Simulation of criticality control in aqueous homogeneous reactor for Mo-99 production using MCNP

Tc-99m is very useful radioisotope in medical diagnostic procedure. Tc- 99m is produced from Mo-99 decay. Mo-99 can be produced in the aqueous homogeneous reactor. MCNP was used to study the aqueous homogeneous reactor reactivity control. We varied the height of uranyl nitrate (20% enrichment) solution and calculated the k_eff using MCNP. We identified the uranyl solution height that produces slightly super critical condition. The insertion height of control rod was varied and the MCNP code was run to calculate k_eff to ensure the reactor can be safely controlled. The case when the reactor core is completely filled with fuel solution was studied, and it was shown that the control rods are able to shut down the reactor. From MCNP simulation, we confirm that this reactor can be operated safely for long periods. The criticality of the reactor can be maintained using two methods: control the height of the liquid solution to address slow reactivity changes and the control rods for transient reactivity changes. (authors)