ML20214S542
| ML20214S542 | |
| Person / Time | |
|---|---|
| Site: | McGuire, Mcguire  |
| Issue date: | 06/08/1987 |
| From: | Tucker H DUKE POWER CO. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| NUDOCS 8706090311 | |
| Download: ML20214S542 (6) | |
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1 DuxE POWER COMPANY P.O. EBOX 33189 CHAmLOFTE, N.C. 98949 HALB.TUOMER (704) a"n>4 eat N'
.LTJLRA. P.O.tMyrgo.
June 8,.1987 U.S. Nuclear Regulatory Commission
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- Desk
'WaishligtBn;~DTC.* 20555
Subject:
McGuire Nuclear Station-
. Docket Nos. 50-369, -370 Technical Specification 3.4.9.3 and 6.9.2 Gentienen:-
Please find attached a special report submitted pursuant to Technical Speci-fication-(T.S.) 6.9.2 as specified by T.S. 3.4.9.3.
The report concerns the opening of a Pressuriser Power Operated Relief valve due to a pressure transient.
This event is considered to be of no significance with respect to the health and safety of the public.
Very truly yours, Hal B. Tucker SEL/68/jgm Attachment xc Dr. J. Nelson Grace INPO Records Center Regional Administrator, Region II 1820 Water Place U.S. Nuclear Regulatory Commission Atlanta, Ga 30339 101 Marietta St., NW, Suite 2900 Atlanta, GA 30323 Mr. Darl Hood Mr. W.T. Orders U.S. Nuclear Regulatory Commission NRC Resident Inspector Office of Nuclear Reactor Regulation McGuire Nuclear Station Washington, D.C.
20555 AD
$6
DUKE POWER COMPANY McGUIRE NUCLEAR STATION Pressurizer Power Operated Relief Valve 2NC-34A Opened Due To A Pressure Transient Which Occurred Af ter A Reactor Coolant Pump Was Started INTRODUCTION:
On May 3,1987 at 1042, Unit 2 Pressurizer Power Operated Relief Valve (PORV) 2NC-34A automatically opened due to a pressure transient in the Reactor Coolant system. This pressure transient occurred after' Reactor Coolant Pump 2B was started with the Reactor Coolant system water solid. PORV 2NC-34A closed approximately 3 seconds after opening.
Unit 2 was in Mode 5, Cold Shutdown, at the time of the incident.
This incident has been attributed to a pressure transient in the Reactor Coolant system which occurred after a Reactor Coolant pump was started.
The parameters which contributed to the transient could not be quantified to determine their effect on the incident. In addition, the operating pressure margin between the Low Temperature Overpressurization Protection setpoint on the Pressurizer PORVs and the pressure required to operate Reactor Coolant pumps is too narrow to prevent operation of the PORVs during certain plant operations.
EVALUATION:
Background
At the beginning of each refueling outage, with the unit in Mode 5, a chemical cleanup of the Reactor Coolant (NC) system is performed to reduce system contamination. This " Crud Burst" operation involves injecting hydrogen peroxide into the system to free the adherent layer of activated corrosion products on the inner walls of the NC system piping and in the Steam Generators (S/Gs). NC pump operation is required to recirculate the hydrogen peroxide for effective cleaning. Contamination is then removed via the Chemical and Volume Control system domineralizers. A Crud Burst helps to minimize radiation exposure during outage maintenance activities. The NC system is maintained water solid during the chemical additions to maximize the surface area exposed to the chemicals and enable operation of the NC pump (s).
NC pump operation requires suf ficient NC system pressure to prevent damage to the NC pump seals. The Controlling Procedures for Unit Shutdown, OP/1,2/A/6100/02, instructs the Operators to maintain NC pump #1 seal differential pressure greater than 200 paid. Operations maintains a minimum differential pressure of approximately 250 psid across the three seals to prevent seal damage.. operations utilizes approximately a 75 psig operating margin, or approximately 325 psig NC Loop Pressure for NC pump operation during cooldown.
The NC system is to be protected from exceeding pressure and temperature limits as specified in 10CFR50 Appendix G, and defined by McGuire Technical
, Specifications Figures 3.4-2 and 3.4-3.
This required protection is presently accomplished by PORVs NC-34A and NC-32B located on the Pressurizer. When the NC system temperature is below 300*F, the Low Temperature Overpressurization Protection (LTOP) setpoint for the PORVs are enabled by manual key-lock switches. This key-lock is activated when the NC system pressure has decreased to 325 psig, as required by procedure.
Technical Specification 3.4.9.3 requires both PORVs to be operable in Mode 4 Hot Shutdown, with Cold Leg temperature less than or equal to 300*F and Mode 5 and Mode 6, Refueling, with the Reactor Vessel Head on.
