ML13308B507
| ML13308B507 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 07/15/1974 |
| From: | Gould W SOUTHERN CALIFORNIA EDISON CO. |
| To: | Oleary J US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| NUDOCS 8103060664 | |
| Download: ML13308B507 (7) | |
Text
Southern California Edison Company P 0. BOX 800 2244 WALNUT GROVE AVENUE
)N LAM R. GOULD ROSEMEAD, CALIFORNIA 91770 TELEPHONE Scu yE vICE PRESIDENT 213-572-2268 July 15, 1974 Mr. John F.
0' Leary,. Director 721 Directorate of Licensing REGULATION U. S. Atomic Energy Commission Washington,. DC 20545
Dear-Mr. O'Leary:
Docket No. 50-206 San Onofre Unit 1 Reactor Trip via Nuclear Overpower Indication July, 7, 1974 The following report is submitted to describe the July 7 reactor trip from a nuclear overpower indication.
This discussion will amplify the July 8, 1974, notification to the Region V Regulatory Operations Office.
,-In summary, a reactor trip occurred due to an indicated overpower condition on nuclear instrumentation channels 1206 and 1207.
These power range channels failed from water intrusion caused by leakage from control rod drive mechanism cooling fan A8SS.
Fan A8SS is one of three fans serving the same purpose and is located approximately 40 feet above and 18 feet north of the reactor vessel.
The fan cooling water leaked through two cooling coil head gaskets and a portion flowed through the fan duct to the reactor head area where the nuclear in strumentation detectors are located.
Approximately 140 gallons of cooling water collected in the detector thimbles and an additional 40 gallons collected in the reactor cavity sump. In contrast to our earlier notificationto the Region V Regulatory Office, subsequent investigation indicated that most of the cooling water leakage collected in the A8,,
A8S and A8SS fan housings and ultimately entered the sphere sump by paths other than the reactor cavity.
The gasket failures were caused by a pressure fluctuation when water flow was disturbed during a routine monthly autostart test of the turbine plant cooling water pumps which provide cooling water for these fan coolers. Nuclear instrumentation source range channels 1201 and 1202 and intermediate range channel 1204 also failed during the event.
Other thimbles containing nuclear instrumentation utilized for axial offset monitoring were also flooded.
Our letter, dated July 15, 1974, further amplifies this item.
I
~
3643
-2 The following addenda are included with this letter:
- 1.
A chronological sequence of events.
- 2.
A summary of the reactor plant safety analysis.
- 3. Investigation and conclusions.
- 4. Corrective action.
Repairs, testing, inspection and a design change to prevent recur rence were accomplished by July 9, 1974, and the unit was returned to service.
Sincerely, Enclosures C: R. H. Engleken, Director Region V, Regulatory Operations Office
ADDENDUM 1 SAN ONOFRE NUCLEAR GENERATING STATION SEQUENCE OF EVENTS July 7, 1974 10:15 AM A routine auto start test of the turbine plant cooling water pumps was in progress. During this test, flow was momentarily disturbed. This resulted in a pressure fluctuation in the system which failed two cooler gaskets in control rod drive mechanism cooling fan A8SS. There was no knowledge of the failures at this time.
10:50 AM Received "NIS detector high temperature" alarm and station D.C. ground alarm as water entered the detector thimbles. NIS channels checked satisfactorily, but a 100% positive DC ground was indicated. The sphere temperature recorder was checked at this time and a sharp drop from 190 0 F to 100aF was noted in the control rod cooler inlet temperature. A control rod drive mechanism fan cooler leak was suspected.
10:53 AM Nuclear instrumentation channel 1207 output became erratic. All other nuclear instrumentation channels were normal.
Personnel began prepara tions to make a containment entry.
11:15 AM It was noted that the turbine plant cooling water tank level had decreased about 3" in 15 minutes.
11:33 AM Personnel first entered the containment to evaluate and correct cooling water leakage. Two subsequent entries were made before the leakage was secured.
11:34 AM Reactor tripped on overpower indication from NIS channels 1206 and 1207.
Normal shutdown sequence occurred. Source range channel 1202 and inter mediate range channel 1204 failed during the shutdown.
11:45 AM Received alarm indicating operation of reactor cavity sump pump.
11:47 AM Received alarm indicating operation of west sphere sump pump.
ADDENDUM 2 SAFETY ANALYSIS A comparison of reactor parameters and their corresponding Tech nical Specification limits prior to and after the trip was accomplished in accordance with Station Order S-A-7. The following items were verified as not exceeding technical specification limits:
- a. Reactor coolant system pressure
- b. Combined temperature and pressure versus power level
- c. Reactor coolant and pressurizer heatup and cooldown rates
- d. Reactor coolant activity
- e. Control rod insertion limits
- f. Reactor coolant system leakage The reactor tripped correctly from an indicated overpower condi tion when nuclear power range channels 1206 and 1207 failed from water intrusion. Power range channels 1205 and 1208 remained operable through out the occurrence and verified that no nuclear overpower occurred.
