Technical development on burn-up credit for spent LWR fuels had been performed at JAERI since
1990 under the contract with Science and Technology Agency of Japan entitled ‘Technical Development on
Criticality Safety Management for Spent LWR Fuels.’ Main purposes of this work are to obtain the
experimental data on criticality properties and isotopic compositions of spent LWR fuels and to verify burnup
and criticality calculation codes. In this work three major experiments of exponential experiments for

The Interim Staff Guidance on burnup credit issued by the U.S. Nuclear Regulatory Commission's Spent Fuel Project Office recommends restricting the use of burnup credit to assemblies that have not used burnable absorbers. This restriction eliminates a large portion of the currently discharged spent fuel assemblies from cask loading, and thus severely limits the practical usefulness of burnup credit.

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 Nuclear Waste Policy Act of 1982, as amended (the Act), assigns the Federal Government the responsibility for the disposal of spent nuclear fuel and high-level waste. Section 302a of the Act authorized the Secretary to enter into contracts with the owners and generators of commercial spent nuclear fuel and or high level waste. The Standard Contract for Disposal of Spent Nuclear Fuel and or High Level Radioactive Waste (Standard Contract) established the contractual mechanism for the Department's acceptance and disposal of spent nuclear fuel and high level waste.

The "Summary Report of Commercial Reactor Critical Analyses Perfonned for the Disposal Criticality Analysis Methodology" contains a summary of the commercial reactor critical (CRC) analyses used to support the validation of the criticality models for spent commercial light water reactor (LWR) fuel.

This report investigates trends in the radiological decay properties and changes in relative nuclide importance associated with increasing enrichments and burnup for spent LWR fuel as they affect the areas of criticality safety, thermal analysis (decay heat), and shielding analysis of spent fuel transport and storage casks. To facilitate identifying the changes in the spent fuel compositions that most directly impact these application areas, the dominant nuclides in each area have been identified and ranked by importance.

The purpose of this study is to provide insights into the neutronic similarities that may exist between a
generic cask containing typical spent nuclear fuel assemblies and commercial reactor critical (CRC) state-
points. Forty CRC state-points from five pressurized-water reactors were selected for the study and the
type of CRC state-points that may be applicable for validation of burnup credit criticality safety
calculations for spent fuel transport/storage/disposal systems are identified. The study employed cross-

The validity of the computation of pressurized-water-reactor (PWR) spent fuel isotopic
composition by the SCALE system depletion analysis was assessed using data presented in the report.
Radiochemical measurements and SCALE/SAS2H computations of depleted fuel isotopics were
compared with 19 benchmark-problem samples from Calvert Cliffs Unit 1, H. B. Robinson Unit 2,
and Obrigheim PWRs. Even though not exhaustive in scope, the validation included comparison of
predicted and measured concentrations for 14 actinides and 37 fission and activation products.

Uncertainties in the predicted isotopic concentrations in spent nuclear fuel represent one of the largest
sources of overall uncertainty in criticality calculations that use burnup credit. The methods used to
propagate the uncertainties in the calculated nuclide concentrations to the uncertainty in the predicted
neutron multiplication factor (keff) of the system can have a significant effect on the uncertainty in the
safety margin in criticality calculations and ultimately affect the potential capacity of spent fuel transport

The Interim Staff Guidance on burnup credit (ISG-8, revision 2) for pressurized-water-reactor spent
nuclear fuel in storage and transport casks, issued in 2002 by the U.S. Nuclear Regulatory Commission’s
Spent Fuel Project Office, recommends an out-of-core burnup measurement to confirm the reactor record
and compliance with the assembly burnup value used for cask loading acceptance. This recommendation
is intended to prevent unauthorized loading (i.e., misloading) of assemblies due to inaccuracies in reactor