Physics of Neutron Interactions with 238 U: New Developments and Challenges R. Capote, 1, * A. Trkov, 2 M. Sin, 3 M. Herman, 4 A. Daskalakis, 5 and Y. Danon 5 1 NAPC–Nuclear Data Section, International Atomic Energy Agency, PO Box 100, Vienna A–1400, Austria 2 Joˇ zef Stefan Institute, Jamova cesta 39, Ljubljana SI–1000, Slovenia 3 Nuclear Physics Department, Bucharest University, Bucharest-Magurele RO–077125, Romania 4 National Nuclear Data Center, Brookhaven National Laboratory, Upton, NY 11973–5000, USA 5 Gaerttner LINAC Center, Rensselaer Polytechnic Institute, Troy, NY 12180, USA The latest release of the EMPIRE-3.1 system (codename Rivoli) is being used in the advanced modeling of neutron induced reactions on the 238 U nucleus with the aim of improving our knowl- edge of neutron scattering. The reaction model includes: (i) a new rotational-vibrational dispersive optical model potential coupling the low-lying collective bands of vibrational character observed in even-even actinides, (ii) the Engelbrecht-Weidenm¨ uller transformation allowing for inclusion of compound-direct interference effects enhanced by a dispersive treatment of the optical model poten- tial, (iii) a multi-humped fission barrier with absorption in the secondary well as described within the optical model for fission, and (iv) a modified Lorentzian model (MLO) of the radiative strength function. Impact of the advanced modeling on elastic and inelastic scattering cross section is being assessed by both comparison with selected microscopic experimental data and integral criticality benchmarks (e.g. FLATTOP, JEMIMA and BIGTEN assemblies). Benchmark calculations provide feedback to improve the reaction modeling and reduce both model and model-parameters uncertain- ties. Additionally, neutron scattering yields on 238 U measured accurately at RPI by the time-of-flight technique at 29, 60, 112 and 153 degrees have been used as a further constraint on the incident en- ergy dependence of elastic and inelastically scattered neutrons. Improvement of scattering cross sections in existing libraries is discussed. I. INTRODUCTION 238 U constitutes more than 90% of nuclear fuel in power reactors, being one of the most important isotopes for neutron transport calculations in the active zone of a re- actor. Recently an IAEA technical meeting on “Inelas- tic Scattering and Capture Cross-section Data of Ma- jor Actinides in the Fast Neutron Region” was held at IAEA Headquarters, Vienna, Austria to review the sta- tus of nuclear data libraries for these cross sections, the status of the experimental results by which these data can be tested, and to evaluate what advances in nuclear modeling and measurement technique may improve our knowledge of these cross sections [1]. Recently evalu- ated nuclear data files of the major actinides (e.g. JEFF- 3.1 [2], CENDL-3.1 [3], JENDL-4.0 [4], and ENDF/B- VII.1 [5]) show significant differences in the inelastic scat- tering cross sections over the fast neutron energy range. Attendees at this IAEA meeting concluded that advances in modeling are substantial, ... and that significant im- provement can be made in reducing modeling uncertain- ties for capture and inelastic scattering [1]. Such reduc- tion of the uncertainty of neutron scattering data on 238 U ∗ Corresponding author: r.capotenoy@iaea.org in the fast neutron region has also been requested by WPEC-SG26 [6], motivating new evaluation efforts. Fast neutron induced reactions on 238 U can be an ideal tar- get to perform a compound-nucleus cross-section inter- comparison, as the fission cross section below 1 MeV is almost negligible and above 1 MeV is a neutron cross sec- tion standard [7]), the capture cross section is known to very high accuracy (from the neutron standard fit [7]), and the total cross section is expected to be very well de- fined by the experimental data. Therefore, the differences among the model calculations concern the partitioning of the compound-nucleus formation cross section over the elastic and inelastic scattering channels [1]. The present work addresses these issues, and a report is provided of on-going evaluation of neutron induced reactions on 238 U in the fast neutron region. II. NUCLEAR REACTION MODELING Given the limited accuracy and availability of experi- mental inelastic scattering data on 238 U, nuclear reaction modeling plays a central role in the evaluation of inelas- tic scattering cross sections. Therefore improvement and benchmarks of the modeling and the model parameters are crucial for better prediction of the unknown inelas- Available online at www.sciencedirect.com Nuclear Data Sheets 118 (2014) 26–31 0090-3752/2014 Published by Elsevier B.V. www.elsevier.com/locate/nds http://dx.doi.org/10.1016/j.nds.2014.04.003