ML20136A751
| ML20136A751 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 12/24/1985 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20136A719 | List: |
| References | |
| NUDOCS 8601020264 | |
| Download: ML20136A751 (5) | |
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%,'\\d+... + 4t' SAFETY EVALUATION BY TWE OFFICE OF MUCLEAR REACTOR REGULATI SUPPORTING AMENDMENT NO. 78 TO FACILITY OPERATING LICENSE N SACRAMENTO MUNICIPAL UTILITY DISTRICT RANCHO SECO MUCLEAR GENERATING STATION DOCKET NO. 50-312 1.
INTRODUCTION A.
DESCRIPTION OF PROPOSED ACTION -
The proposed action would amend item 3 of' Table 4.1-1 of Appendix A
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of the facility Technical Specifications. This item relates to the calibration requirements for the power range nuclear instrumentation amplifiers.
Instead of the present requirement that calibration be performed "...whenever indicated neutron power and core thermal power differ by more than 2 percent and daily during non-steady-state operation...", the requirement would be changed to "...whenever the Nuclear Instrumentation indication is 10% above the core thermal power or 2% below the core thermal power."
B.
BACKGROUND INFORMATION By letter dated June 6, 1983 the Sacramento Municipal Utility District (the licensee) reque,sted anendment of Facility Operatine License No. OPR-54 for Rancho Seco Nuclear Generating Station (the facility) such that the power range nuclear instrumentation would only require calibration when it read low - that is, when the calibration was not conservative.
1 Supplementary information concerning this request was provided by the licensee's letters of June 29, 1983, and April 3, 1984.
As a result of review of this request by the NRC staff, the licensee was requested to provide additional information justifying the safety of the proposed change and placinq an upper limit on the amount the instrumentation could deviate from the core thermal power on the high side (the conservative direction).
This information was provided by the licensee's letters of July 11 and November 28, 1984, and February 8 and April 3,1985.
II. EVALUAT[0$
As a result of inquiries by and discus'sions with the NRC staff, the changes requested by the licensee are as follows:
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.1.
Change the requirement for calibration of the nuclear instrumentation power range amplifier from "...whenever indicated neutron power and core thermal power differ by more than 2 percent..." to "...whenever the Nuclear Instrumentation indication is 10% above the core themal power or 2% below the core thermal power."
2.
Increase the frequency of performing a heat balance check from daily to once per shift.
3.
Delete the present requirement for daily calibration during non-steady-state operation.
Regardin'g Item 1, the licensee states this change is needed because power range nuclear instruments (NI) that are within calibration limits at or near full power will read high (with respect to' thermal power) at reduced power levels (a conservative error). However, if the NI are brought within calibration limits at reduced power, they will then read low (a non-conservative error) when the reactor returns to full power.
This condition will then exist until the next calibration.
Typically this period would be several hours because the facility Technical Specifications presently require a heat balance daily.
This behavior occurs because neutron leakage from the reactor vessel changes with reactor power. This occurs,in part, because the re' actor control system is designed to provide a Constant T-Average value above 15% power (Note:
T-Average is the average temperature of the reactor coolant in the hot leg, Th, and the cold leg, Tc, of the reactor coolant Thus Tavg = (Th + Tc)/2). The other reason this occurs is system.
vall is water from the cold leg of'the reactor coolant sy Therefore, this water is at a temperature, Tc.
Since the temperature rise which occurs in passing through the core must increase with power level, it is clear that with constant Tavg control, Th must increase and Tc must decrease with power level.
Thus, at full' power Tc will have its minimum value and at lower power levels (greater than 15%), Tc will have higher values.
The result of the above is the water in the downcomer annulus i minimum operating temperature when the reactor is at full power.
water density, therefore, is greatest at full power and permits theThe smallest fraction of the generated neutrons to leak from the vessel and reach the out-of-core neutron detectors.
At lower power levels, Tc is higher, the water in the downcomer annulus is less dense, and a larger fraction of the generated neutrons can escape from the vessel.
From the above, it is clear that neutron channels calibrated on the basis of a heat balance at full power will provide high readings at lower power levels due to the increased neutron leakage.
the licensee notes, that if the neutron channels are calibrated atIt is also clear, as reduced power (when neutron leakage is high), the channels will read low This, of course, is an obviously non-conservative error persist until the next heat balance check was performed.
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3-On the basis of the foregoing, we conclude the present requirement to calibrate the neutron channels whenever the difference between the i
neutron-indicated power and the heat-balance power ~ is' greater than 2% can lead to non-conservative calibration if it is performed at reduced power.
