The size, quantity and weight of the graphite blocks made the task unsuitable for manual handling, and the only existing lifting appliance was a 3t single speed overhead crane which was installed with the reactor in 1946. The only entry and egress from the reactor was through a 3m square roof opening 13m above ground level. The graphite blocks covered a base area inside the reactor of over seven square metres, which prevented most of the blocks from being lifted from outside of the reactor.
UK crane builder RS Cranes supplied and installed a single girder light crane system with power driven hoist, long travel and cross travel motions inside the reactor and suspended from the internal roof structure. The crane was manufactured in sections in order that it could be passed through the 3m roof opening and assembled inside the reactor. Once fully installed the crane provided an eight metre square coverage inside the reactor.
To lift the graphite blocks, a drill and tap machine was suspended from the crane and lowered onto each block. To eliminate any trailing power feed cable, a cable reel was attached to the crane bridge. The machine had three special drill bits, which drilled into the graphite and then tapped the drilled holes and locked into the graphite to allow the block to be lifted via the hoist. The graphite drill dust was collected and used for checking the radioactivity of the blocks.
The control for the drill and tap machine was incorporated into the crane radio remote controls by RS Cranes in order that the crane and drill machine could be controlled remotely by a single operator.
As each layer of blocks was removed from inside the reactor the access to the internal crane for service or maintenance purposes would become limited. In order to overcome this the control panels for the crane and radio control systems were mounted separately from the crane, outside the reactor.
The internal crane system was used to lift individual graphite blocks into fabricated steel baskets capable of holding up to 20 blocks at a time. The baskets were then lifted out from inside the reactor using an inverter controlled power slew jib crane, which was installed onto the outside roof of the reactor. This could then pick up from inside the reactor, slew round and then lower the baskets onto electric powered hydraulic scissor tables 13m below for further processing.
The problem then arose of how to handle the blocks for processing, as each block required checking on all sides for radiation.
A free standing runway system was installed above the scissor/process tables that could be withdrawn as the jib crane on the roof of the reactor lowered the baskets onto the tables, and then positioned over the table again to remove the blocks from inside the baskets.
Running along the runway system was a vacuum-lifting device with a special suction cup with a head width of under 100mm designed for handling the blocks. This unit was fitted with an extension leg to allow the operators to lift each block out of the baskets and transport them along the process table for monitoring, inspection and labelling and then palletising prior to moving onto the second process area.
As one basket was being lowered another was being filled inside the reactor. The process was carried out continually until the 13,500 blocks had been removed – a process that took six months. At that rate, about 80 blocks were moved a day.
A second vacuum lifting device was suspended from a free standing jib crane with a roller bed table. This allowed the blocks to be lifted from pallets and manoeuvred into the crushing machine before being contained in drums.
The original 3t overhead crane
was fitted with an inverter drive
control system by RS Cranes. It was fitted with a basket and used to lower personnel into, and lift them out
of the reactor.