The energy release from fission products

Abstract

A simple method has been used to estimate mean beta and gamma decay energies of the short lived fission products, for which only half-life measurements and total decay energy estimates are available. This has enabled calculations to be made of beta and gamma decay heating from fission products which are in agreement with experimental results to better than 10 per cent for decay times as short as 1 s. Calculated beta and gamma decay energy following a long irradiation of 335U (thermal fission), is found to reproduce to better than 3 per cent a currently accepted curve of beta + gamma decay energy (SHURE, 1961) following an infinite irradiation of ²³⁵U, for decay times 1 ≤ t ≤ 10⁸ s. These calculations also gave values $\text{B (∞, 0)=6·60}\text{Me/fission}$, $\text{G (∞, 0)=6·56}\text{Me/fission}$, $\text{D (∞, 0)=21·44}\text{Me/fission}$for the total beta, gamma and total kinetic energy release respectively, from thermal neutron fission of ²³⁵U. A study of the calculations of ENGLAND (1970) and comparisons made, do not support his suggestion that the data of SHURE (1961), which is widely used in industry, underestimates beta + gamma decay energy by ∼ 20 per cent. This suggestion would have serious implications on reactor design criteria; however, it is implied on the basis of comparisons presented in this paper, that the results of ENGLAND (1970) over-estimate by ∼ 15–20 per cent the beta + gamma decay energy following fission for decay times 10 < t ≤ 10⁶ s.

Therefore the results of the present work reject the idea, implicit in the results of ENGLAND'S calculations, that reactor systems have been underdesigned with respect to decay heat removal.

In addition, the present calculations show that the code FISP, together with its data library, provide a reliable means of estimating fission product decay energy release at all decay times after fission.