M J Sanders, R A Peckitt.
Contact: K F Collett, Marketing and Sales Director,
NUKEM Nuclear Ltd
Kelburn Court, Daten Park, Risley, Warrington, WA3 6TW
Tel: 44(0)1925 858226 Fax:44(0)1925 811867

The paper describes the activities associated with the Decommissioning and Demolition of the Harwell Incinerator Complex to "green field" status. The decommissioning was carried out by WasteChem Ltd under contract to UKAEA following a competitive tender exercise. The project is an excellent example of how a private sector nuclear services company can work closely with a nuclear liabilities management organisation to achieve safe nuclear decommissioning of a redundant nuclear facility at economic cost.

The Harwell Incinerator operated between 1960 and May 1994. During its long operational life it incinerated combustible Low Level Waste (LLW) produced at various UKAEA sites and from external organisations. The main building was a three storey brick construction with corrugated asbestos cement and perspex cladding to the south and west sides encompassing a structural steel frame and steel sheet flooring, with a concrete ground slab and foundations. It was nominally 20m x 15m x12m high. Attached to the southern side was a single storey change, shower and mess room The Harwell Incinerator facility included the following items of process equipment: waste feed glove boxes; incinerator burning chamber; regenerative heat exchanger; quench tower; venturi scrubber; cyclone separator; exhauster; waste stack; associated water circuits, ventilation and filter systems. Other components of the facility included: building structure; a change room; the Doris building; external delay tanks and pump houses,

The decommissioning comprised the safe removal of the contaminated plant from the facility and the drainage system including pump houses and delay tanks. To minimise the volume of waste arisings requiring disposal, decontamination was carried out where appropriate and all radioactive wastes from the project were packaged for disposal. Finally, the complete controlled demolition of the building fabric was carried out followed by site reinstatement, thus returning the area to an unrestricted green field status. The project was successfully completed in the eight month period between August 1996 to April 1997.

The Harwell Incinerator went into operation in 1960, with a revised off-gas clean-up system being installed in 1965. It ceased operation in May 1994. During its 34 year operational life it incinerated large quantities of combustible Low Level Waste (LLW) produced at various UKAEA sites and from external organisations. The incinerator has been completely decommissioned to ‘Green field’ and the project was undertaken by WasteChem Ltd (a member of the NUKEM Group) under contract to UKAEA, following a competitive tender exercise.

The Harwell Incinerator is located on Harwell’s Liquid Effluent Treatment Plant (LETP) site, which is a small separately controlled site outside Harwell’s main licensed site. The Harwell Incinerator included the following items of equipment:-

• Waste Feed Gloveboxes
• Incinerator Burning Chamber
• Regenerative Heat Exchanger
• Quench Tower
• Venturi Scrubber
• Cyclone Separator
• Exhauster
• Waste Stack
• Associated Water Circuits, Ventilation and Filter Systems

Additionally, the other components of the facility include:

• B336.17 Building Structure
• B336.27 Change Room
• B336.30 Doris Building
• External Delay Tanks and Pumphouses, B336.17A, B, C & D

The main building (B336.17) was a three storey brick construction with corrugated cement based asbestos and perspex cladding to the south and west sides encompassing a structural steel frame and steel sheet flooring, with a concrete ground slab and foundations. It was nominally 20m x 15m x12m high. Attached to the southern side lay a single storey change, shower and mess room (B336.27). The main building also had a Goods Lift including Lift Motor room above the level of the main roof.

The plant had undergone Post-Operational Cleanout (POCO) prior to the work described here.

B336.30 which contained DORIS had been decontaminated to leave an empty structure, although still with areas of fixed contamination in the floor and drain gullies. The building was a single storey steel framed structure clad with corrugated cement based asbestos, reinforced glazing and brickwork, nominally 35m x 10m x 6m high.

The scope of work was to safely remove the contaminated plant from the facility and effluent system, including pumphouses and delay tanks prior to complete controlled demolition of the building fabrics and finally site reinstatement. A radiological end-point survey was carried out to demonstrate that "green field" status was achieved.

