Hideout of sodium salts in tubesheet crevices: Final report

The hideout of sodium hydroxide and sodium chloride in tubesheet crevices was studied in single tube model boilers at CADARACHE. Radioactive Na24 was used for the measurements. Crevices of 0.2 mm width were empty or packed. A hydraulically expanded crevice and a rolled crevice were also studied. The concentration rate depends on the bulk water concentration but the equilibrium concentration appears to be related only to thermal conditions. The equilibrium value of 25 moles per liter in the crevice was extrapolated, the sodium concentration in the bulk water being in the range of 5 to 50 ppM. This equilibrium will be obtained with an ''integrated pollution'' of 5000 to 10,000 ppM x hours. Flushing produced by the boiler depressurization was shown to be an effective way to remove salt from the crevice. Sodium hydroxide was shown to concentrate even in a residual crevice of few μm width