A UK company has developed a mobile decontamination system that uses foam technology and could aid the lengthy process of decommissioning Sellafield.
Consisting of two components — a wheelie bin-sized stainless steel pressure vessel that is the foam generator and a separate 1m x 1m x 1.5m encapsulation receptacle — Cheshire-based Nexia Solutions' air-powered system can be wheeled into a plant and attached to contaminated equipment.
'At Sellafield, we have a lot of old chemical plant with contamination in the pipe works and vessels, so the foam decontamination system is a novel way to carry out in situ decontamination,' said Richard Taylor, head of technical resource for decontamination at Nexia Solutions.
'We have carried out active trials with radioactive material and have achieved 95 per cent decontamination efficiency on small test pieces, such as small pennies,' he added. 'We have had quite a lot of interest from the site, and we are looking to progress to our first active deployment as soon as possible in Sellafield.'
While foam technology for cleaning already exists, Nexias' foam has been specially designed to carry an acidic liquid decontaminant through the system.
'This means we have a foam generating liquid and the foam is generated with air. There is a lot of technology associated with getting the right characteristics of the foam, the strength and size of the bubbles and so on, so that it remains stable.
'The advantage is that the foam can be pumped quite a distance without breaking down. It will scour the inside surfaces of quite high volume systems, remove the contaminant and is then collected.
'You can add a further chemical that collapses the foam once it has done its work, which produces a small volume of liquid effluent which is much easier to treat. The collapsed foam is perhaps two orders of magnitude smaller in volume than the foam itself. You can then treat the effluent locally and encapsulate it,' said Taylor.
In trials, Nexia filled a 700-litre stainless steel vessel with its foam system, with contact across all internal pipework, fittings and surfaces. On collapse, it found that the volume of collapsed foam was 16.5 litres, or approximately 2.4 per cent of the equivalent liquid reagent volume that would normally be used in non-foam decontamination systems.
According to Taylor, the type of decontaminant carried in the foam ranges from the simple, nitric or hydrochloric acid-based, to more complex decontaminant mixes for more specialist jobs. Substances that the foam is said to be capable of removing includes dust and liquids.
'If a process plant has been used over a number of years, you will get contamination within the pipes that might just be particulates of whatever radioactive material that have adhered to the inner surface of pipes, or you can get bits of sludge or dust that is on the bottom of the vessel. The foam can also pick up liquid contamination that is in the bottom of the vessel and absorb it into it as it is moving along,' said Taylor.
The foam is said to be able to move along, not with a pump, but purely under displacement of new foam being generated within the generator. At the receiver end of the system, the foam can be collapsed in the receptacle module itself.
'A volume of the foam collapses through contact and reaction with the substrate being decontaminated,' said Taylor.
'Residual foam may be collapsed by a number of mechanisms; we have used proprietary anti-foams and alcohols as sprays to reduce surface tension of the bubbles causing them to collapse, and have considered physical means of collapsing foam, such as microwaves or heating. We have also designed foam systems which are self-collapsing, that is they have a 'half-life' where they will begin to break down after a period of time with no additional inputs.'
So far, Nexia Solutions has only carried out active trials on a small scale, but hopes to roll out the system in the near future.