ML18058B348
| ML18058B348 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 01/19/1993 |
| From: | Slade G CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| Shared Package | |
| ML18058B349 | List: |
| References | |
| NUDOCS 9301270195 | |
| Download: ML18058B348 (11) | |
Text
consumers Power GB Slade General Manager POWERiNii MICHlliAN'S flROliRESS Palisades Nuclear Plant: 27780 Blue Star Memorial Highway, Covert, Ml 49043 January 19, 1993 Nuclear Regulatory Commission Document Control Desk Washington, DC 20555 DOCKET 50-255 - LICENSE DPR PALISADES PLANT - EMERGENCY TECHNICAL SPECIFICATIONS CHANGE REQUEST - CONTROL ROD DRIVE MECHANISM TESTING REQUIREMENT Enclosed is a request for a change to the Palisades Technical Specifications to change the surveillance interval for testing two control rod drive mechanisms, CRD-20 and CRD-31, from "Every Two Weeks" to once in March 1993. This request is in followup to the granting of a temporary waiver of compliance (TWOC) on January 14, 1993 as described in the NRC's letter of January 15, 1993.
Below is the basis for your consideration for granting an emergency or exigent amendment to the Palisades Technical Specifications describing, in accordance with 10CFR50.91, why the situation occurred and why we could not avoid the situation. We understand that with the TWOC in effect consideration of an emergency or exigency remains necessary as stated in Thomas E. Murley's February 22, 1990 memorandum on temporary waivers of compliance.
Control rod drive mechanism (CROM) biweekly testing, as currently required by Technical Specifications Table 4.2.2, item 2, is believed to be aggravating the leakage of least 2 CROM seals. Repeated testing of a leaking seal has been shown to shorten seal life and increase CROM seal leak rate, which can lead to forced shutdown due to excessive PCS leakage. Two CROM seals are currently leaking. One CROM which had been declared inoperable thereby removing the testing requirement, has not exhibited signs of further leakage increases during the time period it was not tested. The other CROM which has been tested as required exhibits signs of increased leakage following each test.
Electing to shut down to replace leaking seals may not be the most prudent course of action. This would subject the plant to additional cooldown and heatup transients and it would not be consistent with maintaining radiation exposures ALARA. In addition to our efforts to determine the root cause of the CROM seal leakage problem we have engaged ABB-Combustion Engineering to assist in this determination. Action to address the root cause will be subsequently developed.
~ 950115 9301270195 930119
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~DR ADOCK 05000255 PDR A CMS' ENERGY COMPANY
2 As similarly described in our request for the TWOC, without a change to the Technical Specifications, biweekly testing of operable CRDMs with leaking seals must continue. The resulting expected accelerated seal degradation would be expected to result in forced shutdown within a matter of weeks due to excessive PCS leakage. Delayed processing of this request would have the same effect as disapproval since biweekly testing must continue unless the Technical Specifications are changed.
If reducing the surveillance testing of CRD-20 and CRD-31 is allowed, the expected rate of seal degradation will be reduced and may facilitate continued operation until the next scheduled refueling outage. Excessive leakage may still force a maintenance outage prior to refueling, but such a shutdown will be less probable with the reduced testing requirement.
The following Attachments are included in support of this request:
- 1) Attachment 1, Proposed Technical Specification page
- 2) Attachment 2, Existing Technical Specification page marked to show the proposed changes It is requested that this change request be effective immediately upon approval.
In addition to this Technical Specifications change for the control rod drives which are currently exhibiting leakage, CPCo is preparing a Technical Specifications Change Request to revise the test frequency for all control rod drives from biweekly to quarterly. The analysis which will provide part of the basis for this change is in draft form and is currently being reviewed. The final report is expected by early February, and the Technical Specifications Change Request should be submitted to the NRC shortly thereafter.
Finally, should the plant enter cold shutdown before the 1993 refueling outage the CRDM seals for CRD-20 and CRD-31 will be replaced.
