(November 19, 1993) One of the effects of having increased quantities of spent fuel in storage at older reactors is that opponents begin paying more attention to fuel handling procedures, operating hazards at storage pools, and other features of fuel management within the reactor complex.
(402.3914) WISE Amsterdam - An example is the Palisades reactor in Michigan (US), where a unique type of vertical concrete dry cask, 2.6 meters in diameter and 6 meters tall, is being deployed. At US $300,000 each, these VSC casks are relatively inexpensive, but concrete is not a good heat transfer material, and opponents have complained that this cask has been designed to operate within 4°F of safe thermal limits.
Eight of these casks have been fabri-cated at Palisades so far, and three have been loaded and set out on a concrete pad 150 meters from the shore of Lake Michigan. Citizen opposition has increased to the point that Frank Kelley, the Michigan attorney general, has joined the battle and has called for a national task force of state officials to deal with spent fuel issues.
One hotly contested legal concern is that, in the absence of any federal backup system for managing spent fuel beyond these at-reactor projects, local utilities and their ratepayers are becoming permanent custodians of growing High Level Waste inventories. This issue has moved through the courts and into the legislature in the neighboring state of Minnesota, where the operators of the Prairie Island nuclear plant are seeking to place at least 17 spent fuel casks on the shore of the Mississippi River. The casks in this case are 100-ton steel TN-40 models made by Transnuclear. It is known that the plant operators want to expand the project to include at least 90 casks. Even larger projects are under way at a dozen other US reactors.
At Palisades, however, citizen monitoring has revealed that two serious fuel incidents occurred during refueling this summer. On 30 June plant operators discovered that a fuel rod had dropped out of its assembly to the bottom of the defueling cavity near the pressure vessel. The 5-meter long rod had developed a lengthwise split two meters long and all but a few of the uranium pellets had fallen to the bottom of the cavity. Most, but not all, of the highly radioactive pellets were recovered.
Then, on 8 July another fuel assembly that was being lifted from the core became stuck for two days to the Upper Guide Structure. This incident had no relation to the broken rod event. However, when an Augmented Inspection Team from the Nuclear Regulatory Commission arrived onsite, analysis of the root cause of the two events began.
One condition prevailing within the Palisades core that led to the incidents was the operators' practice of keeping old assemblies in the core for as many as five refueling cycles, or two cycles beyond the maximum length of stay in the core for which the fuel was designed. The reason for this practice was a strategy of moving older assemblies to the outside periphery of the core in order to protect the aging and progressively embnittled pressure vessel. The NRC's analysis also attributed the fuel problems to the fact that Siemens Power, the fuel manufacturer, has had little experience with fuel that has been through so many cycles of re-use. (Siemens has been making major inroads into fuel fabrication in the US. In September, the company won four new contracts worth US$124 million and has begun a $20 million expansion of its plant at Hanford, Washington.)
In an inspection report released by the NRC on 31 August it is also clear that the Palisades operators have instituted a high fuel burn up policy to limit the heat and radiation load on the spent fuel pool. With progressive re-racking of fuel assemblies closer together in the pool, fuel managers had discovered that at Palisades, as at about 30 other US reactors, the boron shielding placed between assemblies has been degrading under heavy neutron and gamma radiation. Degradation of these shielding strips, known by the manufacturer's name of Boraflex, has caused serious variations in criticality calculations in spent fuel pools at other US reactors (e. g., Millstone, South Texas). In heavily loaded pools, failure of Boraflex shields can take the fuel inventory from an 'acceptable" level of 96 percent critical on up to 98 percent of full criticality. 100 percent would mean the startup of a sustained fission chain reaction in the pool.
Source: US NRC, Inspection Report 50-255, 31 Aug. 1993.
Contact: Palisades Watch, 925 Davis Street, Kalamazoo MI 49008 USA; tel: + 1-616-345-0360.