Criticality experiments for validation of cross sections: the ²³⁷Np case

L.S. Leong^1a, L. Tassan-Got¹, L. Audouin¹, C. Paradela², J.N. Wilson¹, D. Tarrio², B. Berthier¹, I. Duran¹, C. Le Naour², M.O. Sornein¹ and C. Stéphan¹

¹ Institut de Physique Nucléaire, CNRS/Unviersité Paris-Sud, 91406 Orsay, France
² Facultad de Fisica, Universidade de Santiago de Compostela, 15782, Spain

^a e-mail: leong@ipno.in2p3.fr

Published online: 14 February 2012

Abstract

²³⁷Np 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 ²³⁷Np of 6 kg surrounded by enriched uranium ²³⁵U, so as to reach criticity with fast neutrons. When introducing the n_TOF data for the fission cross section of ²³⁷Np, 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 ²³⁵U 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 ²³⁷Np.