At Point Lepreau NGS the Health Physics Department looked for alternative decontamination methods that could be applied to reduce the dose expenditure as a result of maintenance on the primary side of our steam generators. Due to the characteristics of the CANDU design (i.e., primary circuit being high purity D2O) introduction of foreign contaminants was also a concern. Other technologies available such as, blasting with CO2, use of solid blast media, and high pressure water were explored, but for various factors, were not ideally suited for this particular application.
CO2 being a dry process requiring large quantities of CO2 and process air, would increase the probability of airborne contamination and generate an oxygen deficient atmosphere. Blasting with other types of solid media generates an excessive amount of solid wastes and dusts requiring prolonged exposures for cleanup and disposal. High pressure water, although more attractive, requires large volumes, in this case D2O, resulting in a substantial cost and limited availability.
The IceBlast process provides an economical, dust free alternative to conventional methods, and permits “in situ” cleaning or decontamination of system components. Ice being the blast medium, starts as a solid and ends as a liquid. Although ice particles fracture rather than abrade, they provide physical agitation on impact creating a mechanical rubbing action that removes most non-bonded foreign material. When the ice melts (following impact) it flushes the surface free of debris leaving no substrate damage.
The source of ice, for this particular application, is a minimal volume of high quality (reactor grade) virgin D2O. Virgin D2O is used to prevent an increase in airborne tritium during operation, down-grading the purity of the main circuit inventory, and contaminating the IceBlast machine internals. These considerations allowed the process to be used for other non-radiological applications as well.
Ice as a blast media performs three types of impact work. Water in its liquid state is not an ideal blast media because it lacks the frictional scrub capability, even though it can be made to generate a very high momentum for displacement work.
Case study - Ice Blast technology for steam generator decontamination