ML18142A365

Proposed Tech Spec Changes,Reducing Min Number of Thimbles from 38 to 26 to Obtain Flux Map for Routine Monthly Surveillance of Hot Channel Factors

ML18142A365
Person / Time
Site:	Surry
Issue date:	04/15/1985
From:	VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:
Shared Package
ML18142A364	List: ML18142A363 ML18142A365
References
TAC-57471, TAC-57472, NUDOCS 8504230485
Download: ML18142A365 (13)

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e e ATTACHMENT 1 TECHNICAL SPECIFICATIONS PROPOSED CHANGES 850-4230485 8 50415 PDR ADOCK 05000280 P PDR

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e e TS 3.12-3 B, Power Distribution Limits

1. At all times except during low power physics tests, the hot channel factors defined in the basis must meet the following limits:

FQ(Z) ~ 2.18/P x K(Z) for P) 0.5 FQ(Z) ~ 4.36 x K(Z) for P ~ 0.5 N

FlH ~ 1.55 [1 + 0.3 (1-P)] for three loop operation

~ 1.55 [1 + 0.2 (1-P)] for two loop operation where Pis the fraction of rated power at which the core is operating, K(Z) is the function given in TS Figure 3 .12-8, and Z is the core height location of FQ.

2. Prior to exceeding 75% power following each core loading and during each effective full power month of operation thereafter, power distribution maps using the movable detector system shall be made to .confirm that the hot channel factor limits of this specification are satisfied (see Specification 4 .10. B).

For the purpose of this confirmation:

a. The measurement of total peaking factor ~eas shall be increased by 8% to account for manufacturing tolerances, measurement error and the effects of rod bow if at least 38 thimbles are used. When less than 38 but at least 26 thimbles are use& for monthly surveillance (see Specification 4.10.B), F Meas measurements shall be Q

further increased by 2%. The measurement of enthalpy rise hot channel factor FtH shall be increased by four percent to account for measurement error if at least 38 thimbles are used. When less than 38 but at least 26 thimbles are used for monthly surveillance (see Specification 4.10.B), FtH measurements shall be further increased by 1%. If any measured hot channel factor exceeds its limit specified under Specification 3.12.B.1,

e TS 3.12-3a the reactor power and high neutron flux trip setpoint shall be reduced until the limits under Specification 3.12.B.1 are met using data from core maps with at least 38 thimbles (see specification 4.10.B). If the hot channel factors cannot be brought to within the limits of N

FQ(Z) ~ 2.18 x K(Z) and F H ~ 1.55 within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, the 6

Overpower 6T and Overtemperature 6T trip setpoints shall be similarly reduced.

TS 3.12-7

a. The hot channel factors shall be determined from a movable detector system map with at least 38 thimbles within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and the power level adjusted to meet the requirement of Specification 3.12.B.1, or

b. If the hot channel factors are not determined within two hours, the power level and high neutron flux trip *setpoint shall be reduced from rated power 2% for each percent of quadrant tilt.

c. If the quadrant to average power tilt exceeds+/- 10%, the power level and high neutron flux trip setpoint will be reduced from rated power 2% for each percent of quadrant tilt.

7. If, except for physics and rod exercise testing, after a further period of* 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, the power tilt in Specification 3.12.B.5 above is not corrected to less than 2%:

a. If design hot channel factors for rated power are not exceeded, an evaluation as to the cause of the discrepancy shall be made and reported as a reportable occurrance to the Nuclear Regulatory Commission.

b. If the design hot channel factors for rated power are exceeqed and the power is > 10%, the Nuclear Regulatory Commission shall be notified and the Nuclear Overpower, Nuclear Overpower LlT, and Over temperature LlT trip set-point shall be reduced 1% for each percent the hot channel factor exceeds the rated power design values.

c. If the hot channel factors are not determined, the Nuclear Regulatory Commission shall be notified and the Overpower

TS 3.12-11 F. Misaligned or Dropped Control Rod

1. If the Rod Position Indicator Channel is functional and the associated full length control rod is misaligned from its group step demand position by more than+/- 12 steps (indicating position) and cannot be realigned, the hot channel factors must be shown using an incore map of at least 38 thimbles to I be within design limits as specified in Specification 3.12.B.1 within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. If the limits of Specification 3.12.B.1 cannot be met, then power shall be reduced to< 75% of permit-ted power within one hour, and the High Neutron Flµx trip setpoint shall be reduced to~ 85% of rated power within the next four hours.

