ML20207Q065
| ML20207Q065 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 01/09/1987 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20207Q064 | List: |
| References | |
| NUDOCS 8701210460 | |
| Download: ML20207Q065 (4) | |
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ENCLOSURE SAFFTY EVALHATTnN DEDUCED NUMRED OF T4THRLES FnR FLUX MAPDINA
'l0RTH ANNA PougD STATION, UNITS NO. 1 AND No. 9 INTRODUCTION By letter dated April 1, 1085, Virginia Flectric and Power Company (VEDCO) proposed an amendment in the form of changes to the Technical Soecifications to Operatino licenses Nor-4 and NPC-7 for North Anna Station Unit Nos. I and 2.
The proposed Technical Specification chances would reduce the minimum number of thimbles required for monthly surveillance flux mapping from 75% to 50%.
In support of the reauested amendment, VEDC0 provided a Topical Report, "A Studv of the Effects of a Reduced Number of Thimbles on the Results of Incore Flux Map Analysis," VEP-NOS-8, October 1983.
EVALUATION Westinchouse reactors operating with Standard Technical Specifications have a specification requiring availability of at least 75% of the detector thimbles.
However, we advocate maintenance of as close to 100% coerability of the incore detector system as is possible. We believe that this is required to be able to identify and evaluate possible power distribution or reactivity anomalies which might occur during the operation of power plants. An example is the burnable poison rod leachino problem that occurred in St. Lucie 1 where the incore in-strumentaticn was essential in identifyino and understanding the problem.
The 75% operability requirement was chosen to allow a reasonable amount of failures of the incore detectors, but to encouraoe the licensees to strive for j
as near to 100% as possible. Technical Specification changes to reduce the number to 50% might result in a lack of incentive to keep the system operating l
as close to 100% as possible. This could result in an unacceptably degraded ability to detect anomalous conditions in the core. We find the proposed change unacceptable primarily for this reason.
8701210460 870109 PDR ADOCK 05000338 P
A recent study by our consultants at BNL* on the impact of failed detectors on the ability to detect abnormal conditions indicates that the number of thimbles provided in the reactor design is marginal for detection of some core abnormalities. The worst conditions involve icw detector density and location of the abnormality in the same area of the core. These results strongly support the conclusion that a reduction in the number of detector thimbles reouired to be operable is not prudent.
The proposed change for the North Anna power plants would allow a minimum of
?6 (50%, rounded up) thimbles to obtain a flux map fer routine monthly surveillance of hot channel factors. The uncertainty allowance for the number measurements and 1% for of thimbles between 26 and 38 is increased by 2% for F0 F
measurements. These uncertainty increases are the result of the analysis oH contained in VEp-NOS-8.
In the proposed amendment in addition to the increased uncertainty allowance when less than 75% of the detector thimbles are used are the reautrements that a minimum of four detectors per quadrant must be available and control rods must be positioned within 12 steps. The latter is required by other specifications anyway. We do not consider that this would compensate for our concern that anomalous conditions may not be detectable with the reduced number of thimbles.
In addition, the bases provided for the proposed N have exceeded their limits measuring of amendment state "when Fg (Z) or Fgg F (Z) or F must be made with a full incore map of at least 38 thimbles."
0 gg N, but with the constant axial This may be pertinent to the monitoring of F4H offset control and cycle specific peaking factor analyses in use at the North Anna power plants, it is not possible for a steady state incore flux rap to indicate the F limit has been exceeded.
g
- Memorandum from P. Neogy and A Prince (BNL) to J. F. Carew (BNL), " Impact of Failed Detectors on the Measurement of Normal and Anomalous Power Distributions," August 8, 1986.
There have been two requests to reduce the number of thimbles required by the Technical Specifications in recent years, one by Duquesne Light for Beaver Valley Unit 1 and the other by Omaha Public Power District for Ft. Calhoun.
These were found unacceptable in letters from L. S. Rubenstein to G. Lainas "Peaver Valley Unit 1, Evaluation of Thimble Deletion Technical Specification Change", August 17, 1983 and L. S. Rubenstein to G. Lainas, March 2?, 19P,3, respectively.
In other related actions, there have been occasions, for various reasons, when failures in operating PWRs have approached or exceeded 25%, and relaxation of the 75% requirement has been permitted for the duration of affected reactor cycles. This has generally been allowed either with increased surveillance of some sort (such as increased frequency of flux mapping), or in most cases because there is substantial margin (usually late in a cycle) to Technical Specification peaking factor limits. Other than such requests for temporary relaxation of the Technical Specification requirements, we are not aware of any problem in meeting the 75% requirement. We, therefore, believe that temporary relief could be provided in any situation requiring it, and prefer to do that rather than allowing a permanent change in detector availability requirements.
The above conclusion has been reached on the basis that we do not know if a thorough study of detector failures has ever been conducted. We do not know detector failure rates, problems encountered at specific plants, freauency of thimble cleaning required, etc. The answer to such questions should be detemined before any further consideration is given to relaxation of detector availability requirements.
In addition, we would recommend that any further consideration of a permanent change in detector requirements should be done on a generic basis.
In our review of the Topical Report provided in support of the VEPC0 amendment request, we find three areas of difficulty. The report provides a statistical basis for determining the increase in uncertainty allowance to be applied to the measurement of F and F with a reduced number of detectors. Our finding 0
y of unacceptability of the proposed amendment is on another basis. We believe that the flux mapping system can provide information about conditions that are not normal, which is not realated to the routine measurement of the peaking
4 factors. The three problems with the report relative to determining anomalous conditions are:
- 1) The uncertainties are determined for a variety of core conditions, all statistically combined. We would expect larger uncertainties for the most abnormal conditions considered alone, which is the time we would really want the best information. 21 The uncertainties for various failure locations and numbers of failures are statistically combined. This washes out the large uncertainty for the case where the failures occur around an area in which there is an abnormal condition. At one time Westinghouse reported at a meeting that they were working on a system which detennined the uncertainty based upon the number of failures in a given area of the core. Such an approach would seem more acceptable. 3) The study does not address non-random failures of the detectors.
CONCLUSION As discussed above, our difficulties with the proposed reduction in the number of incore thimbles required by the Technical Specifications are:
(1) We are concerned that the ability to detect anomalous conditions in the core will be seriously degraded.
(?) The language of the proposed change would allow the system to be degraded indefinitely.
(3) There are technical problems with the statistical treatment supportino the increased uncertainty allowance for reduced detector thimble availability.
(4) Operating data are not available to indicate whether there is a plant specific cr generic problem involved with maintaining detector thimble availability.
(5) The licensee could obtain temporary Technical Specification relief if needed when the number of available detectors approached the limit.
For these reasons, but primarily the first listed, we find the proposed Technical Specification change unacceptable.