05000382/LER-2002-001
| Waterford Steam Electric Station, Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3822002001R00 - NRC Website | |
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REPORTABLE OCCURRENCE
On January 30, 2002, it was determined that during certain testing or maintenance activities on the Shield Building Ventilation System (SBVS) [VC] or the Controlled Ventilation Area System (CVAS) [VF], both systems trains would be rendered inoperable. This resulted in a condition prohibited by Technical Specification (TS), reportable per 10CFR 50.73(a)(2)(i)(B), as the trains were inoperable beyond the allowed outage time of TS 3.0.3. Because the trains were inoperable, a condition existed that could have prevented the fulfillment of the safety function of the systems to control the release of radioactive material, reportable per 10CFR 50.73(a)(2)(v), as neither system could achieve required vacuum to preclude the release of radioactive gases during certain accident conditions. In addition, the configuration of these systems, while testing or performing maintenance activities, caused two independent trains to become inoperable in a single system, reportable per 10CFR 50.73(a)(2)(vii).
INITIAL CONDITIONS
Upon discovery of this event, Waterford 3 was operating in mode 1 at 100% reactor power. There were no major systems, structures or components that were inoperable at the time of discovery that contributed to the condition.
SYSTEM DESCRIPTION SUMMARY
The Shield Building Ventilation System (SBVS) is a safety related system and is designed to preclude annulus pressure from exceeding atmospheric pressure following a Loss of Coolant Accident (LOCA). It is an engineered safety feature system and is not normally in operation.
During accident conditions this system operates to maintain a partial vacuum inside the shield building annulus to prevent unfiltered leakage of airborne radioactivity to the atmosphere. This system provides holdup and removal of fission products by recirculation and filtration using two redundant filtration trains and two 100 percent capacity fans which can exhaust back to the annulus or to the plant stack.
Figure 1 depicts the SBV system. The SBV system has two basic flowpaths used for two phases of operation. The phases of operation are the exhaust phase and the recirculation phase. The flowpath from the annulus through the filter trains and exhaust fans is the same for both phases of operation. Air is drawn through circumferential headers located in the upper and lower annulus regions. Air flows to filter trains 1A and 1B via inlet motor operated butterfly valves SBV-101A and SBV-101B. Inside the filtration unit, the air flows through a demister, electric heating coil, prefilter, high efficiency particulate air (HEPA) prefilter, charcoal adsorber, and HEPA after filter before exiting through motor-operated, butterfly valve SBV-110A(B). Locked open butterfly valve SBV- 109 cross-connects the filter trains just upstream of these motor operated valves which permits one filtration unit fan to draw a small percentage of air flow through the second inactive unit for removal of the decay heat generated by radioactive isotopes that have collected in the standby filtration unit. Downstream of SBV-110A(B), two centrifugal fans SBV-1A(1B) discharge through independent ducts via gravity dampers SBV-111A(B). Downstream of the gravity dampers, two flowpaths are provided to enable the two phases of operation. In the exhaust phase, the filter train discharges through the respective motor operated valve SBV-114A(B) to the plant stack. In the recirculation phase, no air is exhausted to the plant stack. Instead, air is recirculated back to a header via motor-operated valve SBV-113A(B).
The purpose of the Controlled Ventilation Area System (CVAS) is to provide high efficiency filtration and iodine adsorption for all air exhausted from the controlled ventilation areas during normal conditions and following a design basis accident (LOCA).
Figure 2 depicts the CVAS system. The CVAS filtration units draw air through ductwork to establish two air flowpaths. Each flowpath is associated with an A or B filtration unit. Following a Safety Injection Actuation Signal (SIAS), all isolation, suction and inlet valves of each redundant system position as necessary to provide flowpaths from the controlled ventilation areas. A cross connection duct, which includes a manually operated butterfly valve (HVR-312) that is locked one notch open, permits one filtration unit fan to draw a small percentage of air flow through the second inactive unit for removal of the decay heat generated by radioactive isotopes that have collected in the standby filtration unit.
