Experimental Measurement of Vacuum Assisted Drying of Spent Nuclear Fuel

Abstract

A prevalent issue within extended long term dry storage units for spent nuclear fuel has always been fuel and cask contamination. This contamination can be the result of the helium within the cask leaking into the atmosphere or inadequate vacuum drying techniques. Once the cask integrity has been compromised, the helium starts to leak, and the resulting space once occupied by helium in the casks is replaced with ambient air. One of the other prominent gases found within ambient air besides oxygen is water vapor which can be a result of both helium leaking and poor vacuum drying techniques. Contact between water and the fuel rods/assemblies for a prolonged amount of time can result in corrosion of the fuel cladding, and the canister if exposed. The potential of corrosion of the fuel cladding increases risk of radioactive fission fragments contaminating the environment, increases the radioactive period of spent nuclear fuel, and decreases the potential for fuel rod repurposing within the future if U.S. law permits. With literary findings showing liquid water within the inner cask in a long term storage unit of fifteen years or longer, proper drying techniques have not been fully developed. There are a number of projected theories about how water is entering the cask without an external crack or imperfection within the inner cask walls. This case study aims to solve this issue by inspecting the vacuum drying process of the fuel rods/assemblies from the temporary on-site storage pools to their respective long term dry storage casks. The purpose of this case study is to conduct a laboratory experiment of a scale replica of one dry storage cask and the vacuum drying process before long term storage. The experiment will be focused around the process of applying several cycles of vacuum and backfilling the cask with Helium. The purpose of several cycles of backfilling gas is to simultaneously introduce more of a pressure gradient for water evaporates to depart the pressure vessel and to avoid thermodynamic temperatures that would otherwise freeze the top layer of water. To do this, the vacuuming process must be properly understood, as pulling a vacuum drops pressures instantaneously. There are possibilities of freezing water vapor into its solidified form due to its thermodynamic triple point during this vacuum process. Once water is trapped under a layer of ice within the vessel, water will remain throughout storage time due to restrictions to its own geometries. The importance of developing a scale model and improving the drying process that precedes long term storage of spent nuclear fuel is a necessary solution to existing contamination results for practical future applications within the United States and other countries moving towards long term storage of spent nuclear fuel.