ML20236X732
| ML20236X732 | |
| Person / Time | |
|---|---|
| Issue date: | 05/01/1987 |
| From: | Beckjord E NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | Murley T Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20195F761 | List:
|
| References | |
| FOIA-87-714, REF-GTECI-093, REF-GTECI-NI, TASK-093, TASK-OR IEB-85-001, NUDOCS 8712100193 | |
| Download: ML20236X732 (6) | |
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MEMORANDUM FOR: Thomas E. Murley, Director Office of-Nuclear Reactor Regulation FROM: Eric S. Beckjord, Director Office of Nuclear Regulatory Research
SUBJECT:
RESOLUTION OF GENERIC ISSUE 93, " STEAM BINDING OF AFW PUMPS" Generic Issue 93 is concerned with the pctential disabling of the auxiliary feedwater (AFW) pumps by steam binding caused by backleakage of main feedwater (MFW) past the isolation check valves interfacing the AFW and MFW systems. The key safety significance of the issue lies in the potential vulnerability of most AFW systems to common mode steam binding failure of the redundant pumps.
This vulnerability is inherent in the piping configurations used, which allow for cross connections between trains via common headers on the suction and discharge sides of the pumps.
To reduce the probability of pump failure by steam binding if backleakage does occur, IE Bulletin 85-01 (October 1985) required - je licensees and CP holders, who had not already done so in response t. previous NRC and INPO recommendations, to implement procedures both for monitoring the AFW piping temperatures as an indication of possible backleakage and for restoring the pumps to operable status if steam binding were to occur or appeared likely to do so. The Bulletin's recommended monitoring frequency of once per shift, compared with the previous one of effectively once per month (when the pumps were tested), provided for a factor of about 90 reduction in pump unavaila-bility due to steam binding.
Generic Issue 93 has been resolved and no further study is required. The basis for this conclusion is described in the attached Regulatory Analysis. To pro-vide an empirical basis for this analysis, a review was made of the backleakage experience obt::ined in operating plants since the start of systematic monitoring (about April 1985 for a majority of the plants). The Task Manager contacted-each of the operating PWRs via the resident inspectors to determine the status of this concern at each plant. The results showed two distinct groups in regard to backleakage,.with the dominant majority of plants (about 90%) showing a low incidence of backleakage occurrences (less than one per reactor year) while the other plants showed a far higher event rate by a factor of roughly 100. It is important to note that none of the backleakage events during the review period resulted'in steam binding of an AFW pump, because the various monitoring methods employed appear to be effective in preventing steam binding with a high degree of assurance. For the plants with a high backleakage event rate, utility installation of a continuous monitoring system with control room alarm was instrumental in providing for early warning to the operator and timely corrective action. .
As shown in the enclosure, conservative estimates of the contribution of steam binding to core melt frequency indicates this to be small: a frequency of ;
about 1x10 7/RY for the 90% group of plants and about 1x10 S/RY for the second roup. The corresponding public risks, int grated over the remaining plant N b 8712100193 871204 Q/
L 1
. lifetimes, are estimated to total 36 person-rems and 44 person-rems for the respective groups. These results do not support any new requirements or further study of this issue.
Because the risk is low, the plants that have not experienced backleakage over at least several months of operation could be allowed some relaxation in monitoring frequency (f,or example, to once per week). This would be beneficial in permitting a shift of effort to other areas of plant maintenance.
Conversely, the second group of plants that have backleakage problems should continue efforts toward resolving their specific leaky check valve problems.
Generic Issue 93 is resolved and no further study is required. However, to keep the risk low, it is important for the plant operators and our inspectors to continue to be alert to the possible development of leaky check valves, especially as plants age, and be ready to increase the monitoring frequency as needed to assure prevention of pump steam binding. In this connection, it is suggested that inspection modules be modified to include monitoring of possible backleakage degradation that may develop. The coordination of such findings with the relevant sectors of NRR, or of information gained by the resident inspectors on the progress made by the second group of plants in resolving
" 'their specific backleakage problems, would provide the basis for continuing surveillance by NRC on this issue. Additionally, it would be appropriate for the PWR project managers to ensure that the annual FSAR updates include docu-mentation of any plant instrumentation changes made in regard to the monitoring of AFW system piping temperatures so that inspectors will have a basis far inspecting this instrumentation. A draft memorandum covering these points is enclosed for your consideration.
Eric 5. Beckjord, Director Office of Nuclear Regulatory Research
Enclosure:
Regulatory Analysis of GI-93 cc: V. Stello, E00 J. Zerbe, CRGR R. Fraley, ACRS F. Miraglia, NRR R. Starostecki, NRR E. Jordan, AE00 J. Allan, Reg. I J. N. Grace, Reg. II J. G. Keppler, Reg. III .
R. D. Martin, Reg. IV J. B. Martin, Reg. V l
t p nne k UNITED STATES NUCLEAR REGULATORY COMMISSION
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MEMORANDUM FOR: Frank J. Miraglia, Associate Director for Prejects, NRR Richard W. Starostecki, Associate Director for Inspection and Technical Assessment, NRR FROM: Thomas E. Murley, Director Office of Nuclear Reactor Regulation
SUBJECT:
RESOLUTION OF GENERIC ISSUE 93, " STEAM BINDING OF AFW PUNP5" Generic Issue 93 on steam binding of AFW pumps due to backitakage from the
.sain feedwater system has been resolved and no new requirements other than those t.1 ready provided by the issuance of IE Bulletin 85-01 need be imposed on the licensees or CP holders of pressurized water reactor plants.
