Impurity concentration on boiling heat transfer surface in tube-tube support plate crevice. Pt. 3. Estimation of impurity concentration factor on boiling heat transfer surface in tube-tube support plate crevice using electrochemical technique

Local corrosion has been experienced on the secondary side of steam generator (SG) tubes in tube-tube support plate crevice of PWR's, because the slightly dissolved impurities in feed water were concentrated in the crevice. Various chemical analyses have been applied to evaluate the water quality in the crevice, but the in-situ chemical analyses method has not been established yet. In this report, the electrochemical corrosion potential of alloy 600 in the crevice were measured in deaerated sodium hydroxide and sulfuric acid aqueous solutions in a range of 100-270degC under boiling heat transfer condition, and the effects of heat flux and solution temperature on the impurities concentration were examined, and then the impurities concentration factors were calculated from the results of corrosion potential measurements of alloy 600 SG tube in the crevice. The main test results are as follows: (1) Corrosion potential of alloy 600 decreased with the increase of heat flux in the deaerated sodium hydroxide solution, whereas increased with the increase of heat flux in sulfuric acid solution in a range of 100-270degC. from these shift of corrosion potentials, it was estimated that the pH value of the solution in the crevice increased when the heat flux was applied under the caustic conditions, while the value decreased under the acidic conditions, due to the effect of sodium hydroxide concentration or sulfuric acid one in the crevice. Therefore, it was concluded that this electrochemical method was applicable to evaluate the impurities concentration behavior in the tube-tube support plate crevice. (2)from the calculated results of the concentration factors, it was revealed that the factors of sodium hydroxide were about 1,000 to 500,000 in the range of dry and wet condition. And the concentration in the caustic solution was remarkable, compared with the case of acidic solution. (author)