ML20128L618
| ML20128L618 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 10/10/1996 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20128L604 | List: |
| References | |
| NUDOCS 9610150148 | |
| Download: ML20128L618 (4) | |
Text
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4 UNITED STATES g
.g NUCLEAR REGULATORY COMMISSION
'f WASHINGTON D.C. 20565 0001 o,
k.....,o SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT N0.124 TO FACILITY OPERATING LICENSE NO. OPR-42 NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT. UNIT N0. I
1.0 INTRODUCTION
By letter dated July 15, 1996, and supplemented August 22, 1996, the Northern States Power Company (NSP or the licensee) requested an amendment to the Technical Specifications (TS) appended to Facility Operating License No. DPR-42 for the Prairie Island Nuclear Generating Plant, Unit No.1.
The proposed i
amendment would reduce the number of required in-core detectors necessary for continued operation for the remainder of Cycle 18 only.
The Prairie Island Unit 1 Movable Incore Detection System (MIDS) contains a total of 36 instrumentation thimbles in the core.
TS 3.ll.A requires that at i
least 75 percent of the detector thimbles be operable with a minimum of two i
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 determined to be within limits at least once per 31 effective full power days.
Due to the increase in incore detector thimble failures at Prairie Island Unit I during Cycle 18 thus far, Northern States Power has proposed a change that will allow plant operation with the number of operable detector thimbles reduced to 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 and F will be increased whenever the number a
a 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 resulted in the thimbles not being accessible.
Because of these problems the Unit I thimbles were cleaned following Cycle 15 and six thimbles were replaced and the slip clutches were either rebuilt or replaced at the end of Cycle 16.
Following Cycle 17, 35 of the 36 thimbles were eddy-current tested successfully.
However, when the first map for Cycle 18 was taken, only 29 thimbles were accessible. Up to 32 were accessible for some maps, but for the last several maps only 29 have been accessible.
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, 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.
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.
The requirement for maintaining 75 percent of the detector thimbles available provides for a reasonable number of failures of the in-core detectc,s w,ile encouraging licensees to strive for maintaining the system as near to 100 percent available as possible.
TS 3.llA allows continued use of the movable in-core detector system with less than 75 percent of the thimbles available if the measurement error allowance due to incomplete flux mapping is substantiated.
The licensee submitted an analysis performed by the vendor, Westinghouse, which assessed the impacts of a reduction to a minimum of 18 of the 36 movable detector thimbies for Prairie Island Unit 1, Cycle 18.
The analysis indicated and 3 percent for F" are that additional uncertainties of 2 percent for F"$ lies is reduced from $6 to appropriate when the number of instrumented assem 18.
The additional uncertainties should be applied linearly frcm below 75 percent to greater than 50% detector thimble locations.
In addition the Westinghouse analysis assumed random deletion of the thimbles. If the thimbles were systematically deleted from use, the calculated peaking factor uncertainties 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 random and systemic thimble deletion events and establishes the bounds of applicability of tne peaking factor uncertainties.
The licensee has provided the results of core maps for Cycle 18.
These show that there is currently approximately 5 percent margin to the F",the unit does TS limit j
and approximately 9.5 percent margin to the F"ly tend to decrease with burnup, TS limit.
Since not load follow and the peaking factors normal we would expect the margin to increase from now till the end of the cycle.
Another safety concern relating to degradation of in-core 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 j
cycle.
Furthermore, review of the Cycle 18 startup physics test results showed very good agreement between predictions and measurements, thus giving more 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 1
i
, monitored by the excore detectors and would help identify problems not fully detectable with reduced in-core mapping capability.
Furthermore, the core exit thermocouples in the reactor provide a useful supplement to the in-core detectors to detect problems.
According to TS 3.ll.C, if one of the power range neutron flux detectors (excore detectors) is inoperable, power operation above 85 percent may continue if quadrant tilt is monitored by the in-core detectors or by the core exit thermocouples.
During operation with fewer than 75 percent 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 Prairie Island Unit I reactor for the remainder of Cycle 18 with a reduced number of movable in-core thimbles locations to as few as 50 percent indicated adequate margin exists at this time in Cycle 18 and sufficiently increased uncertainty allowances have been made to ensure that TS 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 in-core detector mapping capability.
3.0 TECHNICAL SPECIFICATION CHANGFS 3.1 TS 3.10.B.1. 3.10.8.2. 3.10.B.3.b.l. and 3.10.B.3.d.1 The changes increase the measurement uncertainty for F",75 perc,,ent (27) and and F"
&n W number of operable in-core detector thimbles is between 50 percent (18) of the total number of detectors.
The change in the uncertainty has been justified and is therefore acceptable.
3.2 TS 3.11.8 The change adds a footnote that states that for Unit 1 Cycle 18, with greater than 50 percent and less than 75 percent detector thimbles available, the two thimble 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 Conclusions Based on the staff evaluation in Section 2.0 above, the staff concludes that the proposed TS changes are acceptable.
The licensee has agreed to use the thermocouple option if one excore detector is inoperable and there are fewer than 75 percent of the in-core detector thimbles available.
These changes are for the remainder of Cycle 18 only.
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.
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5.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to installation or use of a facility compcnent 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 (61 FR 40024). 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 in connection with the 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: October 10, 1996 l
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