Urasoko Fault Might Not Be a Line in the Sand

Tsuruga Nuclear Power Plant, at 43, is the oldest nuclear power plant in Japan and is being investigated for active fault lines as part of a safety overhaul following the Fukushima Daiichi disaster. Last month the Nuclear Regulatory Agency (NRA) — Japan’s newly established nuclear watchdog — determined that no nuclear plants could be built on active faults. Under the determination, an “active” fault means one that has shifted in the past 120,000 to 130,000 years. Proving that there has been no seismic activity in the intervening years is difficult and has caused a debate between the NRA and the plant’s operator, Japan Atomic Power Company, over whether Tsuruga’s Unit 2 is indeed sitting on a fault line.

The Asahi Shimbun reports that the fault known as D-1 is located directly under the No. 2 reactor building. Japan Atomic Power officials argue that the D-1 fault was not active because there were no signs of recent movement. However, last December experts working with the NRA detected signs of more recent ground movements at one of the trench sites.

The experts also noted that movement along the nearby Urasoko fault could trigger earthquake activity on the D-1 fault, and the Urasoko fault is known to move every few thousand years. The D-1 fault is made up of earth slippages along several geological layers, which indicates that it has moved in the past, according to experts. For those reasons, the experts’ panel concluded that the D-1 fault was active, but did not conclusively rule out the possibility of restarting the Tsuruga plant.

Japan Atomic Power officials have commissioned an independent review of the available seismic data. There are two teams of reviewers — among them well-known experts in earthquake engineering, risk, geoscience and nuclear power — who specialize in earthquake hazard. The first investigation took place in March, and resulted in a request by independent reviewers for new data on the geology of the site. The two review teams completed the second round of evaluations and presented their findings on August 1.

The teams report that they inspected the rock formations and fractures in the trenches, outcrops and drill cores of the Tsuruga site. The group determined that the two slips within the D-1 fault have not moved in the last 120,000 to 130,000 years. While the independent group was able to establish that Japan Atomic Power sufficiently addressed the specific concerns of the NRA, the data was not a comprehensive geological investigation of all possible seismic hazards that could affect the Tsuruga power plant.

The independent review group recommends that the NRA and Japan Atomic Power consider continuous improvements on the safety evaluation and management of the nuclear power plant, using the Kaizen philosophy. This management should include a living safety assessment, which is continuously updated as new data and techniques arise, and it should follow the recommendations of the International Atomic Energy Agency’s standard safety best practices.

Asahi reports that the NRA panel of experts plans to resume discussions if new evidence is uncovered that casts doubt upon the conclusion that the D-1 fault is active. To date there are no reports that the NRA will reopen the discussion surrounding the Tsuruga fault, but members of the independent group are optimistic.

Public opinion still stands against nuclear power operations, with most voices telling the Japanese government to cut ties to nuclear and move on. The Asahi Shimbun opined that the Tsuruga Power Plant should be closed based on the NRA’s original opinion. While the fear of nuclear explosion and radioactive poisoning captures the worst aspects of the human imagination, there is no scientific evidence that Tsuruga’s nuclear power plant is in danger of doing so.