ML20056H506
| ML20056H506 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 08/31/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20056H504 | List: |
| References | |
| NUDOCS 9309090439 | |
| Download: ML20056H506 (4) | |
Text
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NUCLEAR REGULATORY COMMISSION
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WASHINGTON. D C. 20555-0001 v
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.157 TO FACILITY OPERATING LICENSE NO. DPR-20 t
CONSUMERS POWER COMPANY t
PALISADES PLANT DOCKET NO. 50-255
1.0 INTRODUCTION
By letter dated March 29, 1993, the Consumers Power Company (the licensee) requested an amendment to the Technical Specifications (TS) appended to Facility Operating License No. DPR-20 for the Palisades Plant. The propcsed amendment would change the surveillance interval for Table 4.2.2, Item 2,
" Partial Movement of All Rods (Minimum of 6 In)" from once "Every Two Weeks" to once "Every 92 Days."
It would also delete the footnote to Table 4.2.2, which provides for reduced testing of CRD-20 and CRD-31 during the remainder of Cycle 10.
Finally, it would correct the FSAR references in that table to i
reflect the arrangement of the Palisades Updated FSAR. The licensee submitted additional information to supplement the application by a letter dated April 29, 1993. This letter provided clarifying information and did not change the initial proposed no significant hazards consideration.
2.0 EVALUATION i
The Palisades Control Rod Drive Mechanism (CRDM) is of the Rack and Pinion type.
These CRDMs 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.
The CRDM drive motor is connected to the drive shaft through a reduction gear.
A spring engaged, electrically released brake, is provided to prevent the control rod from drifting when the motor i s not energized.
The motor is a fractional horsepower, single phase, two speed motor, though only a single speed (46 inches per minute) is connected.
The DC brake is energized through separate contacts on the motor contactor.
When the CRDMs are driven outward, the motor and brake are energized, and the motor drives through the gearbox, turning the clutch upper half.
If the clutch is energized (engaged) the clutch lower half is also turned. A cam and roller assembly, concentrically located within the electric clutch, transmits torque in only one direction, and allows the motor to drive the rod inward l
even when the electric clutch is disengaged.
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O The lower half of the clutch is connected to the vertical drive shaft, which turns the horizontal pinion gear through a right angle bevel gear set. The pinion gear drives the rack up and down. The rack assembly is connected to the control rod.
The CRDM rack is guided by a support tube. The rack has a larger diameter section, called a buffer piston, at its upper end. The guide tube has a restricted diameter toward its lower end.
In this restricted diameter region there is a close fit between the buffer piston and the guide tube. When the buffer piston enters this restricted diameter region, water trapped below it acts as a brake to slow the fall of the rod.
l Just below the lower clutch jaw, a small gear set drives the primary position indication shaft.
The primary position indication provides a digital rod position readout by use of a synchro transmitter, and six cam operated limit switches which are used for motor control and position indication lights. A secondary position indication system, using magnetic reed switches, is actuated by a magnet located in the connector nut at the top of the rack assembly.
When a reactor trip signal interrupts power to the CRDM clutch, the clutch t
jaws spring apart, and the control rod falls by gravity into the core. With the clutch disengaged, the CRDM parts below the clutch rotate separately from the gear motor and brake above the clutch. All CRDM parts below the clutch (lower clutch shaft, primary position shaft, mechanical seal, drive shaft, bevel gears, pinion gear, magnet for secondary position indication, and rack) move whenever the rod moves.
The two safety functions of the CRDM 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 CRDM safety function assumed in the analyses.
Second, when a reactor trip signal is generated, a " rod rundown" signal i
energizes all full length CRDM motors to drive their rods in case they j
could not fall freely into the core.
The rod rundown signal is terminated when that rod nears full insertion.
The clutch is designed to allow the i
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 analyses.
Operating history has shown a trend that exercising a CRDM, as required by TS Table 4.2.2, Item 2, often causes seal leakage to increase.