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Gerald B Slad~~
General Manager CC Administrator, Region III, USNRC Resident Inspector, Palisades Attachments
1 l I CONSUMERS POWER COMPANY Docket 50-255 Request for Change to the Technical Specifications License DPR-20 For the reasons hereinafter set forth, it is requested that the Technical Specifications contained in the Facility Operating License DPR-20, Docket 50-255, issued to Consumers Power Company on February 21, 1991, for the Palisades Plant be changed as described below:
I. Change It is proposed that the frequency for control rod exerc1s1ng in Table 4.2.2, item 2, be amended, with a footnote which would read "During the remainder of cycle 10, CRD-20 and CRD-31 will be tested once in March 1993 in lieu of testing once every two weeks."
II. Discussion A.
Background:
The Control Rod Drive Mechanisms (CRDMs) at Palisades are of the Rack and Pinion Drive type. These drives have a drive package, containing a drive motor, position indication equipment, and a releasing clutch, which is outside the Primary Coolant System (PCS) boundary, and a drive shaft, right angle gear set, pinion gear, and rack, within the PCS boundary. The drive package is connected to the drive shaft through a mechanical seal, which forms the PCS pressure boundary.
Leakage through the mechanical seal enters a cavity which is vented to a collection header, and which is sealed at the top by a vapor seal.
Each mechanical seal is provided with a thermocouple to measure the temperature of its leakoff. The leakoff from all 45 CRDMs is collected in a common header and routed to the containment sump.
Two Control Rod Drive Mechanism (CRDM) are exhibiting signs of above normal seal leakage. Operating history has shown a trend that exercising a CRDM, as required by Technical Specifications Table 4.2.2 item 2, often causes seal leakage to increase. One CROM, CRD-20, has been declared inoperable which allows omitting the exercising of that mechanism. Technical Specifications do not allow continued operation with more than one control rod inoperable, so testing of the second CRDM exhibiting leakage has continued.
Increased leakage of a CROM seal is detected by an indicated increase in the CROM leakoff temperature. Individual CROM seal leakoff temperatures are available for review and trending on a chart recorder in the control room. Leakage measurement for individual seals is not possible. Combined leakage from all seals (collected in the common seal leakoff header) can be measured locally inside containment. The
2 seal leakoff header flow is directed into the containment sump, so the combined seal leakoff flow rate can be approximated between actual measurements by observing the rate of sump level increase. Typical leakoff temperatures are in a band of approximately 120-145°F.
Leakoff temperatures on CRDMs with leaking seals increase above the normal band. The collection header is unpressurized, limiting the leakoff temperature to saturation.
The CRDM provides two safety functions. With the exception of the motors ability to move the control rod with the rod rundown signal, the subject surveillance testing does not verify either of these safety functions. The safety functions are:
First, a reactor trip signal de-energizes the clutch and allows the control rod to drop by gravity into the core; this is the only CROM safety function assumed in the safety analyses.
Second, when a reactor trip signal is generated, a "rod rundown" signal energizes all full length CRDM motors to drive their rods in case they should not trip freely into the core. The rod rundown signal is terminated when that rod nears full insertion.
The clutch is designed to allow the motor to apply a torque to the drive shaft, in the "IN" direction, even when the clutch is released. Functioning of the rod rundown feature is not assumed in the safety analyses.
A reduced testing frequency would have no significant effect on the assurance that the CRDM will function properly and would reduce the probability of leakage increasing to the point where a plant shutdown is required.
B. Event History:
The seal leakoff temperature of CRD-20 increased after testing on April 28, May 12, and May 26, 1992, the first three testing dates of the current fuel cycle. Seal leakoff header flow increased on those dates, but dropped on the weeks in between. Leakoff temperature would drop on post-test days and then trend slowly upward until the next test, when another sharp increase would occur. After May 26, CRDM-20 leakoff temperature generally remained above 200°F during plant operation, so the rod was declared administratively inoperable in order to prevent further test-induced damage. No other CRDMs exhibited signs of leakage at that time. In spite of its leakage and regardless of its administrative designation, CRD-20 has remained fully functional and has tripped properly during the 5 plant trips which have occurred since refueling.
The seal leakoff temperature indication for CRD-31 has operated intermittently during this fuel cycle. Its temperature rose from a normal reading of approximately 145°F to 165°F following December 1
3 test, dropped afterward, and rose to 185°F after the December 15 test.
