Almost 200 containers of low-level radioactive waste that once formed part of Britain’s experiment with fast breeder nuclear reactors are being entombed in a purpose-built underground vault at Dounreay.
The containers are stacked 4 high and the spaces filled with grout before being covered by a steel reinforced concrete slab. This slab will become a floor to support more containers going to the vault for disposal.
Low-level waste from the clean-out and dismantling of the site’s reactors, fuel plants and laboratories is being packed into drums, super-compacted to become pucks and stacked inside containers that are filled with grout before being moved to their final resting place in a cavernous below-ground vault. Some larger waste items go into the containers and are grouted alongside the pucks.
Originally, it was planned to half-fill the low-level waste vault with almost 1,000 containers before entombing them in grout and covering with a reinforced concrete floor. This provides a stable base upon which another 1,000 containers can be stacked.
But the project team at Nuclear Restoration Services re-evaluated their methodology and concluded there were practical advantages by adopting a staged approach to carrying out the intermediate floor slab works.
The first stages are now complete, marking the permanent disposal of 192 half-height ISO containers that cover a fifth of the available floor-space and giving the team and their contractors valuable experience.
The original plan was to pour concrete slabs to a depth of 500mm over the full 80m x 50m area once the vault was half full,” explained operations manager Graham Bremner.
We looked again at this and realised there were benefits from moving to a staged approach in line with grouting instead – a significant reduction in downtime during mid-life closure of the vault for waste disposal operations and minimising significant demand of concrete supply that would impact local contractors.
The reinforced concrete floor will eventually support point loads of containers 4 high up to 160 tonnes in weight.
The vault is one of 2 operational disposal facilities, with the other being used for low-level demolition waste.