Examination of iron undersaturation impact on flow accelerated corrosion in the CANDU-6 Primary Heat Transport System
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Date
2023-03
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University of New Brunswick
Abstract
The rate of corrosion can be mitigated by maintaining optimum chemistry conditions and selecting a compatible material for the system environment. Corrosion mitigation is a significant concern across all industries, as fostering an adequate corrosion mitigation program promotes optimum performance, as well as preventing the release of system process fluids into the environment, which is particularly important within the nuclear industry.
From the investigation of the Point Lepreau Nuclear Generating Station (PLNGS) data regarding changes in flow accelerated corrosion (FAC) rate when a fresh lithiated resin column is put into service, it appears that the increase in FAC rate is consistent and was even present prior to the feeder replacement during the 2008 to 2012 refurbishment. Examining this phenomenon further, it appears that the quantity of lithium that is released from the lithiated resin during service could be an indication of the quantity of iron corrosion products being removed by the resin bed as it closely correlates with the changes in the measured FAC rate.
The testing program presented here directly examined the effects of changing purification rate on the corrosion rate observed on carbon steel feeder material exhibiting FAC. Over the duration of approximately two years, CNER’s Loop 1 was reconfigured from a once-through configuration to a recirculating loop, complete with a high-temperature pump. This recirculating configuration more accurately represents the CANDU Primary Heat Transport System (PHTS)1 in terms of simulating inlet feeder and outlet feeder operational temperatures and overall chemistry conditions and allowed for the provision of side-stream purification.
Three sets of experiments were performed and confirmed the direct correlation between the test loop’s purification flow rate and the FAC rate observed on the test probes. Also of note from the experiments was the surprisingly high amount of magnetite deposition observed on the particulate filters, particularly in the high-temperature recirculating section of the loop. The entire test loop is constructed of 316 SS and the only carbon steel present is from the small-bore electrical resistance probe and hydrogen effusion probe located in the test section, representing a small fraction of the overall surface area. It was not expected that significant iron corrosion products would be present or observed from just the test probes alone; thus, a set of “sacrificial” carbon steel tubes with similar dimensions to the monitoring probes were fabricated and installed. They were never used due to both the unexpected high amount of corrosion products observed in the system and in limitations of the high-temperature recirculating pump.
The observed correlation between increased purification flow and increased FAC rates as determined from the loop tests has been confirmed by comparing the experimental results to prior modelling work, which showed a similar trend. Consistent and reproducible correlations were observed from the testing program with nominally identical impact, as evidenced by the slope of the regression curve. While the modelling results show a similar trend, the fact that the modelled change in FAC rate is only about half the magnitude of the observed change from the loop tests potentially indicates that our understanding of total circulating iron and its impact on corrosion and chemistry in the PHTS is still lacking.
1 Registered to AECL