ML20247J950
| ML20247J950 | |
| Person / Time | |
|---|---|
| Site: | Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png |
| Issue date: | 09/30/1989 |
| From: | Israel S NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | |
| Shared Package | |
| ML20247J926 | List: |
| References | |
| TASK-AE, TASK-E907 AEOD-E907, NUDOCS 8909210033 | |
| Download: ML20247J950 (7) | |
Text
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DIVERSION OF SEAL' COOLER PLOW FOR RER PUMPS l
September 1989 Prepared by:
S.
Israel Office for Analysis and Evaluation of Operational Data U.S. Nuclear Regulatory Commission i
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INTRODUCTION Design deficiencies associated with single failures that affect the recirculation mode following a loss of coolant accident (LOCA) have been the subjects of two earlier AEOD reports and information notices. These deficiencies are usually not detected during normal plant operation or surveillance testing because they involve flow networks not replicated during normal or test operations.
The problem discussed in this report was discovered during a probabili-stic risk assessment (PRA) at Haddam Neck.
DESCRIPTION OF PROBLEM A single failure that could disable both residual heat removal (RHR) pumps in response to a LOCA was identified during a PRA study of the emergency core cooling system at Haddam Neck (Ref. 1).
The RER system is used for long term cooling following a LOCA.
Cooling water to the RHR pump seal coolers comes from a supply header off the cooling water inlet lines to each RHR heat exchanger as indicated in Fig. 1.
Normally, the component cooling water system cools the heat exchangers and seals, but during an accident, the component cooling water is isolated and service water is valved in to provide cooling.
If one of the service water valves does not open during alignment for recirculation cooling following a LOCA, one branch (part of a train) of service water would and up providing water to both RHR heat exchangers. Under these conditions, the RHR pump seal coolers would receive inadequate flow. The licensee corrected this problem by installing check valves to preclude one train of service water from feeding two RHR heat exchangers.
DISCCSSION Referring to Fig.
1, line AC, originally without check valves CC-CV-225 AEB, connects the two trains of inlet water to the two RHR heat exchangers.
The check valves shown on this line are not part of the original installation, they were added recently by the licensee to correct the problem under discussion.
Following a LOCA, flow from the component cooling water system is isolated and the valves from the service water system are opened.
If one of the service water valves does not open, then one branch of service water feeds both heat exchangers, one directly, and one via the interconnecting line Ag.
For example, if valve SW-MOV-5 does not open, there is increased flow (above what normally would occur to service just the RHR pump seal cooling) in the line AB because of the open flow path through RER heat exchanger 1B.
The increased flow in line AH results in a higher pressure drop in this section of pipe and a reduced pressure at point B.
The reduced pressure at point B results in l
a reduced flow to RER pump seal coolers.
1
The switch from component cooling water to service water to provida -
cooling.for the RER heat exchangers during an accident is probably unique to Baddam: Neck.
However, even without the evitchover, a similar situation could occur with one branch of cooling water
. feeding two heat exchangers and two pump seal coolers.
A survey of service water systems for about 30 PWRs indicates that most of the plants do not use service water to cool the ECCS pumps directly or they, have a different piping arrangement to preclude this
. situation.
This does not rule out a similar problem for plants using component cooling water to service the RER heat exchangers and'ECCS pump cooling.
The potential for the flow diversion problem depends on the actual piping configuration and the valve alignment for the auxiliary water system during normal and accident conditions.
There have been two earlier ABOD studies ~(Ref. 2 and 3) which have discussed similar function problems with fluid systems when one train or branch has to feed two trains of downstream equipment.
This flow diversion issue is -just another example of failing to anticipate all degraded flow conditions in designing a fluid system.
The licensee added check valves in the crcssover line AQ to eliminate' the problem and has successfully tested the modification.
The use of check valves is very effective in these situations, but requires periodic surveillance to assure that the valves are still performing their intended function.
The water hammer event at san Onofre, Unit 1, in November 1985 (Ref. 4) dramatically illustrated the problems with unmonitored check valves.
The specific design deficiency discussed above has direct safety significance because it represents a single failure that can defeat a safety function, long term cooling, needed to recover from a LOCA.
The licensee is to be commended for identifying this deficiency in his PRA.
CONCLUSION Failure to consider all potential off-design conditions during the design of a fluid system can result in loss of function for systems having interconnected piping configurations.
The interconnections can result in single branches feeding multiple downstream com-ponents.
This type of deficiency generally goes undetected for extended periods because they are not included in periodic surveillance tests.
REFERENCE 8 1.
Connecticut Yankee Atomic Power Company, Licensee Event Report 50-213/88-015, Haddam Neck plant, June 14, 1988.
2.
S. Israel, Engineering Evaluation Report, Inadequate NPSE in Low Pressure Safety Systems in PWRs, AEOD/E710, Oct. 1987.
p 3.
- 8. Israel, Engineering Evaluation Report, Inadequate NPSE in High Pressure Safety Systems in PWRs, AEOD/E803,'Aug. 1988.
I 4.
U.S.
Nuclear Regulatory Commission, Loss of Power and Water Hammer Event at San Onofre, Unit 1, on November 21, 1985, NUREG-1190, Jan. 1986.
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DRAFT I
INFORMATION NOTICE NO.
FLOW DIVERSION OF SEAL COOLING FOR RER PUMPS Purpose q
This notice is provided to inform recipients of potentially significant deficiencies pertaining to the diversion of seal cooling water for RER pumps during a loss of coolant accident.
The diversion of flow would result in inadequate seal cooling' and ultimate failure of the low head pumps needed to provide long term cooling following a LOCA.
It is suggested that the recipients review the information.for applicability to their facilities.
Description of Event:
A single failure that could disable both residual heat removal (RER) pumps in response to a LOCA was identified during a PRA study of the emergency. core cooling system at Haddam Neck. Cooling water for the RER pump seal coolers comes from the coming water inlet lines to each RER heat exchanger as indicated in Fig.
1.
Normally, the component cooling water system provides cooling the heat exchangers and seals, but during an accident condition, the component cooling water is isolated and service water is valved in to provide cooling.
If one of the service water valves does not open during line up for recirculation cooling following a LOCA, one branch (part of a train)of service water would and up providing water to both RER heat exchangers. Under these conditions, the RER pump seal coolers would receive inadequate flow. The licensee corrected this problem by installing check valves (shown in Fig.1) to preclude one branch of service water from feeding two RHR heat exchangers.
Discussion This design deficiency was identified in a plant that switched cooling systems during an accident.
However it could occur in any auxiliary cooling system, such as component cooling or service water system, that provides sizably different cooling needs for different redundant components such as RER heat exchangers and seal coolers.
The important feature is the interconnecting piping between the auxiliary cooling system trains and the piping to the individual components.
In this instance, a single failure of a valve to open caused one branch of auxiliary cooling water to service multiple redundant components.
The resulting redistribution of pressure drop in the system reduced the flow in the small bore piping leading to the seal coolers.
A flow balance analysis must be performed to determine the impact of asymmetric flow balances caused by single failures in these systems.
Simple review of pipig diagrams may
be sufficient to identify systems with potential problems, but a hydraulic analysis is necessary to confirm suspected problems.
No. specific action or written response is required by this information notice.
If you need additional information.regarding this matter, please contact the Regional Administrator of the appropriate. regional office or the technical contact listed below..
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