Consultants' Meeting of INDEN - International Nuclear Data Evaluation Network I - on the Resonance Parameter Evaluation of the Fissile Actinides

8-11 May 2018, IAEA, Vienna

Background

International Nuclear Data Evaluation Network is an activity aimed at streamlining the evaluation activities, taking advantage of expertise in different laboratories in Member States. The activities would follow the pattern of the highly successful CIELO project, organised through the NEA Data Bank. The aim is to minimise duplication of work, except for the testing of different approaches to the evaluation, and to produce evaluated data files with a broad consensus that can be adopted fully or in parts by the National Data Evaluation projects.

Scope

Reactivity prediction of critical thermal assemblies, particularly solution systems, is strongly affected by the PFNS, by nubar energy dependence, and by the detailed shape of the cross sections below 10eV. Newly evaluated PFNS for thermal-neutron induced fission of U-235, Pu-239 and U-233 (within the Neutron Standards Project) resulted in lower PFNS average energy by about 30 keV. It is well established that a softer PFNS significantly increases the reactivity of the high-leakage solution benchmarks by 0.7-1.2 percent.
The Thermal Neutron Constants from the new Standards-2017 also strongly affect the reactivity of thermal systems.
In the recent CIELO evaluation for ²³⁵U the performance of the evaluated data has been preserved by a careful analysis of the measured capture-to-fission ratios at the lower end of the resolved resonance region to determine the resonance parameters of the first few resonances combined with the introduction of resonance nubar. The same has not been done for ²³³U and ²³⁹Pu (although resonance nubar is already used in Pu-239 evaluation). Also, the parameters of the resonances at energies above ~100 eV need a closer investigation.

Objectives of the Meeting

The main objective is to:
- Review the current status of the resonance parameter evaluations of the fissile actinides.
- Check the impact of the new Standards-2017 and the softer PFNS of the fissile actinides on the criticality of thermal systems.
- Define strategies to use new PFNS combined with new RP evaluations for U-233 and Pu-239.
The overall objective is to co-ordinate the activities in different laboratories with the final goal of obtaining consistent evaluated data files that respect the differential data (particularly the thermal constants of Standards-2017, softer PFNS and new cross section measurements) and preserve good performance in criticality benchmarks, especially regarding the trends as a function of the above-thermal leakage fraction (ATLF).