Decommissioning Bunkers at Hunterston A

By Phil Reade, SAWBR Project Manager, BNS Nuclear Services

Hunterston A power station is located on a promontory of the Ayrshire coast, near West Kilbride, 30 miles southwest of Glasgow, Scotland. It is a twin reactor Magnox power station now shutdown and being decommissioned. The station, Scotland’s first civil nuclear generating station and the largest in operation anywhere in the world when it came online, had a generating output of some 360 MW.

The station comprised two Magnox-fueled, graphite-moderated, steel pressure vessel reactors. One of its unique features was that load refueling operations were conducted from below the reactors. Six 60 MW turbo alternators provided electricity to the grid. Throughout its 25-year operational life, Hunterston A was at or near the top of the World Nuclear Performance charts.

Remote box lid bolt replacement. Photo, BNS Nuclear Services. Click here to enlarge image

Reactor 2 was shutdown on Dec. 31, 1989, followed by Reactor 1 on March 30, 1990. Defueling commenced on Aug. 16, 1990 , and was completed on Jan. 21, 1995, with the last fuel being dispatched from the site on Feb. 8, 1995.

The current preferred strategy for decommissioning Hunterston A, like all U.K. Magnox nuclear power stations, is deferred site clearance, allowing for total site clearance about 100 years after cessation of generation. This strategy minimizes risk to workers, the public and the environment, minimizes waste volumes, is technically straightforward and makes financial sense.

U.K.-based BNS Nuclear Services has completed all the mechanical plant and electrical systems for the U.K.’s Magnox North Hunterston A site solid active waste bunker recovery (SAWBR) project. BNS worked closely with the lead contractor, Costain Oil, Gas and Process Ltd., and Magnox North Sites Ltd., on the design, manufacture and integrated works testing. BNS will also provide engineering support to Costain during site installation and will carry out inactive commissioning at the site prior to handover to Magnox North.

Magnox North is applying its expertise to accelerate the decommissioning work plan, which doesn’t preclude a reduction in the 100-year period to total site clearance.

Scope of the SAWBR Project

The object of SAWBR is the retrieval and processing for storage of intermediate level waste (fuel element debris). Once this objective has been achieved, the decontamination and demolition of the five reinforced concrete bunkers can begin. (This element of the project will be undertaken by Magnox North under the provisions of a separate contract.)

Three of the bunkers were built in the 1950s and two were built in the 1980s. The bunkers contain approximately 3,000 square meters of solid intermediate level waste. Costain has demolished a redundant stairwell and corridor adjacent to the bunkers and is constructing a new building to house the SAWBR plant, which should be completed by the end of 2009 and operational by June 2010.

The bunkers contain several different types of waste. To reduce any risk of fire, the retrieval operations will be carried out in an oxygen-reduced atmosphere. The vent plant was designed and manufactured by Studsvik Alpha Limited, with Costain designing and supplying the oxygen reduction atmosphere (ORA) system.

The Retrieval Process

Two Brokk remotely operated vehicles (ROVs) are the first stage of the retrieval process. The ROVs have been successfully operated at a number of sites, including Trawsfynydd in Wales and Sellafield in Northern England.

The first ROV will be bunker-based and will rake and sweep waste to fill a waste bucket. The second ROV will collect the filled waste bucket and deliver it to the wall-mounted conveyor. To achieve the plant throughput criteria, the ROV operators must retrieve enough waste to fill a 3-square-meter radioactive waste management directive (RWMD) box every four hours.

Remote waste retrieval. Photo, BNS Nuclear Services. Click here to enlarge image

At an intermediate position along the conveyor, the waste bucket stops and the activity of the waste is monitored. This allows the operator to ensure that each 3-square-meter box does not exceed the maximum activity limit set for export from SAWBR to storage. The waste bucket is then conveyed to the elevator/tipper.

BNS has integrated an industrial loader tipper from Lodematic Ltd. The waste bucket engages onto its forks and the bucket is then automatically elevated and tipped, with its contents being fed into a 3-square-meter box via a chute. The chute is designed to contain the material and guide it into the box.

When the operator has filled the box to the desired level, a roof-mounted industrial robot is deployed and performs a pre-programmed routine to level the waste pile to ensure that the box lid can be effectively replaced. To ensure the sealing system is not compromised by the build-up of dust or debris, the robot has the facility via a tool changer to vacuum clean around the box-filling aperture and around the critical sealing faces of the waste chute and delidder. There is also a facility for the operator to remotely attach a gripper and in manual mode, control the robot to remove any rogue items of waste that may prevent the box lid from being replaced. The operator has to be satisfied that there is a sufficient void at the top of the box for the grout cap to be added later and that the box is cleanly filled with no contamination on the top edge or sides.

Once a box is filled, the operator initiates an automatic sequence to raise the waste chute, the delidder closes and automatically replaces and releases the box lid. The box is then automatically returned to the box preparation area, where the swabbing and bolting robot replaces the lid bolts. A local gamma monitor performs a final dose check of the activity in the box to confirm that it is within acceptable limits for export. The robot also performs a number of swabbing routines to confirm that there is no loose contamination on the external surfaces of the box. To meet RWMD requirements, even the base of the box can be swabbed, necessitating the opening of the gamma gate and the use of the import/export overhead crane to lift the box to enable the robot to swab the base.

The operator has closed-circuit television of all the retrieval and process operations via the ROV’s on-board cameras, as well as via the overview cameras strategically placed to follow the entire process.

BNS was responsible for the design and supply of the modular control room and the associated power distribution and control ISO containers. They have been fully tested during integrated testing of equipment at their works and will be delivered and situated adjacent to the SAWBR building for interconnecting cabling to be installed.

Transportation

The SAWBR processing facility anticipates producing approximately 1,000 boxes of intermediate level waste (ILW). Once the box has passed both dose and contaminant checks, it is available for export across the Hunterston A site in a purpose-built cross-site shielded transporter.

The vehicle is reversed into position below the import/export shielded gamma gate. To guarantee position accuracy, the vehicle is fitted with an automatic guidance system. When in position, the control of the vehicle is handed over to the control room and the vehicle is elevated before the doors of the onboard shielded overpack are opened. The box-handling crane lowers the box into the overpack and releases the grapple. The crane then rises to its parking height and the gamma gate closes, as does the gate on the shielded overpack. Control of the vehicle is then transferred back to the driver. The driver is then able to lower it off its stabilisers and vacate the building.

Magnox North and Hunterston A

Magnox North is responsible for the safe delivery of the operations and decommissioning programmes on five sites in the U.K. Besides the decommissioning of Hunterston A, the company’s site operations include electricity generation at Oldbury in Gloucestershire, Wylfa and Maentwrog in North Wales, and defuelling and decommissioning of the former generating power stations at Chapelcross in Dumfriesshire and Trawsfynydd, also in North Wales. Magnox North is the management and operations contractor responsible for the day-to-day operation of its site under contract to the U.K.’s Nuclear Decommissioning Authority.

Having successfully defueled Hunterston’s twin reactors and dispatched the irradiated fuel to Sellafield for reprocessing, Magnox North is currently midway through the care and maintenance preparations of the site’s decommissioning programme.

Author: Phil Reade is SAWBR project manager, BNS Nuclear Services. He has 25 years of experience with nuclear engineering at leading U.K. firms. He has worked as a project manager during his last eight years with BNS and its predecessor, INS. The projects he led during this time include the design of high active cells for the BNFL Technology Centre and equipment design for the Berkeley Active Waste Vault Project.

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