2. To increase power above 75% of rated power with a full length control rod more than+/- 12 steps (indicated position) out of alignment with its group step demand position, an analysis shall first be made to determine the hot channel factors and the resulting allowable power level based on the limits of Specification 3.12.B.1.

Basis The reactivity control concept assumed for operation is that reactivity changes accompanying changes in reactor power are compensated by control rod assembly motion. Reactivity changes associated with xenon, samarium, fuel depletion, and large changes in reactor coolant temperature (operating temperature to cold shutdown) are compensated for by changes in the soluble boron concentration. During power operation, the shutdown groups are fully withdrawn and control of power is by the control groups.

A reactor trip occurring during power operation will place the reactor into the hot shutdown condition. The control rod assembly insertion limits provide for achieving hot shutdown by reactor trip at any time, assuming the highest worth control rod assembly remains fully withdrawn, with sufficient margins to meet the assumptions used in the accident analysis. In addition, they provide a limit

e TS 3.12-15 It should be noted that the enthalpy rise factors are based on integrals and are used as such in the DNB and LOCA calculations. Local heat fluxes are obtained by using hot channel and adjacent channel explicit power shapes which take into account variations in radial (x-y) power shapes throughout the core. Thus, the radial power shape at the point of maximum heat flux is not necessarily directly related to the enthalpy rise factors. The results of the loss of coolant accident analyses are conservative with respect to the ECCS acceptance criteria as specified in 10 CFR 50. 46 using an upper bound envelope of 2 .18 times the hot channel factor normalized operating envelope given by TS Figure 3.12-8.

When an measurement is taken, measurement error, manufacturing tolerances, and the effects of rod bow must be allowed for. Five percent is the appropriate allowance for measurement error for a full core map taken with the movable incore detector flux mapping system using at least 38 thimbles, including a minimum of 2 thimbles per core quadrant, monitored. Three percent is the appropriate allowance for I manufacturing tolerances; and 5% is the appropriate allowance for rod bow. These uncertainties are statistically combined and result in a net increase of 1.08 that is applied to the measured value of FQ. When an F Q measurement is taken for monthly surveillance, with less than 38 but with at least 26 thimbles (see Specification 4.10.B), a minimum of 4 thimbles per core quadrant shall be monitored and the measured value of FQ shall be further increased by 2%.

In the specified limit of F!H, there is an 8% allowance for uncertain-ties, which means that normal operation of the core is expected to result in F~H ~ 1.55 [l + 0.3 (l-P)]/1.08. The logic behind the larger uncertainty in this case is that: (a) normal perturbations in the radial power shape (e.g., rod misalignment) affect F~H' in most cases without necessarily affecting FQ, (b) the operator has a direct influence on FQ through movement of rods and can limit it to the desired N

value; he has no direct control over F~H' and (c) an error in the pre-dictions for radial power shape, which may be detected during startup physics tests and which may influence FQ, can

e e TS 3.12-16 be compensated for by tighter axial control. Four percent is the appropriate allowance for measurement uncertainty for F~H obtained from a full core map taken with the movable incore detector flux mapping system, using at least 38 thimbles, including a minimum of 2 thimbles per quadrant monitored. When an incore map is taken for monthly surveillance with less than 38 thimbles but with at least 26 thimbles (see Specification 4.10.B), a minimum of 4 thimbles N

per core quadrant shall be monitored and F~H allowance for measurement uncertainty shall be further increased by 1%.

Measurement of the hot channel factors are required as part of startup physics tests, during each effective full power month of operation, and whenever abnormal power distribution conditions require a reduction of core power to a level based on measured hot channel factors. The incore map taken following core loading provides confirmation of the basic nuclear design bases including proper fuel loading patterns. The periodic incore mapping provides additional assurance that the nuclear design bases remain inviolate and identify operational anomalies which would, otherwise, affect these bases.

For normal operation, it has been determined that, provided certain N

conditions are observed, the enthalpy rise hot channel factor F~H limit will be met. These conditions are as follows:

1. Control rods in a single bank move together with no individual rod insertion differing by more than 15 inches from the bank demand position. An indicated misalignment limit of 13 steps precludes a rod misalignment no greater than 15 inches with consideration of maximum instrumentation error.

2. Control rod banks are sequenced with overlapping banks as shown in TS Figures 3.12-lA, 3.12-lB, and 3.12-2.

3. The full length control bank insertion limits are not violated.

4. Axial power distribution control procedures, which are given in terms of flux difference control and control bank insertion limits are observed. Flux difference refers to the difference

TS 4.10-1 4.10 REACTIVITY ANOMALIES Applicability Applies to potential reactivity anomalies.

