Dealing With a Pa-231 Contaminated Ventilation Duct

Cambridge University had worked in the 1960s and 1970s with Pa-231, a decay product of U-235. The fume cupboards discharged into ventilation ducting made from asbestos cement. The university wished to refurbish the laboratory and the RPA had negotiated over many years with the Environment Agency to set up a project to remove the ducting both to reduce the radiological hazards and as part of a programme to remove unwanted circuits and upgrade the ventilation system to modern standards. Contamination levels were significant and low dose rates were measurable on the external surface. The aim was to be able to remove the ducting and treat it as asbestos waste, rather than to have to treat the debris as asbestos contaminated radioactive waste. The age of the contaminant was such that a large fraction of the decay chain had grown in, giving a mixture of alpha, beta and gamma emissions. The most useful nuclides for surface monitoring were Pb-211 and Tl-207, both of which are energetic beta emitters. A wide energy range beta detector was used, but it was fitted with a filter to absorb any alpha radiation which otherwise would have contributed to the signal for good surfaces but not for dusty, damp or rough surfaces and would have contributed to the uncertainty in the activity assessment. Samples were checked using gamma spectrometry to confirm that only Pa-231 and its progeny were present in significant quantities. The gamma spectrum is complicated and this paper describes the difficulties in confirming that the spectrum only contained the Pa-231 decay chain. The vast majority of the contaminated ducting was successfully consigned as asbestos, rather than radioactive, waste. The other problem was dealing with the soft waste produced during the dismantling process. This was monitored using simple equipment and it was possible to demonstrate that it could be disposed of with the rest of the waste under the relevant UK legislation.