Nuclear Technology / Volume 159 / Number 2 / August 2007 / Pages 134-146
Technical Paper / Fuel Cycle and Management
Current risk assessments of spent fuel in storage and transportation casks use the properties of light water reactor fuel below 45 GWd/t U. Fuel is being driven to higher burnups that may influence the source term in cask accidents. To achieve these burnups the manufacturers are introducing new assembly designs and cladding alloys. As a result, at the higher burnups (50 GWd/t U) some of the characteristics of the fuel pellets, cladding, and assembly design used in the safety analysis have changed. The fuel pellet has developed a fine-grained, Pu-rich rim zone on its exterior surface. The source term may increase by up to three orders of magnitude over that expected from the particulate size distribution based on the fracture of the body of the pellets. The actual increase will depend on the fracture characteristics of the rim and number of fracture sites in the cladding. The cladding may acquire hydrogen contents up to 700 parts per million by weight during the increased exposure. Embrittlement of the cladding with subsequent loss of ductility may occur, especially if there is hydride reorientation. As a result, there may be a greater propensity for fracture of the rods upon impact, with subsequent release of fuel particulate and gas. Significantly improved source terms can be developed if additional data on fuel rim fracture as a function of impact energy, the dependence of cladding ductility for Zircaloy and the newer cladding alloys as a function of hydride reorientation, and release characteristics for fractured rods are obtained. Chalk River unidentified deposit spallation characteristics only make a significant contribution to the source term if the rods do not fracture in the accident or if a fire-only accident occurs.