Comparison of ENDF/B-VI and preliminary ENDF/B-VII results for the MCNP criticality validation suite

An initial assessment of the impact of preliminary data proposed for ENDF/B-VII has been made using the MCNP criticality validation suite. Relative to ENDF/B-VI, the data changes primarily involve high-energy elastic and inelastic scattering in the uranium isotopes and 239Pu, as well as resonance parameters for 238U. The criticality validation suite is a collection of 31 benchmarks taken from the International Handbook of Evaluated Criticality Benchmark Experiments. It contains cases for a variety of fuels, including 233U, highly enriched uranium (HEU), intermediate-enriched uranium (IEU), low-enriched uranium (LEU), and plutonium. For each fuel type, there are cases with a variety of moderators, reflectors, spectra, and geometries. The cases in the suite are summarized in Table I. Three sets of calculations were performed for the suite using the MCNP5 Monte Carlo code. The first set employed nuclear data from ENDF/B-VI Release 8, the final release for ENDF/B-VI. The second set employed preliminary ENDF/B-VII data generated by group T-16 at Los Alamos National Laboratory for the uranium isotopes and for 239Pu but retained ENDF/B-VI data for all other nuclides. The third set was the same as the second except that a new set of 238U resonance parameters generated by researchers at Oak Ridge National Laboratory (ORNL) was added to the T-16 evaluation. The MCNPS calculations were run with 5,000,000 active neutron histories for all but two cases in the suite. Only 3,000,000 active histories were used for those cases, SB-5 and Zebra-8H, because they require substantially more computer time per history than the other cases. Nonetheless, the standard deviation for keff from those cases is comparable to those for other cases in the suite. The results from these calculations are presented in Table II. The preliminary ENDF/B-VII data produce marked improvements in keff for bare spheres of 233U (Jezebel-233), HEU (Godiva), and plutonium (Jezebel and Jezebel-240) as well as the other unreflected HEU and plutonium cases (Tinkertoy02 (c-11) and Pu Buttons, respectively). Furthermore, the reactivity swings between those bare spheres and the corresponding Flattop cases (which enclose the sphere inside an annulus of normal uranium) are substantially decreased. The changes also significantly improve keff for BIG TEN and for HEU and plutonium spheres immersed in water (Godiver and Pu-MF-11, respectively). In addition, inclusion of the ORNL resonance parameters for 238U produces a significantly better value for keff for B and W XI (2), a lattice of LEU fuel pins in water, and ORNL-II, an unreflected sphere of uranyl nitrate solution enriched in 233U. At the same time, the preliminary ENDF/B-VII data produce worse results for thermal lattices of 233U and HEU pins in water (SB-2 1/2 and SB-5, respectively), for the bare IEU sphere (IEU-MF-03) and the IEU sphere reflected by graphite (IEU-MF-04), for a plutonium sphere reflected by thorium (THOR), and for a MOX lattice in water (PNL-33). Furthermore, keff for the uranium cases with intermediate spectra remains substantially underpredicted, while keff for the plutonium case with an intermediate spectrum (HISS/HPG) continues to be significantly over-predicted. In conclusion, preliminary ENDF/B-VII data for the uranium isotopes and 239Pu produce improvements for most of the cases with fast spectra, BIG TEN, and the lattice of LEU fuel pins in water. However, improvements still are needed in some areas, particularly those cases with intermediate spectra.