Oxidation and mass transfer in heavy liquid metal loops

Abstract

A simple method has been devised to calculate the mass transfer in liquid metal loops. It is based on the use of mass transfer coefficients which determine the mass flux from the wall into the fluid. These coefficients depend on a dimensionless characteristic thermo-hydraulic number, namely the Sherwood number, which itself depends under forced convection flow conditions on the Reynolds number and the Schmidt number. This is supplemented by the application of the mass conservation law, which allows the calculation of the conditions in the bulk of the fluid. The newly developed kinetic model for mass transfer under forced convection flow conditions has been implemented in the computer code MATLIM. The formation of protective oxide scales on stainless steel components is of prime importance for lead and lead-bismuth loops. As the propensity for mechanical effects like delamination and spalling increases with the thickness of the oxide scale, it is indispensable to have a clear understanding of the physical effects contributing to their growth. Thus, the importance of dissolution effects on oxide scales must be investigated and quantified, especially the dependence on the oxygen content in the liquid metal. This determines also the mass transfer from the hot leg to the cold leg in these loops. At higher temperatures dissolution attack of stainless steel specimens has been observed in some cases for oxygen contents in lead-bismuth alloy normally sufficient for oxide scale formation. Dissolution effects on pre-existing oxide or other protective scales are also of great importance for lead-lithium loops. These loops are operated at very low oxygen contents in order to avoid the formation of Li 2 O. With the loss of protective scales heavy dissolution attack of stainless steel components commences.