The decommissioning operations were carried out in a series of seven phases:-

Phase 1 - Site mobilisation, initial radiological surveys and, removal of loose waste items.

Phase 2 - Removal of contaminated plant and equipment.

Phase 3 - Health physics clearance of the building structures.

Phase 4 - Building demolition.

Phase 5 - Removal of building sub-structures.

Phase 6 - Radiological end-point monitoring.

Phase 7 - Site remediation.

The Incinerator Facility was handed over to WasteChem in July 1996 and a Permit to Work issued to commence Phase 1.

The work involved in this phase comprised the preparation of the B336.17 area for decommissioning, including a radiological survey of the area, asbestos survey and removal, COSHH assessment and report, removal of loose waste items stored in the facility. The required site and building-specific training was also undertaken.

Following the asbestos surveys, asbestos was removed from the cyclone separator and some sundry pipework.

The initial HP Surveys indicated that loose contamination was present in the building, particularly within the metal floor decking, which could become airborne during dismantling activities. Although contamination was known to be present in the ground floor area the level of fixed contamination could not be determined due to the high background count rates from the existing plant and equipment prior to their removal.

The high radiological background readings adjacent to some of the plant and equipment also meant that a low background area had to be identified for monitoring items for free release.

Waste management procedures for the project were prepared by WasteChem, working closely with UKAEA project and health physics staff. These included procedures for LLW packaging and also criteria for the free release of items.

During Phase 1 the site team developed the plan for dismantling the plant and equipment by segregating the facility into a number of Work Zones around which containment’s could be erected in the most practicable manner.

The removal of the contaminated plant and equipment commenced in earnest following removal of the asbestos. Phase 2 work packages were completed in fourteen weeks, some four weeks longer than planned. Delays to the programme were caused for a number of reasons outlined below but these were mitigated by increasing the labour resources. The main delaying features were:

• More LLW and clean waste than estimated.
• Increased workload in size reduction/decontamination processes, packaging of LLW and free release materials.
• Unexpected contamination of the ground floor slab under the exhauster
• More contaminated insulation brickwork from incinerator than anticipated.

Health Physics clearance of the Building Structure, following removal of the plant and equipment was completed to allow demolition to ground slab level. One of the tasks during this phase of the works was to remove the steel plate and open meshed floors in order to radiologically clear both the floor plates and the supporting steelwork. The open-mesh floor gratings posed the problem of how to ensure that HP monitoring procedures were effective in clearing the material as ‘free release’; this was overcome by taking samples in order to confirm the efficiency of the HP instruments.

To maintain continuity for the demolition work, HP clearance of the main building was given top priority, over and above the removal of the roof contamination, thus enabling the demolition contractor to commence removal of the asbestos wall sheeting at the earliest moment; this excluded the roof and a number of small areas of fixed contamination on structural steel members which would be cut out by the demolition contractor during his work activities.

The Building Demolition commenced and was completed to ground slab level in ~ three weeks. All waste arisings were monitored, samples taken for gamma spectrometry analysis and clearance certificates were raised for each wagon load removed from site. No problems were encountered during the monitoring of the waste arisings for off - site disposal as ‘free release’ material.

Following completion of the demolition work all contaminated areas of the slab were removed working within a small containment tent supplied with a mobile filtration unit and respiratory protective equipment was worn.

During the breaking up of the concrete ground slab further unknown structures were encountered and dealt with to allow the removal of the active drain to proceed. These included:

• A lower slab pre-dating the DORIS structure
• An old manhole, found to be contaminated.
• An area of contamination in the ground beneath the drainage channel
• A large uncontaminated concrete base (3.0m x 3.0m x 3.0m) and subsequently, three more similar structures
• An un-chartered cess pit filled with concrete.
• Contaminated screed
• An area of contaminated soil adjacent to the Delay Tank
• An un-chartered 3" cast iron water pipe surrounded in concrete

A former drainage sump was located and was found to be contaminated. This was removed and packaged as LLW.

