Limits on subcritical reactivity determination using rossi-alpha and related methods

The subcritical reactivity, ρ, of a neutron multiplying system is of interest to many factions of the nuclear community. The subcritical reactivity in a nuclear system can be inferred from measurements of the prompt neutron decay constant, α. Near critical, the reactivity is linearly proportional with α, but the linearity between these quantities does not hold in systems with low multiplication or significantly above prompt critical. This work examines a large range of subcritical prompt neutron decay constant measurements on a thermal HEU metal system. This system is designed to mimic a solution system and to have the optimal H/235U ratio to minimize the critical mass. The reactivity of these measurements varies between 0$ and 70$ below critical. In addition to experimental measurements on the thermal system, simulations are performed on both the thermal HEU system and a fast HEU metal system. Both the fast and thermal results are compared to baseline simulations performed using criticality eigenvalue (k-code) mode in MCNP R6.2. These simulations are used for comparisons on the linearity between α and ρ. Prompt neutron decay constant measurements in this work are performed using the Rossiα method. The measurements are taken using a 3He neutron detection system and a Los Alamos National Laboratory designed list-mode data acquisition system. A specially designed computer program determines α from list-mode data. The α-eigenvalue, or α at delayed critical, for the thermal HEU system is −199.4±4.4 s−1 as measured during this work. The α-eigenvalue for the fast HEU system was previously measured to be −8.5 × 10−5 s−1. The results from the thermal HEU system study suggest that the relationship between ρ and α is linear when keff > 0.80 based on the simulation results. The experimental results also agree with this result, but have less fidelity. No experimental results are available for the fast HEU system