ML20207U014

Ro:On 861030,pressurizer PORV INC-34A Automatically Opened. Caused by Design Deficiency.Control Room Instrumentation Inadequate to Accurately Monitor Sys Pressures.Pressurizer Heater Manual Controls Adjusted

ML20207U014
Person / Time
Site:	Mcguire
Issue date:	12/01/1986
From:	Tucker H DUKE POWER CO.
To:	Grace J NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
References
NUDOCS 8703240504
Download: ML20207U014 (4)

Text

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DUKE POWER GOMPANY v.o.nox canso guantorre, N.c. 28242 HALH.TUCKEH .' *C

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g gn4 u December 1, 1986 Dr. J. Nelson Grace, Regional Administrator U.S. Nuclear Regulatory Commission - Region II 101 Marietta St. NW, Suite 2900 Atlanta, Georgia 30323

Subject:

McGuire Nuclear Station, Unit 2 Docket Nos. 50-370 Technical Specification 3.4.9.3/6.9.2 PORV Opening Special Report

Dear Dr. Grace:

Pursuant to Technical Specifications 3.4.9.3, please find attached a Special Report pursuant to T.S. 6.9.2 regarding the opening of a PORV at McGuire on October 30, 1986.

The event was considered to be of no significance with respect to the health and safety of the public.

Very truly yours, f,

Hal B. Tucker JBD/130/j gm xc: INPO Records Center Suite 1500 1100 Circle 75 Parkway Atlanta, GA 30339 Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555 Mr. W.T. Orders 8703240504 861201 NRC Resident Inspector ADOCK 0500 0 hDR McGuire Nuclear Station ll0 reaA

, DUKE POWER COMPANY McGUIRE NUCLEAR STATION Pressurizer Power Operated Relief Valve INC-34A Opened Due to Inadequate Instrumentation On October 30,1986, at 2147, Pressurizer Power Operated Relief Valve (PORV)

INC-34A automatically opened while in the low pressure overpressurization protection mode. Operators had increased Reactor Coolant (NC) system pressure to approximately 360 psig to increase the differential pressure (D/P) across the Reactor Coolant Pump (RCP) seals. Immediately after PORV INC-34A opened, the Pressurizer heater controls were adjusted to augment NC system depressurization.

PORV INC-34A closed 29 seconds after opening and NC system pressure stabilized at approximately 325 psig.

Unit I was in Mode 5, Cold Shutdown, with NC system temperature at approximately 180*F and cooling down at the time of this incident.

This incident has been attributed to a Design Deficiency, because control room instrumentation was inadequate to accurately monitor NC system pressures near the Pressurizer PORVs low pressure trip setpoint.

Background

The Pressurizer PORVs provide low pressure overpressurization protection during periods of water solid operation during unit startup and shutdown. This protection prevents the NC system from exceeding pressure and temperature limits specified in 10CFR50, Appendix G, as defined in McGuire Technical Specifications Figures 3.4-2 and 3.4-3. Analyses have shown that one PORV is sufficient to prevent violation of these limits due to anticipated mass and heat input transients. However, redundant protection is provided through the addition of low pressure setpoints for two PORVs, NC-34A and NC-32B. The low pressure setpoints are enabled at NC system temperatures below 300*F, The low pressure setpoints are interlocked with NC system temperature to minimize the possibility of inadvertent actuation.

Pressurizer PORVs INC-34A and INC-32B automatically open on high pressure signals generated by wide range pressure transmitters (0-3000 psig) INCPT5120 and INCPT5140, respectively. Pressure transmitter INCPT5120 monitors pressure on the NC Loop D Hot Leg and INCPT5140 monitors pressure on the NC Loop C Hot Leg. The low pressure mode setpoints on these transmitters are calibrated such that actuation signals are generated at NC system pressures between 365 and 400 psig.

Both wide range pressure transmitters have corresponding control room indicators and INCPT5140 also provides an analog computer point on the Operator Aid Computer (OAC). Additional NC Loop C Hot Leg pressure indication is provided by a control room indicator and an OAC analog computer point, both of which receive signals from low range pressure transmitter (0-800 psig) INCPT5141.

Description of Incident On October 30, 1986, Unit I was in Mode 5 with NC system temperature and pressure at approximately 180*F and 300 psig, respectively. Operators were cooling down the NC system and the NC system overpressurization protection was in the low pressure mode.

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". Page 2 As NC system temperature decreased, the D/P across the RCP seals dropped and was approaching 200 paid. An RCP seal D/P between 200 and 300 psid is required to ensure adequate cooling of the pump seals. The Pressurizer heater manual controls were adjusted to increase NC system pressure and RCP seal D/P. Operators used the OAC wide range pressure indication provided by 1NCPT5140 to monitor NC system pressure because they knew the Pressurizer PORVs were actuated by wide range pressure signals. NC system pressure increased to approximately 360 psig as indicated by the OAC.

At 2147, a Hi NC System Wide Range Pressure alarm was received in the control room and a wide range pressure of 376 psig on the NC Loop C Hot Leg was recorded on the alarm typer. Three seconds later, Pressurizer PORV INC-34A automatically opened.

Operators immediately adjusted the Pressurizer heater manual controls to augment NC system depressurization. PORV INC-34A automatically closed 29 seconds after it opened and NC system pressure stabilized at approximately 325 psig. Operators continued cooling down the NC system without further problems.

Conclusion Pressurizer PORVs INC-34A and INC-32B open on high wide range pressure signals from the NC system Loop D and C Hot Legs, respectively. However, only the wide range pressure on Loop C can be accurately monitored in the control room at low NC system pressures. The wide range pressure indicator for Loop D has insufficient resolution to accurately monitor lower pressures. Therefore, as NC system pressure increased, operators had no accurate indication of wide range pressure on Loop D. Also, operators knew that the maximum low pressure mode setpoint on the PORVs was 400 psig, but were unaware the PORVs could open at pressures as low as 365 psig. Loop D wide range pressure apparently exceeded the low pressure mode setpoint and PORV INC-34A automatically opened. The exact pressure at which PORV INC-34A opened is unknown. However, Loop D wide range pressure instrumentation was tested on August 18, 1986, and was found to generate a low pressure actuation signal at a pressure of 388 psig.

This incident has been attributed to a Design Deficiency, because control room instrumentation is inadequate to accurately monitor NC system pressures near the Pressurizer PORV low pressure actuation setpoint.

A review of past incidents at McGuire found one other incident (report dated April 15, 1986, amended September 5,1986) in which inadequate instrumentation caused a Pressurizer PORV to pen with the NC system in the low pressure overpressurization protection mode. Therefore this incident is considered recurring.

Corrective Actions Immediate: The Pressurizer heater manual controls were adjusted to augment NC system depressurization.

Subsequent: None

.. Page 3 Planned: 1) Station Problem Reports (SPRs) for Units 1 and 2 will be written to add NC system Loop D computer points on the OAC.

. Safety Analysis Pressurizer PORV INC-34A apparently opened and closed as designed with the NC system overpressurization protection in the low pressure mode. The low pressure mode setpoints on the PORVs protect the NC systen during periods of water solid operation. The NC system was not water solid at any time during this incident; therefore, the Pressurizer was capable of accommodating any design basis transients without pressure relief from the PORVs.

Section 15.6.1 of the McGuire Final Safety Analysis Report (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. This incident occurred with the unit in Mode 5 at low NC system pressure and temperature.

Therefore, in the event that Pressurizer PORV INC-34A had not closed properly the resulting NC system transient would be bounded by the FSAR Section 15.6.1 accident analysis.

The health and safety of the public were not affected by this incident.

i I

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