Deconstructing Berkeley29 June 2001
Berkeley Power Station is the UK’s first commercial plant to be decommissioned. The cooling ponds building decommissioning project was divided into three phases, the second of which was carried out for BNFL by Rolls-Royce Nuclear Engineering Services and its subcontractors.
Berkeley was commissioned in 1962 and operated until 1989. Its decommissioning began with the removal of fuel from the reactors. The de-fueling was finished in March 1992 and various activities followed, including removal of the gas ducts, boilers, non-active ancillary plant, asbestos and refurbishment of the reactor buildings.
The cooling ponds were formally used for the storage of irradiated fuel elements prior to their despatch for reprocessing. Phase 1 was carried out by Jordan Engineering between June 1994 and April 1996. In this initial phase, redundant plant and equipment were removed, as well as loose and near surface contamination within the pond tanks to a depth of 2mm. Following drainage of the ponds, ultra high pressure water jetting was applied using a remotely operated arm to traverse the pond walls and remove the pond tank coating.
Between April 1997 and May 2000, Rolls-Royce Nuclear Engineering Services carried out phase 2, the largest of the three phases of the project. This involved the design, manufacture installation and operation of equipment to remove the contaminated material from Cooling Ponds 1 and 2, the burst slug pool, irradiated fuel transfer tunnels, sumps and building walkways. The ponds building and transfer tunnel ventilation system was completely removed, as was any other area within the building and transfer tunnels where contamination had occurred.
The ponds were situated at ground level between both reactors in a purpose built crucifix-shaped building, approximately 77m by 42m. The building had a reinforced ground slab supported on beams and piles. Above ground, the building contained steel portal frames with precast concrete roof units providing a weatherproof enclosure. The external walls were mainly solid brickwork and the below ground structures were made of reinforced concrete.
The two main ponds situated at the north end each had a depth of about 8m. In the centre of the building there was a smaller pond, the Burst Slug Pond (BSP), for the receipt and isolation of suspect fuel elements. Adjacent to the BSP was the Sludge Tank Chamber and Dry Canning Machine Plinth.
At the south end the layout of the building provided for road transport flask receipt and dispatch and collection of operational wastes via a throughway. An extension of the west side of the building accommodated an active effluent tank for the collection of pumped arisings from both reactors and the pond building drainage system. A 50t gantry crane was provided to lift the fuel transit flasks through access shafts into the main bay of the pond building, and to lift the road transport flasks from the ponds and transfer them through the building.
After completion of the phase 1 operations a sampling programme was carried out within the ponds building to assess the extent of contamination ingress within the main areas. Removal of contaminated concrete down to specified depths was required to enable clearance monitoring of the ponds building. The specified depths were best estimate, and it was thought likely there may be certain areas where contamination might have penetrated beyond the designated depth. These areas included concrete surfaces where cracking had occurred, civil construction joints and regions where surface coating had deteriorated.
A remotely operated dry concrete planing removal process was used to minimise waste volumes, avoid processing of wet wastes and to obtain an even concrete surface to assist in the radiological monitoring operations. The planing equipment was designed, developed and remotely operated by the main civil subcontractor, Longholme Construction. Initial trials of the planing equipment were carried out within the burst slug pool, which resulted in modifications to the equipment in preparation for concrete removal within the cooling ponds.
The modified planing equipment was able to remove concrete at the specified depths (60mm for pond 1 and 40mm for pond 2) from the wall surfaces within the pond areas. The planing head was supported by a frame sited on two rails, one at the pond floor and the other located at the walkway level. The planing head travelled vertically throughout the height of the wall while the frame travelled horizontally along the length of the wall.
Due to the deployment framework, the planing head could not access the corners of the pond walls. Concrete in these areas had to be removed manually with jackhammers. The floors of the ponds were planed at the specified depths by attaching the planing head to a skid steer unit that was driven over the pond floors.
As the concrete removal works progressed within the ponds areas and as knowledge of the contamination ingress in certain structures increased, it became apparent that removal of the surface concrete from walls associated with the pumphouse and wet sumps had not been sufficient to meet the radiological end point (REP). Additional concrete removal to achieve the REP for these structures required the removal of large areas of reinforcement bar which would have resulted in structural instability. The following structures needed to be removed completely:
•Pond 2 destrutter wall.
•Wet sump walls.
•Pumphouse walls down to 5m from pond floor.
•Destrutter bay plinth areas within ponds 1 & 2.
•West and east parapet walls in ponds 1 & 2 respectively.
The first major structure removed was the pond 2 destrutter wall. Due to the access restrictions it was considered that conventional demolition of this structure was out of the question and procedures were put in place to dismantle the wall in a controlled manner. The equipment used would be a mixture of hydraulic crunching and drill and burst techniques.
Scaffolding was erected around and over the wall, which in turn acted as the framework for the containment structure. A lifting beam located on the scaffold directly over the wall was used to support the hydraulic cruncher. The cruncher sequentially demolished the wall followed by the reinforcement bar down to the pond floor level.
