ML18057A774
| ML18057A774 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 02/27/1991 |
| From: | Slade G CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9103130025 | |
| Download: ML18057A774 (4) | |
Text
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consumers Power POW ERIN&
MICHl&AN'S PRD&RESS Palisades Nuclear Plant:
27780 Blue Star Memorial Highway, Covert, Ml 49043 February 27, 1991 Nucl~ar Regulatory Commission Document Control Desk Washington, DC 20555 GB Slade General Manager DOCKET 50-255 - LICENSE DPR PALISADES PLANT - ADDITIONAL INFORMATION -
CONTAINMENT AIR COOLER LEAKAGE Consumers Power Company letter dated January 16, 1991 entitled "Request for Relief From Specific ASME B&PV Code Requirement - Containment Air Coolers",
requested relief from a code requirement that the containment air coolers exhibit zero leakage in a post repair condition. This request was revised by our February 11, 1991 submittal.
NRC staff reviewers have requested additional information. Their questions and our responses are provided below.
NRC QUESTION:
The licensee states that an administrative limit of 4 gpm, service water leak rate will be established. Specifically what actions will be taken if the leak rate limit is reached?
CPCO RESPONSE:
Once the administrative limit of a total of 4 gpm has been verified as a service water leak in the containment from the containment air cooler system, the limiting condition for operation in Technical Specification 3.4.2 for containment cooling will be entered. This allows one cooler to be out of service for up to seven days before a plant shutdown is initiated.
For the purposes of this scenario a total leakage of 4 gpm from all containment air coolers will be treated as one inoperable cooler to be consistent with the applicability of the Technical specification.
NRC QUESTION:
At what point will the plant be shutdown?
A CMS ENERGY COMPANY
//DtJ I I ID
CPCO RESPONSE:
The plant will be shutdown in accordance with the requirements of Technical Specification 3.4.2 which requires that the reactor be placed in a hot shutdown condition within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> if the cooler or coolers could not be restored to an operable condition within seven days.
NRC QUESTION:
Specifically, how will repairs be qualified and found acceptable?
CPCO RESPONSE:
All repairs or repair attempts of the leaks identified on the containment air coolers to date have been accomplished by brazing the leak connections consistent with the original manufacturing methods.
In our relief request we identified that one of the actions we would pursue in conjunction with the relief request would be to investigate cold type repair techniques (eg epoxy or liquid steel).
We have not reached a resolution or decision at this time as to whether or not an alternate repair technique of this type could be accomplished or that it would be something that we would want to pursue.
We are aware that a cold type repair would have to be shown to function through a design basis accident and we also do not want to introduce material to the cooler tube material that could be detrimental to a future brazing repair.
NRC QUESTION:
The licensee has not adequately addressed the structural integrity of the brazed tube joints. What is the root cause failure? Is it age related (i.e.,
fatigue problem or radiation embrittlement)?
CPCO RESPONSE:
The root cause of the containment air cooler tube leaks is discussed in a reference in the relief request (PDFlenner to DDCrabtree dated February 17, 1987).
The reference concludes that poor workmanship during the original brazing of the cooling coils is the primary cause of the identified leaks.
The poor brazing was compounded by the cooling coil design.
Erosion of the braze material may have also led to some of the leakage.
Fatigue and radiation embrittlement are not known to be contributors to the leakage root cause.
NRC QUESTION:
What confidence is there that the brazed joints will not fail during a MSLB or LOCA event such that the design basis leak rate of service water from the containment air coolers is exceeded during the period of time following initiation of the event that the coolers are required to be operable?
CPCO RESPONSE:
As discussed in the safety evaluation included in the relief request, containment air cooler coil leakage can occur following pressure transients in the service water system which generally occur as a result of system testing during plant shutdown.
During plant operation the containment air coolers
normally operate without system pressure transients occurring.
We therefore, do not expect any significant increase in air cooler leak rate during normal operation.
During a MSLB or LOCA scenario, a safety injection signal initiates the opening of the 8-inch outlet valves on the containment air coolers. This*
allows for maximum flow through the coolers in order to remove the additional accident heat load, and would result in a lower system pressure in the air coolers. If a loss of offsite power were also to occur, the service water pumps would be load shed and then sequenced back on the diesel generators.
The A and B service water pumps start at 9 seconds and the C service water pump starts at 26 seconds after diesel start. Because the 8-inch outlet valves are opened before the pumps start we expect that the surges in this accident condition would be smaller than surges from the pump starts under normal testing conditions. Variations in service water system pressure would be dependent on whether a loss of off-site power occurs and the number of diesel generators and service water pumps starting in an accident scenario.
No quantification has been made of the as found leak rates that have resulted from the system pressure transients which led to the leaking cooling coils.
By subjective evaluation, the leakage observed this past weekend, when compared with known flow rates, has been estimated at two gpm or less. These latest leaks were judged to be among the worst that we have experienced.
Normally observed leakage is far less. Our goal in performing maintenance is to reduce these leaks to zero if practical. Post maintenance leak rates are determined by an air drop* test at a pressure of 90 psig.
The maximum 4.0 gpm leak rate, proposed by the relief request, assumes that flooding of required long term post accident environmentally qualified equipment will occur if the 4.0 gpm of service water leakage were to continue for more than 70 hours8.101852e-4 days <br />0.0194 hours <br />1.157407e-4 weeks <br />2.6635e-5 months <br /> following a LOCA (which is more limiting for containment flooding level than a MSLB).
This scenario assumes one emergency diesel and two service water pumps (A and C) operating and three containment air coolers and one spray pump operable for containment heat removal.
If during this 70 hours8.101852e-4 days <br />0.0194 hours <br />1.157407e-4 weeks <br />2.6635e-5 months <br /> offsite power were returned or the second diesel generator was operable or another source of emergency power established (to provide power for containment spray pump operation) then the service water to the containment could be isolated if necessary (assuming the containment water level was showing an increasing trend and service water isolation was necessary) and containment sprays used to remove heat in containment.
Therefore, even if a pressure transient during the MSLB or LOCA were to increase the amount of leakage to even double the proposed limit (8 gpm) we would still have 35 hours4.050926e-4 days <br />0.00972 hours <br />5.787037e-5 weeks <br />1.33175e-5 months <br /> (70/2) to return offsite power or start the second diesel generator to service or possibly provide an alternate source of emergency power to power the containment spray pumps, and isolate the coolers to prevent further flooding of equipment in containment.
Based on our low as left leak rate and assuming that some gradual increase in the leak rate occurs during plant operation (which isn't typically experienced), and based on past history, we believe that the end of the cycle air cooler leakage will be at less than 1 gpm.
Any pressure transients experienced during a MSLB or LOCA would not likely cause the leak rate to exceed 4.0 gpm.
We believe therefore, that 4.0 gpm represents a reasonable limit which balances the need to provide safety margin with the need to allow
response time for maintenance activities prior to plant shutdown.
Additionally, based on the low probability of a catastrophic failure of a tube during a MSLB or LOCA (i.e. the allowed leakage hasn't contributed to this failure probability) and the low probability of a design basis MSLB or LOCA requiring emergency operation of the containment air coolers, there is enough margin in time to conclude that a hypothesized increase in cooler leakage during a MSLB or LOCA could be addressed and curtailed prior to flooding of required long term environmentally qualified equipment.
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Gerald B Slade General Manager CC Administrator, Region III, USNRC NRC Resident Inspector - Palisades