The regulatory analysis of this issue is included in the RES memorandum from E. Beckjord to me, copies of which have been sent to you and to the Regional Administrators. Within the framework of the existing Bulletin 85-01 require-ments, the asses:sd risk associated with this issue is found to be low.
However, because of the known experience regarding the general unreliability of check valves in nuclear power plants it is important that the project managers be eiert, via the resident inspectors, to possible AFW check valve leakage problems tntt say develop in their plants, in order to ensure that the plant response to the proble is such as to assure the prevention of AFW pump steam binding.
The attached generic information letter should be sent to all licensees and CP
. Solders within your scope of responsibility to advise them of the resolution of Generic Issue 93. Furthermore, all PWR project managers sre to ensure that the annual FSM updates include documentation of any instrumentation changes made in regard to the monitoring of AFW systems piping temperatures so that inspectors will have a~ basis for inspecting this instrumentation.
l Thomas E. Murley, Director Office of Nuclear Reactor Feaulation
Enclosure:
Draft Letter to All Licensees -
UNITED STATES
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'o NUCLE AR REGULATORY COMMISSION I u s. e cton.o.c.rosss Enclosure S.
DRAFT TO ALL LICENSEES, APPLICANTS FOR OPERATING LICENSES, AND HOLDERS OF CONSTRUCTION PERMITS FOR PRESSURIZED WATER REACTORS Gentlemen:
SUBJECT:
RESOLUTION OF GENERIC SAFETY ISSUE 93, "5 TEAM BINDING OF AUXILI ARY FEEDWATER PUMPS" This letter is to inform you of the staff's resolution of the subject safety issue which concerns the potential disabling of auxiliary feedwater (AFW) pumps by steam binding caused by backleakt.ge of main feedwater (MFW) past the The significance isolation check valves interfacing the AFW and MFW systems.
of the issue stems from the potential vulnerability of most AFW systems to common mode steam binding failure vf the redundant AFW pumps, this vulner-ability being inherent in the piping configurations used, which allow for cross connections between trains via common headers on the suction and discharge sides of the pumps. I f
To reduce the probability of pump f ailure by steam binding if backleakage does occur, IE Bulletin 85-01 (dated October 1985) required those licensees '
and CP holders, who had not already done so in response to previous NRC and INPD recommendations, to implement procedures both for monitoring the AFW piping temperatures as an indication of possible backleakage and for restoring the pumps to operable status if steam binding were to occur or appeared likely to do so. The Bulletin's recomended monitoring frequency of once per shift, compared with the previous once per month check when the pumps were tested, proviced for a f actor of about 90 reduction in pump unavailability due to steam binding.
As a basis for the staff's regulatory analysis of this issue, a limited review l
was made of the backleakage experience in operating plants since the start of The systematic monitoring (about April 1985 for a majority of the plants),
usults showed a double-humped distribution in regard to backleakage, with the i
dominant anajority of plants showing a low incidence of backleakage occurrences I
(a mean of less than one per reactor year) while the remaining plants showed a far higher event rate by a factor of roughly 100. None of the backleakage events during the review period appeared to have resulted in steam binding of an AFW pump, indicating that the various monitoring methods employed can be highly effective in providing for the prevention of steam binding if backleakage occurs. For the plants with a high backleakage event rate, the installation of continuous monitoring systems with control room alarms was instrumental in providing for early warning to the operator and timely corrective action.
The results of the regulatory analysis indicated that, within the frame-work of the existing Bulletin 85-01 requirements, the contribution of AFW pump steam binding to core melt frequency and public risk was sufficiently low to warrant the finding that no new requirements beyond those specified in IE Bulletin 85-01 need be imposed.
Further,. the staf f concludes that for plants that have not experienced backleakage over at least several (e.g., four) months of operation, some reduction in the frequency of monitoring (e.g., Som once per shift to once per day or to once per week) might be beneficu, !. ;seraitting a shift of effort to other areas of plant maintenance. Conversely, for those plants that have had frequent backleakage it is considered important thst plant efforts to resolve specific AFW check valve leakage problems receive the degree of emphasis appropriate to a safety system of high risk importance.
Furthemore, the FSARs should be updated to include documentation of any
,; instrumentation changes made in regard to the monitoring of AFW systems piping temperatures.
Although the staff has concluded that the currently assessed risk associated with this issue is ' reasonably low and that some relaxation in monitoring frequency might be appropriate under certain conditions, it is nevertheless I
considered, in light of the generally unsatisfactory check valve reliability experience obtained to date in operating plants, that plant operators continae to be alert to the possible development of malfunctioning check vahes,
) especially as the plant ages, and be prepared to increase the monitoring l
frequency as needed to ensure that steam binding failure of the AFW pumps does not occur.
O No written response or specific action is required by this letter, therefore, no clearance from the Office of Management and Budget is required. If you have any questions on this matter please contact your project manager.
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