ML20086Q472

SR-S2-75-02:during Refueling,Fuel Assembly N-20 Incurred Damage to Two of Four Holddown Springs on Top Nozzle Assembly.Fuel Assembly Acceptable for Power Operation in Cycle 2

ML20086Q472
Person / Time
Site:	Surry
Issue date:	06/02/1975
From:	VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:
Shared Package
ML20086Q378	List: ML20086Q392 ML20086Q415 ML20086Q472
References
SR-S2-75-02, SR-S2-75-2, NUDOCS 8402270562
Download: ML20086Q472 (6)

Text

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rm f, k.s b l SPECIAL REPORT SR-S2-75-02 REPORT ON FUEL ASSDIBLY N-20 DOCKET No. 50-281 LICENSE NO. DPR-37 JUNE 2, 1975 SURRY POWER STATION VIRGINIA ELECTRIC AND POWER COMPANY e

8402270562 750630 PDR ADOCK 05000281 S PDR I

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I. INTRODUCTION During the refueling operations for Cycle 2 of Surry Unit No. 2, fuel assembly N-20 incurred damage to two of the four hold-down springs on the top nozzle assembly.

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II.

SUMMARY

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When fuel assembly L-16 was being removed from core location B-8, l l

two adjacent locking fingers on the fuel handling crane failed to engage the top nozzle, so that the fuel assembly was only supported by the re-maining two fingers. When the assembly was pulled clear of the core, it was free to pivot about the axis formed by the two engaged fingers.

The axis of rotation was perpendicular to the coolant flow maintained through the vessel by the residual heat removal system. This coolant flow caused the lower end of the fuel assembly to drift, and the misalignment of the fuel assembly with the direction of lift cauced it to bind in the crane mast. This binding caused an increase in the nominal load normally required to withdraw a fuel assembly from the core, prompting the crane operator tc cease the fuel assembly withdrawal before an overload condition was reached.

Unaware of the cause of the increased load, the crane operator lowered fuel assembly L-16, the bottom of which had drifted over fuel assembly N-20 which was in core location C-8. The bottom nozzle assembly of L-16 contacted and came partially to rest on the top nozzle assembly of N-20.

Upon contact, the crane operator noted a decrease in the load and then stopped the crane. Two of the bottaa pedestal feet of L-16 were then partially resting on the hold-down springs on faces two and four of fuel assembly N-20.

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both fuel assembly L-16 and N-20 were removed to the spent fuel  !

i pit and subjected to a preliminary binocular visual observation and a

[ subsequent visual' examination using remote TV inspection equipment and

• l videotape. The inspections established that the hold-down springs on i faces two and four of assembly N-20 had been plastica 11y deformed such

) that the springs on faces two and four had a permanent set of 1.0 and i

1.1 inches, respectively, relative to their original free height. The hold-l down springs on faces one and three of N-20 showed no evidence of contact.

No additional damage other than the hold-down springs on N-20 was observed i

on either N-20 or L-16 which is not scheduled for operation during Cycle 2.

l Vepco requested Westinghouse, the fuel assembly designer, to evaluate 1

the extent of the damage and the possibility of adverse consequences of con-i tinued operation of fuel assembly N-20 during Cycle 2 as schedule. An in-dependent consultant was also retained to review the results of the West-l t

inghouse evaluation.

The Westinghouse evaluation consisted of an analysis of the minimum expected hold-down capability of the undamaged springs coupled with the i maximum expected lif t forces during normal cold startup, normal hot operation, and postulated abnormal conditions of reactor coolant pump overspeed. In-l -

I puts to the calculations included uncertainties in coolant'and component temperature, component dimensions and tolerance stack-ups, thermal expansion coefficients, fuel assembly growth during Cycle 1 irradiation, spring material properties, and core pressure drop. As-built information was_used where it l

was available and-pertinent. Quality control / quality assurance records for fuel assembly N-20 were reviewed to ensure that there were no deviations accepted during fabrication which would influence the results of the analysis.

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a __A Upon completion of the Westinghouse analysis, personnel from Vepc6 "

and their consultant met with Westinghouse to review and evaluate tJs results and conclusions. The results of the analyses are discussed below.

The analysis shows that the minimum expected hold-down capability of the springs, when computed at normal power operation and at a.95 per cent x 95 per cent confidence level, exceeds the maximum expected lift forces, (also computed at a 95 per cent x 95 per cent confidence level), thus demonstrating that the remaining hold-down capability is adequate to prevent the assembly from lifting off the lowe. ore plate during normal power operation. The probability of not lifting the assembly is thus gredter than 95 per cent at the 95 per cent confidence level.

The analysis of the heat-up conditions shows to a greater than 95 'peo , ' 's cent x 95 per cent confidence level, that lift forces will not exceed the ~

u hold-down forces with all three reactor coolant pumps operating for reactor coolant temperatures greater than 360 degrees F. In addition, also to greater than 95 per cent x 95 per cent confidence level, a postulated s reactor coolant pump overspeed transient condition of 110 per cdnt or lese during normal power operation will not lift the assembly to the extent that further plastic deformation of the hold-down springs will result. After experiencing a transient in excess of 110 per cent pump overspeed, the two undamaged springs may no longer be capable of providin3 enough hold-down force to keep N-20 on the lower core plate during normal pober operation.

Ill. CONCLUSION

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Fuel assembly N-20 is acceptable for power operation in Cycle 2 of -s Surry Unit No. 2 as a result of the following:

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1. Ilold-down forces are adequate to prevent lift-off of the fuel assembly during normal power operation provided a pump overspeed in excess of 110 per cent has not occurred.

2. Operating procedures will be established to rack out the breaker of a reactor coolant pump in order to prohibit operating with more than two reactor coolant pumps unless the reactor coolant temperature is 360 degrees F or greater.

This will preclude fuel assembly lift-off during startup.

3. A design feature of the Surry Power Station prohibits the reactor coolant pumps from running at overspeed conditions.

This feature is an electrical interlock which disconnects the power to the, reactor coolant pumps from its own generator whenever the generator is not connected to the Vepco trans-mission system. (Where it ir synchronized to 60 liz by the

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, -, In order to prove tha.t the interlock operates properly when needed, the speed of the reactor pumps and/or reactor coolant flow in all three loops will be recorded continuously during Cycle 2 operation. If the interlock fails to function properly and an overspeed in excess of 110 per cent of synchronous speqd occurs, the reactor will be shutdown and further evaluation

' "' condbeted before continuing operation, s -

s 1 The above conclusions have been reviewed by both Westinghouse and s, Vepco personrel (including Vepco's independent consultant) and are con-curred with by all parties. The results and recommendations have also been s

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i reviewed and concurred with by the Station and System Nucicar Safety and Operating Cocunittees. The refueling operation is proceeding, and the reactor will be returned to service on or about June 9, 1975, with the conditions described in items I through 3 above imposed.

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