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High resolution gas phase spectroscopy at solid / solid interfacial regions
[Thesis]. Manchester, UK: The University of Manchester; 2015.
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Abstract
Understanding the behaviour of polymers which are located in the presence of nuclear materials is important in order to predict the lifespan of the materials. Artificial ageing experiments are undertaken at elevated temperatures to infer how the materials may age. This study was concerned with the monitoring of trace gases (H2O, CO2, CO and acetic acid) within a materials ageing vessel which contained ethylene-vinyl acetate (EVA) polymer and uranium in order to deduce the rate of polymer degradation and/or uptake of the gases by uranium.A novel circular multi-reflective (CMR) cell was designed, developed and deployed in situ in order to extend the optical pathlength within the vessel to improve detection limits of the trace gases. One cell was located at the 6 millimetre solid / solid interfacial region between cylindrical coupons of the EVA polymer and uranium, to enable representative sampling in proximity to where the gases were evolved, adsorbed or reacted. The unique planar star-like beam profile of the CMR cell was crucial in enabling detection within this narrow interfacial region. A second CMR cell was incorporated within the vessel headspace, above the two material coupons, to address a specific research problem which aimed to ascertain whether differences in the gaseous composition existed between the two regions, which would indicate poor gas mixing. Two spectroscopic techniques were employed in conjunction with the CMR cells to monitor the trace gases: these comprised broadband absorption spectroscopy (BBAS) and tunable diode laser absorption spectroscopy (TDLAS). Near-infrared (IR) radiation sources, in the form of diode lasers, a superluminescent light emitting diode (SLED) and supercontinuum (SC) source were utilised in BBAS experiments. TDLAS was used for the detection of CO2, CO and H2O, whilst BBAS was used for the detection of acetic acid, and other potential unknown species. The requirement for using near-IR radiation was a consequence of using flexible silica-based optical fibres to remotely monitor the vessel which was located within a temperature controlled chamber. As a result, this was the first demonstration of CMR cells being used in conjunction with near-IR radiation sources.An optical pathlength of 69 cm was achieved within the materials ageing vessel, which led to the following limits of detection at 75 °C, 150 Torr with a 10 second averaging time: H2O = 3 ppm; acetic acid = 157 ppm; CO2 = 596 ppm and CO = 37500 ppm. Manufacturing issues with the cell optics, coupled with monitoring weak ro-vibrational absorption features led to considerably higher limits of detection than expected.The CMR spectroscopic system was used successfully to observe the outgassing trend of partially cured EVA polymer, which was shown to depend on cure time. A key finding of this research, however, was the observation of a difference between the interfacial gaseous composition versus the headspace gas in a system that contained both a source and sink material (i.e. one that evolved, and one that adsorbed gases). This was only made possible by using the CMR spectroscopic system. This observation was also supported by a computational fluid dynamic (CFD) model.