Experimental investigation of denting phenomenon in the steam generators of the nuclear power plant

Abstract

One of the most detrimental corrosion problems occurred in the steam generators of the nuclear power plant is the denting phenomenon which occurs in tube/support plate crevices. Denting means reduction in the diameter of the Inconel steam generator tubes due to the accumulation of corrosion products caused by accelerated carbon steel corrosion by acid chloride. To identify the denting phenomenon, corrosion rates occurred in the simulated tube/support plate crevices examined by using 7 model boilers at 161.7 psia. The model boilers were operated with all volatile treatment (AVT)(morpholine + hydrazine) with 15 ppm and 100 ppm chloride concentrations as 10\% $FeCl_2$, 30\% NaCl and 60\% $CaCl_2$. Heat was provided by electrical heaters with heat fluxes, 3725, 7450 and 11175 Btu/hr-$ft^2$. The simulated support plates were made of carbon steel and Type 304 stainless steel. Corrosion rates are increased with the increase in heat flux. A model is proposed to explain this observation and the mechanism that corrosive materials are accumulated in the tube/support plate. This model is expressed by the following equation for empty heated crevices when the clearance factor is unity. $$\frac{Cc}{Cb}=\exp[(1.5q".s/\rho w \cdot \triangle H \nu \cdot D)^f]$$ $$f = \left[ \begin{array}{ll} 0.515, & q" \ge 25000 \qquad Btu/hr-ft^2\\ 0.465, & q"<25000 \qquad Btu/hr-ft^2 \end{array} \right. $$ Several experiments were conducted to clarify effects of boric acid as a denting inhibitor when boric acid co-exists with Cu ion. The experimental results show that boric acid is an effective denting inhibitor in the environments with Cu ion.