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ABSTRACT

Photon dose rates as a function of fission product decay times have been calculated for spent fuel assemblies typical of MTR-type research and test reactors. Based upon these dose rates, the length of time that a spent fuel assembly will be self-protecting (dose rate greater than 100 rem/h at 1 m in air) can be estimated knowing the mass of fuel burned, the fraction of fuel burned, and the fuel assembly specific power density.

The calculated dose rates cover 20 years of fission product decay, spent fuel with up to 80% U-235 burnup and assembly power densities ranging from 0.089 to 2.857 MW/kgU235. Most of the results are unshielded dose rates at 1 m in air with some shielded dose rates at 40 cm in water. Dose rate sensitivity estimates have been evaluated for a variety of MTR fuel assembly designs and for uncertainties in both the physical and analytical models of the fuel assemblies.

The Monte Carlo dose rates reported in this paper are a revision of the dose rates reported in Ref. 1. These dose rates are the result of a revised energy group structure used for the MCNP calculations, and the fluence-to-dose conversion factors for the new group structure. The photon dose rates in air (water) are a factor of 0.85 (0.98) of the old photon dose rates. In addition, this paper presents an analytical method for estimating the photon dose rate of spent fuel, assuming a line-source model of the fission products in a fuel assembly.

Ref. 1: R.B. Pond and J.E. Matos, "Photon Dose Rates from Spent Fuel Assemblies with Relation to Self-Protection", Proc. XVIII International Meeting on Reduced Enrichment for Research and Test Reactors, Paris, France, 17-21 September 1995.

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