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Safe work access for nuclear containers

Safe work access for nuclear containers
Dounreay was Great Britain\'s experimental centre for fast reactor research and development from 1954 to 1994. Now at the end of its life Dounreay is the largest single nuclear decommissioning project in Scotland and second largest in the UK. As part of this huge decommissioning program legacy radioactive liquid waste from the site\'s reprocessing era has to be processed and moved into storage facilities. 

Dounreay was unique among nuclear facilities in the UK as it managed the fuels and wastes from its own power plants. Drums containing waste pass through the import / export section of the cementation facility where they need to be removed from their transportation container. Before this can happen the lid of each container is removed or attached. In order to carry out safe operations while working around the container for lid removal / attachment process workers need a safe access platform.

A container, containing a drum of nuclear waste is positioned under the platform by a special transport vehicle. So that a platform can be lowered over the container the middle section of the platform is removed. With the platform in the lowered position a team of workers can walk onto the platform and manually unbolt and remove a series of locking bolts at the top of the container, which hold the container\'s lid in position. The team then retire from this work and the platform is raised to its original position. The container (now minus the locking bolts) is transported to another more secure area where the container lid is removed, the drum is then removed from the container and taken to another area where its nuclear waste is treated.

Drums of stabilised waste that require long term storage are exported from the facility in sealed containers that have their lid sealed using the platform. One type of drum can contain 500-litres of cemented waste made up of 178 litres of liquid waste and 610kg of cement to create a "monolithic" (ie minimal cracking of the cement block) waste package weighing 1.25 tonnes.

The mechanism used to lift the access platform is a screw jack system manufactured by Power Jacks. Four screw jacks are arranged in an "H-configuration" with each screw jack at a corner of the platform. The screw jacks are mechanically linked together using bevel gearboxes, drive shafts and couplings so that they can be driven by one electric motor.

The screw jacks used are based around Power Jacks E-Series metric machine screw jack model TE1805. This is an upright translating screw jack where the screw translates through the screw jacks gearbox, however in this application the design is altered so the screw remains stationary and the gearbox climbs the screw. The complete travel of the screw jacks is 3500mm. As safety is paramount at Dounreay the screw jacks each have integrated safety features including safety nut, wear monitor and additional screw guides. The safety nut provides full load holding capability in the unlikely event of the main lifting nut thread failure. The wear monitor allows the customer to inspect the screw jacks to check for the amount of wear on each lifting nuts thread. The additional screw guides provide increased lateral rigidity for a stable working platform. As the gearbox is moving up and down the screw a bellows boot is fitted on either side of the gearbox to prevent ingress of debris onto the screw threads.

The bevel gearboxes used to link the screw jacks mechanically are ultra-compact from Neeter Drive. These bevel gearboxes have a small working envelope and superior gear support thanks to a central bearing support inside the gearbox. For this installation Series 37 and 38 gearboxes in 3-way solid shaft configuration were used to distribute mechanical power to the screw jacks. The drive shafts, screw jacks and gearboxes in the system are connected together using steel geared continuous sleeve couplings.
The platform lift is part of Dounreay\'s maintenance and safety programme and is used up to 4 times per day, 5 days per week as part of the de-commissioning process.

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