Light Water Reactor Equilibrium Cycle Search Methodology for Assembly-Level Fuel Cycle Analysis

We have developed a light water reactor (LWR) equilibrium cycle search algorithm that is similar to the REBUS-3 fast reactor methodology but with depletion capabilities typically employed for LWR analysis. Our LWR methodology projects the original coupled nonlinear isotopic balance equations to a series of equations that are piecewise linear in time. Iterations are performed on microscopic reaction rates until the linearized isotopic balance equations yield an ultimate equilibrium state. We further reduce the computational burden associated with LWR analysis by approximating global depletion calculations with assembly-level, collision probability calculations performed by the CASMO-3 code. We demonstrate the benefits of our equilibrium cycle methodology by calculating the true equilibrium Pu inventory of two configurations: a heterogeneous assembly configuration that contains both low enriched UO₂ and mixed oxide (MOX) fuel pins and a homogeneous configuration comprising a 2 × 2 colorset arrangement of MOX and low enriched UO₂ assemblies. For each configuration our methodology yields a true equilibrium Pu inventory with only 12 CASMO-3 lattice physics calculations. As a validation, an inventory extrapolation technique is used to arrive at a quasi-equilibrium cycle for both LWR configurations. The extrapolated technique yields a similar Pu inventory and isotopic composition but requires 65 lattice physics calculations.