Information Notice 1991-52, Nonconservative Errors in Overtemperature Delta-Temperature Setpoint Caused by Improper Gain Settings

Nonconservative Errors in Overtemperature Delta-Temperature Setpoint Caused by Improper Gain Settings

ML031190373
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	08/29/1991
From:	Rossi C Office of Nuclear Reactor Regulation
To:
References
IN-91-052, NUDOCS 9108230188
Download: ML031190373 (3)

I

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 August 29, 1991 NRC INFORMATION NOTICE NO. 91-52: NONCONSERVATIVE ERRORS IN OVERTEMPERATURE

DELTA-TEMPERATURE (OTAT) SETPOINT CAUSED BY

IMPROPER GAIN SETTINGS

Addressees

All holders of operating licenses or construction permits for Westinghouse

• (W)-designed nuclear power reactors.

Purpose

This information notice is intended to alert addressees to potential problems

resulting from improper voltage gain settings applied to components in the

overtemperature delta-temperature (OTAT) reactor protection system trip

circuitry at Westinghouse (W)-designed nuclear power reactors. It is expected

that recipients will review the information for applicability to their facili- ties and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice do not constitute NRC require- ments; therefore, no specific action or written response is required.

Description of Circumstances

On July 17, 1991, Duke Power Company notified the NRC of a problem with the

OTAT reactor trip circuit at McGuire and Catawba Nuclear Stations which may

have prevented the system from operating correctly over its entire operating

range for reactor coolant loop average temperature (T-avg). Engineers dis- covered the problem at McGuire while attempting to rescale their OTAT setpoints

for the upcoming new fuel load. The engineers found that the gain applied to

the lead/lag amplifier for the T-avg portion of the OTAT circuit was set too

high (Westinghouse 7300 series process equipment card TY412A), based on the

current scaling of the hardware. This caused the T-avg input to "saturate" at

approximately 597 0 F for McGuire and 5920 F for Catawba which prevented the OTAT

circuit from reducing the OTAT setpoint as the T-avg increased above these

temperatures. The required input temperature range for T.-avg is 5300 F to

630'F. Therefore, the OTAT setpoint calculation was in error above these

"saturation" temperatures since the lead/lag amplifiers would not produce a

proportional output.

Following discussions with Westinghouse Electric Corporation, the licensee

corrected the problem at both McGuire and Catawba by redistributing the gain

associated with the T-avg term of the OTAT setpoint. This was accomplished by

replacing resistors in the lead/lag circuit to ensure that the gain was not so

high that it could cause "saturation" of its output over the entire input

9108230188 PV gtTVX

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IN 91-52 August 29, 1991 at

range. On July 19, 1991, the licensee modified the OTAT instrumentation

both facilities.

had

On July 31, 1991, the Commonwealth Edison Company informed the NRC that it

reviewed the gain settings for the OTAT circuits at its Braidwood and Byron

Stations and had discovered conditions similar to those at McGuire and Catawba.

Discussion:

The OTAT reactor trip protects against departure from nucleate boiling (DNB).

The trip function operates by comparing indicated loop delta-temperature (AT)

to the calculated setpoint, causing a reactor trip when two or more AT channels

..

exceed their setpoint. Analog circuitry for each reactor coolant loop contin- uously calculates the setpoint for this trip according to the temperature in

the loop, the neutron flux distribution in the reactor, and reactor coolant

system (pressurizer) pressure. The principal concern for the function of the

OTAT trip is for certain rod withdrawal events in which OTAT could be the

primary safety-related trip mitigating the event. It also functions as a safety- related backup for several other transients.

The problem involves the scaling used in the OTAT hardware. If the hardware is

scaled such that the output gain on the T-avg lead/lag amplifier is greater

than unity (1.0), the amplifier will saturate at some T-avg and prevent

further adjustment of the OTAT setpoint. The equation for the OTAT setpoint at

McGuire and Catawba includes as parameters lead-lag and lag constants, a bias

(K1), and gains (K2 and K3). Gains K2 and K3 are the adjustments for the

effects of temperature and pressure, respectively, on DNB limits.

The licensee indicated that the OTAT hardware had operated in the degraded

condition since 1983 for McGuire and since startup for Catawba. The original

0 F for McGuire. The hardware

1K2 value, provided by Westinghouse, was 0.0133/

was scaled by Westinghouse such that the lead/lag amplifier gain corresponding

to this K2 was less than 1.0. With a gain setting of less than 1.0, the ampli- fier does not overrange since the gain output voltage cannot exceed 10 volts.

was introduced at McGuire when the licensee rescaled the

The degraded condition

to allow

OTAT trip hardware as a function of performing a core reload analysis

for the use of Westinghouse "optimized" fuel. As part of this change to the

0

OTAT parameters, K2 was increased to its current value of 0.0222/ F. Using the

Westinghouse scaling methodology, however, the licensee scaled the OTAT hardware 1.0,

such that the output gain on the T-avg lead/lag amplifier was greater than

which introduced the problem.

Preliminary information concerning Catawba indicated that the original K2 value, supplied by Westinghouse, was also 0.0133/°F. Sometime in 1983 or 1984, and

following a McGuire transient which indicated the need to modify the OTAT current

overpower delta-temperature (OPAT) setpoints, K2 was increased to its

OTAT

value of 0.02401/ 0F. It was then that the licensee incorrectly scaled the

was greater

hardware such that the output gain on the T-avg lead/lag amplifier

than 1.0.

At some plants licensees have avoided the problem by using different scaling

methodologies or have identified the problem during the scaling process and

IN 91-52 August 29, 1991 modified the hardware (redistributing the gain) similar to the corrective

measures performed by Duke Power Company.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please call one

of the technical contacts listed below or the appropriate NRR project manager.

harX{e+s orssi

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical Contacts: William Orders, RII

(803) 831-2963 Thomas Koshy, NRR

(301) 492-1176 Attachment

Attachment: List of Recently Issued NRC Information Notices IN 91-52 August 29, 1991 Page I of I

LIST OF RECENTLY ISSUED

KRCINFORMATION NOTICES

Information Date of

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86-14, Overspeed Trips of AFW. 08/26/91 All holders of OLs or CPs

Supp. 2 HPCI and RCIC Turbines for nuclear power reactors.

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Programs for nuclear power reactors.

91-50 A Review of Water Hamner 08/20/91 All holders of OLs or CPs

Events After 1985 for nuclear power reactors.

91-49 Enforcement of Safety 08/15/91 All Nuclear Regulatory Com- Requirements for Radiog- mission (NRC) licensees

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sources for Industrial

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formance Provided by West- for nuclear power reactors.

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Fire Barrier aterial to for nuclear power reactors.

Pass Fire Endurance Test

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Supp. 2 of Material Supplied to for nuclear power reactors.

the Defense Department by

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Westinghouse ARD,BFD, and for nuclear power reactors.

NBFDRelays, and A200 DC

and OPC 250 Magnetic Con- tactors

OL

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