Steam Generator Cleaning Campaigns at Bruce A: 1993-96

Abstract

Boiler chemical cleaning (BOCC) and high-pressure water lancing operations were performed during the Bruce A 1993 Unit 4, 1994 Unit 3, 1995 Unit 1 and 1996 Unit 3 outages to remove secondaryside deposits. High-pressure water lancing focused on three boiler areas: tube support plates (TSPs) to remove broached hole deposits, hot leg U-bend supports (HLUBS) to dislodge deposits contributing to boiler tube stress corrosion cracking (SCC) and tube sheets with the aim of removing accumulated sludge piles and post BOCC insoluble residues. The chemical cleaning processes applied -were modified versions of the one developed by the Electric Power Research Institute/Steam Generator Owners Group (EPRI/SGOG). During these BOCC operations, corrosion for several key boiler materials was monitored and was well below the specified allowances. Boiler chemical cleaning and tube sheet lancing removed about 6,250 and 2,200 kg of deposit respectively from Unit 4 boilers in 1993. The BOCC process involved an initial copper removal cycle, magnetite removal step, a final copper cycle and passivation step in that order. Tube sheet lancing operations, done after chemical cleaning, removed accumulated sludge from the cold leg and outer hot leg areas down to the tube sheet. However, hard tube deposits roughly 18 cm (7 inches) above the tube sheet were left in the hot leg central regions. High-pressure water lancing of the HLUBS, before or after BOCC, had limited success in cleaning the scallop bar surfaces of these U-Bend supports and left most hourglass (tube land) areas filled with deposit. In contrast, the combined effect of water lancing and BOCG left most TSP broached holes in an almost "as new" condition. Minor deposits remained in the TSP land areas. The BOGC operations performed on Unit 3 in 1994 dissolved about 6,800 kg of deposits. For this BOCG campaign, the cleaning sequence was the same as applied in Unit 4, but the magnetite step was lengthened from 40 to 100 hours to enhance cleaning of the U-bend supports. The combination of pre and post BOCC lancing left HLUBS scallop bars cleaner than those in Unit 4, but did not remove the hourglass deposits. Tube sheet lancing before BOGC dislodged roughly 1,770 kg of sludge, compared with 875 kg of material after BOCG. Tube sheets were left in a similar condition to those in Unit 4 the year before; cold leg and outer hot leg areas were cleaned down to the tube sheet and hard tube deposits about 15 cm (6 inches) high remained in the hot leg central areas. As in Unit 4 a year earlier, water lancing and BOGC left TSP broached holes nearly deposit-free with minor deposits in the tube land regions. To enhance cleaning of the HLUBS hourglasses and TSP land areas, the BOCG process was modified for Unit 1 in 1995; the low temperature magnetite step was shortened to about 24 hours and followed by a 67.5-hour EPRI/SGOG-type crevice cleaning step applied at 121 degrees C. The latter step involved ~1-minute steam drum vents every hour to induce boiling and enhance crevice deposit dissolution. The sequence of pre and post BOGG lancing activities was the same as performed during the 1994 Unit 3 outage. In total, the Unit 1 BOGG process dissolved almost 8,500 kg of deposits. Post water lancing inspections of the HLUBS after BOGG showed that more scallop bar surface and hourglass deposit had been removed compared with the Unit 3 operations a year earlier. However, the hourglasses were only partially cleaned out. About 2,000 and 520 kg of tube sheet sludge was removed by water lancing before and after BOGG respectively. Tube sheets were left in virtually the same condition as those in Unit 4 and Unit 3 after BOGC and water lancing; hard tube scale about 18 cm (7 inches) above the tube sheet remained in the hot leg central regions. The combined cleaning operations left TSP broached holes nearly deposit free, but again, minor residues remained in the tube land areas. In an effort to maximize deposit removal from the HLUBS hourglasses, a modified BOGG process was qualified for recleaning Unit 3 in 1996. The BOGC process performed involved a copper removal step, crevice clean and a proprietary copper/passivation step in that order. The crevice solvent was applied for 100 hours at 107 degrees C while performing 2 vents per hour. Since the TSP broached holes were left nearly deposit free after the 1994 cleaning activities, pre and post BOCG lancing was limited only to the HLUBS and tube sheet areas. Post lancing visual inspections of the HLUBS after BOGG showed scallop bar surfaces and hourglasses to be almost free of deposits; only thin residues remained in the hourglass areas. The BOGC operations removed about 2,530 kg of deposit, while the pre and post BOCC tube sheet lancing activities dislodged approximately 162 and 46 kg of tube sheet sludge respectively. The height of the hard tube scale in hot leg central areas above the tube sheet, was reduced by 2.5-6 cm (1-4 inches) in most Unit 3 boilers by the BOCC process. Hard sludge piles about 8 cm (5 inches) high were evident in some of these hot leg regions. As expected, TSP broached holes appeared almost "as new" and minor residues remained in the land regions.