The method of selecting time interval in the calculation of subcritical α eigenvalue

Two methods are widely used to calculate the α eigenvalue for the moment. The first one is to search a α value satisfying the condition that k(α) is equal to 1, which is introduced to neutron transport equation in advance. It is frequently invalidated in subcriticality because that cross section is less than 0. The second one is called dynamic fit method. Neutron density with time N(t) is calculated by MCNP program before-hand, and then α value is fitted according to its definition α=dN/(Ndt). Some experience or repetitions are required to estimate the truncation time of neutron density and select the time interval during which α value is fitted. the answer can be badly affected by the unapt selection of time interval. This paper gives a means to solve the problem by the prompt neutron lifetime (τ). fitting α value between 30 τ and 50 τ is recommended in this paper. (authors)