On the other hand, the licensee's proposal to require calibration only when the heat balance power _ exceeds the neutron-indicated power by 2% would appear to resolve this temporary non-conservative condition.
To explore the issue further, we asked the licensee to address the possibility that xenon transients could alter this typical behavior.
In response, the licensee stated that xenon transients could introduce calibration errors in either the conservative or non-conservative directions.
The licensee also stated, however, that such changes have always been smaller than the thermal effects discussed above.
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licensee also noted that even if a transient were to cause the neutron channels to give a non-conservative indication, a recalibration would be required under both the existing and proposed Technical Specifications.
We also asked the licensee to address the effect of neutron channel L
readings more than two percent higher than thermal power on accidents
- considered in the FSAR and Technical Specification limits.
The-licensee responded that the accidents considered in the FSAR are terminated or limited by the Safety Limits and Limiting Conditions for Operation set forth in the Technical' Specifications.
The licensee cited reactor maximum power limit, power imbalance limits sud control rod insertion limits as examples of parameters which are dependent upon reactor power.
The licensee also correctly' notes that power imbalance and-control rod insertion limits become. more restrictive as power level increases.
neutron channel indications which are high impose. stricter and more Thus, conservative limits on reactor operation. In addition, we note that in the event of a power transient, neutron channels which read high would i
provide reactor trip at a lower thermal power, and thereby produce a less severe transient in terms of integrated energy release.
Based on the foregoing, we were unable to identify any situation where the licensee's failure to calibrate the power range neutron instrumentation while it was indicating higher than the true thermal power could cause or increase the severity of an accident. Further, we note the plant operators -have a significant incentive to avoid such a condition since it could cause inadvertent plant trip. Nevertheless, from the human factors standpoint. of retaining operator confidence in instrument indications, we considered it inadvisable to a' pprove a request that would allow an unlimited ceiling on the amount that the neutron instrumentation could be out of calibration on the high (conservative) side.
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Accordingly, the licensee was requested to revise their originial request, which provided no upper limit for calibration errors on the.high side, to provide a finite upper bound. This revision was provided by the licensee in a letter dated February 8,1985.
In-this letter the licensee proposed an i
upper limit of 10% for the maximum amount the indicated neutron power may i
exceed the core thermal power, i
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-4 Based on discussions with the licensee's representatives, the value of 10% is sufficient to encompass typical out-of-calibration situations which occur during power reductions and thereby correct the situation which led'to this request.
At the same time,10% is sufficiently close to the true power to resolve human factors concerns related to unbounded out-of-calibration conditions.
Therefore,'on the basis that Item I would eliminate a calibration requirement that can lead to a non-conservative instrument setting, because no 'ondition has been identified which indicates the need for
-correction of a high neutron channel reading, and because the licensee has proposed an appropriate upper limit for out-of-calibration conditions on the high side, we conclude the proposed change identified as Item I is acceptable.
As noted previously, Item 2 would increase the minimum required frequency for performing heat balance checks from daily to once per shift.
We conclude that because this change would provide more frequent monitoring of the calibration of the power range neutron instrumentation, this is an improvement to the' safety.of operations and is acceptable.
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l Regarding Item 3 which would eliminate the requirement for daily calibration of the power range neutron instrumentation during non-steady-state operation, we agree this is en unnecessary requirement.
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The reason this requirement is unnecessary is because the proposed new specification would require a heat balance check (comparison of indicated neutron power and core thermal power) at least once each shift.
This, in turn, would require calibration whenever the nuclear instrumentation indication was 2% below core thermal power or 10% above core thermal Power.
These requirements, therefore, address any calibrations needed
-the present requirement.during steady-state or non-steady-state operation Accordingly, we conclude the present requirement for daily calibration of the power range nuclear 4
the proposed revised specifications,and the deletion requirement is acceptable.
III. ENVIRONMENTAL CONSIDERATION This amendment involves a change in the installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20.
We have 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 no public coment on such finding. Accordingly, this amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b), no environmental impact i
statement or environmental: assessment need be prepared in connection with the issuance of this' amendment, t
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IV. ' CONCLUSION We have concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (2) such activities will be conducted in compliance with the Commission's regulations and the issuance of the amendment will not be inimical to common defense and security or to the health and safety of the public.
Dated: December 24, 1985 Principal Contributor:
G. Zwetzig
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