The last recorded temperature prior to the December 29 test was 182°F on December 23. At that time, indication was lost until it started functioning again about 6:00 pm December 29 (after testing), the indicated leakoff temperature was 215°F. At that time CRD-20, indicated leakoff temperature is also about 215°F. These two CRDMs are well separated on the reactor head, eliminating the possibility of leakage from one affecting the temperature indication on the other.
Since both CRD-20 and CRD-31 exhibited leakage on December 15 and 29, CRD-31 was selected for testing and CRD-20 continued to be declared inoperable for three reasons: first, based upon prior trending of temperature and leak rate data we concluded that the CRD-20 leak rate exceeded that of CRD-31; second, CRD-20 was expected to degrade more rapidly than CRD-31 if tested; and third, it was theorized that exercising might flush the CRD-31 seal which had exhibited leakage for a shorter period and reduce its leakage.
C. Effect of Proposed Change on CRDM Performance:
The Technical Specifications Bases do not explicitly define the purpose for biweekly control rod surveillance testing. We believe the primary basis for the biweekly surveillance is to verify that each, rod is moveable (i.e., not mechanically bound) and therefore the test provides some measure of increased confidence that it is trippable.
This interpretation is consistent with the basis for comparable surveillance requirement SR 3.1.5.5 in the Restructured Standard Technical Specifications (RSTS) for Combustion Engineering ~lants, NUREG 1432. The surveillance frequency for RSTS Surveillance 3.1.5.5 is quarterly. The Bases for SR 3.1.5.5 also indicates that other information is available and other surveillance are performed which add to the determination of operability of a control rod.
CRDM seal leakage does not increase the likelihood of an untrippable control rod. In order to do so, leakage would have to cause the clutch to fail to release, or cause mechanical binding of the driveshaft between the lower clutch face and mechanical seal, because all components above the lower clutch face are disengaged from the driveshaft upon a trip, and normally wetted components inside the PCS boundary will not be mechanically bound by leakage effects.
Clutch: In order to hinder trippability, the lower section must either fail to disengage or else jam between the shaft and some stationary component. Plausible failure modes cause the clutch to disengage (thus causing a rod trip), not remain engaged. Original clutches employed a splined sleeve which was prone to binding, but current clutches use a spring bellows and jaw faces which do not depend upon sliding action. When electrical power is removed, the upper face springs away from the lower one, an action which is not prone to mechanical jamming. Even if the vapor seal failed,
4 leakage would not prevent rotation of a disengaged lower clutch element.
Bearings: There are three sets of ball bearings between the clutch and vapor seal. To prevent a rod trip, one or more of these sets would have to bind sufficiently to resist dropping of a weight in excess of 200 pounds, or else degrade badly enough to allow gross driveshaft misalignment. The vapor seal protects the bearings from a corrosive atmosphere, and leakage limitations reduce the likelihood of vapor seal failure. Leakage limitations are not changed by the proposed waiver. In the past, even bearings filled with boric acid have performed properly. There is no reason to believe that any currently installed bearings have been exposed to steam or boric acid.
Vapor Seal: This is an elastomeric cup seal with a metal backing ring. The steam impingement washer protects it from erosion, and the vapor seal in turn protects drive components above from leakage. Operating temperature depends upon seal leakoff pressure as long as flashing occurs in the leakoff cavity. The collection header is unpressurized. The elastomer is designed for high temperature operation, and there is no metal-to-metal contact between stationary and rotating parts. If the vapor seal were to fail, it would not itself prevent shaft rotation.
Steam Impingement Washer: This thin stainless washer fits loosely around the driveshaft immediately below the vapor seal, at the top of the seal leakoff cavity. It cannot bind between the shaft and housing while remaining around the shaft, and plausible leaks will not break it.
Seal Assembly: The rotating element is inside the PCS boundary, so leakage will not corrode or bind small internal parts. There is ample clearance between the stationary assembly and driveshaft.
Shear forces will prevent binding at the seal boundary itself, as seal contact area is very small and materials were selected for low friction operation. Leak-induced temperature increase can degrade the three static 0-rings, but this will not prevent rotation.
