An equilibrium core depletion algorithm for pebble-bed reactors in the Griffin code

We present a novel deterministic equilibrium core pebble-bed depletion algorithm. It differs from most other algorithms by distinguishing pebble age directly by burnup instead of using the number of completed passes through the core. Using burnup allows for a more physically realistic modeling of the pebble discharge process, especially for low pass cores. The major innovation of this algorithm is to define pebble fractions and number densities as a distribution over both space and burnup and then define an evolution operator for these quantities. The algorithm is implemented in the Multiphysics Object-Oriented Simulation Environment based reactor physics code Griffin. Numerical results show good agreement (eigenvalue to within 150 pcm, power peaking factor to within 0.2%) for the HTR-PM when compared to published Very Superior Old Programs (VSOP) results.