Nuclear instrumentation source range channel 1202 and intermediate range channel 1204 also failed during the occurrence. Later it was noted that the output from source range channel 1201 was decreasing. Since one source range and one intermediate range channel are required to maintain hot shutdown, the reactor coolant system was borated to greater than 10%
AK/K shutdown margin in accordance with the Technical Specifications.
Water intrusion into the nuclear instrumentation power range detector thimbles will cause short circuits or grounds. In either case, the detector current indication will increase toward the high levelttrip point. This type failure is in the conservative direction as the indi cated nuclear overpower will trip the reactor.
e
%2:00 PM Noted the output of source range Channel 1201 decreasing and initiated a boration of reactor coolant system to greater than 10% AK/K shutdown margin.
ADDENDUM 3
INVESTIGATION AND CONCLUSIONS During a routine monthly autostart test of the turbine plant cooling water pumps, two cooling coil head gaskets in control rod drive mechanism cooling fans A8SS failed due to a pressure fluctuation in the turbine plant cooling water system., This system supplies cooling water to all the air conditioning fan units inside the containment as well as turbine plant equipment.
Fan A8SS is one of three that cool the control rod drive mechanisms by pulling containment air across the area above the reactor head into a torus which surrounds the head and up a duct which is common to all three fans.
-Fan A8SS is located about 40 feet above the reactor flange and attached to the west side of the "B" steam generator and pres surizer enclosure.
Leakage from the fan A8SS cooler collected in the intake plenum and overflowed into the duct leading to the other two fans and the common duct leading to the reactor head area. Air flow up this duct caused water to collect in the other two fan intake plenums.
Most of this water leaked from the plenums to the operating deck, evaporated, orwas carried into the containment atmosphere by the air flow.
After the fan intake plenums flooded, some water flowed through the duct to the reactor head area.
This water contacted the fan air inlet thermocouple which is mounted on the side of the duct near the fans and caused a drop in recorded temperature from 190 degrees F to 100 degrees.F.
This alerted operators to the possibility of a fan cooler leak.
During the initial containment entry, operators noticed water leaking from all three fan inlet plenums, the connecting ductwork, from the torus surrounding the outside of the reactor head insulation and to the area where the nuclear instrumentation thimbles are located.
Water from these sources made it difficult for the operators to pinpoint and isolate the source of leakage.
A total of approximately 3400 gallons of turbine plant cooling water (as calculated by tank level change) leaked through the failed gaskets at a rate of approximately 15 gallons per minute.
Of this amount, about 2900 gallons was ultimately collected in the sphere sump via drains from the operating deck.
Another 40 gallons collected in the reactor cavity sump and about 140 gallons collected in the detector thimbles.
The remaining 320 gallons evaporated into the containment atmosphere.
Water intrusion in the detector thimbles caused failure of nuclear in strumentation power-range channels 1206 and 1207, intermediate range channel 1204 and source range channel 1202 prior to or during the reactor shutdown.
-Several hours later nuclear instrumentation source range channel 1201 also failed.
The initial alarm, which indicated "NIS Detector Hi Temperature", was caused by water having shorted the associated temperature detector.
ADDENDUM 4 CORRECTIVE ACTION A. The following items were accomplished prior to unit startup.
- 1. All nuclear instrumentation detector packages were removed and the water was pumped from the detector thimbles.
- 2. Inspections, tests and repairs to the NIS channels were accomplished as listed below:
Channel Action 1201 Replaced detector and cable 1202 Replaced detector and cable. Repaired preamplifier 1203 Inspected and tested satisfactorily 1204 Replaced detector and cables 1205 Inspected and tested satisfactorily 1206 Replaced detectors and cables 1207 Replaced detectors and cables 1208 Inspected and tested satisfactorily
- 3. In accordance with Edison's commitment to the AEC in the June 27, 1974, letter from R. N. Coe to R. H. Engleken, a special preopera tion test program was conducted on all four NIS power range channels.
- 4. The cooling coil head gaskets on fan A8SS were replaced and all three control rod mechanism cooling fans (A8, A8S & A8SS) were leak tested satisfactorily.
- 5. A design change was accomplished to install drains from the A8, A8S and A8SS fan inlet plenums. This will prevent water accumu lation in these plenums from overflowing to the duct and the reactor head area.
- 6. All other containment cooling fans supplied by the turbine plant cooling water system were inspected and found in satisfactory condition.
B. Additional Item An engineering evaluation has been initiated to determine if additional equipment modifications are warranted.