A radical rethink of the refuelling system won DML the contract.
Under Devonport's existing refuelling system, fuel is lifted relatively high and far via an overhead crane. The correspondingly higher risk of an accident resulting from failure in the system called for expensive equipment specification. Under the new system, refuelling will be carried out at ground level and rods lowered a minimal distance into submarines.
New and spent fuel rods are stored in a central low level refuelling facility. They are conveyed to the head of a dock via a dedicated low level rail system and transferred to a reactor access house. This three- storey, 3,000t structure straddles the dock and is mounted on rails. The RAH is moved by incremental hydraulic jacks. From the head of the dock the RAH moves to position above a sub's reactor. A 90t capacity crane then lowers rods through an interconnecting collar into the vessel.
The Low Level Refuelling Facility consists of two interconnecting land- based and 'island' structures. The island building, which is under construction, stands at the north end of the main basin close to 14 Dock.
Contractor Taylor Woodrow started construction with erection of a cofferdam. This was dredged using air lift and Venturi methods in preparation for placement of a 27m wide, 46m long and 10m deep mass concrete plinth.
The plinth was cast underwater to prevent the dock bed from lifting. Joints were jet-cleaned to ensure a good bond. The 40N hydrocrete was developed by Taywood Engineering with 75% ground granulated blast furnace slag replacing Portland cement. The mix contained 450kg/m3 cement overall and additives to stabilise the mix and prevent fines washing out in the water.
Taywood used local limestone as aggregate because it demonstrated good thermal characteristics, enabling thick pours to be achieved. It placed 3,600m3 in the biggest of four pours. Concrete trials started last November and construction began in February this year. The final pour took place at Easter. Once 10m thickness had been achieved there was enough mass to counteract buoyancy and the site was dewatered.
The plinth was anchored to underlying rock by subcontractor PSC Freyssinet with 112 high carbon steel, 75mm diameter McCalls anchors. Because the rock stratum slopes across the site, anchors vary in length between 37m and 40m with 200t loading. Some failure was expected in the 200t anchors and redundancy was built in. However, the subcontractor experienced only one failure.
Casting of the LLRF foundation is under way and has reached 11m above the plinth. This will support a steel portal framed building containing stainless steel lined storage facilities and a 90t crane.