Workshop 5 – Cross Section, Nuclear Data, and Uncertainties

Abstract

The Cross Section, Nuclear Data, and Uncertainties Workshop met with 30 participants. There was a lively discussion that covered several different topics. Some of the topics had resonance with topics brought up in other workshop at this symposium. One major issue was how to assign an a priori uncertainty to the trial spectrum used in spectrum adjustment techniques, or, more specifically, the need for a more rigorous and automated coupling of uncertainties into the radiation transport codes that are used to generate the a priori (trial) spectrum that is used in neutron spectrum adjustment approaches. Approaches based on discrete ordinate transport provide a means of generating sensitivity coefficients for specific variations, but fall short of the required task. While some Monte Carlo-based radiation transport codes permit the user to vary the cross sections for specific reactions or to vary material densities, a more universal and automated variation of all relevant inputs to the radiation transport code is required. One approach mentioned was a Total Monte Carlo (TMC) approach provided by a coupling into radiation transport codes of the recently generated statistical sample of random ENDF nuclear data evaluations produced by the TALYS code and incorporated into the TENDL library (for some isotopes). This would provide a set of self-consistent nuclear data files that can be sampled in a transport calculation. This variation of nuclear data would need to be coupled with a variation in material composition, density, and spatial dimensions. Another discussion topic was the need for the community to revisit the consistency of current approaches to spectrum unfolds/adjustment, i.e. a new round of the older REAL-84 and REAL-88 exercises to assess our status. While the dosimetry community has made great strides with the IRDFF library in establishing an international community consensus for dosimetry cross sections and associated nuclear data, the differences within the community in the treatment of the uncertainty in the trial spectrum and the availability of new spectrum adjustment techniques, such as Maximum Entropy codes like MAXED and the use of genetic algorithms, warrant a revisit of these international exercises to assess the current consistency (within stated uncertainty bonds) of approaches within the community to the characterization of neutron spectra and towards expressing the uncertainty in integral metrics based on these spectrum uncertainties. Since the IAEA/NDS has done such a good job in coordinating an international consensus in dosimetry cross sections and nuclear data, and since they coordinated the older REAL-8X exercises, we look to them to coordinate a new international assessment of our consistency in treating spectrum adjustments and characterizing the resulting uncertainties.