Following removal of this sump the surface water drain connection was found to be contaminated; this was removed in small sections and the internal surfaces of the pipe decontaminated, broken up, monitored and disposed of. The area was HP cleared and the remaining surface water drain and manhole materials removed for free release.

The surface water drain and the surface water drain from the road gully were plugged at the site boundary.

Over a four week period, the site had been cleared of all substructures and contaminated ground except for: the delay tank; the adjacent concrete base and surrounding contaminated ground beneath the floor slab. Decontamination work to these areas took four weeks to complete by which time the excavation had reached a depth of 5500mm below ground floor level.

The delay tank was contaminated internally on all sides and externally on three sides. The internal contamination was removed using air powered tools within a ventilated containment. The ground around the Delay Tank was then excavated and any contaminated soil removed and packaged as LLW. All loose contamination on the external sides was removed, remaining contamination was fixed and identified using a spray paint. The delay tank was then removed in one piece by the excavator and laid on its ‘clean side’ at ground level. The excavation was then surveyed and any further contaminated ground removed. The contamination on the sides of the delay tank was removed using air powered tools inside a ventilated containment erected along each side of the delay tank. The underside of the delay tank was found to be clean.

The concrete base adjacent to the delay tank was found to be contaminated on two sides. The concrete base at one time supported a stack which became corroded due to condensation, subsequently contaminated liquid dripped onto the ground slab, the external wall and through the ground between the building substructures and this concrete base.

The contamination in the ground reached a depth of 5500mm this was 1000mm below the bottom of the concrete base. It was removed by excavating, initially by hand, down the side of the base.

HP surveys and clearance certificates were issued at various stages throughout the project as areas of the building and work activities were completed and cleared. The most significant Radiological End Points were as follows:-:

• HP clearance of the DORIS area to allow demolition to commence issued on 13.1.97.
• HP clearance of B336.17 internal superstructure issued on 22.1.97.
• HP clearance of B336.17 roof allowing demolition of all superstructures issued on 27.1.97.
• HP clearance of B336.17 site excavations issued on 25.3.97.
• Final HP clearance of B336.17 site and its environs following removal of all Waste Packages, Plant and Equipment 24.4.97.

Site reinstatement by top-soiling and grass seeding was completed.

The Harwell Incinerator complex has been completely removed including the contaminated plant, drainage, pumphouses and delay tanks. All radioactive contamination has been removed within the boundary of the B336.17 site and all substructures removed.

A Radiological End-point Survey was carried out to demonstrate that "green field" status was achieved.

‘As-built’ service drawings have been prepared to show the extent of services, which remain on the site and the locations at which services have been capped at the boundaries of the site.

The project was completed by top soiling and grass seeding the site.

The doses received compared to the Dose Restraint Objectives provided in the UKAEA Safety Case.

The contract was very well managed from the Safety aspect. There were no incidents having a corporate incident category. The site was inspected weekly by the WasteChem Site Safety Co-ordinator and regularly inspected by the UKAEA, Harwell Safety Representative and on one occasion by the UKAEA Corporate Safety Representative. No adverse reports were received during these visits.

The decommissioning of the B336 Incinerator Building had a few technical problems, in particular, areas of contamination which had not been foreseen at tender stage. The problems were primarily due to radiological contamination being found in areas previously reported as clear or of unknown status. As work progressed the background activity levels in areas reduced, thus allowing more detailed monitoring to be carried out. This revealed previously undetected areas of contamination which were immediately brought to UKAEA’s attention.

Loose contamination was detected on surfaces in the building. Reports provided indicated that the structure of the building was clear and loose contamination was confined to the plant. Areas of loose contamination were cleaned as they were found.

Fixed contamination was found in the floor of Zone 6. The background levels from the exhauster, cyclone separator and associated plant prevented this being detected until late in the building clearance. The contamination affected a significant area of the floor and was removed by scabbling.

