Fission cross section ratios of U233, U235, and Pu239 and capture rates in Mn55 and In115 have been measured in an NRX type uranium metal rod irradiated in a heavy water moderated reactor. The results have been compared to the values obtained using a model designed for NRX burnup calculations. Agreement was obtained using an “incurrent” spectrum composed of a Maxwellian thermal component and an epithermal component of the form obtained from neutron spectrometer measurements. Changes in spectrum caused by the light water coolant were also studied.