Near-IR Laser Ablation of Simulated Radiologically Contaminated Oxides on Carbon Steel Feeder Pipes

As nuclear power plants age and retire from service, many countries face significant challenges concerning the safe long-term storage and disposal of large volumes of low and intermediate level radioactive wastes (L&ILW). The volumes of metallic waste are of particular concern, as when metal corrodes it produces hydrogen that could lead to pressure build-up in interim storage and disposal. In Canada, a significant fraction of the metallic wastes for Canada Deuterium Uranium (CANDU) nuclear reactors are out-of-core reactor components, such as carbon steel (CS) feeder pipes. The radioactive contamination is expected to be largely confined to the surface oxide layers on these pipes. The removal of radiologically contaminated surface oxide layers from these materials could make the metallic wastes free-releasable, or at the least, convert them from intermediate level to low level waste. If the main concern is not the extension of their service lives, but safe decontamination and disposal of radioactive wastes, non-chemical methods such as mechanical abrasion or stripping using a laser or focused ion beam can efficiently remove radioactive surface oxides without generating additional wastes. The development and application of innovative surface decontamination technologies like laser ablation will reduce costs associated with transportation, storage, and waste minimization. The first objective of this thesis was to simulate oxides that form on CANDU reactor CS feeder pipe surfaces. The variability in the physical characteristics of oxides along the length of the feeder pipes has not been investigated thoroughly. In this thesis, combinations of different solution parameters (temperature, pH, γ-radiation, and ionic strength) were used to corrode CS samples to grow oxides with varying thickness, composition, and morphology. To simulate radiological contamination of feeder oxides, inactive Co2+ ions were used as a surrogate for the radioactive 60Co found in actual feeder oxides. After simulating oxides on CS samples, the subsequent objective was the optimization of operating parameters (laser irradiance, number of scans, pulse overlap, and cleaning angle) for an ytterbium doped fiber laser (1064 nm) for the laser cleaning of corroded CS samples. Laser cleaning tests demonstrated that laser irradiance was the most important operating parameter determining cleaning efficacy. At laser irradiances > 100 MW/cm2, the formation of a high temperature plasma ejects oxides as large fragments, resulting in the highest oxide removal efficiencies. A HEPA filter was used to collect the ablated oxides, and collection efficiencies of up to ~ 95% were observed for the highest-grade HEPA filter tested. The results in this thesis show that laser ablation using an ytterbium doped fibre laser (1064 nm) is a promising decontamination technique for radiologically contaminated CS feeder pipes.

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