ML20216J408

Safety Evaluation Supporting Amend 106 to License DPR-3

ML20216J408
Person / Time
Site:	Yankee Rowe
Issue date:	06/26/1987
From:	Office of Nuclear Reactor Regulation
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NUDOCS 8707020382
Download: ML20216J408 (4)

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1 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION j SUPPORTING AMENDMENT NO. 106 TO FACILITY OPERATING LICENSE NO. DPR-3 YANKEE ATOMIC ELECTRIC COMPANY YANKEE NUCLEAR POWER STATION DOCKET NO.50-029 1

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INTRODUCTION By letter dated January 29, 1987, the Yankee Atomic Electric Company (YAEC) submitted a reouest for changes to the Yankee Nuclear Power Station Technical Specifications (TSI. I The amendment modifies the TS reouirement for the minimum number of operable neutron detector thimbles from twelve, with at least two per quadrant, to nine, with at least one per quadrant for Cycle 19 operation.

DISCUSSION The incore instrumentation systen is required to be in operation to obtain a power distribution map at least once per 1000 effective full power hours.

Factors are applied to the measured data prior to comparison with the TS operating limits. One such factor accounts for the uncertainty in the power 1 distribution measurement. The Technical Specifications currently require a minimum of twelve incore instrumentation neutron detection thimbles with two per core quadrant to be operable. At present there are 13 detectors operable; 1 the change was requested in case two additional detectors failed. When fewer detectors are operable, a larger uncertainty factor is applied to the ,

measurements.

On November 24, 1978, the minimum number of operable detectors was reduced from 17 and 12. At. that time, the measurement uncertainty was increased from 5 percent to 6.8 percent. In 1982 during Cycle 15, after failure of a detector, only 12 detectors were operable. A Technical Specification change to allow use with fewer than 12 detectors was requested, and approved on March 8, 1982 for the remainder of Cycle 15. The licensee repaired neutron detector thimbles during the next refueling outage and Cycle 16 began with 17 detectors operable. Two detectors failed in the following month. Two more detectors have failed since then, leaving 13 operable.

A Technical Specification change to allow fewer than twelve operable detectors was requested and approved on December 1,1986, for the balance of Cycle 18 operation. Because of the failure record of these incore detectors, the licensee is installing fixed incore detectors in the 8707020382 870626 9 DR ADDCK 0500

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J core. Once these detectors are operational and are correlated with the f moveable incore detectors, the fixed incores would then be used as the primary means of obtaining the power distribution measurements. In the interim, YAEC ]

l has requested that the Technical Specification changes in effect for Cycle 18  !

operation be continued for Cycle 19. That is, the minimum number of operable i detectors would be nine, with at least one per core quadrant. .When fewer than twelve detectors are operable, the measurement uncertainty factor shall be 8%.

This is the factor that was used for previous cycles (Cycles 15 and 18) with )

fewer than twelve detectors operable. 1 EVALUATION I

i The staff has previously approved the uncertainty factor for use with fewer )

than twelve detectors after the start of the cycle, (Amendment 100 to the licensee, December 1,1986) and this factor (8%) remains acceptable for Cycle 19.

4 The issue to address is possible operation for a full cycle with fewer than  !

12 operable detectors. The staff has historically resisted reque.sts to I reduce the number of available detectors on a permanent basis. The reason is that the studies involving the increase in uncertainty as a function of the  ;

number of available detectors are statistical in nature. They consider l normal and abnonnal power distributions and different patterns of failed I detector locations. The results then define the uncertainty with a probability (and confidence factor) that the power distribution is known. l They do not isolate the extreme cases where there are blind spots which  !

cannot be detected but which could result in unsafe power distribution or where there are severe power ma1 distributions for which the uncertainty is much larger than allowed for in the statistical studies.* These conditions can be postulated with full as well as partial availtbility of incore detector locations, but are less likely to escape detection when more  !

detectors are available. Extreme power distribution are unlikely to occur; however, prudence would indicate detection of an extreme situation is worth the effort of maintaining high fractions of incore detector locations. This has been our objective in requiring a high fraction of detector location availability (usually _75%) and in resisting attempts to lower operability requirements on a permanent basis.

The present proposal for Yankee does not represent a permanent reduction in the number of incore detector locations required for operation. although it is a request for a reduction at the beginning of a cycle. It likewise does not represent a continually degrading system in which failures and relaxation of requirements will continue to occur. At end of Cycle 18 operations, 13 detector locations were operable. The proposed Technical Specification change will not be used unless there are further failures in the existing operable detector locations. Most importantly, however, the proposed change is interim because the licensee is in the process of refitting the detector locations in the core with fixed neutron detectors. For Cycle 19, six of the possible 22 thimble locations are planned to be replaced with fixed detector strings. However, when first installed, fixed detector strings cannot be considered as substitues for inoperable moveable incore locations. However, data from the fixed detector strings will be collected Memorandum trom P. Neogy and A. Prince (BNL) to s1. F. Carew (BNL), " Impact of Failed Detectors on the Measurement of Anomalous Power Distributions",

August 8, 1986

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i and cross-correlated with the moveable detector data to develop normalization I factors and appropriate system measurement uncertainties. Thus, better incore l neasurement capability then the minimum allowed by the proposed Technical Specification changes should be available during Cycle 19. The licensee has stated in his proposal that "Given success of the fixed system, Technical Specification changes based on a combination system will be proposed in the near future with a total conversion to fixed detectors being the ultimate goal."

As noted above, a major consideration for incore detector availability at the beginning of a cycle is detection of power maldistributions. The licensee has addressed the measures used to detect core misloadings, which we agree are adeouate without reliance on incore mapping. In addition, there are other j means to detect possible power distribution anomalies in the reactor core. q These include incore thermocouple data, excore detector signals, and loop <

temperature indications. Because of these, and the hiahly unlikely probability of occurrence of an extreme power distribution problem, the staff concludes that the proposed Technical Specification change will not result in a loss of safety margins in Cycle 19 operation of the Yankee power plant. Finally, the staff intends to follow operation during the cycle so that we could initiate appropriate action should there be evidence of anomalous conditions. Based on l

the above considerations, the staff finds this proposed Technical Specification change acceptable.

ENVIPONMENTAL CONSIDERATION j This amendment involves a change to a requirement with respect to the installation or use of facility components located within the restricted area as defined in 10 CFR Part 20. The staff has determined that the amendment in-volves 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 hn been no public comment on such finding. Accordingly, this amendment meets the eligi-bility 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 this amendment.

CONCLUSION The staff 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, and (2) such activities will be conducted in compliance with the Commission's regulations and the issuance of this anendment will not be inimical to the common defense and security or to the health and safety of the public.

ACKNOWLEDGEMENT Principal Contributor: M. Dunenfeld Dated: June 26, 1987

i June 26, 1987 Dated AMENDMENT NO.106 TO FACILITY OPERATING LICENSE DPR-3, YANKEE NUCLEAR POWER STATION Distribution:

Docket File 50-029 Y NRC PDR Local-PDR PDI-3 Reading )

S. Varga l E. McKenna {

M. Rushbrook'~ 3 t OGC-Bethesda D. Hagan E. Jordan J. Partlow T. Barnhart (4) l W. Jones E. Butcher )

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ACRS (10) l GPA/PA l i

ARM /LFMB M. Dunenfeld I

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