Decay heat calculations based on theoretical estimation of average beta- and gamma-energies released from short-lived fission products

Fission product decay heat of 235U was calculated for short cooling-time on the basis of a preliminary version of a new decay data library recently completed by the Japanese Nuclear Data Committee. It was shown that a full adoption of recent publications of decay schemes to derive average energies of β- and γ-rays, average E sub(β) and average E sub(γ), leads to a large underestimation of the γ-ray component of the decay heat and to an overestimation of the β-ray component. In order to remedy this, theoretical values of average E sub(β) and average E sub(γ) were introduced for high Q-value decays, which were obtained with a gross theory of β-decay. It improved remarkably the agreement between calculation and experiment not only for the 235U decay heat but for 239Pu and 241Pu as well. It was concluded that a large part of decay schemes recently published for high Q-value nuclides are inappropriate to use in calculations of average E sub(β) and average E sub(γ), because they fail to reproduce the effect of β-strengths at high excitations, which makes average E sub(β) small and average E sub(γ) large. The use of the gross theory introduces this effect correctly into the values of average E sub(β) and average E sub(γ) and, hence, leads to a quite good prediction of both β- and γ-ray components of the decay heat. (author)