ML20058K666
| ML20058K666 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 11/25/1974 |
| From: | Mauer L NORTHERN STATES POWER CO. |
| To: | Oleary J US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| AO-263-74-24, NUDOCS 9102130430 | |
| Download: ML20058K666 (3) | |
Text
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NORTHERN STATES POWER COMPANY Minneapolis, Minnesota 55401 Mr. J. F. O' Leary, Director Directorate of Licensing Office of Regulation U S Atamic Energy Commission Washington, D. C.
20545 ABNCRML OCCURRENCE REPORT TO THE AEC Premature Actuation of Saferv/ Relief Valve "E" 1.
Report Nunber: A0 263/74-24 2a.
Report Date: November 25, 1974 2b. Occurrence Date: November 15, 1974 3.
Facility: Monticello Nuclear Generating Plant (DPR-22)
Monticello, Minnesota 55362 2
Identification of Occurrencc:
This report conce=s the premature actuation of safety / relief valve "E" during normal power operatien.
5.
Conditions Prior to Occurrence:
Steady State Power - The plant was at 81% of rated power at the time at tne occurrence.
6.
Description of Occurrence:
On November 15, 1974, during normal power operation, safety / relief valve "E" pre ~.aturely actuated at a reactor pressure of 981 psig. The valve rc=ained open for approxinately 6 seconds resulting in a reactor pressure loss of 11 psig. The actuaticn occurred at a time when the plant was operating with seven safetv/ relief valves (this is the minimum number of operable safety / relief valves reauired by the Technical Specifications).
Since the cause of the actuaticn was unknown, the valve could not be considerable operable, therefore, the reactor was brought to the cold shutdown condition.
7.
Designation of Apparent Cause of Occurrence:
All eight safety / relief valves were bench checked, disassembled and inspected. The "E" valve and three other valves were found to have excessive leakage of the first stage pilot. The remaining four valves were found to be in good condition. The four leaking valves had a 1?'nD 9102130430 741125 CF ADOCK 05000263 CF
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buildup of foreign material on the pilot seating surfaces and the i
surfaces were steam cut. The accumulation of foreign material is i
believed to have caused pilot leakage which eventually become aggra-vated to the extent that the valve actuated.
The degree of leakage and foreign material buildup correlated with the number of actuations of the first stage pilot valves on steam.
l During the Spring 1974 refueling outage, five rebuilt and three l
new valves, including the "E" valve, were installed. The new valves j
i had each been tested on steam by the vendor. The total number of vendor tests perfomed is not known, however, records indicate at 2
least four actuations of each valve. Two of the new valves and one rebuilt valve also were actuated once following a turbine trip. The new valves were found in worse condition than the rebuilt valve which had actuated on steam only once. None of the four valves which were found in good condition had excerienced operation of the first stage pilot on steam.
S.
Analysis of the Occurrence:
j There was one previous similar malfunction when safety / relief valve "G" was prematurely actuated on November 11, 1974, during a plant i
startup with reactor pressure at 911 psig. The valve actuated for
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a only 3 seconds then reseated properly. The plant was shutdown, the q
safety / relief valve was removed, and the plant was returned to nomal operation.
It is noted that safety / relief valve "G" was also one of the l
new valves,
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e Eased on the inspections discussed above, it is cencluded that all of the safety / relief valves were operable and would have actuated to prevent overpressurization of the reactor vessel if required. This occurrence had no effect on the public health, and safety.
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Corrective Action:
Seven of the eight safety / relief valves were refurbished under the direction of a vendor representative and reinstalled. The pilot seat of the eighth safety / relief valve was lapped to the extent that insuffi-l cient stellite remained on the seat. This valve was not reinstalled pending further repair work.
l Results of the investigation discussed above indicate that, unless a i
valve is actuated on steam, the probability of a spurious actuation is i
minimal.
Increased surveillance of the safety / relief valve discharge i
-temperature has been initiated to provide early detection of deteriorat-l 1
ing seat ccnditions. Means of improving the sensitivity of the leak l
detection system are being investigated.
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10, Failure Data:
Safety Relief Valve "E" Serial No. 119 Safety Relief Valve "G" Serial No. 122 Target Rock Model 67F Pilot Operated Safety Relief Target Rock Corporation Y 0-f/
L. O. Mayer, PE f
Director of Nuclear Support Services ID4/kik cc: J G 1:eppler G Charnoff Minnesota Pollution Control Agency ATIN: E A Pry:ina l