Experimental Evaluation of Drying Spent Nuclear Fuel for Dry Cask Storage Through Vacuum and Forced Helium Dehydration

Currently approximately 80,000 tons of spent nuclear fuel (SNF) are in either dry or wet storage at nuclear power plants or designated spent fuel storage facilities. Over 2,000 tons are added to that sum each year. All SNF must be dried to an accepted Nuclear Regulatory Commission (NRC) drying criteria before being placed in dry storage. Inadequately drying SNF increases the risk of detrimental effects to the fuel while in dry storage. Two commonly used drying operations in the nuclear industry are vacuum drying and forced helium dehydration (FHD). Although these drying processes have been used for many years, there have currently been no experimental work with full-scale fuel assemblies confirming the residual water following the two drying operations. The purpose of this work is to experimentally evaluate the performance and drying effectiveness of FHD and vacuum drying on SNF. Experimental drying tests were conducted on a full-size Light Water Reactor (LWR) fuel assembly (Framatome Atrium 10A Boiling Water Reactor (BWR) assembly) consisting of depleted uranium rods, 12 heater rods to simulate decay heat of SNF, and an interchangeable rod position to examine key features of concern such as failed fuel rods, BWR water rod, and Pressurized Water Reactor (PWR) guide thimble. The LWR assembly was housed inside a vacuum chamber with structures simulating baskets and rails that are found in industry drying canisters to center the assembly. Additional drying tests were conducted with Holtec International drying equipment on a full-size Type 1a basket containing 10 mock aluminum-clad fuel assemblies (ASNF) mimicking fuel used in Idaho National Laboratory’s (INL) Advanced Test Reactor (ATR). The potential of freezing in spacer discs was also evaluated in both drying setups through a simulated spacer disc. In vacuum drying tests, the formation of ice was prevented when increasing the hold time at each pressure sequence hold from 5mins to 15mins. Ice formation was also seen in areas the volume of water was large relative to the surface area of the water. Faster drying times were achieved with increase of decay heat. Industry drying criteria was sometimes found inadequate, leaving upwards of 18.5mL of bulk water. Consistency was found in complete dryness when both the vapor pressure did not rise more than 1 Torr during the final hold and the dew point inside the canister at the start of the final pressure hold was between -8 and -16°C. For FHD, the effectiveness of drying was observed to be directly proportional to the mass flow rate and temperature differential across the canister. Although improvements are needed in facility hardware to better represent industry FHD conditions, results did show the FHD drying criteria is adequate and improvements on drying time are seen when treating the siphon as an inlet rather than an outlet. Overall, the capability to control the fuel temperature through concurrent fuel cooling gives FHD the ability to further decrease drying times without exceeding fuel cladding temperature limitations