Possible Origin of the Gamma-ray Discrepancy in the Summation Calculations of Fission Product Decay Heat

Abstract

In order to identify the origin of the ubiquitous and long-standing discrepancy seen in the γ-ray component of the FP decay heat in the cooling time range 300-3,000 s, a comprehensive analysis of the differences between the summation calculations and the experiments has been carried out. There may be some missing of the β-strength in the high energy region of the FPs in the mass region A=100-110. Especially, ¹⁰²Tc, ¹⁰⁴Tc, ¹⁰⁵Tc and ¹⁰⁸Rh are potentially responsible for the γ-ray discrepancy seen in the three major fissioning nuclides, ²³⁵U, ²³⁸U and ²³⁹Pu, systematically. The β-strength functions are theoretically calculated in order to validate this possibility. It is proved that the chance for finding the additional β-strength required to solve the discrepancy is not so large but still exists, and the exact β-feed from Tc to the highly excited levels in Rb should be identified experimentally. Finally, the impact of the β-ray discrepancy on the reactor-core decay heat is evaluated quantitatively for the first time. It may introduce 0.5–1% underestimation of the total FP decay heat from 1 to 2,000 s after reactor shutdown, and the underestimation may reach 3% at the maximum when the γ-ray component of the decay heat is separately taken into consideration.

KEYWORDS:

• decay heat
• after-heat
• fission products
• beta decay
• beta-strength function
• gross theory
• branching ratios
• fission yield
• loss of coolant