Abstracts Engineering

The safety and commercial implications of polymer seal degradation in a nuclear power generation application

by Christopher P Porter

An industrial seal manufactured from James Walker Co. and made from an acrylonitrile-butadiene based synthetic elastomer, reinforced with carbon black and incorporating glass cloth, brass wire and a sulphur based curing system has undergone a variety of tests to identify its physical and mechanical properties, how these properties change under exposure to gamma radiation and the effect pressurised CO2 has on this radiation induced degradation. The seal was found to have a Δ(M%) in toluene of 46% ± 2 , Tg of -24.9 ℃ ± 0.7 and a Retention Factor of 0.413 ± 0.004. Δ(M%) decreased with radiation and Tg and RF increased suggesting crosslink formation. SEM also revealed the development of microcracks on the outside curvature of the seal. Following exposure to gamma radiation under 32 bar pressurised CO2 the same changes were seen in all properties. These results were compared against three commercially available nitrile rubber sheets sourced from Whitby and Chandler Ltd; at hardness grades BA40, BA50 and BA60. Analysis of peak heights found in IR spectra suggested that degradation in BA40 proceeded through the ionisation of the BDN monomer and crosslinking between cis C=C and C=CH2 followed at higher doses by ionisation of both BDN and ACN monomers and crosslinking between trans C=C and C≡N groups. In BA50 it appeared that degradation of both monomers occurred concurrently. Those samples with a higher BDN content also displayed an increased proclivity for crosslink clustering, reducing the detrimental impact of gamma radiation. The IR spectra for samples irradiated under 32 bar pressurised CO2 showed no significant change. Service life has been simulated on a custom built Test Rig. This has shown an exponentially decaying increase in static friction with time, an increase in force required to move the seals with exposure to CO2 and the deleterious effects of grease accumulation in flowrate sensing ports.