Mechanistic understanding of CRUD deposition in accelerated high-temperature water as a function of water chemistry

Abstract

Corrosion products build-up is well known to compromise the safety and efficiency of light water reactors such as the Pressurised Water Reactor (PWR) and the Boiling Water Reactor (BWR). In the primary cycle of a PWR, corrosion build-up is known under the acronym Corrosion Related Unidentified Deposit (CRUD). Locations where hydrodynamic perturbation of the flow occurs, such as under the acceleration observed in steam generators, are subjected to an enhanced CRUD build-up. Under these conditions, electrokinetic effects driven by the deflection of streaming current and supported by faradic reactions have been postulated as responsible for the CRUD deposition. In the present work, the effects of flow, material, water chemistry and cathodic polarisation on CRUD build-up were investigated and a comprehensive understanding of the deposition phenomenon was achieved. The results showed that CRUD build-up, induced by a wall current, has only a marginal contribution in the overall deposition at neutral pH where the deposition is independent of the materials tested and is controlled by mass transport; this latter effect depends on the amount of soluble and particulate species in solution and is strongly affected by the hydrogen content. Conversely, at alkaline pH, where the solubility of the oxide is reduced, and the magnitude of the zeta potential is greater, there is an increase in electrokinetic deposition contribution on the overall build-up. Such an increase was however only found in SS304L and Alloy 690 and not in Zirlo or Zirconia which do not have a good electrically conductive oxide or substrate. Finally, polarisation tests showed that CRUD deposition is suppressed by a cathodic polarisation and it was postulated that this is due to a combination of factors, which include a counteraction on the electrokinetic component, as well as, the mechanical removal effect associated with the evolution of hydrogen.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Fabio Scenini (Supervisor) & Michele Curioni (Supervisor)