Pressurizer PORVs NC-34A and NC-32B are designed to automatically open on high pressure signals generated by wide range pressure transmitters (0-3000 psig)
NCPT5120 and NCPT5140, respectively. Pressure transmitter NCPT5120 monitors pressure on the NC Loop D Hot Leg, and pressure transmitter NCPT5140 monitora pressure on the NC Loop C Hot Leg. Technical Specification (T.S) 3.4.9.3 requires the Pressurizer PORVs to open prior to NC system pressure reaching 400 psig. The LTOP mode setpoints on these transmitters are calibrated per the Reactor Coolant System Wide Range Pressure Calibration procedure, IP/0/A/3000/01A, such that actuation signals are generated at NC system pressures between 365 and 395 psig. This range is the result of a -30, +0 psig tolerance band developed from the manufacturer's specifications.
Both wide range pressure transmitters have corresponding Control Room indicators, but only NCPT5140 (which controls PORV NC-32B) provides an analog computer point on the Operator Aid Computer (OAC). The program on the OAC which displays the Loop C Wide Range NC pressure is updated only every 5 seconds. Nuclear Station Modification (NSM) MG-12010 was written to provide a new OAC analog point from pressure transmitter NCPT5120 (which controls PORV NC-34A) to more clearly indicate NC Loop D pressure. This modification is currently scheduled for the end of the cycle 5 refueling outage for Unit 2 and at the end of the cycle 6 refueling outage on Unit 1 (Unit 2 is presently at the end of cycle 3 and Unit 1 is currently in cycle 4).
Additional NC Loop C Hot Leg indication is provided by a separate low range transmitter (0-800 psig) which supplies a signal to a Control Room indicator and an OAC analog computer point. This transmitter does not provide any control function for the Pressurizer PORVs.
Description of Incident on May 3, 1987, preparations were under way for a chemical cleanup (Crud Burst) of the Unit 2 NC system.
Unit 2 was in Mode 5 with the NC system water solid, pressure was at approximately 320 psig without any NC pumps running, and temperature wsc at approximately 150*F.
Pressure was being maintained by controlling the charging and letdown flow to and from the NC system.
In preparation for starting NC Pump 2B, charging flow was decreased and letdown flow was increased causing NC pressure to decrease. At 1041, NC Pump 2B was started to establish flow to the Steam Generators (S/Gs). Af ter 15 seconds, the pump reached 100% speed and induced a pressure transient on the NC system.
Charging flow was further decreased to compensate for the rising NC system pressure, but at 1042:01, a High NC System Wide Range Pressure alarm was received in the Control Room.
Pressurizer PORV 2NC-34A automatically opened 7 seconds later, remained open for 3 seconds, and then closed.
NC system
. pressure had increased to a minimum of 368 psig and was decreased to approximately 300 psig as a result of the PORV opening. This pressure was maintained and the Crud Burst was continued.
Conclusion There were several factors which could not be quantified to determine a sole root cause of the incident. When the NC pump was started, a pressure transient occurred in the NC system from one or both of the following: 1) the actual slug of pressurized water produced by starting the NC pump which is forced through the NC system and/or, 2) the expansion of the cool seal injection flow water that remained in the loop until heated via the Reactor Core. Some factors which affect the severity of this expansion are the stored heat energy in the NC system, and the amount of time that the cooler seal injection water (at temperatures between 70*F and 110'F) has been flowing into the NC system prior to starting an NC pump.
In addition to these factors, the operating pressure margin between the low end setpoint on the LTOP PORVs and the pressure requirements for NC pump operation are narrow.
During the incident that occurred on May 3, 1987, after the NC pump had started.
Operations had throttled down on the charging flow valve to the point of closing it, but still had leakage flow into the NC system. This leakage flow was due to the high differential pressure caused by the high charging pump discharge pressure. The effects of each item on the pressure transient could not be quantified.
A possible reason that PORV 2NC-32B did not open is because this PORV transmitter is located on an NC loop with an idle NC pump. Also, the pressure transmitters for the two PORVs are of a different manufacturer. PORV 2NC-32B uses a Barton transmitter while PORV 2NC-34A uses a faster responding Rosemont transmitter.
It may be possible for the Barton transmitter not to detect a quick pressure transient in a situation where the Roseront would.
There were two other separate but similar incidents involving automatic Pressurizer PORV actuations due to NC system pressures at the boundary of the allowable low end range setpoints. These two incidents were not reported earlier due to responsible station personnel not recognizing the incidents as reportable during their review of the Operations logbooks. The cause for the following two Pressurizer PORV actuation incidents is unknown because the NC system pressure was below the PORV open setpoint for LTOP and no apparent transients were induced on the system prior to the PORV actuations.
On August 27, 1986 Unit I was in Mode 5 with NC system temperature at approximately 180*F and pressure at approximately 360 psig with a bubble in the Pressurizer. NC Pumps 1B and 1C were running and the LTOP system had been activated for overpressurization protection. While NC system temperature and pressure were being maintained, Pressurizer PORV INC-34A opened for approximately 20 seconds and reduced NC pressure from approximately 360 psig to approximately 320 psig. The Pressurizer heater manual controls were adjust..d to augment NC system depressurization.
NC system pressure was stabilized at approximately 320 psig and was later increased to approximately 350 psig without additional cycling of the Pressurizer PORVs.
