nureg/cr-6700 - Oak Ridge National Laboratory

Section 5
Radiation Shielding
The neutron source was further evaluated to identify the major actinides that contribute to the spontaneousfission
neutron source. The relative contributions of the dominant actinides are shown in Figures 20 and 21.
A common feature for all cooling times investigated is the dominance of 244 Cm at high burnup and the
significant increase in the relative contribution of 244 Cm (and 246 Cm) and the decreasing relative importance
of the Pu isotopes to the dose rate with increasing burnup. At lower burnups the contribution from 240 Pu is
significant (particularly at 100-years cooling, where it is the dominant nuclide) although its relative
importance diminishes rapidly with increasing burnup as the curium isotopes build up.
These results demonstrate the high relative importance of neutrons, specifically the spontaneous fission
neutrons, to cask shielding studies for high-burnup fuel and cooling times greater than 5 years. For shorter
cooling times the fission products represent the higher fraction of the dose. However, neutrons only become
a dominant fraction of the total dose rate for the steel and lead cask designs analyzed in this study, which
were less effective than concrete in shielding neutrons. The concrete storage cask has the smallest neutron
dose rate component (60% after 100-years cooling).
5.2 Radionuclide Rankings
The fractional contribution of individual radionuclides to the total dose rate for the three different cask
designs was calculated for the enrichment and burnup combinations and cooling-time ranges of interest to
obtain the importances used in the ranking process. A review of the dominant nuclides for each of the
application regimes (cask design, fuel enrichment, burnup and cooling time) found that the majority of the
dose rate was attributed to relatively few nuclides for all regimes studied. The nuclides listed in Table 10
(11 actinides, 8 fission products, and 1 activation product) account for more than 99% of the total dose rate
for all cask designs and fuel regimes (enrichments and burnup) analyzed.
Table 10 Most important nuclides in cask shielding calculations
U-238 Pu-242 Cm-246 Y-90 Ba-137m
Pu-238 Am-241 Cm-248 Rh-106 Eu-152
Pu-239 Cm-242 Cf-252 Ag-110m Eu-154
Pu-240 Cm-244 Co-60 Cs-134 Pr-144
The nuclide importance, expressed as the fractional contribution of each nuclide to the total dose rate at the
cask surface, is illustrated in Figures 22 and 23 (steel cask), Figures 24 and 25 (concrete cask), and
Figures 26 and 27 (lead cask) for 5-wt % PWR spent fuel. The results are presented as a function of
increasing burnup and for cooling times of 5 years and 100 years. The fractional contributions and ranking
for the dominant radionuclides (>1%) are listed in Table 11 for moderate- and extended-enrichment and
high-burnup PWR fuel (5-wt % and 70-GWd/t) for 5- and 100-year cooling times. The lower-burnup results
(3-wt % and 20-GWd/t) are included for comparison and as verification with previous calculations in Ref. 9.
A comparison of the dominant nuclides and their contributions to the total dose rate for the lower-enrichment
and low-burnup fuel is consistent with previous ranking studies that assessed more conventional-burnup fuel.
41