ML17349A915
| ML17349A915 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 06/15/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML17349A914 | List: |
| References | |
| NUDOCS 9306210282 | |
| Download: ML17349A915 (6) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, O.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO TECHNICAL SPECIFICATION CHANGE TO REDUCE THE NUMBER OF RE UIRED IN-CORE DETECTORS FOR REMAINDER OF CYCLE 13 OPERATION FLORIDA POWER AND LIGHT COMPANY TURKEY POINT UNIT 3 DOCKET NO. 50-250
1.0 INTRODUCTION
By letter dated April 13, 1993 Florida Power and Light Company (FPL) requested changes to the Technical Specifications for Turkey Point Unit 3.
These changes would reduce the number of required in-core detectors necessary for continued operation for the remainder of Cycle 13 only.
The Turkey Point Unit 3 Movable Incore Detection System (MIDS) contains a
total of 50 instrumentation thimbles in the core.
Technical Specification 3.3.3.2 requires that at least 38 detector thimbles be operable with a minimum of two detector thimbles per quadrant when used for monitoring F~H, Fq, and Fxy.
Due to the increase in incore detector thimble failures at Turkey Point Unit 3 during Cycle 13 thus far, FPL has proposed a change which will allow plant operation with the number of operable detector thimbles reduced to a
minimum of 50%.
To compensate for the increased uncertainty as the number of operable detector thimbles is reduced, the measurement uncertainty for F~H and Fq will be increased whenever the number of detectors is between 38 and 25.
Also the minimum number of detector thimbles per quadrant will be increased from two to three whenever the number of operable thimbles is less than 38.
Two sets of quadrants are defined:
- 1) quadrants formed by the vertical and horizontal axes of the core and
- 2) quadrants formed by the two diagonals of the core.
- 2. 0 EVALUATION Essentially all PWR Technical Specifications contain a requirement for operability of 75/ of the incore detector locations for mapp',ng of the core power distribution.
On a number of occasions, for various reasons, failures of detector thimbles in operating PWRs have approached or exceeded
- 25K, and relaxation of the 75/o requirement has been permitted for the remainder of the affected operating cycle.
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In-core detector data is used to calculate power peaking factors which are used to verify compliance with fuel performance limits.
As the number of inoperable detector segments increases, the uncertainties in the power distribution calculation increase.
A generic analysis of the peaking factor uncertainties with a reduced number of operable detector thimbles was performed by Westinghouse for Turkey Point Units 3 and 4 in 1987.
An update was provided in 1988.
In order to ensure that the analyses are applicable for Unit 3 Cycle 13, FPL performed calculations based on the operational data for Turkey Point Unit 3 Cycle 13 through Harch 1993.
The Westinghouse analysis used flux maps from three different three-loop reactors with incore thimble patterns identical to the Turkey Point Units.
A total of seven flux maps with five separate random thimble reduction cases were used for the study.
The measured peaking factors in the 35 reduced thimble maps were compared with the reference maps.
The resulting uncertainties for peaking factors with only 50% of the thimbles operable were 1.043 for F~H, 1.057 for Fq and 1.054 for Fxy.
These values were rounded up to 1.05 for F~H and 1.07 for Fq and Fxy.
As an additional conservatism, the peaking factor uncertainties due to the reduction of thimbles were doubled.
Thus the peaking factor uncertainties for deletion of 50% of thimbles are 1.06 for F~H and 1.09 for Fq and Fxy.
These uncertainties are applied to measurements using a ramp function for maps taken with between 50% and 75% of the total number of incore detector thimbles.
The uncertainty analysis assumed the thimbles were randomly deleted.
If the thimble deletion is not random, the calculated uncertainties do not apply.
A computer simulation showed that when 50% of the thimbles remain after random
- deletion, at least three thimbles should be left in each of the eight quadrants.
If less than three thimbles are left in a quadrant, then the thimble removal was probably not a random process and the peaking factor uncertainties previously calculated would not apply.
Thus the restriction that at least three thimbles must be present in each of the eight quadrants was added.
