NTS were asked to help with the movement of un-irradiated fissile materials (‘Exotics’) from the site managed by Dounreay Site Restorations Ltd (DSRL).

The inventory contained 36 PFR sub-assemblies, twelve of which required transportation in specialist M4/12 flasks. The flasks are designed with a ‘Multiple High Standard Water Barrier’ (MWB) providing secondary leak-tight tested seals in the M4/12 cavity. The role of NTS role was to demonstrate the criticality safety for the PFR sub-assemblies within the MWB transport package.

The solution 

Criticality calculations were undertaken for damaged and undamaged package conditions, with further assessment determining the impact of temperature changes.  

Calculations demonstrated the criticality safety of the package under normal and fault conditions. The majority of calculations undertaken were demonstrably conservative to give added assurance. 

The outcome 

The criticality assessment undertaken by NTS supported a successful application for Special Arrangement Approval by ONR, allowing the transport of PFR sub-assemblies to be completed safely and securely. This was the UK’s first ‘Multiple High Standard Water Barrier’ for transport. 

This project supported the UK Government with their strategic objective to consolidate fissile materials onto a single site for safe disposal.