ML20111B357
| ML20111B357 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 02/21/1985 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20111B266 | List: |
| References | |
| NUDOCS 8503130069 | |
| Download: ML20111B357 (3) | |
Text
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3,d UNITED STATES NUCLEAR REGULATORY COMMISSION o
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WASHINGTON, D. C. 20S55
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPP0PTING AMENDMENT NO. 86 TO FACILITY OPERATING LICENSE NO. DPR-28 VERMONT YANKEE NUCLEAR POWER CORPORATION VERP0NT YANKEE NUCLEAR POWER STATION
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DOCKET NO. 50-271 1.0 Introduction By letter dated March 26, 1984, Vermont Yankee Nuclear Power Corporation (the licensee) requested an amendment to the Technical Specifications, Appendix A, of Facility Operating License No. DPR-28 for the Vermont Yankee Nuclear Power Station. Additional information and clarification were provided by letter dated September 7, 1984 and teleconferences. The amendment would change the trip setting for the high main steam line flow instruments from the present 120% to 140% of rated steam flow. This change will reduce the probability of inadvertent reactor isolations and possible challenges to safety / relief valves.
The licensee considers this change as one.of the methods to satisfy the requirements of TMI Action II.K.3.16,
" Reduction in the Challenges and Failures of Relief Valves." The amendment also would increase the power limit for quarterly main steam isolation valve (MSIV) full closure testing from 50% to 75% of rated power. The change will allow quarterly MSIV testing at 75% power and will minimize the thermal cycling currently required by the reduction to 50% power.
This change also provides an economic benefit to the licensee since the power need not be reduced to 50% for testing.'
2.0 Evaluation The primary function of the high flow instrumentation is to detect a main steam line break. The steam flow trip setting, in conjunction with the flow limiters and MSIV closure, limits the mass inventory loss such that the fuel is not uncovered. The analysis for the main steam line break outside containment is given in the FSAR, Section 14.6.5.
The analysis assumes that 200% of rated steam flow is passing through the break, which is the maximum permitted by the steam line flow limiters. A 0.5 second i
delay is assumed before tripping the MSIV closure signal. This 0.5 second delay accounts for the response time required for the MSIV closure. The 1
MSIVs are assumed to be closed within 10 seconds. The assumptions used in the-FSAR analysis, which are based on 200% rated steam flow, are still i
conservative for a 140% high flow isolation setpoint. Even with the instrument uncertainty of 2%, there is a wide margin between the MSIV isolation setpoint of 140% and the analysis value of 200%.
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_ The high flow isolation signal is generated so quickly that the change in high flow isolation setpoint from 120% to 140% will not significantly affect the assumed 0.5 second delay time.
In addition the assumed 10 second MSIV closure time is conservative, since the closure time specified in the plant technical specifications is between 3 and 5 seconds. Thus
- the 10.5 second assumption used in the FSAR analysis is still conservative with a 140% main steam line high flow setpoint. The proposed change does
.not impact the FSAR calculated results for the worst case main steam line break outside the primary containment.
The licensee performed the enalysis for a spectrum of break sizes. The analysis performed by GE for the licensee indicates that the radiological consequences of the bounding small break are conservatively bounded by the radiological conseouences for the design basis main stean line break accident given in the FSAR. The small break analysis assumed that the steam line, takes no credit for high temperature or high steam line flow trip, and assumes a 10 ninute operator action time before isolation.
NRC approved models were used in the analysis.
Quarterly MSIV testing is performed at nresent with a 50% power reduction.
This reduction was imposed to avoid inadvertent trips with a 120% high flow isolation setpoint.
Simulation of a single MSIV closure transient at 75% power showed that testing can be performed without reaching the high steam line flow (140%), high neutron flux, or high pressure trip.
To account for the instrument uncertainties, lower limits of the allowable values for high flow, APRM scram and high pressure scram were used. The results showed sufficient margin such that a trip would be avoided.
The marain to an inadvertent trip on high steum flow during a single MSIV closure is reduced from 60% of rated steam flow to 40% of rated flow, with the new 75% power restriction. However, for the weekly. testing of MSIVs during full power, the margin is increased from 20% to 40%. According to the licensee, the MSIVs were often closing more than the desired 10%
during testing thereby causing pressure fluctuations. The new high flow trip setpoint is expected to reduce main steam system disturbances during the weekly testing.
The 75% power analysis described above was performed with the Vermont Yankee RETRAN model. During the teleconference on October 18, 1984, the licensee stated that their RETRAN model was used for the reload submittal for Cycle-9 and the staff SER was issued subseouently.
Results using the same modeling techniques were shown to be conservative for pressurization transients when compared to the Peach Bottom turbine trip test data. Therefore, the use of the RETRAN model for 75% pewar analysis is acceptable.
a 3-3.0 Summary Based on the analysis provided by the licensee, we have concluded that the proposed changes to reflect a change in the. trip setting for the high main steam line flow instruments from the present 120% to 140% of rated steam l
flow, and the increase in reactor power limit for quarterly MSIV full closure testing from 50% to 75% of rated power are acceptable.
4.0 Environmental Considerations This amendment involves changes in the installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes in surveillance requirements. The staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that this amendment involves no significant hazards consideration and there has been r.c public comment on such finding.
Accordingly, this amendment meets the eligibility criteria for categorical 3
i exclusion set forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared I
in conndction with the issuance of this amendment.
1 5.0 Conclusion We have concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the will not be endangered by operation in the proposed manner, and (2) public such activities will be conducted in compliance with the Commission's regulations and the issuance of this amendment will not be inimical to l
the common defense and security or to the health and safety of the public.
1 Principal Contributor:
G. Thomas Dated:
February 21, 1985 i
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