engleski

Radiation shielding analysis of the HI-STORM FW storage cask

This paper presents radiation shielding analysis of the HI-STORM FW storage cask designed by the Holtec International, Inc. for the Nuclear Power Plant Krsko Spent Fuel Dry Storage (SFDS) project. The first part of the analysis is addressing the problem of a particle source generation. For each spent fuel assembly (SFA) neutron and gamma source intensities and spectra were calculated using ORIGEN-S module of the SCALE 6.2.3 code package. The second part of the analysis is considering neutron and gamma dose rates calculation, for which hybrid shielding methodology based on Monte Carlo (MC) code MCNP6.2 and ADVANTG3.0.3 code was used. The dose rates for the reference case included the contributions of fuel neutron, fuel gamma, secondary gammas from (n, γ) reactions, as well as gammas from activated SFA hardware components due to Co-60 presence. Expectedly, the results have shown that the dose rates around the cask have nonuniform behavior due to a specific design of the cask. The heavy concrete body of the cask presents a good attenuator for all types of radiation, however, a considerable streaming through the top and bottom ventilation openings was noticed. It was also observed that an important role was played by the isotope Co-60 which emits two lines of high energy deep penetrating gammas. The convergence of dose rates and associated relative uncertainties during MC calculations have shown that significantly more histories were required for gamma calculations compared to neutron calculations. To investigate the impact of different SFA content in the casks on the dose rates, shielding analysis was made for 16 casks that will be loaded in the first campaign. For comparison, the calculations of a single HI- STORM FW cask were additionally performed using MAVRIC module of the SCALE 6.2.3 and the obtained differences in external dose rates of 20–30% are expected. The calculation cases showed the differences in casks doses due to different casks loading within loading plan criteria, reduction provided by heavy concrete in cask body (more important for gammas), importance of careful design of lower and upper casks openings to keep increase of dose rates, compared to closed configuration, within one order of magnitude, and compromise needed when describing details of intensity and spectrum of SFA sources. The increased calculation mesh density was used to show that selected discretization is appropriate, and the limited influence of storage pad thickness was demonstrated by performing calculation with thick concrete slab corresponding to Dry Storage Building (DSB) floor foundation thickness.

Spent fuel dry storage ; Storage cask ; Radiation shielding ; Hybrid shielding ; Monte Carlo

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