Impurity concentration behaviors in a boiling tubesheet crevice - Part I. Open crevice

Abstract

In a locally restricted geometry on the secondary side of steam generator (SG) in a pressurized water reactor (PWR), impurities in bulk water can concentrate by boiling processes to extreme pH that may then accelerate the corrosion of tubing and adjacent materials. To simulate a real SG tubesheet crevice, a high temperature/high pressure (HT/HP) crevice simulation system was constructed. The simulated crevice area was monitored with thermocouples and electrodes for the measurement of temperature and electrochemical corrosion potential (ECP), respectively, in the crevice as well as at the free span. A secondary solution composed of 50 ppm Na and 200 ppb hydrogen (H-2) was supplied. In an open tubesheet crevice with 0. 15 mm radial gap, axial distribution of temperature and ECP were measured as a function of time and available superheat, DeltaT. The sodium hydroxide (NaOH) concentration process in the crevice was characterized with temperature and ECP data. It was observed from temperature results that the liquid penetration depth decreased and the dry region expanded from the bottom of the crevice as DeltaT increased. Measured ECP data showed a behavior similar to the hydrogen electrode potential behavior predicted by the Nernst equation. From measured values of boiling point elevation, the maximum crevice concentration factor was estimated to be about 500 at DeltaT = 20degreesC and about 1000 at DeltaT = 25degreesC across the tubing wall. (C) 2003 Elsevier B.V. All rights reserved.