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The Effect of Water Chemistry on Corrosion Product Build-Up under PWR Primary Coolant Conditions
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
The corrosion of structural materials in PWR primary circuits causes the release of metal ions into solution and the formation of suspended particulate corrosion products. The deposition and release of these corrosion products and the precipitation of dissolved ions onto internal surfaces in the primary circuit can cause a number of issues that affect the efficiency and safety of plant operation. Such deposits have been termed CRUD. CRUD deposits preferentially form in regions of the primary circuit where flow acceleration occurs such as orifice plates and the entrance to steam generator tubing. The mechanism that drives this phenomenon is thought to be due to electrokinetic effects caused by flow acceleration. Coolant flow shears ions at the metal/solution interface causing the formation of a streaming current. In accelerated flow, the increasing magnitude of the streaming current is supported by wall currents which facilitate the oxidation of dissolved ferrous ions in solution, and precipitation on the material surface occurs. In this work, this postulated mechanism of CRUD deposition was investigated by recreating accelerated flow regions under PWR primary circuit conditions. In the primary circuit of a PWR, various chemical additives are present in solution in an attempt to minimise degradation mechanisms as well as control the reactivity of the core. The effect of these chemicals on CRUD formation in accelerated flow regions of the circuit is not fully understood, and so the effect of water chemistry on CRUD deposition has been investigated. In particular, the effect of H2, Li, Fe and flow velocity on the build-up rates of CRUD was investigated. The main constituent of CRUD in the primary circuit of a PWR is Fe. Therefore, the concentration of Fe in primary coolant is of great interest when investigating CRUD deposition. With this in mind, Fe concentrations were analysed by ICP-MS as a function of temperature and water chemistry. To complement this investigation, a computational model was designed and implemented which described the transport of CRUD around a simulated primary circuit loop. The experimental results presented in this thesis provide data on the effect of water chemistry on the build-up rates of CRUD in accelerated flow regions, which can help to design conditions to minimise the build-up. It also provides evidence for the postulated mechanism thought to drive CRUD deposition, and the mechanism has been applied to describe the observed effects of water chemistry on CRUD build-up.