nureg/cr-6700 - Oak Ridge National Laboratory

Criticality Safety Section 3
The fission product exhibiting the most rapid increase with burnup is the stable nuclide 155 Gd. Its precursor
155 Eu has a 4.75-year half-life, and so 155 Gd has not reached equilibrium even for a 70-GWd/t exposure. The
importance of 155 Gd increases by an order of magnitude between 20 and 60 GWd/t for cooling times greater
than about 5 years, and it is one of the larger fission-product absorbers in high-burnup fuel. Another stable
nuclide, 153 Eu, exhibits a similar, although less-pronounced effect, as its precursor 153 Gd and has a 241.6-day
half-life. However, 153 Eu ranks considerably lower than 155 Gd in high-burnup fuel for cooling times more
than 5 years.
The criticality safety nuclide rankings were recomputed for the PWR fuel using a SAS2 model in which the
specific power was 25 MW/t. In general, the nuclide rankings were not affected, except that (rarely) the
order of two nuclides could be reversed. In those cases, the absorption rates for the nuclides whose order
changed were essentially identical. The only noticeable difference in the actinides for decay times from 5 to
100 years was an increase of about 6% in the absorption fraction for 238 Pu. The amount of 238 Pu increases
because its precursor 237 U (produced both by capture from 235 U to 236 U and from (n,2n) from 238 U) undergoes
relatively more decays to 237 Np when the power (and therefore flux) is lower. The biggest changes in
fission-product absorption rates were for the stable 149 Sm (−11%) and 157 Gd (−15%) nuclides that have large
capture cross sections.
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