Homogeneous Slowpoke Reactors for Replacing SLOWPOKE-2 Research Reactors and the Production of Radioisotopes

Abstract

Inspired from the inherently safe SLOWPOKE-2 research reactor, the Homogeneous SLOWPOKE reactor was conceived with a double goal: replacing the heterogeneous SLOWPOKE-2 reactors when they reach end-of-core life to continue their missions of neutron activation analysis and neutron radiography at universities, and to produce radioisotopes such as ⁹⁹Mo for medical applications. A homogeneous reactor core allows a much simpler extraction of radioisotopes (such as ⁹⁹Mo) for applications in industry and nuclear medicine.

The 20 kW Homogeneous SLOWPOKE reactor was modelled using both the deterministic WIMS-AECL and the probabilistic MCNP 5 reactor simulation codes. The homogeneous fuel mixture was a dilute aqueous solution of Uranyl Sulfate (UO₂SO₄) with 994.2 g of ²³⁵U(enrichment at 20%) providing an excess reactivity at operating temperature (40°C) of 3.8 mk for a molality determined as 1.46 mol kg^-1 for a Zircaloy-2 reactor vessel. Because this reactor is intended to replace the core of SLOWPOKE-2 reactors, the Homogeneous SLOWPOKE reactor core had a height about twice its diameter. The reactor could be controlled by mechanical absorber rods in the beryllium reflector, chemical control in the core, or a combination of both. The safety of the Homogeneous SLOWPOKE reactor was analysed for both normal operation and transient conditions. Thermal-hydraulics calculations used COMSOL Multiphysics and the results showed that natural convection was sufficient to ensure adequate reactor cooling in all situations. The most severe transient simulated resulted from a 5.87 mk step positive reactivity insertion to the reactor in operation at critical and at steady state at 20°C. Peak temperature and power were determined as 83°C and 546 kW, respectively, reached 5.1 s after the reactivity insertion. However, the power fell rapidly to values below 20 kW some 35 s after the peak and remained below that value thereafter. Both the temperature and void coefficients are significantly more negative than the corresponding coefficients in SLOWPOKE-2. The simulations of the reactor in transient states showed that the temperature and power levels attained never compromised the integrity of the reactor.