A New Method for Generating Probability Tables in the Unresolved Resonance Region

A new method for constructing probability tables in the unresolved resonance region (URR) has been developed. This new methodology is an extensive modification of the single-level Breit-Wigner (SLBW) pseudo-resonance pair sequence method commonly used to generate probability tables in the URR. The new method uses a Monte Carlo process to generate many pseudo-resonance sequences by first sampling the average resonance parameter data in the URR and then converting the sampled resonance parameters to the more robust R-matrix limited (RML) format. For each sampled set of pseudo-resonance sequences, the temperature-dependent cross sections are reconstructed on a small grid around the energy of reference using the Reich-Moore formalism and the Leal-Hwang Doppler broadening methodology. The effective cross sections calculated at the energies of reference are then used to construct probability tables in the URR. The RML cross-section reconstruction algorithm has been rigorously tested for a variety of isotopes, including ¹⁶O, ¹⁹F, ³⁵Cl, ⁵⁶Fe, ⁶³Cu, and ⁶⁵Cu. The new URR method also produced normalized cross-section factor probability tables for ²³⁸U that were found to be in agreement with current standards. The modified ²³⁸U probability tables were shown to produce k_eff results in excellent agreement with several standard benchmarks, including the IEUMET- FAST-007 (BIG TEN), IEU-MET-FAST-003, and IEU-COMP-FAST-004 benchmarks.