Driveshaft: One end of the driveshaft is inside the PCS boundary, so component material was selected to withstand PCS effects.
Driveshaft upper end alignment is maintained by the lower clutch shaft which rides in three sets of ball bearings above the vapor seal. The drive shaft lower end bearings are within the PCS boundary.
We have concluded, therefore, considering the system design and acceptable performance of the refueling outage trip test, that the control rods are trippable and therefore, can meet their functional
5 requirements.
D. Effects of Continued Operation without Proposed Change:
The combined CROM seal leakoff flow through the leakoff header increased considerably during December, commencing shortly after December 1 testing. On December 29, leak rate rose from about 870 mvm~ a couple of hours prior to testing to 920 mvm~ eight hours after testing. Approximately 600 mi/min is attributed to CRD-20, based upon trending when CRD-31 temperature was near normal, and most of the balance to CRD-31. Palisades Technical Specification 3.l.5b mandates shutdown when total identified PCS leakage equals 10 gpm (37,850 mi/min), or unidentified leakage equals 1 gpm (3,785 mi/min).
Gross control rod drive seal leakage eventually can cause the vapor seal to fail, allowing leakage to flash and escape the leakoff piping near the drive package. This leakage can eventually damage drive package internals, and contaminate the surrounding area. To prevent these effects Palisades Off Normal Procedure ONP 23.1, Section 4.8, requires reactor shutdown when leakage is confirmed to be a CRDM seal failure in excess of 2 gpm ( 7, 570 ml/ min)
- This leak rate is we 11 within the leakoff header flow capacity, so the limitation effectively protects vapor seal integrity.
E. Actions being taken to Reduce Future Leakage:
Palisades operating experience has demonstrated that there is a connection between biweekly CRDM exercising and seal leakage. For some time we have been investigating ways to justify a reduction in test frequency. During the course of that investigation we became aware of an effort by the Combustion Engineering Owners Group to develop generic justification for reducing rod drive test frequency from biweekly to quarterly for CE plants with magnetic jack type control element drive mechanisms (CEDMs). The CE designed plants with magnetic jack CEDMs are vulnerable to dropping rods during the exercise test and therefore, in large part this was the reason for the frequency reduction. In August, CPCo submitted a purchase order to ABB-Combustion Engineering to develop a report to apply to the Palisades rack-and-pinion type drives, and to also develop a safety evaluation for a Technical Specifications Amendment request. The draft of this Palisades-specific report was sent to CPCo in December, less than a week after CRD-31 began to exhibit leakage. CPCo is currently reviewing that draft report. Following resolution of CPCo comments and receipt of the final report we will submit a proposed Technical Specifications change to adopt a quarterly rod testing surveillance frequency.
6 III. Analysis of No Significant Hazards Consideration Consumers Power Company finds the activities associated with this proposed Technical Specifications change involve no significant hazards and accordingly, a no significant hazards determination per 10CFRS0.92(c) is justified. The following evaluation supports the finding that operation of the facility in accordance with the proposed change would not:
- 1. Involve a significant increase in the probability or consequences of an accident previously evaluated.
The proposed Technical Specifications change does not alter any plant systems, instrument settings, or operating methods. Its only potential effects would be to reduce expected CROM seal Leakage and to reduce the assurance, normally provided by biweekly testing, that a CROM has not become mechanically bound to the point where it cannot be moved by its motor. Mechanical binding of a CROM is not classed as an "accident".
Therefore, operation of the facility in accordance with the proposed change to the Technical Specifications would not involve a significant increase in the probability of an accident previously evaluated.
The intent of the biweekly control rod exercise surveillance test is to detect control rods that are stuck and demonstrate that control rods can move freely over a small range of movement (minimum of 6 inches). The current Palisades surveillance frequency of every two weeks was apparently based on engineering judgement. Operating experience has demonstrated that this surveillance is not a principal method for detecting stuck control rods. The ability to trip the control rods, i.e., the operability of the rods, is not affected by decreasing the surveillance frequency. Operability (trippability) of the rods is demonstrated by the refueling outage surveillance test.
