Determination of the thermal degradation rate of polystyrene-divinyl benzene ion exchange resins in ultra-pure water at ambient and service temperature

Abstract

Polystyrene divinylbenzene resins used for purification in heavy-water moderated nuclear reactors are subject to thermal, oxidative and radiation-induced degradation. The rates (relative or absolute) of these processes are unknown. This work addresses the need to develop a test for the thermal degradation of resins. Resin samples were submitted to temperature bracketing service conditions (25 to 70 °C) in a high purity system. The resin degradation rates were obtained by measuring the Total Organic Carbon (TOC), to obtain rate constants for different nuclear-grade resins, cationic and anionic beds, and also anionic resins converted to different counter-ions. The test showed reproducible results (replicates from a batch, batch-to-batch variations), and the rate constants obeyed the Arrhenius principle. The results indicated that the anionic resins degraded ∼1.6 to 7 times faster than the cationic resins, while the presence of common anions as counter-ions on the resin (chloride, nitrate, sulfate) lowered the degradation rate by up to 2.3 times. Calculations for the full scale of a reactor suggested that thermal degradation alone, producing TOC in a closed-loop circuit, would be a minor factor in saturating a column during normal service.