nureg/cr-6700 - Oak Ridge National Laboratory

Section 7
Recommendations
Conclusions and
7 CONCLUSIONS AND RECOMMENDATIONS
The importance rankings in this report are designed to facilitate validation efforts related to the prediction of
spent fuel isotopics and radiological decay properties of high-burnup spent fuel by identifying the nuclides
that have the greatest impact in the application areas of interest and illustrate the variation in nuclide
importance with burnup. These results also have potential application to establishing the degree of
applicability of existing validation data to the high-burnup regime and may be useful to help identify the
challenges that high-burnup fuel presents to existing computational methods and nuclear data.
The availability of experimental data for code validation in the high-enrichment and high-burnup regimes is
likely to be limited in the near term to isotopic assay measurements that are either underway or are being
planned in support of criticality burnup credit activities in the United States and in other countries. Through
these activities the experimental assay programs are seeking to compile a comprehensive database of spent
fuel isotopics that is representative of the fuel types and assembly designs from operating LWRs.
Because the experimental database for decay heat and radiation source-term validation data is likely to
remain limited to low- and moderate-enrichment and burnup fuels in the near term, a separate-effects
approach to code validation currently used in burnup credit may be a valuable, if not necessary, option for
validating code predictions of spent fuel radiological decay properties in the high burnup regime. This
approach relies on independently validating the depletion analysis methods used to predict the spent fuel
compositions using isotopic assay data, and validating the isotopic response (e.g., isotopic reactivity worth in
burnup credit applications, or energy release rate per decay for decay heat applications). Such a strategy for
validating decay heat generation and radiation source-term predictions would be simplified by the fact that
relatively few radioisotopes are responsible for the responses at high burnup. For example, the decay-heat
generation rate for high-burnup fuel is dominated by fewer than 10 radionuclides that are responsible for
about 95% of the total decay heat at 5-years cooling. After 100 years just four radionuclides account for
about 85% of the decay heating. A similar trend is observed for the radionuclides important to radiation
shielding.
A separate-effects approach would also be simplified by the fact that the nuclide decay data parameters
important in these application areas (recoverable energy per decay for decay heating and radiation emission
spectra for source terms) are already relatively well established by measurements. Consequently, the
accuracy of computer code predictions could be established to a high degree of confidence using measured
isotopic inventory data alone. Unfortunately, many of the nuclides important to the burnup credit are for the
most part different than those important to decay-heat and radiation-source-term analysis. However, some
overlap does exist.
It is recommended that consideration be given to expanding the isotopic assay measurements currently being
planned within the United States in support of the burnup credit activities to include additional radionuclides
important to decay heating and radiation-source-term applications. A review of the findings from this study
suggests that relatively few new radionuclides are needed to provide a relatively comprehensive
characterization of high-burnup spent fuel in the respective application areas. Also, several key
radionuclides exist as parent-daughter pairs (e.g., 137 Cs− 137m Ba, 90 Sr− 90 Y, 144 Ce− 144 Pr) and therefore only one
radionuclide in these pairs would require measurement since they exist in secular equilibrium (i.e., the
activities of the parent and daughter product are equal). The selected parent or daughter could be chosen to
optimize the coverage of different application areas.
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