A survey of the incinerator roof showed it to be significantly contaminated. The building had 5 unfiltered roof vents which were contaminated with loose activity. Investigations showed this to be in the top surface only, and the material was removed, drummed and consigned as active waste. The roof was then shown to be clear of contamination.

The Internal Surfaces of the building were generally clear of fixed contamination. Isolated areas of contamination were scabbled off, drummed and consigned as active waste.

The asbestos cement sheets were removed in an agreed, HSE approved and safe manner. Despite this, occupants of neighbouring buildings on the site believed asbestos fibres were being released and were invited to view the demolition process and to follow up the investigation. The invitation was taken up and the result of the investigation vindicated WasteChems responsible approach to the work.

The remaining building structure was demolished in a controlled manner by the demolition contractor without incident.

Once the structure of the building had been demolished the rubble was sampled, shown to be clear it was removed from site by truck as free release material.

The exposed slab was decontaminated where appropriate. The localised area of contaminated concrete screed in the former source store was removed, drummed and consigned as active waste.

The remainder of the slab was broken up, sampled, surveyed and consigned as clean waste. The slab was not constructed in a single layer but appeared to have been laid on top of any existing slab. During the excavation, 5 large (i.e. each the size of a small car) concrete substructures were unearthed. Each block was surveyed and sampled prior to being released from site.

When surveying the soil a number of areas were found to be contaminated. Each area was sampled to determine the level of contamination and for fingerprinting of isotopic content; the original level of contamination decreased rapidly with the depth of the excavation. The excavation was surveyed then backfilled and finally the surface of the land was then surveyed and sampled to show all contamination had been removed.

The total waste arisings (clean waste and LLW) from the plant and equipment was much greater than expected. The total LLW arisings exceeded the original estimate by a factor of ~ 12.

The primary objective of the project to decommission the Harwell Incinerator facility and return the site to "green field" status was achieved.

The waste management (LLW and Free Release) and monitoring procedures developed and enhanced during the project by WasteChem have proved to be effective and reliable in the detection, segregation and packaging of radiologically contaminated items.

The operation of a size reduction and decontamination area within a segregated containment proved highly successful in both the decontamination of items to greatly reduce the total amount of LLW produced and also in reducing the actual volume of LLW generated.

The estimated waste arisings (LLW and free release) for the incinerator plant and equipment was greatly exceeded and additional resources were required to size reduce, decontaminate, package and monitor the waste arisings.

The estimated total radiological inventory for the Incinerator Plant and equipment was greatly exceeded. The high levels of contamination found on the brickwork in the incinerator furnace was the major contributory factor.

The presence of radiological contamination within the building structures and substructures were unexpected. Their removal extended the project programme.

The presence of extensive uncharted concrete substructure prolonged the work programme by causing a delay to WasteChem being able to establish the radiological end point.

The decommissioning was carried out by WasteChem Ltd under contract to UKAEA following a competitive tender exercise. The joint objective of both client and contractor was to achieve project completion safely, to time and cost, whilst minimising the production of radwaste and to a large extent, this objective was achieved. Inevitably, in almost all decommissioning projects, there is a risk of unforeseen problems being discovered which are outside the originally specified scope of work and this project was no exception. However, an excellent Client-Contractor relationship based on mutual trust and reasonableness enabled these challenges to be efficiently managed.

The Harwell Low Level Waste Incinerator Decommissioning Project is an excellent example of how a private sector nuclear engineering services company can work closely with a nuclear liabilities management organisation to achieve safe decommissioning of a nuclear facility at economic cost.

This work has been funded under the DRAWMOPS programme of the UK Department of Trade and Industry (DTI). The results of this work form part of the UK Government programme on decommissioning and radioactive waste management, but do not necessarily represent Government policy.

The authors gratefully acknowledge the interest and support given by Dr K F Langley and Mr R B Bull of UKAEA DRAWMOPS Directorate, Harwell

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