• Energy Research

Abstract The new paradigm of nuclear reaction evaluation postulates that differential and integral experiments along with the reaction models should be used in concert to produce the evaluated data files. The results of a previous assimilation project are summarized as a proof of principle that adjusting reaction model parameters to the results of integral experiments is feasible. The new paradigm requires vast modernization of the nuclear data infrastructure. Such modernization is actually carried out by several national and international efforts. We describe those dedicated to: (i) handling of differential and integral data, (ii) providing and operating the sensitivity profiles, (iii) ensuring reproducibility of the evaluations, and (iv) enabling automated verification of the entire library. We also point to the importance of the nuclear reaction modeling and discuss advantages and disadvantages of the new paradigm.

We have performed a new combined set of evaluations for 90–96Zr, including new resolved resonance parameterizations from Said Mughabghab for 90,91,92,94,96Zr and fast region calculations made with EMPIRE-3.1. Because 90Zr is a magic nucleus, stable Zr isotopes are nearly spherical. A new soft-rotor optical model potential is used allowing calculations of the inelastic scattering on low-lying coupled levels of vibrational nature. A soft rotor model describes dynamical deformations of the nucleus around the spherical shape and is implemented in EMPIRE/OPTMAN code. The same potential is used with rigid rotor couplings for odd-A nuclei. This then led to improved elastic angular distributions, helping to resolve improper leakage in the older ENDF/B-VII.1β evaluation in KAPL proprietary, ZPR and TRIGA benchmarks. Another consequence of 90Zr being a magic nucleus is that the level densities in both 90Zr and 91Zr are unusually low causing the (n,el) and (n,tot) cross sections to exhibit large fluctuations above the resolved resonance region. To accommodate these fluctuations, we performed a simultaneous constrained generalized least-square fit to (n,tot) for all isotopic and elemental Zr data in EXFOR, using EMPIRE's TOTRED scaling factor. TOTRED rescales total cross sections so that the optical model calculations are unaltered by the rescaling and the correct competition between channels is maintained. In this fit, all (n,tot) data in EXFOR was used for Ein>100keV, provided the target isotopic makeup could be correctly understood, including spectrum averaged data and data with broad energy resolution. As a result of our fitting procedure, we will have full cross material and cross reaction covariance for all Zr isotopes and reactions.

We describe the next generation general purpose Evaluated Nuclear Data File, ENDF/B-VII.0, of recommended nuclear data for advanced nuclear science and technology applications. The library, released by the U.S. Cross Section Evaluation Working Group (CSEWG) in December 2006, contains data primarily for reactions with incident neutrons, protons, and photons on almost 400 isotopes, based on experimental data and theory predictions. The principal advances over the previous ENDF/B-VI library are the following: (1) New cross sections for U, Pu, Th, Np and Am actinide isotopes, with improved performance in integral validation criticality and neutron transmission benchmark tests; (2) More precise standard cross sections for neutron reactions on H, 6 Li, 10 B, Au and for 235,238 U fission, developed by a collaboration with the IAEA and the OECD/NEA Working Party on Evaluation Cooperation (WPEC); (3) Improved thermal neutron scattering; (4) An extensive set of neutron cross sections on fission products developed through a WPEC collaboration; (5) A large suite of photonuclear reactions; (6) Extension of many neutron- and proton-induced evaluations up to 150 MeV; (7) Many new light nucleus neutron and proton reactions; (8) Post-fission beta-delayed photon decay spectra; (9) New radioactive decay data; (10) New methods for uncertainties and covariances, together with covariance evaluations for some sample cases; and (11) New actinide fission energy deposition. The paper provides an overview of this library, consisting of 14 sublibraries in the same ENDF-6 format as the earlier ENDF/B-VI library. We describe each of the 14 sublibraries, focusing on neutron reactions. Extensive validation, using radiation transport codes to simulate measured critical assemblies, show major improvements: (a) The long-standing underprediction of low enriched uranium thermal assemblies is removed; (b) The 238 U and 208 Pb reflector biases in fast systems are largely removed; (c) ENDF/B-VI.8 good agreement for simulations of thermal high-enriched uranium assemblies is preserved; (d) The underprediction of fast criticality of 233,235 U and 239 Pu assemblies is removed; and (e) The intermediate spectrum critical assemblies are predicted more accurately. We anticipate that the new library will play an important role in nuclear technology applications, including transport simulations supporting national security, nonproliferation, advanced reactor and fuel cycle concepts, criticality safety, fusion, medicine, space applications, nuclear astrophysics, and nuclear physics facility design. The ENDF/B-VII.0 library is archived at the National Nuclear Data Center, BNL, and can be retrieved from www.nndc.bnl.gov .