The US Nuclear Regulatory Commission has issued Interim Staff Guidance (ISG) to US nuclear power plants to ensure proper implementation of three Orders the agency issued in March, in response to lessons learned from the Fukushima Daiichi nuclear accident.

The ISGs represent acceptable approaches to meeting the Orders’ requirements before their 31 December 2016 compliance deadline, NRC said. Although the ISGs are not mandatory, US nuclear power plants would have to seek the regulator’s approval if they wanted to follow a difference compliance approach.

The first Order requires all US plants to better protect portable safety equipment put in place after the 9/11 terrorist attacks and to obtain sufficient equipment to support all reactors and spent fuel pools at a given site simultaneously. The ISG for this Order endorses the industry’s updated guidance for dealing with a scenario in which all of a plant’s alternating current electric sources are knocked out. The updated approach includes the use of backup power supplies for devices that would be able to burn off any hydrogen generated by the accident before it could accumulate to explosive levels.

The second Order applies only to US boiling-water reactors that have “Mark I” or “Mark II” containment designs. Mark I reactors must improve installed venting systems that help prevent core damage in the event of an accident; Mark II reactors must install these venting systems. The ISG for this Order provides more detailed technical information on the vents, as well as how vent designs and operating procedures should avoid, where possible, relying on plant personnel taking actions under hazardous conditions.

The third Order requires all plants to install enhanced equipment for monitoring water levels in each plant’s spent fuel pool. The ISG defines in more detail the water levels the new equipment must accurately report, as well as standards for equipment mounting, powering and testing, personnel training and other criteria. The final ISG notes several areas, including instrument qualifications and instrument protection from falling debris, where the industry revised its initial approach. An exception in the staff’s endorsement sets specific seismic criteria to ensure the instruments will survive an earthquake.