Production of Probability Tables for the Unresolved-Resonance Region Using the AMPX Cross-section Processing System

The DOE Laboratory Project and Cost Proposal for NRC Work (JCN W6479) identifies various subtasks for upgrading the AMPX code system to process ENDF/B-VI data. The overall objective of the upgrade is to facilitate independent processing of Version 6 formats. Task 8 of the cost proposal addresses the development of the capability to generate probability tables for neutron transport applications in the unresolved-resonance region (URR). In accordance with the reporting requirements for Task 8, this letter report documents development and testing of the capability to generate probability tables for the URR. A new module for the AMPX code system has been developed to satisfy the objective of Task 8. The new module is named PURM (Probability tables for the Unresolved Region using Monte Carlo). PURM calculates probability tables in the URR using Monte Carlo procedures to sample the resonance spacings and widths. The Monte Carlo procedure used in PURM is based on the methodology in the URR code that was developed at ORNL in the late 1980s. PURM samples pairs of resonances surrounding a reference energy for each probability table. The resonance spacings are sampled from a Wigner distribution. PURM uses the {Delta}{sub 3}-statistics test that was developed by Dyson and Mehta to determine the number of resonances to sample for each spin state. For each resonance, PURM samples the resonance widths from a {chi}{sup 2}-distribution for a specified number of degrees of freedom. Once the resonance parameters are sampled, PURM calculates the total, capture, fission and scatter cross sections at the reference energy using the single-level Breit-Wigner formalism with appropriate treatment for temperature effects