Department of Physics

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Browsing Department of Physics by Author "Ackermann, Tarryn"

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    Methods for the radiological characterisation of the FiR 1 TRIGA research reactor decommissioning waste
    (Stellenbosch : Stellenbosch University, 2017-03) Ackermann, Tarryn; Newman, Richard T.; Kotiluoto, P.; Stellenbosch University. Faculty of Science. Dept. of Physics
    ENGLISH ABSTRACT : Every nuclear reactor will eventually reach the end of its life time. With more than 60 new power reactors currently under construction, the need for understanding the safe decommissioning of a nuclear reactor is as important as ever. Among those reactors at the end of their life time is the VTT owned, FiR 1 TRIGA Mark II Research Reactor in Espoo, Finland. The FiR 1 decommissioning project is reviewed as a whole, as well as the requirements set by regulatory authorities relating to decommissioning waste and the environment. Special focus is placed on full site characterisation in terms of activity measurements. During this part of the procedure, an activity inventory of all activated components and structures is determined. The activity inventory has been determined computationally through the use of MCNP and ORIGEN-S, and this is explained on a general level. However, the results thereof require validation through physical measurements. These physical measurements are performed using γ spectroscopy for three different types of samples (concrete, Fluental and aluminium) using the In Situ Object Counting System (ISOCSTM). In addition, 13 barrels containing ion-exchange resin are characterised and compared with accepted release levels. Measurements are performed and interpreted using GenieTM 2000 software, to determine activity concentrations for validation of computationally determined activities. Algorithms used in the Genie 2000 software are outlined to better understand how the activity concentrations are determined. For the concrete sample, measured and predicted activities for 60Co and 152Eu agree with an uncertainty of 11% and 2%, respectively. While unsystematic differences are initially found for the cases of Fluental and aluminium, it is understood that the most probable cause is the assumed material compositions in the calculations. After ICP-MS leading to more accurate knowledge of the Fluental and aluminium sample compositions, there is much better agreement between computational and measured results. The differences are still large, but systematic. The calculated values are consistently one order of magnitude larger than the measured values, except for 59Fe in the aluminium sample, for which the values agree to the same order of magnitude. The 13 γ spectra for ion-exchange resin barrels all look similar, with some barrels more active than others, as would be expected for an inhomogeneous sample. In every barrel, 60Co and 137Cs is found, with the 137Cs suspected to come from a leaking fuel rod. Although much time and effort was spent on the analysis of a small number of samples relative to the number of samples in the full reactor decommissioning, it is a necessary exercise to learn lessons that may save time in the future. Most important is the study of the methods of measurement and calculation that will be applied later during full decommissioning