Tulloch, Sam (2011)
Eng.D. thesis, University of Birmingham.
Restricted to Repository staff only until July 2041.
Environmental degradation promoted by the presence of sludge piles in the steam generators of Pressurised Water Reactors (PWR) can pose a threat to their safe and continuous operation. The use of dispersants can reduce the rate at which sludge accumulates. Polyacrylic acid (PAA) is currently the only dispersant used in PWRs. Settling rate tests identified several dispersants with the potential to outperform PAA, notably Hydroxyethylidene-1,1-diphosphonic Acid (HEDP). To estimate the dispersant concentration required during plant operation, optimum concentrations were identified for both PAA and HEDP. Nuclear magnetic resonance spectrometry was used to investigate the thermal stability of HEDP between 230 and 270oC, revealing that HEDP decomposes more rapidly than PAA. The dominant HEDP decomposition product was shown to be orthophosphate but several other long lived intermediate products were detected.
The effect of dispersants on the environmental degradation of grade 316 stainless steel was determined by electrochemical methods and by constant extension rate tests. Rates of general corrosion measured by linear polarisation resistance and electrochemical impedance spectroscopy were shown to be very low (on the order 10-5 mm/year) under aerated and deaerated conditions both at room temperature and at 70oC. Corrosion rates were slightly reduced in the presence of PAA and HEDP. Constant extension rate tests demonstrated that neither PAA nor HEDP promote stress corrosion cracking at 250oC. It was concluded HEDP would not be suitable for use in PWRs due to its rapid thermal degradation rate. The decomposition products were shown to rapidly concentrate in steam generators thereby preventing accurate control of water chemistry.
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