After used nuclear fuel (UNF) assemblies are removed from the reactor, they are immediately stored in cooling pools to lower their radioactivity and heat generation rates. After a few years, they are transferred to dry storage casks for interim storage. During the transition from wet storage (pool) to dry storage (cask), different non-oxidizing gases, such as helium, nitrogen, or argon, may be used to drain the water. Following the drying process, vacuum drying is often used to remove any water that may remain in the cask. During this operation, gas pressure is reduced below 400 Pa to promote evaporation and water removal. A temperature-jump thermal resistance at the solid-gas interfaces may cause the temperature of the fuel rods to considerably increase during vacuum drying. At the achievement of the technical specification for vacuum drying, as specified by the 2013 Interim Staff Guidance \[1\], the cask is backfilled with the same non-oxidizing gas to a pressure of 3-7 atm.

In this work, a two-dimensional (2D) geometrically-accurate model of a TN-32 cask is generated in ANSYS/Fluent to investigate the effect of backfill gases and their pressures on peak cladding temperatures. This model is similar to the cask being used in high-burnup (HBU) spent fuel data project lead by the Electric Power Research Institute (EPRI). Air, helium, argon, and nitrogen backfill gases are investigated at pressures ranging from atmospheric (\~10^5^ Pa) to 100 Pa. Steady-state computational fluid dynamics (CFD) simulations that include the effect of gas rarefaction (temperature-jump) at the gas-solid interfaces are conducted. The peak cladding temperature as a function of heat generation rate and pressure is reported as well as the heat generation rate that brings the cladding to the radial hydride formation limit. The results shows that there a competing effects between the temperature-jump and the thermal conductivity of the gas to increase the fuel rods’ temperature. The low pressures significantly increased the peak cladding temperatures, with the increase being most significant for the helium backfill.


Temperature Prediction of a Used Nuclear Fuel Cask With Different Gas Backfills

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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