Turning the WAGR mountains in to mole hills

Completion of the decommissioning of the Windscale Advanced Gas Cooled Reactor (WAGR) at Sellafield in mid-2011 marked the end of a 20 year programme.

Hertel’s involvement was the removal of the asbestos to access the Reactor Pressure Vessel. When the asbestos had been initially installed it had been in three layers of magnesia flat blocks which contained amosite and chrysotile asbestos. This was held in place by steel wire mesh, overlaid with a cement-based asbestos fibre (chrysotile) with a reinforced armouring compound and finished with 1.6mm aluminium cladding.

Once removed the asbestos would be stored in 200 litre drums which would be placed in ISO containers before being taken to be stored at the National Low Level Waste Repository (LLWR) site.

Challenging project

The asbestos was cut away from the side of the vessel using oxy-propane. The removal team found that the asbestos broke away from its original block in a greater volume than anticipated. This was due to the asbestos turning into a powder-like substance which meant that it created considerably more volume than expected.

Ralph Pattinson, decommissioning operations team leader, says; “This had an impact on the filtration system being used in the cutting operations. An unexpectedly large quantity of sub-micron particles was passing through the through a pre-filtration system of momentum separators and cyclones. The result was a premature blinding of the building’s HEPA filters.

“With the particulate matter being mildly radioactive the blinded filters, which were standard 950 litre cylindrical filter 623mm high and 518 mm outer diameter, once removed they were compacted, double bagged and removed to a storage area. During the whole removal campaign around 750 filters were used.”

Collaborative safety approach

The amount of asbestos waste being generated came from a small initial area of 11m³. This produced in excess of 15m³ of waste. The concern was that if this continued then it would markedly increase the calculated number of drums needed to store the waste. At this level waste it was estimated that more than 130m³ of asbestos would be produced during the whole project which would increase the storage room required at the LLWR. A solution needed to be found to minimise the total amount of asbestos to be stored.

Discussions within the Hertel project team, with the UKAEA and consultants CH2M Hill looked at the various options for dealing with the excess waste. The options considered were:

Send the uncompacted wastes to the UKAEA site at Winfrith in Dorset for supercompaction
Bring Winfrith’s supercompactor to the WAGR site;
Carry out low force compaction of wastes at the site;
No compaction – place in a specialist WAGR box which is used to package waste materials and sealed with steel and grout; and
No compaction – dispose of wastes in half or third sized ISO containers

Ralph adds: “After consideration it was felt that option three - low force compaction - was the best way forward. Research had shown that this had the potential to reduce the asbestos waste generated by an average of 60%.

“The added attraction of this option is that the compaction could be carried out close to the site and is cost-effective. It would also allow the inclusion of other low level waste from work from the decommissioning of the WAGR to be stored between the drum pucks in the ISO container.”

It was important to look at the exposure the handlers would have to the removed asbestos. The Health and Safety Executive (HSE) were fully informed about the process being carried out by fully licensed asbestos workers.

Having decided on on-site compaction, various tests were carried out to find the best compactor to deal with the volume of asbestos being created. The Orwak 2500 low level compactor was identified as the most suitable option.

In depth trials

Before compacting started, trials were undertaken using man-made mineral fibre. This found that the filtration system gave a higher than anticipated level of fibre escaping into the enclosure which would present problems with air borne contamination. There was also a filtration system which was placed on the inside of the drum lid and the initial tests showed that this could be damaged as the drum filled.

To overcome this design amendments were made to the drum lids and two designs of filtration systems tested. The first used a filter screwed into a threaded hole in the drum lid but it had problems in not being able to displace enough air during the compaction process.

It was found that using a standard respirator filter, the same as those used for contamination protection, provided the ideal solution. This was fitted to the underside of the drum lid and was protected with a perforated steel cover to prevent it from being crushed during the compaction cycle.

Developing a fail-safe solution

After deciding on a location for the compactor to provide maximum protection it was placed within an asbestos enclosure with a three-stage air lock. For added protection there was the inclusion of an additional filtration system with HEPA filters to provide a fail-safe system that would prevent asbestos fibres escaping should there be a system failure.

All drums were brought to the compactor, within the asbestos enclosure, using a pallet handler. This was fitted with stops which allowed the drum to be located whilst maintaining operator distances from any potential source of radiation.

Constant air sampling was carried out at all stages of the process ensuring that there was no additional particulate burden caused by the compaction process. As each drum was filled it was also checked to ensure that its fabric had not been breached.

A 500cfm extractor system was also included within the asbestos enclosure to minimise the opportunity for asbestos to escape. Should a breach happen a protocol was established which used a specialist vacuum to shadow vacuum the area. The problem area on the drum would then be sealed and the drum itself double sealed in bags prior to being stored in the ISO container.

Continuous safety monitoring

Once the process was established, continuous air monitoring took place and this showed that there was no measurable increase in particulate matter which came from the compaction process. An important part of the process was to monitor personal and background radiation. The results from this showed that with the precautions taken it was below the levels of detection.

The personal protection equipment used by the team working with asbestos and drums, including latex gloves, overshoes, respirator masks were also placed in the ISO containers. The HEPA filters which had been removed were also placed in the container.

During the campaign a total of 130m³ of asbestos was removed from the reactor vault. Some 460m³ of hard waste was also removed including the steel structures. This waste was also stored in the ISO containers and was used to fill the voids around the drums so making use of all available space.

The process established during the decommissioning of the WAGR has set a template for the solution to the issue of removing asbestos insulation to give access to key equipment and structures as they are dismantled.