Further evidence this cycle has been the five reactor trips that have occurred in which all control rods including CR0-20 (which has evidenced leakage since April 1992) have tripped. Reactivity control, therefore, through control rod tripping or through boration is not affected by this change in the surveillance frequency.
The FSAR reactivity events consider that the most reactive control rod remains fully withdrawn from the core during a reactor trip. Because the trippability of the control rods are not degraded by this surveillance frequency changes the consequences of these reactivity events have not been increased.
The control rod rundown feature, which is not required to mitigate an accident, will also not be degraded by the change in surveillance frequency. Control rod indication would not be affected by the change. Additionally the mechanical or electrical reliability of the control rods would not be degraded
7 by the change in frequency of the surveillance for the leaking control rods. Therefore, combined with the ability of the control rods to remain trippable (operable}, the probability of occurrence of an accident previously evaluateq has not been significantly increased.
The effect of CROM seal leakage on CROM components has been reviewed to determine if trippability of the control rods is affected. That review of the components (described above) leads to the conclusion that seal leakage will not affect the trippability of the control rods.
Therefore, operation of the facjlity in accordance with the proposed change to the Technical Specifications would not involve a significant increase in the consequences of an accident previously evaluated.
- 2. Create the possibility of a new or different kind of accident from any previously evaluated.
The proposed change in surveillance frequency for the leaking control rods would not alter the equipment design or operation. Therefore, operation of the facility in accordance with the proposed change to the Technical Specifications would not create the possibility of a new or different kind of accident from any previously evaluated.
- 3. Involve a significant reduction in a margin of safety.
Review of control rod events at Palisades and Fort Calhoun (the only other plant with Palisades-style CROMs) back to 1971 has shown no instances in which biweekly testing detected untrippable rods. Fort Calhoun data was obtained from NPROS and was not verified with OPPO.
Inability to drive rods via the rod rundown feature was discovered in some cases, generally caused by brake, drive motor, or relay contactor
- failure. Such occurrences could have prevented control rod rundown capability, but since affected components were all above the clutches, the ability to trip the control rod was not affected. The review also indicated that there were 33 instances of untrippable or sticking control rods (of which 22 were attributable to three common failure modes which have since been resolved). Of these, 4 were discovered prior to initial criticality, 29 during tests other than biweekly exercising, 2 during scrams, and 2 by failure to withdraw during startup, but none by biweekly testing (some of the occurrences fit in multiple categories). There were 2 other events in which trippability was not ascertainable from records reviewed, but neither occurred during biweekly testing.
Biweekly testing of the control rods over the 20 years of Palisades operating history has not detected any instance where control rods have not been trippable. The control rods were demonstrated trippable (operable) by the control rod drop timing test during the last
8 refueling outage and by their successful operation during the five reactor trips since refueling. Therefore, even with the presumed most reactive rod being stuck during an FSAR reactivity event, there is not a reduction in the margin of safety with respect to limiting reactivity additions during any of these FSAR events.
Therefore, the proposed change to the Technical Specifications would not involve a significant reduction in a margin of safety.
IV. Environmental Consideration Consumers Power has also reviewed this application in accordance with IOCFRSI.22 and has concluded that it would not result in a significant change in the types or significant increase in the amounts of any effluents that may be released offsite. Additionally the proposed change would not result in a significant increase in cumulative or occupational exposure. Accordingly, since the change would also not involve any significant hazards the categorical exclusion requirements of IOCFR51.22(e)(9) are satisfied and an environmental review is not required.
V. Conclusion The Palisades Plant Review Committee has reviewed this Technical Specifications Change Request and has determined that the change involves no significant hazards consideration. This change has been reviewed by the Nuclear Performance Assessment Department. A copy of this Technical Specifications Change Request has been sent to the State of Michigan official designated to receive such Amendments to the Operating License.
CONSUMERS POWER COMPANY To the best of my knowledge, information and belief, the contents of this submittal are truthful and complete.
P Hoffman, Vice Nuclear Operati Sworn and subscribed to before me this /:J"'o,ay of~ 1993.
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Notary Public
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