Objective To require evaluation of applicable reactivity anomalies within the reactor.

Specification A. Following a normalization of the computed boron concentration as a functon of burnup, the actual boron concentration of the coolant shall be compared monthly with the predicted value. If the difference between the observed and predicted steady-state concentrations reaches the equivalent of 1% in reactivity, an evaluation as to the cause of the discrepancy shall be made and reported to the Nuclear Regulatory Commission per Specification 6.6.

B. During periods of power operation at > 10% of rated power, the hot channel factors identified in Specification 3 .12 shall be deter-mined during each effective full power month of operation using data from limited core maps. If these factors exceed their limits, an evaluation as to the cause of the anomaly shall be made.

Provided at least 38 thimbles with a minimum of 2 thimbles per core quadrant are operable, all required surveillances shall be performed using the full number of operable thimbles. However, for normal monthly surveillance an incore map can be taken using a minimum of 26 thimbles if, in fact, less than 38 thimbles are operable. If between 26 and 38 thimbles are monitored, a minimum of 4 thimbles per core quadrant must be monitored and all full length rods must be OPERABLE and positioned +/-12 steps (,indicated position) of their group step counter demand position.

Peaking Factors

• TS 4.10-3 A thermal criterion in the reactor core design specified that "no fuel melting during any anticipated normal operating condition" should occur.

To meet the above criterion during a thermal overpower of 118% with additional margin for design uncertainties, a steady-state maximum linear power is selected. This then is an upper linear power limit determined by the maximum central temperature of the hot pellet.

The peaking factor is a ratio taken between the maximum allowed linear power density in the reactor to the average value over the whole reactor. It is of course the average value that determines the operating power level. The peaking factor is a constraint which must be met to assure that the peak linear power density does not exceed the maximum allowed value.

During normal reactor operation, measured peaking factors should be sig-nificantly lower than design limits. As core burnup progresses, measured designed peaking factors typically decrease. A determination of these peaking factors during each effective full power month of operation using incore maps with either at least 38 thimbles (including a minimum of 2 thimbles per core quadrant) or, if less than 38 thimbles are accessible, at least 26 thimbles (including a minimum of 4 thimbles per core quadrant) is adequate to ensure that core reactivity changes with burnup have not significantly altered peaking factors in an adverse direction.

• e ATTACHMENT 2 SAFETY EVALUATION

• SAFETY EVALUATION The Surry Technical Specifications currently require a full core map to consist of a minimum of 75% of the monitored detector thimbles (38 thimbles).

However, if a full core map is necessary, and a. malfunction of the flux mapping system hardware results in a condition where less than 38 thimbles are accessible, valid hot channel *peaking factor measurements can still be made.

Based on a study of the effects of reduced number of thimb 1es on hot channel peaking factor measurements 1, uncertainty values, which must be applied in addition to the current measurement and manufacturing uncertainty values, were derived. They are an additional 1% for F~H' and 2% for FQ.

These uncertainty values are valid if there is a minimum of 26 monitored thimbles with at least four thimbles per core quadrant (two thimbles per core quadrant are required for a minimum of 38 thimbles). Furthermore, only monthly hot channel factor measurements will be performed using less than 38 thimbles (each monthly surveillance interval is 31 effective full power days).

If less than 26 detector thimbles are accessible, the incore detection system will not be used for hot channel peaking factor (FQ and F:H) verification.

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This proposed change does not pose an unrevi ewed safety question or a significant hazard consideration. The probability of occurrence or the consequences of *a malfunction of equipment important to safety and previously evaluated in the UFSAR is not increased, and the possibility of a different type of accident or malfunction than was previously evaluated in the UFSAR has not been created because this proposed change only modifies a surveillance hardware requirement without reducing the capability of the hardware to 1 . .

A .Study of the Effects of a Reduced Number of Thimbles On the Results of Incore Flux Map Analysis, VEP-NOS-8, D. L. Reid, October, 1983. (Attached)

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its intended function. The margin of safety as described in the Bases section of any part of the Technical Specifications is not reduced since the addi-tional uncertainty values which will be applied to the measured hot channel peaking factors yield a result that is equivalent to or conservative with respect to flux map analyses results obtained in accordance with the require-ments of the current Technical Specifications.

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• ATTACHMENT 3 VEP NOS - 8