Leaking seals can result in shortened seal life and increased CRDM seal leak rate, which can lead to forced shutdown due to excessive PCS leakage. Due to this increase in leakage, a previous amendment (Amendment No. 155), dated January 29, 1993, has changed the surveillance interval in Table 4.2.2, Item 2, for testing two
s 4 CRDMs, CRD-20 and CRD-31, from once "Every Two Weeks" to once in March 1993, until the end of Cycle 10. The proposed amendment changes the frequency for all control rod exercising in Table 4.2.2, Item 2, from the current frequency of once "Every Two Weeks" to once "Every 92 Days."
Th? licensee has provided an analysis to demonstrate that CRDM exercising is not sufficient to ensure mechanical freedom over the full insertion distance.
The analysis also demonstrates that the rod exercising test has never detected any of the occurrences where mechanical binding of mechanical components has prevented or excessively slowed full control rod insertion.
In operating experience with CRDMs, there have been 12 reported instances involving a total of 29 control rods which could not be fully inserted by gravity within the required time. Of these 12, 10 were discovered during startup testing and 2 on reactor trips.
In neither case was the problem detectable by rod exercising since the rods did not stick fully out of the core.
CRDM seal faces are finely lapped, are constantly pressed together, are without relative motion (except when the rod is moved), and are in a warm acidic environment.
Such conditions make them most susceptible to bonding.
However, operating experience suggests that seal face Ia.: ding does not occur.
This is supported by the following experiences:
- 1) The part length rods are fully withdrawn prior to reactor startup, are not exercised, and typically remain motionless throughout the operating cycle. These rods have shown no sign of seal face bonding either during operation or during disassembly for maintenance.
- 2) On several occasions, the plant has operated for extended periods with one full length rod declared inoperable due to drive package or seal problems.
Inoperable rods are typically not exercised. No rod has failed to trip or shown other signs of seal bonding.
- 3) Disassembled CRDM seals typically show bright polished contact surfaces except in those areas eroded by continued severe leakage. There have been no signs of bonded or sticky seal faces due to corrosion or material migration.
Without a change to the TS, biweekly testing of the CRDMs must continue.
Experience has shown that this biweekly testing of the CRDMs has resulted in accelerated seal degradation.
The wear of the CRDM seals, magnetic brakes and other related mechanisms has led to plant shutdowns and cooldowns in order to i
perform repairs. These cooldowns lead to unfavorable plant transients and i
additional wear on plant equipment. The required maintenance also increases
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radiation exposure. With reduced surveillance testing, the expected rate of
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seal degradation, and its consequences, will be reduced.
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(s s The NRC staff has reviewed the licensee's analysis and concluded that the proposed change is acceptable. The proposed longer testing period will reduce personnel radiation exposure and the wear of the CRDM seals, magnetic breakers, and other selected mechanisms without diminishing the capability of i
detecting mechanical binding of the CRDM. The granting of this amendment is consistent with both NUREG-1366 (item 4.2.1), and NUREG-1432, Standard Technical Specifications - Combustion Engineering Plants (SR 3.1.5.5),.which suggest a quarterly and 92-day interval for CROM exercising, respectively.
Based on the above, the staff has determined that this amendment should be granted.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Michigan State Official l
was notified of the proposed issuance of the amendment. The Michigan State Official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to the installation or use of
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a facility component located within the restricted area as defined in 10 CFR Part 20. The staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendment involves no significant hazards consideration and there has been no public comment on such finding (58 FR 28054). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR i
51.22(c)(9).
Pursuant ta 10 CFR 51.22(b), no environmental impact statement i
or environmental assessment need be prepared in connection with the issuance of the amendment.
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5.0 CONCLUSION
The staff has concluded, based on the considerations discussed above, that:
(1) there is ' reasonable assurance that the health and safety of the public
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will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor: Mohammed A. Shuaibi Date:
August 31, 1993
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,I DATED:
August 31. 1993 AMENDMENT NO. 157 TG FACILITY OPERATING LICENSE NO. DPR-20-PALISADES 1 Docket File NRC & Local PDRs j
J. Roe i
J. Zwolinski W. Dean C. Jamerson i
A. H. Hsia l
OGC-WF i
D. Hagan, 3302 MNBB i
G. Hill (2), PI-37 l
C. Grimes, ll/F/23 4
R. C. Jones, SRXB L. Phillips, SRXB ACRS (10)
OPA OC/LFDCB W. Shafer, R-III cc:
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