The availability of the Monte Carlo-based code MCNP 4A has made possible the simulation of the low-enriched uranium (LEU)-fueled SLOWPOKE-2 reactor using a probabilistic approach. The reactor core and its surrounding pool can be modeled in three dimensions with numerous details included in the representation. Significant improvement from previous modeling attempts was obtained with the MCNP 4A simulation, with the discrepancy between the calculated and experimental values of the excess reactivity at 20°C reduced to only 0.2 mk. The analysis suggests the error of the MCNP 4A-calculated excess reactivity as between 1 and 2 mk.

The SLOWPOKE-2 reactor was then simulated with its single control rod at various degrees of insertion in the core: The reactivity worth of the rod was calculated as 7.85 mk, only 2.4 mk above the measured value. MCNP was then used for predicting the temperature effects on the excess reactivity. Although the inherent safety of the SLOWPOKE-2 reactor was confirmed in the simulation, the temperature dependence of the excess reactivity could not however be accurately predicted, due for the most part to the lack of appropriate cross-section libraries available at the time of this work. The potential of MCNP 4A is nevertheless clearly demonstrated for the simulation of the LEU-fueled SLOWPOKE-2 reactor, once the missing cross sections become available for the low temperatures at which the reactor operates.