ML17285A075
| ML17285A075 | |
| Person / Time | |
|---|---|
| Site: | Columbia  |
| Issue date: | 11/03/1988 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML17284A614 | List: |
| References | |
| TAC-55579, NUDOCS 8811230217 | |
| Download: ML17285A075 (3) | |
Text
ENCLOSURE I
8811230217 881103 PDR ADOCK 05000397 P
PDC SAFETY EVALUATION OF INSTRUMENTATION SETPOINT METHODOLOGY FOR WASHINGTON PUBLIC POWER SUPPLY SYSTEM WNP-2, DOCKET NO. 50-397
1.0 INTRODUCTION
/BACKGROUND The scope of this safety evaluation is limited to the methodology being used to establish setpoints for the safety-related instrument measuring systems in service at WNP-2 power plant.
These instrument actuation systems primarily consist of a plant process sensing device that employs a mechanical switch which provides a contact output to the reactor protection system trip logic circuitry.
Typically, the contact output instrument sensing devices are provided by the manufacturers of Barton, Barksdale, Magnetrol, and Static "0" Ring instruments.
The functions of these sensor s include monitoring selected plant process parameters and initiating reactor trip actions when predetermined abnormal conditions exist.
Additionally, the mechanical type design is typically associated with the "earlier" GE BWR design (i.e. licensees with construction permits before 1976) whereas the later GE BWR design tends to utilize the analog output transmitter trip unit (ATTU) type of instrument measuring system.
The analog sensing systems are generally provided by the Foxboro, Gould, Rosemount, etc., instrument manufacturing companies.
The setpoint methodology used to establish reactor trip setpoint values for the ATTUs is described in. GE Topical Report NEDC-31336 dated October 1986.
- However, this setpoint methodology is considered outside the scope of this safety evaluation, and will be evaluated by SICB as a generic issue at a later date.
2.0 Evaluati on/Conclusion With respect to the GE setpoint methodology being used to establish setpoint values for the safety-related actuation systems in service at the OP-2 plant, the lice'nsee has provided to the staff via Docket Letter 602-85-238, dated.
May 6, 1985 GE Design Specifications 22A5261 and 23A1900AA for review and evaluation.
These specification documents present the GE methodology that the licensee had utilized in determination of reactor-trip setpoint values for safety-related instruments which are listed in the WNP-2 Technical Specifica-tions.
The SICB staff review of the GE design specifications indicates that this methodology utilizes two standard deviations in instrument drift and
- accuracy, and calibration calculations..
Two standard deviations means that there is a 95% certainty that a reactor safety trip function will occur within the allowable margin at which the trip action was set to operate.
This degree of certainty was determined following careful consideration of such contributing error factors as those associated with instrument drift, instrument accuracy, calibration calculations, repeatability, and measuring system range.
According.
to the licensee, these error components are considered statistically independent and as such are combined in a group by calculating the errors by using the Square Root of the Sum of the Squares of errors to determine instrument channel setpoint values.
The same basic methodology has been utilized in the determination of trip setpoints for balance-of-plant (BOP) instrumentation listed in the Technical Specifications.'he difference being:
- 1) the BOP designer, Burns and Roe numerically summed or took the square root of the sum of squares for combined instrument inaccuracies and 2) when no data concerning drift was available for a particular instrument, a standard 1% drift was assigned.
Specific instrument operating history data are being periodically reviewed to adjust drift allowances as necessary.
Based on the above findings, SICB believes that the methodology described in the GE Design Specifications and the methodology for the BOP instrumentation as acceptable methods for establishing setpoint values at WNP-2 provided the following conditions for acceptance are met.
The first condition for acceptance relates to the limited scope of this evaluation which deals only with those instrument errors that are required to operate at or near the onset of the LOCA event being analyzed.
In this regard, the staff finds the setpoint methodology as presented in the subject GE Design Specifications to be an acceptable method by which to establish setpoint
- values, provided the licensee can demonstrate by analysis that the instrument actuation systems under review are designed and intended to operate only at or near the beginning of the LOCA event being analyzed.
Supporting documentation confirming these licensee determinations should be maintained and made available for staff review and examination on request.
The second condition for acceptance involves the instruments that are required to operate during the course of the LOCA event (e.g.. post accident monitoring instrumentation) and which may be subjected to "harsh environmental effects."
To cope with this staff concern, the licensee has comnitted to undertake a
Margin Reevaluation Program which will address the timing of the environmental impact as it relates to the safety function of the instrument under review.
According to the licensee, the most limiting (seismic or environmental) event will be verified.
Following completion of the Margin Reevaluation
- Program, revisions to the setpoints will be made, as necessary.
A copy of the report containing the results of this reevaluation program effort will be submitted to the NRC for staff review and appropriate action.
To summarize, SICB concludes that the WNP-2 methodology, as discussed, conservatively assigns instrument setpoint valjkes; and, therefore is considered acceptable under the conditions stated herein.
Also, SICB believes that based on review of the information provided in the above mentioned May 6, 1985 licensee submittal. the methodology being used to establish setpoints at WNP-2 is consistent with the requirements and guidelines specified in GDC-13.
ISA Standard S-67.04,
- 1982, and Regulatory Guide 1.105 Revision 1, 1976.