Cladding Hoop Stresses in Spent PWR Fuel – Determination of Rod Internal Pressure as Initial Condition for Dry Storage

The end-of-reactor-life (EOL) rod internal pressure (RIP) is the primary protagonist for several evolutionary changes during long-term dry storage that affect cladding resistance to failure when spent fuel assemblies are subjected to normal and accident conditions of transport. At the maximum temperature attained either during vacuum drying or dry storage, EOL RIP determines the maximum stress state in the fuel rod cladding, which in turn sets the initial conditions for potential time-dependent changes in the hydrides structure in the cladding as the latter undergoes slow cooling in the inert environment of the storage cask. An extensive literature search was conducted for pressurized water reactor (PWR) standard and Integral Fuel Burnable Absorber (IFBA) fuel to collect RIP and void volume data for the entire spent fuel population of Zircaloy- 4, ZIRLO and M5 cladding designs. Only the non-proprietary data set was analyzed; no IFBA data were retained because only a small number of proprietary data points were identified. Use of bounding values for EOL RIP, would typically be of limited value, given that such values must be modified for the temperature and void volume distribution in the fuel rod that exists during vacuum drying or at the beginning of dry storage. Using the non-proprietary data set, a simple model was developed for calculating rod internal pressure, which can be derived from the universal gas law using rod gaseous contents (number of moles), void volume, and temperature profiles compatible with dry storage operations and conditions. Examples of calculations of initial stress cladding conditions for spent PWR fuel stored under vacuum inside a vertically positioned cask are provided taking advantage, in a simplified manner, on experimentally determined temperature profiles inside the storage system. For rod-average burnup of less than ~60 GWd/MTU, the examples show that initial cladding stress conditions are estimated to be less than 100 MPa for the vast majority of spent PWR fuel for which non-proprietary data are available.