Design and Implementation of a Tile Hole Drying System for Water Removal from a System Containing Immobile Degraded Spent Fuel and Container
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Abstract
Canadian Nuclear Laboratories (CNL), formerly known as Atomic Energy of Canada Limited (AECL) stored irradiated natural uranium fuel rods in vertical in-ground structures known as Tile Holes. These were below-grade concrete cylindrical structures that were vertically positioned on a concrete slab and that surround 10 inch (25.4 cm) diameter mild-steel pipes approximately 15 feet (4.57 m) in length. Nine fuel rods were placed in an 8 inch (20.3 cm) diameter mild-steel pipe, referred to as a “canoe can,” which was closed on each end, but remained open along one side. This canoe can was placed in the tile hole and covered at ground level with a steel flange and a lead shield plug.
One tile hole in particular had a history of water ingress from the top of the structure due to precipitation and snow melt. Tile hole dewatering campaigns were carried out in 1984, 2009 and 2010 to remove the free standing water, however not all of the water could be removed at the time. Attempts to retrieve the fuel and canoe can were not successful due to excessive corrosion build-up on the 10-inch steel pipe and the canoe can. A 2018 camera inspection of the tile hole revealed the extent of corrosion on the steel pipe, fuel can, aluminum cladded metallic uranium. There was therefore a need identified to place the fuel in a stable and retrievable state.
A process of evaporative mass transfer was proposed using heated argon gas as the carrier of water vapour from the tile hole bottom region. A portable Tile Hole Drying System (THDS) was designed and constructed to remove the bulk water from the tile hole by evaporation. A safety evaluation determined that operation of the THDS did not impose unreasonable risks to site personnel and/or members of the public. Successful operation and performance of the THDS for removal of the bulk water was demonstrated in the field and was confirmed by camera inspection of the bottom region of this tile hole.