To ensure that the generic analysis is applicable to Turkey Point Unit 3 Cycle 13, a calculation was performed using the four 100% power flux maps available from Cycle 13.
Four subsets were generated for each flux map by randomly deleting 50% of the thimble traces.
The uncertainties were evaluated in the same manner as in the generic study.
The peaking factor uncertainties were lower than those obtained in the generic study.
Thus it was concluded that the original analyses are applicable to Unit 3 Cycle 13.
Another safety concern relating to degradation of incore mapping ability is the ability to detect anomalous conditions in the core.
One of these is inadvertent loading of a fuel assembly into an improper position.
Since this is a loading problem, it is not of concern for the remainder of the operating cycle.
Other anomalous conditions are conceived to produce either an axial or radial effect, which would cause either a change in quadrant tilt ratio or axial offset ratio.
These are monitored by the excore detectors and would help identify problems not fully detectable with reduced incore mapping capability.
Furthermore, the core exit thermocouples in the reactor provide a
useful supplement to the incore detectors to detect problems.
If one of the power range neutron flux detectors (excore detectors) becomes inoperable, power operation may continue if the power level and the trip setpoint are reduced or the quadrant tilt is monitored by the incore detectors every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />, or the quadrant tilt is monitored by the thermocouples.
During such operation with fewer than 75N of the incore detectors
- operable, the licensee has indicated that the thermocouple option would be the one chosen.
Our review of the suitability of operation of the Turkey Point Unit 3 reactor for the remainder of Cycle 13 with a reduced number of movable incore thimble locations to as few as 50 percent indicated that adequate margin exists at this time in Cycle 13 and sufficiently increased uncertainty'llowances have been made to ensure that Technical Specification peaking factor limits will be met.
In addition, there are adequate supplemental indicators of anomalous conditions to preclude an unsafe condition from escaping detection in the absence of full incore detector mapping capability.
3.0 TECHNICAL SPECIFICATION CHANGES Technical Specifications 4.2.2. 1 and 4.2.2.5 The change increases the measurement uncertainty for Fq when the number of operable incore detector thimbles is between 75N (38) and 50N (25) of the total number of detectors.
The change in the uncertainty has been justified and is therefore acceptable.
Technical Specifications 4.2.2.3 and 4.2.2.4 - The change increases the measurement uncertainty for the base load uncertainty factor (U, )
and the radial burnup uncertainty factor (U
) when the number of operabte incore detector thimbles is between 75% (35 and 50% (25) of the total number of detectors.
The new uncertainty has been justified and is acceptable.
Technical Specification 4.2.3.2 - The change increases the measurement uncertainty for F~H when the number of operable incore detector thimbles is between 75N (38) and 50No (25) of the total number of detectors.
The new uncertainty has been justified and the change is acceptable.
Technical Specification 3.3.3.2 - The change allows a decrease in the number of required operable incore detector thimbles to 50N (25) of the total number of thimbles consistent with an increase in peaking factor uncertainties, and
increases the minimum number of thimbles required per quadrant from two to three.
The proposed change has been justified and is acceptable.
4.0 STATE CONSULTATION
Based upon the written notice of the proposed amendments, the Florida State official had no comments.
- 5. 0 ENVIRONMENTAL CONSIDERATION
,The amendments change the surveillance requirements.
The NRC staff has determined that the amendments involve 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 the amendments involve no significant hazards consideration, and there has been no public comment on such finding (58 FR 28066).
Accordingly, the amendments meet 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 statement or environmental assessment need be prepared in connection with the issuance of the amendments.
- 6. 0 CONCLUSION Based on the staff evaluation in Section 2.0 above, the staff concludes that the proposed Technical Specifications changes are acceptable.
The licensee has agreed to use the thermocouple option if one excore detector is inoperable and there are fewer than 75N of the incore detector thimbles available.
These changes are for the remainder of Cycle 13 only.
The Commission has 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, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
Principle Contributor:
H. Chatterton, SRXB L.Raghavan,
- PDII2, DRPE Date:
June 15, 1993
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