Gleep in the dark28 July 2004
At the UKAEA's Harwell site, the decommissioning of Gleep, the oldest reactor in Western Europe, has reached the end of a crucial stage.
Harwell International Business Centre in Oxfordshire, UK is one of Europe’s biggest science parks, home to around 100 companies and 5000 employees. What was once the centre of UK nuclear energy research was initially home to racehorses, before conversion to a base for bomber aircraft during the 1930s. Most of the site’s buildings date from that time.
Following World War II, facilities were established under the UKAEA to study neutron scattering and nuclear fission. Among them was the Graphite Low Energy Experimental Pile (Gleep) which was built to prove the physics for the UK nuclear power programme in 1946/47.
Use of the Harwell site peaked in the 1960s, when employment levels reached 6000 before the nuclear industry matured enough to warrant less publicly funded research. From then, the site gradually diversified into more non-nuclear work.
In the 1990s the experimental reactors, such as Gleep, Dido, Lido and Pluto were shut down and contractors were increasingly brought in to carry out decommissioning work. Now, the UKAEA holds only part of the site and intends to delicense some sections for general use. In 2002, the clean-up of Harwell’s Southern Storage Area, which contained 18,000m3 of assorted buried waste from the military and post-war periods was completed and the land has been designated as suitable for alternative use (see NEI July 2002, p22). By 2020, UKAEA hopes it will have delicensed about half of its current licensed area. The removal of Gleep will allow the demolition of two wartime Royal Air Force hangars and clear a portion of the site for new development.
The closure of a radiochemical laboratory in May marked the end of UKAEA’s nuclear research at Harwell, although decommissioning work remains. The Lido reactor has been completely removed, along with 47 other nuclear-use buildings. However, the shells of Dido and Pluto contain cobalt-60 and are being left for 50 years to decay. After that time, the radioactivity of the sites will be greatly reduced and decommissioning will be far easier and cheaper.
After shutdown in 1990, Gleep’s decommissioning began in 1994 with the removal of fuel and control rods. This stage posed quite a challenge as the reactor had only been refuelled once before in 1960 and the total of 11,500 fuel rods accounted for 99% of Gleep’s radioactive inventory. The reactor remained under care and maintenance until April 2003, when Mitsui Babcock were awarded the full decommissioning contract after beating five other shortlisted bidders from a total of 17 firms.
Although Gleep was designed to operate at a maximum of 100kW, it ran for just 18 months at 80kW producing radioisotopes which were used at local hospitals, before spending most of its life running at a nominal 3kW during use for calibration and the testing of materials under irradiation.
The neutron flux at the core centre was extremely stable and was calibrated by the National Physical Laboratory, becoming an international standard. In addition, the alloy used to house Magnox fuel rods was developed using results from Gleep.
Gleep’s core was constructed as a literal pile of 13,500 graphite blocks in layers arranged in a way similar to a parquet floor. The blocks, each weighing 40kg, were chamfered so that fuel and control rods could be simply inserted between the blocks into the heart of the core.
The removal of the core’s graphite blocks was the longest phase of the decommissioning work and ended in May this year. Now comes the four-month process to remove the biological shield before a final survey, a final report and the project’s ultimate completion, scheduled for early 2005.
The graphite pile contained a total activity of 167GBq, of which over 95% came from tritium and carbon-14. This relatively low level meant that it was possible for workers to enter the bioshield, and remove the blocks layer by layer using light protective equipment. However, contact between workers and the blocks was kept to a minimum through the use of semi-remote equipment whereby workers held remote controls for the equipment they used, but did not touch either the blocks or the equipment and all lifting was powered.
The bricks were picked up by a Drill and Tap machine, originally developed for the dismantling of the Windscale Advanced Gas-cooled Reactor pile. The machine would drill three holes into the top face of a block, lock onto it via the holes and allow it to be lifted into a basket. Dust from the drilling was collected with a vacuum collector, tested, and kept as a sample.
When the basket was filled with blocks, it would be lifted out of the bioshield and lowered to a second workplace. There, the blocks’ radioactivity was recorded and they were packaged carefully onto a conventional wooden palette for buffer storage before the next stage, shredding. The use of a remote suction-lift transfer method again ensured there was no manual handling of the blocks.
A standard industrial paper shredder, modified with a new blade and a hydraulic ram, would then shred the blocks into pieces approximately 25mm across, which fell into 200 litre plastic waste drums. The debris of four blocks were put into each drum, which was then stored for a short time in the hangar.
Workers report that there is a surprising variation in the radioactivity of different blocks. Even adjacent blocks were found to vary in activity by as much as tenfold. Despite this, the total radioactivity in each waste drum never exceeded limits appropriate for LLW.
It is possible to trace via a paper trail each block from its original position in the pile to the drum containing it. Samples of the blocks are also retained.
Most of Harwell’s decommissioning waste generated so far, has been LLW and has been shipped off to Drigg in Cumbria. About 2000m3 of ILW will be stored on-site in new facilities. ILW kept in intermediate stores constructed in expectation of a UK national repository has been repackaged in stainless steel drums and put in a new vault store. Some liquid waste has been treated and discharged, while a cementation plant has been built to handle a quantity of highly active liquor.