Methods to determine burst repeatability for Godiva IV

Godiva IV is a fast burst critical assembly constructed of approximately 65 kg of highly enriched uranium (HEU) fuel alloyed with 1.5% molybdenum. It can be used to perform super-prompt critical bursts with temperature rises of up to 250 °C. In many cases, the 'repeatability', or relative change in radiation emission between bursts, is not important to the application. An upcoming shielding benchmark, however, will use pairs of bursts to determine the shielding effect of samples without relying on a determination of the absolute fluence. Therefore, one of three things is needed - the variation in relative source term needs to be small, the uncertainty needs to be known, or a method to normalize the source term between bursts is needed. Various methods for quantifying the source term exist. Temperature rise, activation product counting, and active detectors were used for a recent set of measurements across a set of twelve bursts to determine the relative change in the radiation emission between bursts. Analysis was performed by LANL, SNL, and PNNL on an independent selection of materials. The standard deviation in the highest quality data sets utilizing S, Ni, and Au was 2.7%. Furthermore, there was a high correlation between temperature rise and specific activity. An overview of historic data and the recent measurements will be presented. (author)