EVENT DESCRIPTION
On January 30, 2002, it was determined that during certain testing or maintenance activities on the Shield Building Ventilation System (SBVS) or the Controlled Ventilation Area System (CVAS) both systems trains would be rendered inoperable.
Prior to approximately 1988 and after initial plant startup in 1984, in-place filter testing, such as for those in the SBVS and CVAS systems, was performed during refueling outages. These systems were not required to be operable in mode 5 (cold shutdown) or mode 6 (refueling). Starting in approximately 1988, in-place filter testing for these systems began to be performed while the plant was in mode 1. Technical Specifications 3.6.6.1 and 3.7.7 require that two independent trains of the SBVS and the CVAS be operable in mode 1 (Power Operation), 2 (Startup), 3 (Hot Standby), and 4 (Hot Shutdown) respectively.
As shown in figures 1 and 2, both SBVS and CVAS consist of two units each (A' and 'B' trains), which utilize a charcoal adsorber bed to remove radioactive iodines from the air stream. The CVAS filters, heaters, and demisters are housed in a unit approximately 30 feet long. The unit has three access doors approximately 10 square feet in area and three access doors approximately 12 square feet in area. The SBVS filters, heaters, and demisters are housed in a unit approximately 33 feet long. The unit has four access doors approximately 10 square feet in area and three access doors approximately 12 square feet in area. A cross-connected duct and throttled valve permits the operating unit to draw a small percentage of airflow through the standby unit. This allows for the removal of the decay heat generated by radioactive isotopes that have collected in the standby filtration unit. Upon a start signal to either train, the inlet valves to both trains go open.
The inlet valve on the operating train opens to align the system for proper operation; the inlet valve on the standby train opens to allow air to be drawn through it via the cross connected duct connecting the two trains. A SIAS signal initiates a signal to open inlet valves on both the SBVS and the CVAS. If a unit door or doors are opened for maintenance or testing on the standby unit and the inlet valve is not tagged closed, a start from the other train would open both inlet valves.
This would create an open path from ambient conditions, through the open door, through the open inlet valve, and into the Shielding Building (annulus) or CVAS envelope. This path would prevent the operable train from fulfilling its design requirements of maintaining the desired vacuum in the respective envelope. When the unit doors were opened for filter testing, the inlet and cross-connect valves were not tagged closed. It was not until after the most recent in-place testing that engineering recognized that this condition could render both trains inoperable simultaneously.
ROOT CAUSE
A latent organization weakness was discovered in the review and approval process for work control. When in-place testing was moved from a "refuel activity" to an "on-line activity", the plant failed to recognize the potential for one train to effect the operability of the opposite train. The error was not recognized during subsequent reviews or performance of the surveillance, since 1988.
Contributing Factors
The unique system design contributed to the difficulty in recognizing the effects of the inoperable unit on the operable unit. Multiple groups repeatedly failed to recognize this condition for over 14 years.
CORRECTIVE ACTIONS
To preclude the effect of the door opening in the future, the inlet valves for the trains being tested or maintained will be tagged closed allowing the opposite train to achieve required vacuum conditions.
Current work practices enhance the review and approval process of activities planned to be performed online that were previously performed offline.
1. A multi-discipline team reviews and approves changes in outage work activities. The review is documented and includes, as a minimum, the impact on Technical Specifications.
2. Procedure WM-100, Maintenance Action Item (MAI) Generation, Screening, and Classification, requires a work screening committee to review work performed on-line to ensure the activity does not cause an entry into Technical Specification 3.0.3 action statement, does not require the unit to be in cold shutdown, refueling or defueled, and the activity is not likely to exceed any of the Technical Specifications/Technical Requirements Manual completion times requiring shutdown, etc.
Additional corrective actions to preclude recurrence have been entered, and are being tracked in the plant's corrective action program. (Reference Condition Report CR-WF3-2002-0168)
SAFETY SIGNIFICANCE
This condition rendered the SBVS and CVAS being incapable of achieving required vacuum conditions during accident conditions. This constitutes a loss of safety function and is considered a Safety System Functional Failure (SSFF). Although the systems were not able to achieve required vacuum, resulting from the door openings, this event's safety significance was minimal. Following an accident (LOCA) and subsequent SIAS actuation, initial vacuum conditions (greater than or equal to 0.25 inches of water gauge in the annulus in 1 minute after start signal and greater than or equal to 0.25 inches of water gauge within 45 seconds for the CVAS system) may not have been achievable. No unfiltered release is expected to occur. The train not being tested or maintained would still function as required to filter the air.
