Background and Derivation of ANS-5.4 Standard Fission Product Release Model

摘要

This background report describes the technical basis for the newly proposed American Nuclear Society (ANS) 5.4 standard, Methods for Calculating the Fractional Release of Volatile Fission Products from Oxide Fuels. The proposed ANS 5.4 standard provides a methodology for determining the radioactive fission product releases from the fuel for use in assessing radiological consequences of postulated accidents that do not involve abrupt power transients. When coupled with isotopic yields, this method establishes the 'gap activity,' which is the inventory of volatile fission products that are released from the fuel rod if the cladding is breached. Best-estimate and conservative upper-bound 95/95 tolerance models for predicting the release of volatile radioactive isotopes of krypton, xenon, and iodine are described. The isotope that provides the most significant contribution to equivalent dose to individuals is generally I-131 for accidents that occur during in-reactor operation or shortly after reactor operation (e.g., the fuel-handling accident) due to its dose to the thyroid. The existing conservative upper-bound model for predicting the I-131 release has been shown to be very conservative in comparison to the available measured release data for I-131. The 2009 model proposed herein predicts up to a factor of 2 lower release than the old ANS 5.4 release model.