05000529/LER-2006-006
| Palo Verde Nuclear Generating Station | |
| Event date: | 12-22-2006 |
|---|---|
| Report date: | 03-22-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 5292006006R01 - NRC Website | |
Note: All times listed in this event report are approximate and Mountain Standard Time (MST) unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This LER (50-529/2006-006-01) is being submitted pursuant to 10 CFR 50.73(a)(2)(i)(B) and 10 CFR 50.73(a)(2)(ii)(B), to report an Inoperable Essential Cooling Water (EW), Train B, Heat Exchanger which was determined to be a Technical Specification (TS) 3.7.7 violation. This condition left Unit 2 in an unanalyzed condition that significantly degraded plant safety and challenged the plant's residual heat removal capability.
Specifically, for 78 days, ending on September 27, 2003 when Unit 2 shutdown for a refueling outage, less than optimal chemistry controls for the Spray Pond (SP) System caused fouling on the EW 2B heat exchanger. The degraded condition was significant enough to render the heat exchanger inoperable.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The Spray Pond system (SP) (EIIS Code — BS) is the ultimate safety-related heat sink and is an open water, standby cooling system. The primary function of the spray pond system is to cool the Emergency Diesel Generator (EDG) (EIIS Code — EK) and Essential Cooling Water (EW) (EIIS Code — BI) systems. The primary function of the EW system is to provide cooling for the shutdown cooling system. There are multiple EDG heat exchangers (i.e., jacket water, lube oil, air intercooler and fuel oil) that are cooled by the spray pond system. Additionally, the EW system is used as an alternate means for cooling the spent fuel pool. The design bases for the spray pond system assume the system will function for 26 days without makeup water, losing approximately 85% of its original volume of water to evaporation and drift.
3. INITIAL PLANT CONDITIONS:
On December 22, 2006, Palo Verde Unit 2 was in Mode 1 (power operations), operating at approximately 100 percent power. There were no major structures, systems, or components inoperable at the start of the event that contributed to the event.
4. EVENT DESCRIPTION:
On December 22, 2006, the Palo Verde Nuclear Generating Station (PVNGS) received a Final Significance Determination letter from the Nuclear Regulatory Commission (NRC) for Apparent Violations deceivedduring an NRC Heat Exchanger Performance Inspection. In its Final Significance Determination letter, the NRC concluded that the Unit 2 Essential Cooling Water, Train B, Heat Exchanger was inoperable and a Technical Specification (TS) 3.7.7 violation for a period of 78 days, ending on September 27, 2003 when Unit 2 was shutdown for a refueling outage.
5.� ASSESSMENT OF SAFETY CONSEQUENCES:
APS performed an evaluation to predict the degradation that may have occurred over the operating cycle (2N11) for EW 2B heat exchanger (HX) during the period April 2002 and September 2003. The lowest coefficient of heat transfer (U) value that would be expected for this HX at the beginning of a Design Bases Accident (DBA) was 238.5 BTU/hr ft2 °F. Also the degradation that would be expected during the first day and 26 days of the DBA would result in U values of 234.6 BTU/hr ft2 °F and 231.3 BTU/hr ft2 °F respectively.
An evaluation has also performed to calculate the minimum required U value that would have been necessary for the EW 2B HX to perform its design bases function for the same operating cycle. Based on this calculation, the highest value for the minimum required U is 224 BTU/hr ft2 °F. This value already includes a degradation of 3.9 units of U during the 1st day following a DBA. During the following 25 days, an additional loss of 3.3 units of U is anticipated. Therefore, during the 26 day mission time, the minimum required U would be (224 + 3.3) = 227.3 BTU/hr ft2 °F. Since the predicted U for the HX at the end of the 26 day time-frame of 231.3 BTU/hr ft2 °F is greater than the U needed for a postulated DBA, the EW 2B HX would have been able to perform its intended design bases function during the period of interest (cycle 2N11).