I
. On November 15, 1986, Unit 2 was in Mode 5 with NC system pressure at approximately 350 psig with NC Pumps 2B and 2C running to maintain NC system temperature between 180*F and 200'F.
A bubble had been established in the Pressurizer and the LTOP system was activated for overpressurization protection. At 0723, while maintaining NC system temperature and pressure, Pressurizer PORV 2NC-34A opened after pressure had crept up to the range of 355 to 360 psig NC system pressure. The reason for this slight pressure increase could not be determined.
It is believed that the PORV had opened prematurely because it should open at a 400 psig setpoint. After approximately 17 seconds, the block valve wac closed isolating the open PORV, and then the PORV closed. NC system pressure was reduced to approximately 340 psig by using Pressurizer spray. With the block valve closed, an attempt was made to return PORV 2NC-34A to the automatic mode. PORV 2NC-34A reopened a second time since the NC system pressure had not reached the PORV close setpoint of 335 psig, the PORV was reclosed after approximately 5 seconds.
Operations continued to decrease NC system pressure to approximately 320 psig and then returned PORV 2NC-34A to automatic.
It remained closed and the block valve was opened. Then a stable NC system pressure of approximately 320 psig was maintained.
In both the August and November 1986 incidents, Operations were not subjecting the NC system to an apparent pressure transient prior to the PORV actuations.
The allowable actuation setpoint range, as calibrated, is 365 to 395 psig.
Training materials for Operations traditionally instructed that the setpoint at which the Pressurizer PORVs automatically open, as required by Technical Specifications, is 400 psig. Personnel who were operating the unit at steady state pressures around 360 psig were unaware of the proximity to the true low end PORV open setpoint. Due to the lack of sufficient instrumentation to monitor Loop D NC system pressure, and the unawareness of the low end setpoints, Operations inadvertently allowed NC system pressure to increase toward the actuation setpoint of the PORVs.
As a result of Pressurizer PORV INC-34A opening in the LTOP mode on October 30, 1986, and the subsequent investigation and report, personnel are now trained that the allowable pressure range in which the PORVs open is between 365 and 395 psig.
Westinghouse and the Westinghouse Owners Group (WOG) are pursuing deletion of the LTOP setpoint on the Pressurizer PORVs. This is in conjunction with the removal of the Residual Heat Removal (RHR) autoclosure interlock. The RHR autoclosure interlock has had several inadvertent actuations at other plants which resulted in the temporary loss of the function of the RHR pumps. With respect to the increasing number of Pressurizer PURV actuations, Westinghouse and the WOG are preparing a topical report to present to the NRC to show the autoclosure interlock as detrimental to safety and that the RHR safety relief valves can act as the LTOP instead of the PORVs.
In addition, the basis which established the original Nil Ductility Transition (NDT) curves is being reevaluated to determine if a more realistic higher LTOP setpoint can be utilized to provide additional operating margin when in the LTOP mode.
. a
, An evaluation of the desirability of performing Crud Burst cleaning while the NC loops are half filled is being performed to compare with the Crud Burst cleanings performed while the NC system is water solid. The comparison of the two methods will determine whether water solid operation is significantly more beneficial to the reduction of personnel radiation exposure.
This incident is not reportable to the Nuclear Plant Reliability Data System (NPRDS).
There were no personnel injuries, radiation overexposures, or releases of radioactive material as a result of this incident.
CORRECTIVE ACTIONS:
Immediate:
The charging and letdown flow were adjusted to decrease NC system pressure.
Subsequent:
Stable NC system p ressur e was maintained af ter the PORVs closed to prevent further PORV actuations.
Planned:
McCuire will add a statement to the Unit Startup and Shutdown procedures, OP/1,2/A/6100/01 and OP/1,2/A/6100/02 respectively, and the Reactor Coolant Pump Operation procedures OP/1,2/6150/02, to inform operators of the allowable setpoint range of the Pressurizer PORVs as 365 psig to 395 psig, as calibrated.
SAFETY ANALYSIS:
Pressurizer PORVs opened and closed as designed with the NC system overpressurization protection in the low pressure mode. The low pressure mode setpoints on the Pressurizer PORVs are required for periods of water solid operation with an NC system temperature less than or equal to 300*F.
The NC system conditions described in this report were either water solid or with a bubble established in the Pressurizer.
During the pressure excursions and resulting transients, the Pressurizer PORVs were operable and demonstrated their capability of accommodating the transients without exceeding the limitations described in 10CFR50 Appendix G.
Section 15.6.1 of the McGuire FSAR, entitled Inadvertent Opening of a Pressurizer Safety or Relief Valve, analyzes an accidental depressurization of the NC system assuming reactor power and NC system pressure and temperature are initially at their nominal values. The incidents in this report occurred with I
the units in Mode 5 at low NC system pressure and temperature.
Therefore, in the event that the Pressurizer PORVs had not closed properly, the resulting NC system transient would be bounded by the FSAR Section 15.6.1 accident analysis.
This incident is considered to be of no significance with respect to the health and safety of the public.
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