nureg/cr-6700 - Oak Ridge National Laboratory

Section 6
Summary
6 SUMMARY
This report has presented the importance rankings of the various actinides, fission products, and activation
products that are important to criticality safety, decay-heat analysis, and cask-shielding analysis. The work
is largely an extension of previous ranking studies (Ref. 9) to assess higher fuel enrichment and burnup
regimes, and has focused on the changes in nuclide importance with increasing burnup, rather than cooling
time, in the respective analysis areas. In addition, the present study has restricted the decay times to between
2 and 100 years, the regime relevant to spent fuel transport and interim storage.
Table 13 and Table 14 present summaries of the actinides, fission products, and activation products that have
importance rankings greater than 0.1% in any of the application areas. Nuclides having a relative importance
greater than 10% of the total are designated as high ranking (H), those between 1% and 10% are medium
ranking (M), and those less than 1% are low ranking (L). These designations are provided for a reference
low-burnup fuel having 3-wt % 235 U and 20-GWd/t and high-burnup fuel having 5-wt % 235 U and
70-GWd/t to give a quick indication of large changes in rank with increasing burnup. A reference cooling
time of 20 years was used in generating these rankings. Thus, the tables do not include some of the shortlived
radionuclides important in decay heat and shielding analyses and do not indicate large changes in
nuclide ranking which do not result in a change in the ranking group (i.e., groups are very broad). These
tables should therefore only be used as an approximate guide to nuclide importance for longer cooling times
(about 20 years).
One of the most significant changes in the spent fuel characteristics associated with high burnup, relative to
conventional-burnup fuel, is the dramatic increase in the 244 Cm concentration in extended-burnup fuel, which
has an important impact for both decay-heat rates and shielding applications. For decay heating the
contribution of 244 Cm increases from typically about 1% of the total at low-to-moderate burnup and cooling
times less than 50 years, to over 10% of the total at high burnup. A large increase in the importance of
238 Pu to decay heat is also observed for high-burnup fuel, and it can contribute from 10% to about 30% of the
total decay heat for high-burnup fuel. The effect of the increased 244 Cm inventory is also responsible for the
large increase in the neutron source term (spontaneous fission) at high burnup relative to the gamma rays.
The analysis of a steel transport cask indicates that the contribution from 244 Cm can increase from 3% at
20 GWd/t to over 60% of the total dose rate at 60 GWd/t after 20-years cooling. Consequently, existing
validation data for low- and moderate-burnup fuel may not be representative of extended burnup fuel since it
may not accurately represent the contribution from 244 Cm and other potentially important radionuclides.
Any calculational uncertainties or biases based on validation studies for lower-burnup fuel need to be
reviewed with respect to the changes in the underlying fuel compositions with enrichment and burnup that
contribute to the response.
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