Theoretical treatment of the asymptotic behavior of fission product decay heat toward very short cooling-time

The asymptotic behavior of the FP decay heat has been studied toward the very short cooling-time range down to 0.1s after a fission event. For this purpose we apply the second generation of the gross theory of β-decay and estimate the decay properties of short-lived FP nuclides. In order to reproduce the recent measurements adequately, we introduce a new parameter named the Q₀₀-factor, and optimize it in terms of the calculation of the half-lives and the average β- and γ-ray energy releases per decay (E_β− and E_γ-values). The favorable value of the Q₀₀-factor is 1.0 for the half-lives and about 0.4 for the E_β− and E_γ-values. The E_β and E_γ-values thus calculated for the short-lived nuclides, whose β-decay Q-values are larger than 3–4 MeV, guarantee a good reproduction of asymptotic behavior of the measured decay heat toward the very short cooling-time range. Adoption of the present calculation to data files for the summation calculations is recommended from a practical point of view.