This analysis is prepared by the Mined Geologic Disposal System (MGDS) Waste Package Development (WPD) department to provide a probabilistic evaluation of the potential for criticality of fissile material which has been transported from a geologic repository containing breached waste packages of commercial spent nuclear fuel (SNF). This analysis is part of a continuing investigation of the probability of criticality resulting from the emplacement of spent nuclear fuel in a geologic repository.

Past efforts by the Department of Energy (DOE), the Electric Power Research Institute (EPRI), the Nuclear Regulatory Commission (NRC), and others have provided sufficient technical information to enable the NRC to issue regulatory guidance for implementation of pressurized-water reactor (PWR) burnup credit; however, consideration of only the reactivity change due to the major actinides is recommended in the guidance.

The purpose of this report is to document and validate the external accumulation model that predicts accumulation of fissile materials in the invert, fractures and lithophysae in the rock beneath a degrading waste package containing spent nuclear fuel (SNF) in the monitored geologic repository at Yucca Mountain. (Lithophysae are hollow, bubblelike structures in the rock composed of concentric shells of finely crystalline alkali feldspar, quartz, and other materials (Bates and Jackson 1984 [DIRS 128109], p.

In this study, the long-term geochemical behavior of waste package (WP), containing Pu-ceramic, was modeled. The ceramic under consideration contains Ti, U, Pu, Gd and Hf in a pyrochlore structure; the Gd and Hf stabilize the mineral structure, but are also intended to provide criticality control. The specific study objectives were to determine:
1) the extent to which criticality control material, suggested for this WP design, will remain in the WP after corrosion/dissolution of the initial package configuration (such that it can be effective in preventing criticality), and

Optimization of energy resources suggests increased fuel residence in reactor cores and hence improved
fission product evaluations are required. For thermal reactors the fission product cross sections in the JEF2.2 and
JEFF3.1 libraries plus new evaluations from WPEC23 are assessed through modelling the CERES experiment in
the DIMPLE reactor. The analysis uses the lattice code WIMS10. Cross sections for 12 nuclides are assessed. The
thermal cross section and low energy resonance data for 147,152Sm and 155Gd are accurate to within 4%. Similar data

U.S. Nuclear Regulatory Commission Interim Staff Guidance 8 (ISG-8) for burnup credit covers actinides only, a position based primarily on the lack of definitive critical experiments and adequate radiochemical assay data that can be used to quantify the uncertainty associated with fission product credit.

The Disposal Criticality Analysis Methodology Topical Report (Reference 1) states that the accuracy of the criticality analysis methodology (MCNP Monte Carlo code and cross-section data) designated to assess the potential for criticality of various configurations in the Yucca Mountain proposed repository is established by evaluating appropriately selected benchmark critical experiments.

This study is prepared by the Mined Geologic Disposal System (MODS) Waste Package Development Department (WPDD) to provide input to a separate evaluation on the probablility of criticality in the far- field environment. These calculations are performed in sufficient detail to provide conservatively bounding configurations to support separate probabilistic analyses.

Target accuracy on LWR neutronics parameters is 2 to 5 times lower than the a priori uncertainty (1σ)
due to nuclear data. This paper summarizes the experimental facilities and the integral measurements that are required
for code qualification. The rigorous use of integral information through trend analysis method is described. Trends
on JEF2 data from Keff measurements and P.I.Es are presented. These trends were accounted for in the new JEFF3
evaluations. The role of fundamental experiments, such as worth measurement of separated isotopes, is emphasized.

This report presents the analyses and results for the potential occurrence of external criticality events which could result from plutonium waste forms emplaced in a geologic repository similar to the one being developed at Yucca Mountain. The analyses evaluate both the MOX spent fuel and the immobilized plutonium waste forms in a repository if the waste package has degraded and if the fissile material has migrated to the invert and out into the far-field.