The Development and Initial Testing of the Ice Pig Cleaning Method for Nuclear Reprocessing Plants

There are numerous situations throughout the nuclear industry where it would be advantageous to clean out enclosed systems, e.g. ducts and pipes, to remove sediments and radioactive contamination. However, conventional flushing methods result in the production of significant volumes of contaminated liquor. The wall Shear achieved with water flushing is insufficient to remove and carry dense deposits often found in nuclear facilities. Conventional mechanical pigs can be propelled through the duct by application of a driving medium such as compressed air or water. These are able to achieve very high wall shears, but can also get physically stuck where there is variation in the pipe / duct diameter or small radius bends. The field of application is best suited to straight constant diameter systems with no discontinuities such as Tee pieces, valves and in-pipe instrumentation.
Driven by the desire to clean and displace radioactive materials from complex topology ducts and pipes, without the risk of having conventional pigs becoming lodged in a plant and coupled with the need to minimise effluent volumes, the UK Nuclear Industry led by Sellafield Limited (on behalf of the UK Nuclear Decommissioning Authority), has worked to demonstrate and underpin the use of the ice pigging technology for the nuclear industry.
It is found that high ice fraction slurries flow as very thick liquids that exhibit a number of mechanical properties that offer much higher shear rates. The ‘Ice Pig’ exerts shear rates on pipe walls which are typically 1000 times greater than those achieved with water flowing at the same velocity. The ’Ice Pig’ is able to negotiate almost any topology, flow through pipe networks of vastly differing diameter, through heat exchangers and in-line instrumentation etc.
Experimental data and operational experience from other industries demonstrates that whilst there is high shear, the ‘Ice Pig’ does not damage the surface of the pipe wall. A further attractive characteristic of the ’Ice Pig’ is that it eventually melts rendering it a simple effluent for processing by downstream processes.
Due to the special nature of ice slurry, its unique rheological and physical characteristics, together with the complementary underpinning experimental work undertaken by Sellafield Limited enables the Ice Pigging process to move from an ‘interesting’ technology to one suitable for use by the nuclear industry. The inaccessible nature of radioactive processing plant makes the sector unique, for example, the Nuclear Industry cannot accept technologies which can give rise to blockages or deterioration in heat transfer performance. For example, although a guarantee can be given that the ice pig itself can never get stuck, the materials which the ice pig transports along the pipe could accumulate in a plug-like structure which might then get stuck. This ‘bull-dozing’ effect simply cannot be tolerated in nuclear applications. And so, Sellafield has developed techniques and protocols to mitigate this potential.
Freezing point depressants (FPDs) are used to prolong the longevity of the ‘Ice Pig’ by reducing the temperature of the slurry. In other industries this is typically achieved with common salt (Sodium Chloride). Experiments have been conducted to examine alternatives that are compatible with nuclear facilities and have been shown to be effective. This data coupled with the Generic properties of the ‘Ice Pig’ offer an opportunity for inclusion for baseline plans for Post Operations Clean Out and decommissioning activities.