The Nuclear Regulation Authority (NRA) is set to drastically broaden the definition of active faults to cover those that moved 400,000 years ago or later in its new nuclear plant safety standards, according to a draft outline.
Under current standards, those that moved from 120,000 to 130,000 years ago or later are recognized as such in its atomic power plant safety standards.
The nuclear plant regulator will finalize the outline by the end of this month and draw up the new standards on nuclear plants’ vulnerability to earthquakes and tsunami by July.
The draft, worked out by an NRA expert team, states that faults that repeatedly moved in recent years and have a possibility of moving in the future should be regarded as active faults when designing nuclear reactors’ quake-resistance. It then says “the activity of faults as early as approximately 400,000 years ago should be assessed.”
This is in line with the definition that the government’s Headquarters for Earthquake Research Promotion made in 2010 that faults that moved approximately 400,000 years ago or later are active faults.
The guidelines for seismic-resistance designing of nuclear plants that the government compiled in 2006 recognizes faults that moved 120,000 to 130,000 years ago or later as active.
However, there have since been several cases in which experts cannot specify when faults on the premises of nuclear power stations moved, prolonging assessments.
JRA Commissioner Kunihiko Shimazaki points to the need to change the definition of active faults in nuclear plants’ quake-resistance design. “In some cases, assessing whether faults on the premises of nuclear plants are active have been prolonged. Pointless discussions will disappear if the definition is reviewed.”
The government does not permit the construction of nuclear reactor buildings just above active faults.
The NRA secretariat incorporated the phrase, “Reactor buildings can be built above active faults as long as they do not pose a serious threat to the facilities’ safety,” in a draft of the safety standards it released in December 2012.
However, the phrase was deleted from the draft that the expert team worked out on Jan. 15 after Shimazaki voiced opposition to the inclusion of the phrase on the grounds that methods for assessing how such faults would affect the safety of nuclear plants have not been established.
The expert team is also considering incorporating the introduction of the assessment of active faults near nuclear plants into the outline it will complete by the end of this month. It will also consider specific methods for determining the maximum height of tsunami that each atomic power station can withstand.
– source mainichi.jp