ML20236V407
| ML20236V407 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 07/28/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20236V405 | List: |
| References | |
| NUDOCS 9808030162 | |
| Download: ML20236V407 (4) | |
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UNITED STATES p
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NUCLEAR REGULATORY COMMISSION
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WASHINGTON, D.C. 30seH001 I
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j SAFETY EVALQATION BY THE OFFICE OF NUCI FAR REACTOR REGULATION RELATED TO AMENDMENT NO.136 TO FACILITY OPERATING LICENSE NO. DPR 42 NORTHERN STATES POWER COMPANY i
PRAIRIE ISLAND NUCLEAR GENERATING PLANT. UNIT 1 l
DOCKET NO. 50-282 4
1.0 INTRODUCTION
By letter dated January 15,1998, the Northem States Power Company (NSP or the !icensee) requested an amendment to the Technical Specifications (TS) appended to Facility Operating License No. DPR-42 for the Prairie Island Nuclear Generating Plant, Unit 1. The proposed amendment would reduce the number of required incore detectors necessary for continued operation for the remainder of Cycle 19 only. Supplemental information was provided in a letter dated May 29,1998. This information was within the scope of the original Federal Register notice and did not change the staff's initial proposed no significant hazards considerations determination.
The Prairie Island Unit 1 movable incore detection system (MIDS) contains a total of 36 instrumentation thimbles in the core. TS 3.11.A requires that at least 75 percent of the detector thimbles be operable with a minimum of two detector thimbles per quadrant when performing a flux map to ensure compliance with the peaking factor requirements of TS 3.10.B. The surveillance requirements of Specification 3.10.B.2 require that the peaking factors must be detarmined to be within limits at least once per 31 effective full power days. Due to the increase in incoro detector thimble failures at Prairie Island Unit 1 during Cycle 19 thus far, NSP has proposed a change tbt will allow plant operation with the number of operable detector thimbles reduced *n a minimum of 50 percent. To compensate for the increased uncertainty as the number of operable detector thimbles is reduced, the measurement uncertainty for F,n and Fa will be increased whenever the number of detectors is between 27 and 18. Changes are also proposed to TS 3.11.B to clarify the requirements for the number of detector thimbles required per core quadrant when the number of available detector thimbles is less than 75 percent.
During the last few cycles the moveable incore detectors have had sticking problems which resuted in the thimbles not being accessible. Because of these problems the Unit 1 thimbles I
were cleaned following Cycle 15 and six thimbles were replaced and the slip clutches were l
either rebuilt or replaced at the end of Cycle 16. Following Cycle 17,35 of the 36 thimbles were i
eddy-current tested successfully. However, by the enti r,T Cycle 18, the number of accessible j
thimbles had dropped to 26 of 36 (72 percent). During the refueling outage following Cycle 18, the licensee solvent-flushed all but one of the thimbles that had been previously abandoned.
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9 During post-cleaning tests for obstruction,34 of the thimbles were verified to be unobstructed by inserting a test cable through the entire length of each thimble. Despite this effort only 29 thimbles were accessible for the first flux map (taken at 30 percent power) fokowing the refueling outage. Only 27 thimbles were accessible for the 70 percent power map and all subsequent 100 percent power maps. If one more thimble becomes inaccessible, a plant shutdown will be required in accordance with the Prairie Island Unit 1 TS.
2.0 EVALUATION Essentially all pressurized-water reactor (PWR) TS contain a requirement for operability of 75 percent of the incore detector locations for mapping of the core power distribution. On a number of occasions, for various reasons, failures of detector thimbles in operating PWRs have approached or exceeded 25 percent, and relaxation of the 75-percent requirement has been permitted for the remainder of the affected operating cycle. Prairie Island Cycle 19 is different from the other cases because there were se few, just 75 percent, of the detectors available for the first 100 percent power map of the cycle. This leads to less verification of the core and necessitates more reliance on the results of the remainder of the physics startup tests.
Incore detector data is used to calev! ate power peaking factors whie5 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. The requirement for maintaining 75 percent of the detector thimbles available provides for a reasonable number of failures of the incore detectors while encouraging licensees to strive for maintaining the system as near to 100 percent available as possible. TS 3.11.A allows continued use of MIDS with less than 75 percent of the thimbles available if the measurement error allowance due to incomplete flux mapping is substantiated.