The hydraulic crunching method was also used in conjunction with drilling and bursting for the controlled demolition of the pumphouse and wet sump walls. For this operation a large containment structure was erected around the pumphouse, and was modified to cater for the decreasing size of the pumphouse and various working faces.
Mass concrete removal such as the plinth areas and sumps located on the floor of the ponds was carried out using a hydraulic breaker mounted on a skid steer.
There were various localised contaminated areas, mainly associated with construction joints in the ponds and building floor slabs. Removal of these areas was carried out using jackhammers. This equipment proved ideal for removal of contamination from construction joints. Operator control and awareness negated damage to the water bar that would have allowed water ingress into the ponds – the pond floor in certain areas and conditions was considered to be up to 6m below the water table.
The strategy for phase 2 was to maintain the building as a C2 designated area so as not to restrict operations within the building. A C2 or C3 designated area was determined from the likely airborne contamination levels. The level of airborne contamination denoted the level of protective clothing for the operators. For example, in a C2 designated area, an operator would wear coveralls, overshoes and gloves, while in a C3 designated area, an operator would wear the above protective clothing plus additional coverall and gloves, as well as dedicated footwear and a respirator. By using a dry concrete removal process it was necessary to contain these areas to enclose any generated airborne activity that may have caused the whole of the building to be designated as C3.
The main containment structures with dedicated ventilation were placed over each pond, the pumphouse, pond 2 destrutter wall and the burst slug pool (BSP). With the exception of the BSP, the large containment structures (constructed with scaffolding with associated working platforms) were continuously modified to assist in concrete removal and optimise airborne control measures. The containment areas were designated C3 and each containment had its own change barrier.
Two large containment structures were erected to contain the screed and concrete removal on the building walkways south of the cooling ponds. Smaller localised containment structures were erected to enclose localised concrete removal operations throughout the ponds building.
Concrete wastes were loaded into 200-litre drums at the point of origin and transferred to the ISO loading area. The drums were weighed and monitored prior to the contents being tipped into the ISO container in order to achieve maximum waste packaging efficiency. Over the duration of the works, a significant number of such operations took place resulting in 55 x 1/2 height ISO containers of mixed steel and concrete being filled.
Throughout the works various redundant plant and equipment was removed from the building. The main items were:
•Active drains pipework and ancillary equipment.
•Effluent sumps and associated steel and brick liners.
•Various encast steelwork such as crane rails and flask supports.
•Electrical panels, cables and ancillary equipment.
•Ponds building & transfer tunnel ventilation and filtration plant.
•Ponds building 50t crane.
The dimensions and layout of the transfer tunnels were ideal for the use of grit blasting as it was easy to contain the grit blasting area. The tunnels were segregated into 10m partitions with dedicated ventilation and services that facilitated the retrieval of the used grit from the floor for recycling. From the experience gained in the tunnels, the option for paint removal from the walls within the main building using grit blasting was assessed. However layout of the main building superstructure, with the amount and layout of steelwork and other obstructions around the building, did not allow for easy retrieval of the grit material.
The successful campaign within the tunnels led to grit blasting trials and eventual decontamination works within certain inaccessible active drains pipework within the building. For example, some drains pipework was buried up to 8m under the building base slab and mass concrete.
A containment was set up within the building for grit blasting operations, and was used to decontaminate any plant and equipment used for the works. The greatest achievement was the decontamination of the active ventilation ductwork that reduced the waste disposal volumes considerably. Out of all the active ventilation ductwork, only the welded seams (less than 1% of overall ductwork volume) were deemed too difficult to prove free of contamination and suitable for release as scrap material.
Contamination associated with painted surfaces within the main building was mainly attributable up to 2m above the walkway level and this paint required removal. Air operated scabblers and rotopeens were used for this operation as these methods were considered appropriate to minimise generation of dust levels within the building. Rolls Royce and Trelawney developed a 10” rotopeen supported from the building superstructure beam with a tool balancer. This was able to scabble large wall areas in a short timescale.
End point monitoring
After the specified depths of concrete had been removed extensive health physics monitoring activities were carried out on the ponds walls and floor surfaces to check if the REP had been met. This operation was mainly carried out manually, as 100% surface monitoring was required. Prior to monitoring activities each wall and floor surface was “marked up” into m2 segments to assist clearance-monitoring objectives. The checks were successful and the results also independently verified.
Throughout the work, routine monitoring operations were carried out on a daily basis to:
•Assess airborne levels within the main building.
•Assess airborne contamination levels within contained areas.
•Identify high dose rate areas resulting from concrete removal operations.
•Monitor personnel doses during decommissioning operations.
•Monitoring of personnel through change barriers.
•Monitoring of any potential operator internal dose intake by bio assay.
Phase 2 works proved to be a great success, paving the way for a successful demolition stage. This final phase was carried out between August 2000 and April 2001.