Security & Safety
The US Nuclear Regulatory Commission has rejected a petition by an anti- nuclear group that called for the agency to suspend the operation of the country’s 22 remaining General Electric boiling water reactors with a Mark I primary containment system, which are the same as Units 1, 2 and 3 at Fukushima-Daiichi.
An emergency enforcement petition collected by the Maryland-based Beyond Nuclear group and originally filed in April 2011 asked the NRC to suspend operating licences at the units.
The NRC’s decision was published in the Federal Register on 23 January 2015. The NRC concluded that many of the steps it is in the process of taking to implement the “lessons learned” from Fukushima-Daiichi are already addressing the concerns of Beyond Nuclear.
Had the group’s petition been successful, reactors that would have been shut down include the Tennessee Valley Authority’s Browns Ferry nuclear station in Alabama, Exelon Corp’s Dresden station in Illinois and Entergy Corp’s Pilgrim station in Massachusetts.
As a “minimal safety stopgap” the NRC should have at least required the Mark I reactors to install filters on their hardened vents so that if the vents have to be used to relieve pressure buildup to avoid a meltdown, radiation will not be released, Beyond Nuclear said in a statement commenting on the NRC’s decision.
So far, the NRC has stopped short of mandating the filters. Instead, the agency is requiring plants with designs similar to Fukushima-Daiichi to make modifications so the hardened vents can be operable even in extreme scenarios. For example, BWRs with Mark I or Mark II containments are installing equipment to allow their vents to be operated from the control room even if there is no external power.
The NRC said that in the case of Fukushima-Daiichi, operators were not able to vent the reactors and prevent a dangerous buildup of hydrogen within the containment buildings because flooding knocked out power sources for exhaust systems and other components used to remove the gas.
In addition, the venting system design at Fukushima-Daiichi may have increased the chance of meltdown because the venting pipes at some of the reactors were interconnected, allowing hydrogen to travel from one reactor to another. One of the modifications required by the NRC is that separate venting paths must be installed for each reactor.
In its decision, the NRC cited the modifications as evidence that the agency is already addressing the risks with the Mark I design. In addition, the decision mentioned other new rules from the agency that are meant to ensure that plants can run safety systems even if the station loses power.
For example, the industry is distributing backup power equipment to earthquake and flooding-resistant sites around the country. The goal of this effort, known as the FLEX programme, is to have all of the necessary equipment installed at 70 reactors by the end of this year.
In a memorandum issued in March 2012, the NRC said that if additional backup or alternate sources of power had been available to operate the containment venting system remotely, or if certain valves had been more accessible for manual operation, the operators at Fukushima-Daiichi may have been able to depressurise the containment earlier.
“This, in turn, could have allowed operators to implement strategies using low-pressure water sources that may have limited or prevented damage to the reactor core.”
The NRC said the events at Fukushima-Daiichi showed that reliable hardened vents at BWR facilities with Mark I and Mark II containment designs are important to maintain core and containment cooling.