In its Final Significance Determination letter, the NRC concluded that the EW 2B HX was Inoperable for 78 days, ending on September 27, 2003. The NRC concluded, "...the most appropriate value for the change in core damage frequency associated with these issues is 2.3x10:7 per year, allowing for some incidental cleaning credit, using actual pond and weather conditions during the period of greatest degradation, using midrange credit for operator recovery actions, and using upper bound values (conservative values) for the remaining assumption. To assess uncertainty, the maximum upper bound for the change in core damage frequency associated with these issues was determined to be 1.3x10-6 per year, allowing for no credit for incidental cleaning, and using actual spray pond and weather conditions during the period of greatest degradation, and using a high value for failure probability for operator recovery actions, and applying a maximum dependency on pump room cooling. Given that the majority of core damage frequency lies within the Green region, and considering the uncertainty involved as defined by the significance determination process, we have concluded that the most appropriate characterization of the significance of this issue is Green, i.e., very low safety significance".
For the time frame that Unit 2 operated with the degraded condition, Unit 2 did not experience an event that called for the Essential Cooling Water System to perform its safety function(s) for a DBA. This condition did not result in any challenges to the fission product barriers or result in any offsite releases. Using its Significance Determination Process, the NRC assessed a 'Green' None Cited Violation for failing to meet TS 3.7.7. Additionally, EW 2A was functional for the 78 day period that EW 2B was considered inoperable.
6.�CAUSE OF THE EVENT:
The direct cause of the loss of thermal performance from the EW 2B HX was the formation of an insulating precipitant on the SP side of the heat exchanger surfaces.
Root causes identified were:
- Inadequate Chemistry Control Program — Parameters were not being adequately controlled.
- Managed to Inadequate Chemistry Metrics — Metrics were not established for inorganic fouling
- Inadequate Resolution of Performance Problems — Opportunities to fix problems were missed.
- Over-reliance on a Single Expert — Incorrect actions were taken based on direction from an expert.
- Ineffective Change Management — Multiple changes to the plant, processes and people occurred ultimately causing the formation of the calcium-zinc-phosphate precipitant.
7.�CORRECTIVE ACTIONS:
The action to restore EW 2B HX to an Operable condition was accomplished during the Unit 2 fall 2003 refueling outage (U2R11).
In May, 2006, APS identified that the EDG 2B intake air temperature was higher than the maximum limits specified in the data collection logs. APS declared the EDG inoperable and inspected and cleaned the intercooler. As a result of these issues, APS initiated an investigation team that consisted of individuals from the Chemistry Department (Palo Verde and APS Corporate Chemistry), Engineering Department, Performance Improvement Team members with plant operations experience, and industry consultants. The scope of the investigation included a review of all heat exchangers that are cooled by spray pond water and addressed the lack of dispersant control for the SP from approximately March of 2004 until June of 2005. The investigation team evaluated both past and current plant documents including sampling results, reviewed industry information, obtained independent chemistry analysis and conducted limited inspections of the system to support their investigation and to draw their conclusions.
The APS investigation identified associated problems and actions were developed to resolve a long standing problem with chemistry controls for the EW and SP systems.
The SP system chemistry was corrected as follows:
- Dispersant additions were resumed in all spray ponds.
- Additional acid was added to lower pH in all ponds.
- Feed and bleed was used to reduce calcium and phosphate concentrations in each spray pond.
EW heat exchangers were cleaned in all three units.
EDG intercoolers were cleaned in all three units and an Operations Department Standing Order was issued to increase the EDG test frequency to assess intercooler performance. This standing order continued through September 29, 2006. It was removed after APS gained confidence that the immediate corrective actions were effective.
Procedure 40DP-90P08, "Diesel Generator Test Records," was revised to require a work order to be generated to clean any EDG intercooler if temperature exceeds 120°F in order to ensure the system does not exceed its Design Basis Manual (DBM) limit of 130°F.
The SPs have been cleaned, facilitating improved chemistry control and accessibility for inspections.
Additional cleaning and inspection activities of the EDG and EW heat exchangers have been scheduled to ensure the effectiveness of the completed corrective actions and to ensure there are no unintended consequences of the chemistry changes.
Additionally, modifications will be installed to add higher capacity acid and dispersant pumps.
8.�PREVIOUS SIMILAR EVENTS:
In the past three years, no similar event was reported.