The fast fission factor revisited

Artikel i vetenskaplig tidskrift, 2009

The concept and calculation techniques of multiplicities in nuclear safeguards are applied to the calculation of the traditional fast fission factor of reactor physics. The concept is the assumption that the original source neutrons from spontaneous or induced fission, and the further neutrons given rise through fast fission in the sample before leakage, are considered as being generated simultaneously with the source neutrons. The number distribution of the neutrons arising from such a "superfission" process will be different from that of the nuclear fission process. Concerning the mathematical treatment, in safeguards works the master equation approach is used to calculate the moments of such a distribution. Hence, to follow suite, a derivation of the fast fission factor is given here by a backward master equation. This method has the advantages that the derivation of the fast fission factor becomes more transparent than the traditional one, and that it allows also to determine higher order moments, notably the variance, of the total number of neutrons generated, i.e. when account is taken also of the contribution of fast fission to these moments. The results show that the relative standard deviation increases fast with the increase of the non-leakage probability of neutrons, and hence with the increase of the fast fission factor itself. Also the Diven factor of the superfission process (neutrons from fast fissions included) is significantly larger than that of thermal fission. We argue that the traditional model, in which the Feynman- and Rossi-alpha models are derived, does not account correctly for the extra branching represented by the fast fission process. Hence the Diven factor traditionally used in those formulae should be used in a modified form. We show how the effect of fast fission needs to be included into the model to obtain the correct formula, and give explicit expressions. Some quantitative examples are given for illustration.