PNNL

Abstract

Current risk assessments of spent fuel in storage and transportation casks use the properties of LWR fuel below 45 GWd/MTU. 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/MTU) 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 1 – 3 orders of magnitude depending on the fracture characteristics of the rim. The cladding may acquire hydrogen contents up to 700 wppm during the increased exposure. Embrittlement with subsequent lose 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 were obtained. CRUD 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.