Browsing by Subject "tritium"
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(2020)Due to its exceptional thermal properties and irradiation resistance, tungsten is the material of choice for critical plasma-facing components in many leading thermonuclear fusion projects. Owing to the natural retention of hydrogen isotopes in materials such as tungsten, the safety of a fusion device depends heavily on the inventory of radioactive tritium in its plasma-facing components. The proposed methods of tritium removal typically include thermal treatment of massive metal structures for prolonged timescales. A novel way to either shorten the treatment times or lower the required temperatures is based performing the removal under an H-2 atmosphere, effectively exchanging the trapped tritium for non-radioactive protium. In this thesis, we employ molecular dynamics simulations to study the mechanism of hydrogen isotope exchange in vacancy, dislocation and grain boundary type defects in tungsten. By comparing the results to simulations of purely diffusion-based tritium removal methods, we establish that hydrogen isotope exchange indeed facilitates faster removal of tritium for all studied defect types at temperatures of 500 K and above. The fastest removal, when normalising based on the initial occupation of the defect, is shown to occur in vacancies and the slowest in grain boundaries. Through an atom level study of the mechanism, we are able to verify that tritium removal using isotope exchange depends on keeping the defect saturated with hydrogen. This study also works to show that molecular dynamics indeed is a valid tool for studying tritium removal and isotope exchange in general. Using small system sizes and spatially-parallelised simulation tools, we have managed to model isotope exchange for timescales extending from hundreds of nanoseconds up to several microseconds.
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(2020)AA Sakatti Mining Oy is researching the possibility of conducting mining operations in Sakatti ore deposit, located partially under the protected Viiankiaapa mire. In order to understand the waters in mining development site, the interactions of surface waters, shallow aquifers, and deep bedrock groundwaters must be understood. To estimate these interactions, hydrogeochemical characterization, together with four tracer methods were used: Tritium/helium, dichlorodifluoromethane and sulfur hexafluoride, stable isotopes of hydrogen and oxygen, and carbon-14. Most of the shallow groundwater samples are similar to the natural precipitation and groundwater in their chemical composition, being of Calcium bicarbonate type. B-11-17HYD013 was an exception, containing much more Cl and SO4. The samples from the deep 17MOS8193 all show a very typical composition for this type of a borehole, on the line between the saline Sodium sulphate and Sodium chloride water types. The samples from the 12MOS8102, as well as the river water samples and the Rytikuru spring sample are located between these two end members. The hydrogen and oxygen isotope values divided the samples into two distinct groups: those that show evaporation signal in the source water, and those that do not. The most likely source for the evaporated signal in the groundwaters is in the surface water pools in the Viiankiaapa mire, which have then infiltrated into the groundwater and followed the known groundwater flow gradient into the observation wells near the River Kitinen. Tritium showed no inclusion of recently recharged water in the deep 17MOS8193, and dated most of the shallow wells with screen below bedrock surface to be recharged in the 70’s and 80’s. B-10-17HYD017 had an older apparent age from 1955, and B-14-17HYD006 was curiously dated to be recharged in 2018. 14C gave apparent age of over 30 000 a for the deep 17MOS8193. The slight contents of 14C could be caused by slight contamination during sampling meaning the age is a minimum. The sample M-4-12MOS8102 got an apparent age of ~3 500 a, which could in turn be an overestimate due to ancient carbon being dissolved from the local bedrock fractures. CFC-12 showed apparent recharge dates from 1963 to 1975 in the shallow wells, and no recently recharged water in the deep 17MOS8193, and so was generally in line with the 14C and Tritium results, although some contamination had happened. SF6 concentrations exceeded possible concentrations considering other results, most likely due to underground generation, and the method was dismissed. By trace element composition, all samples from the deep 17MOS8139 are distinct from other samples and saw slight dilution in concentrations of most elements in the span of the test pumping. Other samples are more mixed and difficult to interpret, but some trends and connections are visible, such as the higher contents in wells with screens below the bedrock surface than those with screens above the bedrock surface, and the exceptionally high contents of many elements in B-13-17HYD004. Overall, the study did benefit from the large array of methods, showing no interaction between the deep bedrock groundwaters and shallow groundwaters or surface waters. The evaporated signal from the Viiankiaapa was clearly visible in the samples close to the River Kitinen.
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