Evaluation of the235U resonance parameters to fit the standard recommended values

Luiz Leal,Marian Jandel,Carlos Paradela,Yaron Danon,Ignacio Durán,Laurent Tassan-Got,Gilles Noguere

doi:10.1051/epjconf/201714602021

Abstract

A great deal of effort has been dedicated to the revision of the standard values in connection with the neutron interaction for some actinides. While standard data compilation are available for decades nuclear data evaluations included in existing nuclear data libraries (ENDF, JEFF, JENDL, etc.) do not follow the standard recommended values. Indeed, the majority of evaluations for major actinides do not conform to the standards whatsoever. In particular, for the n + 235 U interaction the only value in agreement with the standard is the thermal fission cross section. A resonance re-evaluation of the n + 235 U interaction has been performed to address the issues regarding standard values in the energy range from 10−5 eV to 2250 eV. Recently, 235 U fission cross-section measurements have been performed at the CERN Neutron Time-of-Flight facility (TOF), known as n_TOF, in the energy range from 0.7 eV to 10 keV. The data were normalized according to the recommended standard of the fission integral in the energy range 7.8 eV to 11 eV. As a result, the n_TOF averaged fission cross sections above 100 eV are in good agreement with the standard recommended values. The n_TOF data were included in the 235 U resonance analysis that was performed with the code SAMMY. In addition to the average standard values related to the fission cross section, standard thermal values for fission, capture, and elastic cross sections were also included in the evaluation. This paper presents the procedure used for re-evaluating the 235 U resonance parameters including the recommended standard values as well as new cross section measurements.

Highlights

IntroductionBenchmark calculations prompted the proposal for a subgroup of the Working Party on International Nuclear Data (WPEC) to investigate the 235U capture problem [7]
The results demonstrated that the evaluation performed poorly mainly a e-mail: luiz.leal@irsn.fr due to concerns with the capture cross-section in the energy region 22.6 Energy Interval (eV) to 454 eV [3] indicating a need for increasing the capture cross section
The novel feature of the work described in this paper, in contrast to previous work on the 235U resonance parameter evaluation, is the addition of new cross-section measurements done at Los Alamos National Laboratory (LANL), Rensselaer Polytechnic Institute (RPI) and at n Time-of-Flight facility (TOF)

Summary

Introduction

Benchmark calculations prompted the proposal for a subgroup of the Working Party on International Nuclear Data (WPEC) to investigate the 235U capture problem [7]. The WPEC recommendation was that rather than re-evaluating the 235U resonance parameters based solely on integral benchmark results new capture cross-section measurements should be made to confirm the findings of the WPEC subgroup. TOF capture cross-section measurements were planned and performed independently at the Rensselaer Polytechnic Institute (RPI) [8] and at the Los Alamos National Laboratory (LANL) [9]. These new measurements were used together with the computer code SAMMY to update the 235U resonance parameters in the energy range from thermal to 2.25 keV. The experimental data base used in the evaluation is described with emphasis on new measurements added to the present evaluation

External energy levels determination

Experimental data base

Conclusions