Long-term repacking3 July 2003
The active waste retrieval programme at Harwell is a collection of projects for the supply of equipment and facilities for the retrieval, repackaging, cementation and storage of intermediate-level waste (ILW). UKAEA is seeking to add a waste encapsulation and treatment plant to fully immobilise waste prior to a decision being made on construction of a national ILW repository.
Originally, the United Kingdom Atomic Energy Authority (UKAEA) was involved in R&D, as well as being responsible for a number of research reactors of differing type. The last operating reactors at Harwell were closed in 1990, changing the focus of UKAEA towards decommissioning and radwaste management.
Consequently, UKAEA has developed an active waste retrieval programme (AWRP), which is part of UKAEA's major projects and engineering division. The AWRP is a collection of projects for the retrieval, repackaging, cementation and storage of intermediate-level waste (ILW) from below-ground storage tubes. The programme has been active for 15 years, based in building B462 at Harwell. The overall objective of the AWRP is to recover and immobilise all waste stored by 2020.
The waste was originally placed in cans, which were then placed in tubes in three tube stores at building B462. Prior to 1988, the cans were constructed of mild steel. Unfortunately, the mild steel has tended to corrode, and some of the cans cracked, spilling their contents into the tubes. In addition, in the early days of disposing of ILW into drums, the requirements for recording the contents were much less stringent than they currently are. As a result, the opportunity is being taken to accurately and fully record the can contents and details in accordance with the current regulatory requirements.
It has therefore been necessary to remove the cans or the debris from the tubes, identify and fully record the details of the contents, repackage the waste into drums, prepare the drums to be encased in a cement grout (subject to final approval) and then send to storage awaiting final disposal.
At present, the facility consists of three tube stores, a vault store, a head end cell, and a control room. UKAEA is currently seeking outline planning permission from Oxfordshire County Council for construction of a new facility, to be called the waste encapsulation and treatment plant (WETP), which would consist of a cementation plant and a waste handling facility.
Waste store B462.2
This contains about 1300 waste cans in 422 tubes that are 2.5m deep, representing about 13% of the total waste stored. Retrieval started in 2002, and all waste is due to be removed to the new facility by 2009. This waste store has a 109t retrieval machine, which is manoeuvred around the store on a low loader.
Unfortunately, this machine is unable to access the tubes located closest to the walls of the building. As a result, a gamma gate retrieval machine will be required. All of the cans located in the edge tubes are made of stainless steel, and thus have not suffered corrosion or spillage.
Waste store B462.9
This contains about 7000 waste cans in 777 tubes that are 4.5m deep, representing about 70% of the total. Retrieval from this store is due to start in 2008, and be completed by 2017. Some of the cans in this store are of mild steel, and thus susceptible to corrosion and fracture.
A retrieval system is required for the store. This retrieval system will have to have a better operational performance than that of the retrieval equipment in the B462.2 waste store. It will need to be operational by 2008, and the installation, commissioning and operation of the equipment must have minimal impact on retrieval operations in the B462.2 waste store.
The current situation on the retrieval equipment is that the option studies have been completed, the customer specifications established, the contract strategy established, and the tender action is in progress for phase 1 (scheme design and production of the preliminary safety report).
Waste store B462.26
This contains about 2000 waste cans in 584 tubes that are 4.8m deep. Retrieval from this store is due to be completed by 2017. All the cans in this store are of stainless steel, so corrosion and fracture, and hence spillage of debris is not an issue.
Head end cells
The head end cells are currently in active commissioning, and a safety case for the cells is due to be submitted to the Nuclear Installations Inspectorate (NII) by the end of 2003. The cells are used to determine and record in detail the contents of the stored cans, in accordance with current regulatory requirements, and the waste is then repacked into 500 litre drums that are then transferred to the vault store.
There are four cells: input cell, assay cell, packing cell and lidding cell. The cans being retrieved are brought into the input cell, where they are given a unique identifier for record purposes. From here, the cans are then moved to the assay cell, where they are subjected to inspections by a segmented gamma scanner and a neutron interrogator. The latter instrument detects the number of neutrons being given off by the waste, and hence how much fissile material is present. This enables the operators to determine whether the can contents should be classified as low-level waste (LLW) or ILW.
Over the years, the activity level in the cans has decayed, and in some cases, this activity has fallen to such a level that the waste can be reclassified. LLW can be packaged up and sent for storage at the LLW repository at Drigg.
The can is then moved to the packing cell, where it is placed on a rack. The waste is removed from the can, and the details of the waste recorded. The contents are tagged as being either divertable (LLW) or remote handled (ILW).
The fourth cell is the lidding cell. An empty 500 litre drum is placed in this cell, and manoeuvred to a port connecting the lidding cell with the packing cell. The port is opened, and the waste in the packing cell is placed inside the drum through the port. The drum is then sealed, and the security of the sealing is checked by swabbing the surface of the drum followed by checking the swab for activity.
The maximum anticipated weight of a filled drum is 1.6t, and the handling weight limit for the drums is 2t.
The vault store contains eight bays, and has 1.4m of reinforced concrete shielding. It has a capacity of 2000 drums, which are stacked five drums deep. The store has an overhead travelling crane, CCTV viewing, and a drum handling grab.
In December 2002, the first successful consignment of repackaged waste into the vault store was carried out. The completion of the vault store crane upgrade signalled the start of active tipping. Active tipping was the final stage of the vault store active commissioning. A total of 20 remote handled waste cans were recovered from the waste stores, inspected and assayed. Eight of the cans were opened, their contents catalogued and tipped into a 500 litre drum.
Several retrieval technologies are being used in the various AWRP projects. Retrieval machines, such as the one in the B462.2 waste store and the one being designed for use in the B462.9 waste store enable the safe retrieval of older, mild steel waste cans, some of them dating back over 40 years. Retrieval machines like these maintain high levels of containment and shielding.
However, for the cans in the B462.26 waste store, such levels of containment were not necessary, since the waste is more recent and was sealed inside stainless steel waste cans. For this kind of waste, UKAEA employs gamma gates. Two are already in use in the B462.26 waste store, and another is currently in design for the retrieval of the stainless steel contained waste in B462.2.
The gamma gate devices are designed to provide shielding while the shield plug is removed from the storage tube, a flask fitted and the waste can retrieved into the flask.
The waste encapsulation and treatment plant (WETP) is due to be in place by 2010, and to enter operation shortly afterwards. It is due to complete all waste treatment by 2020. The WETP will consist of a flexible waste handling facility (FWHF) and a cementation plant.
The WETP has three main functions:
• Encapsulate waste in concrete inside 500 litre drums.
• Monitor and inspect the drums.
• Pre-treatment of wastes not suitable for encapsulation.
It is estimated that 5-10% of the waste from the stores will be waste requiring additional treatment (WRAT). Typically, the WRAT will be in the form of a powder, which cannot be grouted. This has to be treated to turn it into a form that can be emplaced in concrete. Treatment of WRAT will be carried out at the FWHF.
The WETP also has the future requirement of providing an export facility to send the drums off for final disposal.
The current status of the WETP is that the options assessment has been completed, the concept design developed, the required plant flow processes defined, the lifecycle cost estimates developed, the outline planning application submitted, and a voluntary environmental statement produced.
Ongoing activities for the WETP include:
• Refining the risk log.
• Refining cost estimates.
• Developing output specifications.
• Building a project core team.
• Formulating a private finance initiative (PFI) pathfinder case. This case is currently being prepared to present to the Department of Trade and Industry for a gateway review. If the PFI process proves successful, it will be one of the first examples of a PFI being applied to a nuclear project.