Abstract
Monte Carlo–based depletion tools used for the high-fidelity modeling and simulation of the
High Flux Isotope Reactor (HFIR) come at a great computational cost; finding sufficient
approximations is necessary to make the use of these tools feasible. The optimization of the neutronics
and depletion model for the HFIR is based on two factors: (i) the explicit representation of the involute
fuel plates with sets of polyhedra and (ii) the treatment of depletion mixtures and control element
position during depletion calculations. A very fine representation (i.e., more polyhedra in the involute
plate approximation) does not significantly improve simulation accuracy. The recommended
representation closely represents the physical plates and ensures sufficient fidelity in regions with high
flux gradients. Including the fissile targets in the central flux trap of the reactor as depletion mixtures
has the greatest effect on the calculated cycle length, while localized effects (e.g., the burnup of
specific isotopes or the power distribution evolution over the cycle) are more noticeable consequences
of including a critical control element search or depleting burnable absorbers outside the fuel region.
