4.0 Summary of Major Review Topics, DCISC 15th Annual Report - July 1, 2004 thru June 30, 2005
4.16 Steam Generator Performance
4.16.1 Overview and Previous Activities
Steam generator (SG) tube reliability is important to operational safety, because the SG tubes are part of the Reactor Coolant System (RCS) boundary. The nuclear industry has experienced substantial problems with a variety of failure mechanisms, notably tube stress corrosion cracking. The DCISC reviews steam generator performance following each refueling outage.
The SGs are currently scheduled for replacement in the R15 refueling outages, however PG&E is evaluating moving replacements to 2R14 and 1R15. It typically requires 5 years lead time for SG construction.
Long Term Plans for Steam Generators
The long term plans have been reviewed in previous reporting periods. These were:
- Continued improvement in probe technology and repair criteria
- Chemical Cleaning - could mitigate the increasing trend of ODSCC. Performed in 1R12 and 2R12.
- Sleeving - May eventually be required to maintain 15 plugging margin
- Tube Plugging Limit Increase - Westinghouse analysis supports 15 per loop and overall. May need to revise analysis to allow one loop to exceed 15.
During the previous reporting period, the DCISC reviewed the following SG items:
- 1R12 Steam Generator Chemical Cleaning Preview
- Outage 1R12 Steam Generator Chemical Cleaning Results
- Outage 1R12 Steam Generator Tube Inspection Results
During previous periods, the DCPP steam generators appeared to be in adequate health and well within safety limits. PG&E’s monitoring and replacement plan appeared satisfactory.
4.16.2 Current Period Activities
The principal degradation mechanisms affecting DCPP Steam Generators (SGs) include: outside diameter stress corrosion cracking (ODSCC) and primary water stress corrosion cracking (PWSCC) at the hot legs, at the tube sheets and at dented and non-dented intersections; U-bend PWSCC; anti-vibration bar (AVB) wear scarring; fatigue and cold leg thinning (CLT). The degradation requires regular tube inspections during refueling outages.
During the current reporting period, the DCISC reviewed the following SG items:
- Steam Generator Chemical Cleaning
- Plans for New Steam Generators
Steam Generator Chemical Cleaning
The DCISC reviewed the results of the SG chemical cleaning performed during Outage 2R12 at the December 8 & 9, 2005 Fact-finding Meeting (Volume II, Exhibit D.5, Section 3.7).
PG&E Plans to replace its SGs in 2007 and 2008 because of excessive cracking in many of the thousands of tubes across which heat transfer takes place and which separate the primary and secondary systems. PG&E had decided to chemically clean the secondary side of the SGs in Outages 1R12 and 2R12 to remove fouling deposits to improve heat transfer and remove scale to help avoid free-span outside diameter stress corrosion tube cracking. The cleaning process is that developed by the Electric Power Research Institute and Steam Generator Owners’ Group. There have been more than 25 successful applications of this process since the mid-1980s.
PG&E reported that it had completed the cleaning of Unit 1 SGs with no problems other than some minor occurrences of chemical fumes near some cleaning equipment and piping connections. The cleaning process functioned as designed and planned. The tubes were described as “shiny clean” in a post-cleaning visual inspection. A series of pre- and post-cleaning eddy-current tests (ECTs) on 200 tubes indicated no degradation due to the cleaning process. The amount of material (19,313 pounds) removed from the SGs was about half the weight expected. The expected amount was wrong because of a model error by the SG manufacturer and cleaning vendor, Westinghouse.
The Unit 2 cleaning went just as well as Unit 1 with the operation well within the SG cleaning safety plan. The 28,486 pounds of sludge and scale removed was within 10 of the estimate. The amount of materials removed from Unit 2 was larger than for Unit 1 because the deposit loading in that unit was higher.
The Unit 1 average main steam pressure at the SG exits had increased by about 7 psia versus a decline of 6 psia before and after Outage 1R11. The steady-state steam pressure appeared to be about 790 psia, back to its pre-1R11 outage value. Unit 2 had not yet restarted at the time of the Fact-finding meeting.
It appears the Steam Generator chemical cleaning was successful on both Units 1 and 2 during Outages 1R12 and 2R12, respectively. SG tubes appeared shiny, and SG exit steam pressure was improved. Both cleanings were performed with no adverse effects on personnel or equipment.
Plans for New Steam Generators
PG&E reported on the plans for the new Steam Generators (GS) at the February 16 &17, 2005 DCISC Public Meeting. (Volume II, Exhibit B.6). The SG Replacement Project required a separate rate making application concerning financial aspects of the Project be made to the CPUC, which PG&E submitted at the beginning of 2004. PG&E has conducted an environmental review and will submit an Environmental Impact Report concerning the Project.
PG&E plans to store the old SGs on site at DCPP in a storage facility and fabrication and analysis of the new SGs has begun. Westinghouse was selected as the prime contractor and the firm of ENSA, a Spanish company, was selected to fabricate the new SGs. The firm of SGT has been selected as the installer and a contract is currently being negotiated. PG&E also reviewed some of the similarities and differences between the models including the number, material and thickness of the tubes and the number of primary dryers. They stated the thermal power rating will stay the same and the new SGs will fit into the same space as the old. To the extent possible, the new SGs will be replaced under provisions of 10 CFR 50.59, and DCPP is required to show its plans are within the DCPP licensing basis.
PG&E reviewed and discussed improvements, which should result from installation of the new SGs including material improvements with the Alloy 690 tubing, high-strength steel shell components and stainless steel or high chromium content internals. Improvements due to manufacturing techniques should also be realized. Improvements to accident/transient margins will be realized through the smaller tubing size, SG tube rupture analysis will allow for a smaller assumed leak and reduced off-site dose consequences; and due to an integral steam flow restrictor providing a smaller postulated main steam line break area and potentially lower, post accident peak containment pressure; expanded narrow range level indication resulting in lower low water level reactor trip setpoint, enabling the Plant to potentially withstand a loss of one main feed pump without a reactor trip.
The new SGs will have 60-70 lbs. higher steam pressure, due to more heat transfer capability, which will potentially increase the steam plant’s thermal efficiency, especially if the high pressure turbines are replaced. The increased surface area of the new SGs will potentially allow for operation at a reduced reactor coolant hot leg temperature. Better performance should also be realized in improved moisture separation and reduced moisture carryover, while higher circulation ratio and blow down efficiency should also be achieved, reducing sludge accumulation on the tube sheets.
The U-2 SGs are scheduled to be replaced in February 2008, during the 2R14 refueling outage, while U-1 is scheduled to have its SGs replaced in January 2009, during the 1R15 refueling outage.
The DCISC should continue to follow this area and to track potential changes from replacement of the DCPP SGs and any increased safety margins actually achieved.
4.16.3 Conclusions and Recommendations
- Conclusion:
- PG&E has an effective Steam Generator (SG) health program. In addition to examining and plugging questionable tubes and maintaining tight chemistry on both the primary and secondary sides, DCPP’s chemical cleaning was successful on both Units 1 and 2 during Outages 1R12 and 2R12, respectively. SG tubes appeared shiny, and SG exit steam pressure was improved. Both cleanings were performed with no adverse effects on personnel or equipment.
Notwithstanding the attention paid, the SGs are slowly aging, and PG&E plans to replace them in three-to-four years. The DCISC plans to follow the replacement program and to track potential safety changes and any increased or decreased trends in safety margins.
The DCISC will also continue to closely monitor DCPP SG performance and inspections.
- Recommendations:
- None