Parameters Controlling Hydrogen Ingress During Corrosion of Pressure Tube Material

Abstract

In general, pressure tube material (currently Zr-2.5% weight Nb) picks up hydrogen (or its isotope deuterium) in reactor primary heat transport system coolant during aqueous corrosion raising the possibility of hydride formation and delayed hydride cracking at high levels of hydrogen concentration. As a result, the build up of hydrogen in pressure tubes due to corrosion could be life limiting. The objective of this work was to identify the parameters controlling the percentage of hydrogen, generated during corrosion, being absorbed by pressure tube material in order to increase the understanding of the pickup phenomena. Corrosion coupons taken from two different pressure tubes (β-quenched and non-β-quenched Zr-2.5Nb) were corroded under different conditions for 30 days following a test matrix designed to examine the effect of the corroding environment, the lithium concentration and the temperature on the percentage hydrogen uptake. The results show that the percentage uptake is higher in aqueous environment than in steam. However, adding lithium to the aqueous solution results in significant decrease in the value of the percentage uptake with increasing lithium concentration. The percentage uptake also decreases with increasing temperature in the range from 250oC to 360oC, which includes reactor operating temperatures. In general, the percentage uptake appears to decrease as the oxide thickness increases.