the safety of nuclear power plants. Until this point, faults that have shifted in the past 120,000 to 130,000 years have been regarded as active.
Now the NRA says that faults that have shifted at least once in the past 400,000 years should be labeled as active. This new definition will be incorporated into the NRA’s new post-Fukushima safety standards, which are expected to go into force next July.
This change strengthens one of the main pillars of nuclear safety standards. Even under current standards it is prohibited to build reactors and other critical nuclear facilities on active fault lines. Given how quake-prone Japan is, the NRA’s broadening of the definition of active faults is a welcome step in the right direction.
The NRA’s new definition of active faults will likely result in significant changes. It will now be necessary to adopt new assumptions concerning future large earthquakes and to reinforce the structures of nuclear power plants. Furthermore, plants that are found to be located on active faults will be subject to calls for decommissioning.
On the basis of the new definition, an NRA team headed by Mr. Kunihiko Shimazaki, a seismologist and professor emeritus at the University of Tokyo who serves as the NRA’s deputy chairman, has begun to conduct on-site examinations of the geological characteristics of the sites of several nuclear power stations, including Kansai Electric Power Co.’s Oi plant, Japan Atomic Power Co.’s Tsuruga plant, Kepco’s Mihama plant and the Japan Atomic Energy Agency’s fast-breeder reactor Monju, all in Fukui Prefecture, Hokuriku Electric Power Co.’s Shika plant in Ishikawa Prefecture, and Tohoku Electric Power Co.’s Higashidori plant in Aomori Prefecture.
In the past, experts took part in on-site examinations under the now-defunct Nuclear and Industrial Safety Agency of the trade and industry ministry, but bureaucrats made the final judgments. It is hoped that this time final decisions will reflect the experts’ views.
It is important for the NRA to study the geological characteristics in and around the sites of nuclear plants from scratch, free from the influence of past examinations carried out under NISA. Toward this end, the NRA has excluded experts who took part in geological examinations under NISA. This is a reasonable step. The NRA should also consider expanding the scope of the examinations if other experts or even ordinary citizens raise suspicions that active fault lines exist below or near other nuclear plants.
NRA chairman Shunichi Tanaka said if nuclear plants are judged “black” or “strong gray” in terms of their proximity to an active fault line, the NRA will decide to halt their operation. From the viewpoint that priority should be given to ensuring public safety, this policy should be strictly enforced.