Investigation of Nuclide Importance to Functional Requirements Related to Transport and Long-Term Storage of LWR Spent Fuel

The radionuclide characteristics of light-water-reactor (LWR) spent fuel play key roles in the design
and licensing activities for radioactive waste transportation systems, interim storage facilities, and the final
repository site. Several areas of analysis require detailed information concerning the time-dependent behavior
of radioactive nuclides including (1) neutron/gamma-ray sources for shielding studies, (2) fissile/absorber
concentrations for criticality safety determinations, (3) residual decay heat predictions for thermal
considerations, and (4) curie and/or radiological toxicity levels for materials assumed to be released into the
ground/environment after long periods of time. The crucial nature of the radionuclide predictions over both
short and long periods of time has resulted in an increased emphasis on thorough validation for radionuclide
generation/depletion codes.
Current radionuclide generation/depletion codes have the capability to follow the evolution of some
1600 isotopes during both irradiation and decay time periods. Of these, typically only 10 to 20 nuclides
dominate contributions to each analysis area. Thus a quantitative ranking of nuclides over various time periods
is desired for each of the analysis areas of shielding, criticality, heat transfer, and environmental dose
(radiological toxicity). These rankings should allow for validation and data improvement efforts to be focused
only on the most important nuclides.
This study investigates the relative importances of the various actinide, fission-product, and lightelement
isotopes associated with LWR spent fuel with respect to five analysis areas: criticality safety
(absorption fractions), shielding (dose rate fractions), curies (fractional curies levels), decay heat (fraction of
total watts), and radiological toxicity (fraction of potential committed effective dose equivalent). These
rankings are presented for up to six different burnup/enrichment scenarios and at decay times from 2 to
100,000 years. Ranking plots for each of these analysis areas are given in an Appendix for completeness, as
well as summary tables in the main body of the report. Summary rankings are presented in terms of high
(greater than 10% contribution to the total), medium (between 1% and 10% contribution), and low (less than
1% contribution) for both short- and long-term cooling. When compared with the expected measurement
accuracies, these rankings show that most of the important isotopes can be characterized sufficiently for the
purpose of radionuclide generation/depletion code validation in each of the analysis areas. Because the main
focus of this work is on the relative importances of isotopes associated with LWR spent fuel, some conclusions
may not be applicable to similar areas such as high-level waste (HLW) and nonfuel-bearing components
(NFBC).