Information Notice No. 93-58: Nonconservatism in Low-Temperature Overpressure Protection for Pressurized-Water Reactors

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                                July 26, 1993


NRC INFORMATION NOTICE 93-58:  NONCONSERVATISM IN LOW-TEMPERATURE OVERPRESSURE
                               PROTECTION FOR PRESSURIZED-WATER REACTORS


Addressees

All holders of operating licenses or construction permits for pressurized-
water reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a nonconservatism in the low-temperature
overpressure protection (Ltop) setpoint calculation for Westinghouse
facilities.  It is expected that recipients will review the information for
applicability to their facilities and consider actions, as appropriate, to
avoid similar problems.  However, suggestions contained in this information
notice are not NRC requirements; therefore, no specific action or written
response is required.

Description of Circumstance

On October 29, 1992, the Texas Utilities Electric Company, the licensee for
Comanche Peak Steam Electric Station, reported that its existing low
temperature overpressure protection may not have provided the required margins
of safety against reactor vessel brittle fracture under certain overpressure
transients.

The same concern was later reported by the licensees for Byron, Zion, Diablo
Canyon, Kewaunee, Sequoyah and Point Beach nuclear stations.

Discussion

In reactor units designed by Westinghouse, overpressure protection of the
reactor vessel at low temperature conditions is provided by a cold
overpressure mitigation system (COMS).  This system compares pressure and
temperature inputs against a preset setpoint curve and relieves the pressure
when the setpoint is reached.  This protection is necessary because, while at
low temperatures during plant startup and shutdown conditions, certain
transients could cause the reactor coolant system pressure to exceed the
reactor vessel pressure-temperature (P-T) limitations established for
protection against brittle fracture.  A spurious start of a safety injection
pump, reactor coolant pump, or other operational errors could activate this
system.  During such events, the P-T limitations are maintained by opening the
pressurizer power-operated relief valves (PORVs) or safety relief valves in
the residual heat removal (RHR) suction lines to relieve system pressure.  

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The transmitters that provide pressure signals to the COMS are located at the
primary system hot leg piping of the reactor vessel.  During low temperature
operation of the reactor coolant pumps, dynamic pressure in the reactor vessel
would be higher (by the amount of flow loss in the core and vessel outlet)
than that sensed in the hot leg.  Additionally, the static head correction for
the difference in elevation of the sensor to the core region was not
considered.  The resulting pressure difference between the sensor and the
vulnerable location in the reactor vessel could be as high as 790 kPa
(100 psig), depending on the number of reactor coolant pumps in operation and
the location of the pressure-sensing taps.  The Ltop setpoint curve that was
originally developed by Westinghouse did not take these factors into
consideration.

Westinghouse has sent a letter to licensees recommending one of the following
methods to compensate for this pressure increase:  1) reduce the maximum
allowable relief valve setpoint by an amount equivalent to the plant-specific
calculated difference in pressure, 2) maintain RCS pressure below the
heatup/cooldown curves by a value equal to the plant specific difference in
pressure from both flow loss and elevation difference when the reactor coolant
pumps are in operation, 3) restrict the number of reactor coolant pumps and,
therefore, the flow loss error that can be operated below a defined RCS
temperature without drawing a steam bubble in the pressurizer, or 4)
demonstrate that the available margin in the Ltop calculation, taking into
account instrumentation uncertainty, is sufficient to offset the plant-
specific pressure difference. 

The Westinghouse letter describes interim administrative controls as well as
calculational methods to verify setpoint adequacy for addressing the Ltop
concern.  The staff notes that the administrative restrictions described in
approaches (2) and (3) are intended by Westinghouse to provide interim actions
to address the concern only until Ltop setpoints are verified to be adequate
or are revised appropriately in technical specifications.

Although the information in this notice addresses the cold overpressure
mitigation system at Westinghouse designed plants, aspects of this issue may
also be applicable to other PWRs.

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This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.


                                       ORIGINAL SIGNED BY


                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Thomas Koshy
                     (301) 504-1176

                     Chu-Yu Liang
                     (301) 504-2878

Attachment:  List of Recently Issued NRC Information Notices

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