The Effect of Radiation and Dose on Diffusion Pump Oils

Jonas Smith


The goal of this project was to determine the oil which was least damaged by neutron radiation; damage done to their chemical structure was quantified using Fourier-Transform Infrared Spectroscopy (FTIR), interfacial tension (IFT) in air via the Pendant Drop method, and neutron activation analysis (NAA). The radiation hardness of diffusion pump oil will be an important factor if this type of pump is used in the Tokomak reactor design for fusion power plants. Six oils were irradiated by a 2MW research reactor at five groups of tissue equivalent dose estimates: 0.17 , 0.34 , 0.68 , 1.70 , and 3.40 . Each of the samples was mixed with 2% 2-vinylnapthalene and given the same fluence of neutrons. Leybonol LVO 500, a mineral oil, was found to be unsuitable for use at dose estimates equal to or exceeding 1.70 , since it completely solidified at each higher dose estimate tested. FTIR scans identified the generation of an oxidation peak at 965 cm-1 which was indicative of damage at dose estimates as low as 0.17 . Trace activated components were found in the polyphenyl ether oils Santovac 5 and Santovac 9, measuring up to ten times the activity of any other oil after two weeks of cooling time. The unexpected activity is suspected to be 35S and an unknown low-energy beta emitter. Santovac 9 retained its interfacial tension within 2.2% of its initial, unirradiated value, and FTIR scan did not reveal any chemical changes. Silicone oil had the lowest change in interfacial tension all the way up to the highest fluence obtained. It remained within-1peak appeared in Neovac SY starting at a dose estimate of 0.17 . It showed a negligible amount of neutron activation after a cooling time of two weeks. Based on this evaluation, it is hard to definitively distinguish a ‘best’ oil, but it is evident that LVO 500 is not suited for a high radiation environment in oxidizing conditions.