nureg/cr-6700 - Oak Ridge National Laboratory
nureg/cr-6700 - Oak Ridge National Laboratory
nureg/cr-6700 - Oak Ridge National Laboratory
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Section 3 Criticality Safety<br />
3.1 Criticality-Safety Rankings<br />
The fractional contributions of individual actinides and fission-product nuclides to the neutron absorption<br />
were generated to obtain the radionuclide importance to <strong>cr</strong>iticality-safety analyses. The actinide and fissionproduct<br />
plots are presented separately due to the large number of nuclides that contribute greater than 0.1%<br />
to the total neutron absorption (12 actinides and 25 fission products) relative to the other application areas<br />
considered in this study. In terms of nuclide rankings, the PWR and BWR results were found to be very<br />
similar; therefore, plots are shown for only the PWR assembly.<br />
The nuclide importance, expressed as the fractional contribution of each nuclide to the total neutronabsorption<br />
rate, is illustrated in Figures 3 and 4 (actinides) and Figures 5 and 6 (fission products) for 5-wt %<br />
PWR fuel. The fractional contributions and rankings of actinides in the PWR fuel are given in Table 4 for<br />
decay times of 5 and 100 years. The corresponding contributions and rankings for fission products are given<br />
in Table 5. The BWR actinide rankings are listed in Table 6, and fission product rankings are located in<br />
Table 7. Note that where the ranking order of nuclides is different for the BWR than the PWR, that the<br />
absorption values are nearly identical, so that the general trend is still the same.<br />
The dominant actinide absorbers (absorption fraction >0.1) are 238 U, whose absorption fraction remains fairly<br />
constant (at about 0.28) as burnup in<strong>cr</strong>eases: 239 Pu, whose fraction in<strong>cr</strong>eases up to about 50 GWd/t, then<br />
levels out; and 235 U, whose fraction de<strong>cr</strong>eases rapidly with burnup as it is depleted by fission. The next<br />
group of moderately absorbing actinides (0.01 < absorption fraction < 0.1) include 240 Pu, 241 Am, 236 U, and<br />
241 Pu (at shorter decay times). The rate of in<strong>cr</strong>ease of importance for these actinides is fairly rapid up to<br />
about 40 GWd/t, then somewhat less at higher burnups. The importance of the remaining actinides<br />
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