nureg/cr-6700 - Oak Ridge National Laboratory

Section 5
Radiation Shielding
5 RADIATION SHIELDING
The fractional contributions of individual radionuclides to the radiation dose rates from a transport/storage
cask were evaluated for extended-burnup fuel and used to rank nuclides and obtain their importance to
shielding analyses. Three different cask models were used to assess radionuclide importance: a carbon steel
transport cask, a lead transfer cask, and a concrete storage cask. The variation in nuclide importance to
shielding as a function of cooling time has been previously studied in Ref. 9. This study does not attempt to
compile these earlier findings but instead focuses primarily on the changes in nuclide importance for
extended-burnup fuels.
The typical variation in the neutron and gamma dose rate as a function of increasing burnup is shown in
Figures 11 and 12 for the steel cask model and 5-wt %-enrichment fuel. The figure illustrates the rapid
increase in the neutron dose rate component with burnup relative to the gamma-ray component. While the
gamma dose rate (D g ) increases nearly linearly with burnup, the neutron dose rate (D n ) increases
approximately as the burnup (B) to the power of 4 (D n ∝ B 4 ), indicating that neutrons represent an
increasingly important component of the dose rate relative to gamma rays in cask shielding analyses
involving high-burnup fuel.
The profiles in Figures 11 and 12 are representative of the other cask designs and fuel enrichments, although
lower-enrichment fuel produces higher absolute dose rates, attributed to the fact that lower-enrichment fuel
(lower fissile content) requires exposure to a greater neutron fluence per metric tonne (t) of uranium to
achieve the same burnup as higher-enrichment fuel. As a result, for a given burnup the actinide inventory
generated by neutron capture of 238 U increases by an amount nearly inversely proportional to the enrichment.
The effect of enrichment is most evident at longer cooling times and higher burnups, where the actinides tend
to become increasingly important.
The relative importance of neutrons with burnup is illustrated in Figures 13–15, which shows the neutron
dose fraction (fraction of the total dose rate) as a function of burnup for 5-wt % PWR fuel for concrete, steel,
and lead cask models. The different cask models show similar trends, although the magnitude of the neutron
dose component varies by cask design. For the steel cask model, the neutron component increases
dramatically with burnup, from less than 5% at 20 GWd/t to nearly 40% at 60 GWd/t after a 20-year cooling
time. For the lead cask, the increase is from about 5% at 20 GWd/t to more than 60% of the total dose rate at
60 GWd/t. For the concrete cask the relative increases are similar; however, the neutron component is
significantly lower (e.g.,