Criticality experiments for validation of cross sections: the237Np case

L.S Leong,I Duran,M.O Sornein,C Le Naour,B Berthier,J.N Wilson,C Paradela,L Tassan-Got,D Tarrio,C Stéphan,L Audouin

doi:10.1051/epjconf/20122103003

Abstract

237 Np is one of the actinides which is most abundantly produced in present reactors, and it could be a good candidate for incineration due to its long life time. However this requires a good knowledge of neutron cross sections. The n_TOF collaboration has carried out a fission cross section measurement at the n_TOF neutron facility at CERN, spanning a very broad energy range, from a fraction of eV to GeV. When compared to previous measurements the n_TOF fission cross section appears to be higher by 5-7% beyond the first fission threshold. We show however that several previous measurements, although consistent with each other, are dependent through re-normalization procedures. To check the relevance of n_TOF data, we simulate an experiment, performed at Los Alamos, with a sphere of 237 Np of 6 kg surrounded by enriched uranium 235 U, so as to reach criticity with fast neutrons. When introducing the n_TOF data for the fission cross section of 237 Np, the simulation predicts a multiplicating factor k eff in better agreement with the experiment as the deviation of 750 pcm is reduced to 250 pcm. We explore also the hypothesis of possible deficiencies of the inelastic cross section in 235 U which has been invoked by some authors to explain the deviation of 750 pcm. However, the large distortion that should be applied to reconcile the critical experiment with its simulation discredits this option. These outcomes support the hypothesis of a higher fission cross section of 237 Np.

Highlights

IntroductionIts incineration is of interest to reduce on the long term the radiotoxicity of the final disposal
237Np is a long lived (2 My) radioactive isotope which is a abundantly produced in nuclear power plants
Its incineration is of interest to reduce on the long term the radiotoxicity of the final disposal. This could be in principle achieved in fast reactors, where a significant fraction of the neutron energy spectrum extends beyond the first chance fission threshold, it requires a better knowledge of the neutron cross sections specially for fission

Summary

Introduction

Its incineration is of interest to reduce on the long term the radiotoxicity of the final disposal This could be in principle achieved in fast reactors, where a significant fraction of the neutron energy spectrum extends beyond the first chance fission threshold, it requires a better knowledge of the neutron cross sections specially for fission. Some data sets are consistent within the claimed accuracy, which is usually about 3-4 %, some significant discrepancies show up between others, in particular the n TOF recent measurement which exhibits values higher by about 6 % when compared to several previous measurements

Status of 237Np fission cross section

The critical 237Np benchmark

Findings

Conclusion