Direct Doppler broadening in Monte Carlo simulations using the multipole representation

• We propose a new approach for direct Doppler broadening of nuclear data for Monte Carlo simulations.
• Multipole representation is used to replace point-wise cross-section data.
• An approximation is introduced to reduce cost of the multipole representation.
• Storage requirements are reduced by 1–2 orders of magnitude.
• New approach is tested in OpenMC for two LEU critical experiments.

A new approach for direct Doppler broadening of nuclear data in Monte Carlo simulations is proposed based on the multipole representation. The multipole representation transforms resonance parameters into a set of poles and residues only some of which exhibit a resonant behavior. A method is introduced to approximate the contribution to the background cross section in an effort to reduce the number of poles needing to be broadened. The multipole representation results in memory savings of 1–2 orders of magnitude over comparable techniques. This approach provides a simple way of computing nuclear data at any temperature which is essential for multi-physics calculations, while having a minimal memory footprint which is essential for scalable high performance computing. The concept is demonstrated on two major isotopes of uranium (U-235 and U-238) and implemented in the OpenMC code. Two LEU critical experiments were solved and showed great accuracy with a small loss of efficiency (10–30%) over a single-temperature pointwise library.