Modeling and Measurement of a Neutron Generator Driven Sub-Critical Assembly Used for Isotope Production

Abstract

The primary objective of this research was to provide a validated model of Global Medical Isotope Systems’ (GMIS) sub-critical assembly used for isotope production. This model would provide estimates of dose calculations, radioisotope production rates, and neutron flux and energy spectrum A system model was constructed using Monte Carlo N Particle Version 6.1 (MCNP). An initial MCNP model replicated lithium irradiations for tritium production GMIS performed in 2016. MCNP estimated a production rate of 2.259±0.021 Bq g-1 h-1 compared to GMIS’ reported rate of 1.48-1.96 MBq g-1 h-1. Exterior dose measurements were performed to provide initial validation of the MCNP model. The MCNP model and empirical dose agreed within 1σ error for three of five measurements; the other two agreed within a factor of 2. Further validation work was performed using activation wires and the neutron unfolding code SAND-II. The planned series of measurements was cut short by equipment failure and system decommissioning limiting available data. Integral fluxes from MCNP and SAND-II agreed within 20% for three of four cases, the fourth differed by a factor of 2. Using the MCNP model, the subcritical multiplication factor was calculated to be 0.25430 ± 0.00090 corresponding to a system power of 9.78 ± 0.05 mW at standard operating parameters. This corresponds to a total 7 day 99Mo activity production of 11.70±0.02 MBq. This indicates that the system is not a viable method for producing the isotopes of interest as currently designed.