Getting credit
With burn-up credit, nuclear fuel casks designed for truck shipment can carry twice as much spent fuel, and casks designed for rail shipment can carry up to one-third more spent fuel.
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When nuclear fuel is irradiated, or "burned," in a reactor, its reactivity drops as the uranium fuel is consumed and fission products accumulate. The less reactive the burned, or spent, fuel removed from the reactor, the less likely that the fuel can go critical, or sustain a chain reaction.
Until recently, however, the licensing approach used to certify spent fuel transportation packages did not use the credit from the reactivity decrease caused by burn-up (i.e., burn-up credit). ORNL researchers and others have shown that, if burn-up credit were utilized, spent nuclear fuel could be safely packed in more dense arrays, dramatically reducing the cost of transporting, storing, and disposing of spent fuel from nuclear power plants.
"With burn-up credit, casks designed for truck shipment can carry twice as much spent fuel, and casks designed for rail shipment can carry up to one-third more spent fuel," says Cecil Parks of ORNL's Nuclear Science and Technology Division. "You could put 32 spent fuel assemblies in a rail cask with burn-up credit but only 24 spent-fuel assemblies without."
In 1999, the Nuclear Regulatory Commission (NRC) asked Parks to lead a project to investigate and resolve outstanding technical issues related to burn-up credit and then provide a recommended basis for issuing regulatory guidance on the subject. The project combined ORNL staff experience in cask design and operations, reactor physics, criticality safety, and regulatory practice.
An ORNL-developed code system called SCALE, for "Standardized Computer Analyses for Licensing Evaluation," was used to help understand how reactivity decreases in spent fuel vary with reactor conditions, time after discharge from the reactor, the level of knowledge regarding the spent fuel isotopic contents, and cask conditions. Parks says, "We have clarified technical issues and impacts on cask design of burn-up credit and the amount of inventory allowed in the cask."
In September 2002, after ORNL completed its technical documents, NRC published new guidance that established recommended approaches for applicants to use when requesting burn-up credit as a part of the safety basis for cask designs. Currently, a cask vendor is seeking a license from NRC that asks for burn-up credit, referencing the ORNL documents.