Investigating the effect of hydrodynamics and coolant chemistry on CRUD deposition in regions of accelerated fluid flow.

CRUD deposition on the surface of micro-orifice flow restrictions (300 µm in diameter) were conducted in simulated PWR primary water containing either LiOH or KOH water as a function of hydrodynamics conditions and autoclave refresh rate. It was found that the CRUD volume on the front surface of the orifice, and the morphology of the non-soluble species adhering in the outer layer, were dependent on the total flow rate around the loop. FEG-SEM images reveal the CRUD deposit morphology, was consistent with a combination of both particulate adhesion and crystalline particle growth. These results highlight the importance of particulate and metal ion content in the water, released by the structural materials at elevated temperatures. In fact, the CRUD build up rate was the highest when the autoclave was semi-stagnant and the flow rate varied between 4 – 13 l/h, whilst it decreased by a factor of ~5 when the autoclave was fully refreshed with a constant flow rate of 25 l/h. Moreover, the surface build-up was consistently lower by a factor of ~2 in KOH water, when compared to tests conducted in LiOH at comparable flow velocities. This is postulated to be due to an increase in the solution conductivity, originating from the reduced size of the hydrated radius and increased mobility of K⁺ relative to Li⁺, in equivalent pH environments, which affected the deposition process. The mechanism underpinning CRUD deposition on the surface of micro-orifice flow restrictions is also discussed