The licenseo submitted an analysis performed by the vendor, Westinghouse, that assessed the impacts of a reduction to a minimum of 18 of the 36 movable detector thimbles for Prairie Island Unit 1, Cycle 19. The analysis indicated that additional uncertainties of 2 percent for F",a and 3 percent for F"a are approp late when the number of instrumented assemblies is reduced from 36 to 18. The additional uncertainties should be applied linearly when the number of detector thimble locations available is between 50 percent and 75 percent of the total number of locationt,. In addition, the Westinghouse analysis assumed random deletion of the thimbles. If the thimbles were systematically deleted from use, the calculated peaking factor uncertainties l
would not apply. Thus there is an additional requirement that when the number of detector locations is less than 75 percent, there should be a minimum of two thimbles available per quadrant, where quadrant includes both horizontal-vertical quadrants and diagonally bounded quadrants (eight quadrants in all). This requirement improves the ability to distinguish between l
random and systemic thimble deletion events and establishes the bounds of applicability of the peaking factor uncertainties.
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l The licensee has provided the results of core maps for Cycle 19. The results of these maps show approximately 4.5-percent margin to the F,n TS limit and approximately 6 percent margin I
1 to the Fa TS limit. The peaking factors normally tend to decrease with bumup, so we would expect the margin to increase.
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9 Another safety concem 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 concem once the core has been verified at the beginning of the cycle. Unfortunately only 27 thimbles were accessible for the first full power map. The staff has reviewed the remainder of the physics startup test results for Cycle 19 and the results showed excellent agreement between predictions and measurements, thus giving assurance that the core is as designed. 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 power range neutron flux detectors (excore detectors) and would help identify problems not fully detectable with reduced incore mapping capability. Furthermore, the core exit thermocouple in the reactor provide a useful supplement to the incore detectors to detect problems. According to TS 3.11.C, if one of the excore detectors is inoperable, power operation above 85 percent may
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continue if quadrant tilt is monitored by the incore detectors or by the core exit thermocouple.
During operation with fewer than 75 percent of the incore detectors operable, the licensee may still use the thermocouple option to detect anamolous conditions within the core.
Our review of the suitability of operation of the Prairie Island Unit i reactor for the remainder of Cycle 19 with a reduced number of movable incore thimbles locations to as few as 50 percent indicated adequate margin exists at this time in Cycle 19 and sufficiently increased uncertainty allowances have been made to ensure that TS peaking factor limits will be met. In addition, there are adequate supplementalindicators 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 3.1 TS 3.10.B 1. 3.10.B.2. 3.10.B 3.b.1
.d 3.10.B.3.d.1 l
The changes increase the measurement uncertainty for Po and F,n when the number of i
operable incore detector thimbles is between 75 percent (27) and 50 percent (18) of the total number of detectors. The change in the uncertainty has been justified and is therefore acceptable.
3.2 TS 3.11.B The change adds a footnote that states that for Unit 1 Cycle 19, with greater than 50 percent and less than 75 percent detector thimbles available, the two thimbles per quadrant requirement must be met for each of the four horizontal-vertical quadrants and the four diagonally bounded quadrants (eight individual quadrants in total). The proposed change was added to establish the bounds of applicability of the evaluation and is acceptable.
3.3 Conclunians Based on the staff evaluation in Section 2.0 above, the staff concludes that the proposed TS changes are acceptable. The licensee may still use the thermocouple option of TS 3.11.C if one excore detector is inoperable and there are fewer than 75 percent of the incore detector thimbles available. These changes are for the remainder of Cycle 19 only.
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4.0 STATE CONSULTATION
in accordance with the Commission's regulations, the Minnesota State official was notified of the proposed issuance of the amendment. The State official had no comments.
5.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20. The NRC 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 the amendment involves no significant hazards consideration and there has been no public comment on such finding (63 FR 4676). Accordingly, the 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 statement or environmental assessment need be prepared 5 connection with the j
issuance of the amendment.
6.0 CONCLUSION
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 amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor: Margaret S. Chatterton Date: July 28, 1998 l
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