Material Science

Abstract 

Coolant circuit components inside pressurized water reactors (PWRs) are exposed to high temperature flowing water leads to flow-assisted corrosion of the primary circuit materials. This corrosion results in the formation of oxide layer(s) primarily composed of nickel ferrites, nickel oxides, and other nickel-iron-chrome spinel oxides. Being exposed to fluid with high flow rates and pressure, these oxide layers can be eroded, resulting in particulate fouling due to the release of corrosion products (“CRUD”) into the coolant circuit. When CRUD subsequently deposits on fuel surfaces, it negatively affects the fuel performance (heat transfer, and fuel failure); in addition, the particles can undergo neutron activation, which is problematic when the particles detach and travel to out-of-core regions, contributing to worker’s radiological exposure.

Several factors affect CRUD deposition in these environments. As deposition of these particles depends in part on the surface charge of the particles and the nearby surfaces, tuning coolant chemistry and/or the composition of the primary circuit materials has been one of the empirical levers for CRUD mitigation. While the benefits of modified water chemistries, such as Zn addition and using alloys with low Ni composition,  are already seen in some operating PWRs, the underlying mechanisms are not fully understood. I propose a modeling approach to understand the role of water chemistry (effect of Ni composition, Zn addition, and reducing agent) on CRUD deposition.  To ensure the relevance of this model, surface properties of CRUD particles will be measured and incorporated into the simulations, to study the impact of the parameters that are affected by the water chemistry. To this end, a library of particles has been synthesized, covering a range of compositions and water chemistry (Zn addition, and reducing environment). The reaction products were screened for phase purity using X-ray diffraction, and their surface properties as a function of size and composition were evaluated by electrophoretic light scattering. Similarly, stainless-steel coupons representing the interior surfaces of coolant circuit materials will be exposed to hydrothermal conditions at 200°C with varying composition of Zn, Ni  and reducing agents in the water to understand the growth and possible detachment of nanoparticles from these surfaces.

To simulate the transport and deposition process of crud particles, a COMSOL simulation has been designed for a simple geometry representing piping with fluid properties consistent with the PWR environment. In bulk, the transport of these particles is predominately influenced by turbulent diffusion, implemented using a k-ω turbulence model. Near the pipe wall, the particle trajectory is largely governed by electric double layer forces and Van der Waals interactions. To account for the differing length and time scales of the physical processes involved, sequential multiscale modeling was implemented. A macro-scale model handles the fluid flow, traces the trajectory of the particle inside the pipe, and obtains parameters for the constituent fine-scale models as needed. Preliminary results of the fine-scale simulations examining the effect of the surface potential of the crud particles and the pipe surface are consistent with expectations from DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory, which looks at the surface interactions between charged colloidal surfaces. The results obtained from these simulations will be compared to the experimental data from the published test loop experiments that mimics the PWR environment to validate the model and, eventually, predict the efficacy of mitigation strategies.

• Nanoparticle synthesis and characterization
• Colloidal interactions
• Multi-scale Modelling
• Structure-Property correlation