The effect of the removal of steam generator tube ID deposits on heat transfer

The thermal resistance of boiler primary-side tube deposits from the Gentilly-2 NGS (Hydro-Quebec) was evaluated by an experimental comparison of the heat transfer rates between fouled samples and identical, factory-new, 'clean' tubing. The deposits were subsequently removed using either a chemical decontamination process (CAN-DEREM Plus) or a mechanical cleaning process (Siemens SIVABLAST) in two stages. After each removal, the thermal resistance of the remaining deposit was re-measured. The 90- to 150-μm-thick deposits on the inside diameter of steam generator cold-leg tubes were found to pose significant resistance to heat transfer (0.05 to 0.06 m2·K/kW at 210 degrees C). However, the 10- to 30-μm-thick dense layers remaining on the tubes after the decontamination were found to have no measurable effect on the heat transfer. The thin, 2-μm, tube deposit on the steam generator hot leg slightly enhanced heat transfer. The measured thermal resistance results in a calculated thermal conductivity of 1.5 W/m·K for the 90-μm-thick deposit. The 150-μm-thick deposits were found to consist of two layers: an outer surface layer having an average porosity of 50% and a conductivity of 2.3 W/m·K, and an inner layer with an average porosity of 5% and a conductivity of more than 3.0 W/m·K. The previous best estimate of the thermal conductivity was 1.4 W/m·K for the porous magnetite deposits that had formed on the primary side of nuclear steam generators with thickness <90 μm. This work confirms this number but also demonstrates that it is applicable only for porous, unconsolidated deposits. The conductivity increases for thicker deposits because of increasing deposit consolidation, particularly at the most inner layer adjacent to the tube metal. (author)