Fractional heat generation rates in Hanford reactors after shutdown

摘要

The knowledge of the fraction of decay-heat which is absorbed in fuel elements after a reactor is shutdown is important for many reasons. For example, allowable, reduced flow-rates after shutdown are very sensitive to the manner in which the decay-heat is distributed. Also, the temperature-rise in a discharged, uncooled fuel element is dependent on the total heat generated in the slug. Apart from any heat consideration, the escape of rays from a discharged fuel element is also of importance in certain applications. A significant refinement in the knowledge pertaining to decay-heat in irradiated uranium in recent years warrants a complete review of the fractional heat generation in the Hanford reactors. Earlier work was based on very qualitative aspects of fission product decay rates and energy spectra following reactor shutdown. The results of these early calculations on the original, solid, Hanford slug indicated that about 20 per cent of the energy generated in a fuel element from both fission product decay and delayed fission escaped from slug. The results reported here show a much smaller escape-fraction. Also, the time-dependence of the escape-fraction is considered for times greater than 100 seconds after reactor shutdown. Fractional heat-generation rates are calculated for various Hanford fuel types. The results, condensed to the maximum fraction of energy which escapes from the fuel elements over the period 10(sup 2)--10(sup 7) seconds after shutdown, are given in table below. Time-dependent Curves from 100 seconds to 10(sup 7) seconds after shutdown are given in the body of the report.