Corrosion and deposition in PFR evaporator tubes: Pt. 2

Detailed measurements have been made of the thickness of oxide layers in the vicinity of grid plates on the water side of a full length tube removed from a prototype fast reactor (PFR) evaporator unit. Except in a few small areas where oxide spalling has occurred, the metal loss due to corrosion is similar to that in other regions of the tube and independent of heat flux. Large local variations are observed in deposited oxide thickness in grid regions. In the case of supporting grids, the oxide within the grid can be less than half of the thickness of oxide on the surrounding tube, whereas upstream and downstream of the grid, the oxide can be more than twice this thickness. Large circumferential variations in oxide thickness are also seen. Similar, but lesser, effects are present in the region of non-supporting grids. Using a previously determined relationship between deposited oxide thickness and heat flux, it is possible to derive heat flux data from the experimental results. The results can be accounted for by leakage-flows of hot sodium through small gaps between circumferential baffles surrounding the tube bundle. Thus accurate measurements of oxide thickness on evaporator tubes can be used to establish heat flux patterns and hence gain information on sodium-side thermal-hydraulic behaviour. (author)