During testing or maintenance of these systems, craft personnel were informed not to leave the units with the doors open and to close the unit doors in the event the system actuates. � There is a maximum of seven doors on the SBVS, which are all within approximately 33 feet of each other.
There is maximum of six doors on the CVAS, which are all within approximately 30 feet of each other. It is assumed that any and all open doors would be closed within a maximun of five minutes of required system start of either the SBVS or CVAS. This would limit the amount of time the systems could not achieve required pressure conditions. Since this condition is expected to exist for a very short time, i.e., maximum of 5 minutes, this condition is deemed not to have a significant impact on the release of radioactive material to the environment because:
- The releases through the operable train will be filtered.
- The pressure in the shield building annulus and in the areas serviced by the CVAS is not expected to be significantly different than the ambient pressure. Thus, there is no pressure gradient to cause unfiltered release from these areas.
- The post-accident (LOCA) activity release to the containment is in the early stage of the fuel rod gap release. Per NUREG-1465, Accident Source Terms for Light-Water Nuclear Power Plants, in the first 30 minutes, only a small percentage (5%) of the total iodine is released to the containment.
This event is considered a Safety System Functional Failure (SSFF).
SIMILAR EVENTS �
- Record searches from 1999 to present have not identified similar failures with a root cause of latent organization weakness in the review and approval process for work control. A condition similar to this event involving online maintenance work resulting in the inoperability of multiple trains is being reported in LER 2002-002. A discussion of this similiarity is provided in LER 2002-002.
ADDITIONAL INFORMATION
Energy Industry Identification System (EIIS) codes are identified in the text within brackets [ ].
FIGURE 1
SHIELD BUILDING VENTILATION SYSTEM
..,....._ / SBV '101A
SBV
110A SBV ( C(...."- -------- ------, 176 ELEV . ---" , M ✓ H ,
MEPF HEPA
— HECA HEPA m 111A "--- ----. 10 000 ,--,---- ■ "*---
CFM
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F.'
FAI
SBV
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SBV
101B M _ ____y_ HC MEPF ,--
HEPA
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HECA HEPA
109 L.O.
BV
SOB SBV
[RI
FAI
SBV-113B SBV-112B 'X r--1----- FAI SBV-1B --, CFM ` --, 411---ANNULUS
CONTAINMENT
24' ELEV I/1 FAI CAR SBV-114B 206B SBV 113A FROM CAR TO PLANT STACK "All EXHAUST FANS FAt FAI CAR 206A SBV-112A L.---1 SBV-114A M
REACTOR SHIELD
BUILDING WALLS
TO PLANT STACK
FAT
SD-SBV-0 FIGURE 2
CONTROLLED VENTILATION AREA SYSTEM
(FAIL OPEN) HVR 301 -4-{ � 1 � SHUTDOWN HEAT EXCHANGER A & B
VALVE
GALLERY
4 SAFEGUARD PUMPS' � , A & A/B � [SAFEGUARD PUMPSI B
VAULT AREA
RAG
FROM RAB
HVR HVR F-14- NORMAL HVAC 107 106 (FAIL SHUT) HIM � E TI_ D
HVR
3048 H 3038
MAKEUP
DAMPERS
FROM PIPE
PENETRATION AREA
H 303A 7
FROM PIPE
CHASE CO NORMAL RAB HVAC
IVR X-E1 1°9 SHUT) (FAIL It"-D 108 tHi OFtl) VR 302 � Fit
FROM PIPE
PENETRATION
AREA
HVR V-1E1 110 HVR X--E3
RAB HVAC
111
TO NORM
� T HyR L.., 3138 LO � (3B-SB) 312T M HVR E23 313A (3A-SA) 11111
FROMNORM
RAB HVAC
RAD
MONITORS
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