The nuclear industry watchdog is pitching new safety standards on earthquake and tsunami preparedness, which could substantially delay restarts of some idled nuclear plants.
The draft outline of the new standards, presented on Jan. 29 by an expert panel of the Nuclear Regulation Authority, calls for broad measures against the highest possible tsunami.
The highest possible tsunami, called the "reference tsunami," will be set for individual nuclear plants.
Operators will be required to build levees high enough to defend nuclear plants from a reference tsunami and take measures to prevent damage from possible flooding, such as waterproofing buildings that house key equipment.
The draft outline also calls for considering volcano collapses and landslides, in addition to earthquakes, as possible triggers for a tsunami.
The new safety standards are expected to go into effect by July 18. The NRA will use the new standards to screen applications for restarting reactors shut down after the Fukushima nuclear crisis or building new ones.
Utilities have been erecting levees and introducing anti-quake measures at nuclear plants since the accident at the Fukushima No. 1 nuclear power plant.
But some could be forced to take additional measures to defend nuclear plants from a reference tsunami, which would substantially delay their restarts.
The existing earthquake-resistance guidelines for nuclear plants barely mention tsunami because they are merely defined as a phenomenon accompanying an earthquake.
The former Nuclear Safety Commission proposed revisions to the guidelines after the Fukushima No. 1 nuclear power plant was crippled by a tsunami spawned by the March 2011 Great East Japan Earthquake. The NRA, which took over the commission, has been discussing the new standards since autumn.
According to the draft outline, operators may have to consider back to 400,000 years in the past when they determine whether a fault line is active and its seismic risk should be assessed for a nuclear plant.
Currently, faults that have shifted during the past 120,000 to 130,000 years are defined as active.
The draft outline calls for seismic activity up to 400,000 years ago to be examined if a definite decision cannot be made based on data for the past 120,000 to 130,000 years.
The new requirement could affect safety evaluations for the Kashiwazaki-Kariwa nuclear plant in Niigata Prefecture and the Tomari nuclear plant in Hokkaido, where experts say faults may have shifted during the past 400,000 years.
The draft outline also calls for considering the possibility that fault lines close to each other may move in tandem.
According to the draft outline, operators must allow sufficient leeway in estimating seismic movements if an active fault runs in close proximity to key facilities such as reactor buildings. An example of such a fault line is the Urasoko Fault at the Tsuruga nuclear plant in Fukui Prefecture.
The draft outline also clearly says key facilities cannot be built right above a fault line that could shift.
According to the draft outline, operators will be required to conduct detailed studies of subsurface structures that affect how seismic waves are transmitted.
The requirement has been included because localized strong shaking has been observed at the Hamaoka nuclear plant in Omaezaki, Shizuoka Prefecture, and other facilities.
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