The US NRC has ruled that it will not push utilities to speed up transfer of spent fuel out of spent fuel pools and into dry storage.
The subject, which was a key post-Fukushima issue, was settled in a staff requirements note published in late May, after all of the commissioners voted in favour of the staff recommendation to cease work. NRC chairman Alison Macfarlane, however, expressed both assent and dissent to the proposals.
The issue of transfer of spent fuel was most prominent early on in the Fukushima crisis when it was thought that fuel in one of the ponds had become uncovered, risking an explosion, although this was later found to be not the case.
A study carried out by NRC staff and published in October 2013 (ADAMS number ML13256A342) examined the benefits of expedited transfer of spent fuel to dry cask storage. Among many other findings, the study reported that high-density racking makes no real difference to spent fuel pool safety because both types of racks contain the same amount of hotter, recently-discharged fuel.
"Analysis ... shows that for the scenarios and spent fuel pool studied, spent fuel is only susceptible to a radiological release within a few months after the fuel is moved from the reactor into the spent fuel pool. After that time, the spent fuel is coolable by air for at least 72 hours."
In a memorandum dated November 2013, NRC staff discussed how they drew on that study and other operating experience. Based on all of these sources, they concluded that expedited removal would provide only a minor or limited benefit, and that its expected implementation costs would not be warranted.
That study said: "a loss of heat removal from the SFP, which could be caused by a loss of electrical power, produces a slowly evolving event that could be mitigated with a high probability of success by plant staff and availabe equipment. Potentially more significant events involve coolant inventory loss resulting from a loss of pool integrity." It went on to say that although fuel uncovery and zirconium ignition and fire is conceivable, the outcome evolves relatively slowly, "with time for mitigative and/or protective actions to prevent a release or otherwise ensure public health and safety."
Macfarlane calls for further examination of 'larger issue'
In her comments, chairman Macfarlane said she approved of the plan to stop that work as part of the current post-Fukushima programme, in part for practical reasons: "It is not clear whether it is realistically possible to rapidly defuel several thousand assemblies from US plants across the nation within 5 years", she said. But she also said she did not approve of stopping examination of the larger issue. She said that she felt that the staff had not adequately explored the issue of spent fuel management in the pool, since they only considered seismic initiators. They did not consider alternate configurations such as 1x8 (one hot assembly surrounded by eight cool ones), as practiced at Peach Bottom, but only considered a 1x4 arrangement.
In the May staff requirements note, staff were directed to put together an information notice on the benefits of the 1x8 spent fuel pool configuration, and perform several other tasks.