Comparison of LWR Decay Heat Calculations Using ANSI/ANS-5.1-2014 Standard and SCALE

There is growing interest in the nuclear industry for deploying extended enrichment and high burnup fuel assemblies for commercial light water reactors (LWRs). One of the consequential parameters to be addressed for this new type of fuel is the decay heat emitted by fuel assemblies after discharge from the reactor, which is critical for short- and long-term fuel storage. Various computational tools are available to the nuclear engineer for use in decay heat calculations, and standards may also be used: the ANSI/ANS-5.1 decay heat standard offers a methodology for conservative estimations of LWR fuel assembly decay heat. A question of interest is how decay heat calculations using the ANS standard methodology compare to those from detailed neutronics modeling with computational tools such as those in the SCALE code system, both for typical LWR assemblies in operation now and for prospective extended enrichment high burnup assemblies. This work demonstrates consistently higher decay heat calculated with the ANS standard methodology relative to SCALE. Using generic assembly models as well as models of real assemblies from the SCALE decay heat validation basis, relative decay heat differences between the ANS standard and SCALE calculations for enrichments less than 5% range from 3 to 40% over cooling times from 3 days to 300 years. The ANS standard calculations yield higher decay heat relative to the SCALE calculations even as extended enrichment high burnup assemblies exceed the standard's accepted range of applicability. Note, however, that the ANS standard was designed for enrichments up to 5%, and the present study intended to provide some insight on the comparison outside the range of its stated applicability. Given commercial interest in LWR assemblies with enrichments and burnups beyond those investigated in this work or for which the ANS standard was designed, future investigation into the development of a standard decay heat calculation methodology for extended enrichment high burnup assemblies may be of interest.