An Investigation into Vacuum Drying of Spent AGR Fuel through Failed Cladding

In the United Kingdom the spent nuclear fuel produced by the Advanced Gas-cooled
Reactors (AGR) is stored underwater at Sellafield awaiting a decision on the final
disposal. Currently it is considered best practice to continue with the wet storage of the
spent fuel however dry storage is being researched as an alternative should it be deemed
necessary. There is also an acknowledgement that prior to the disposal of spent fuel in
any future Geological Disposal Facility (GDF) will likely require the spent fuel to be
dried. The preferred approach for conducting this drying process is called vacuum
drying.

The vast majority of the spent fuel is expected to remain intact and not pose an
impediment in the transition from wet interim storage to dry interim storage or disposal.
However, under reactor conditions, the cladding can be rendered more vulnerable to
corrosion. If the cladding was to corrode and fail then water from the pond can ingress
into the fuel pin and become trapped. With the failure of the cladding providing a route
for the water to enter the pin, it was suggested that this should also be a route by which
the water could be removed. It is important to remove this water as the radiation from
the spent fuel can cause the trapped water to undergo radiolysis which can produce
hydrogen gas, hydrogen peroxide and other species which are a detriment to the safe,
long-term storage or disposal of the spent fuel.

This thesis presents the results of an investigation into the vacuum drying process and
the production of a model which can be used to provide insight into the conditions
required for the drying. To achieve this representative cracks were required for the water
and water vapour to pass through during the vacuum drying tests. An adapted form of
the drop evaporation technique was used to produce these cracks in stainless steel. In
addition to this work, the flow of air through various defects (pinholes, slits and cracks)
was studied using the fluid flow set up with the results from these experiments
presented in this thesis. Additionally a method to determine the dimensions (length and
average width) of a defect is presented and the data acquired from this process used to
validate the vacuum drying model.