ML19308D777

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Decay Heat Pump
ML19308D777
Person / Time
Site: Crystal River Duke Energy icon.png
Issue date: 07/07/1978
From:
FLORIDA POWER CORP.
To:
Shared Package
ML19308D776 List:
References
NUDOCS 8003130822
Download: ML19308D777 (9)
v • d • e


Text

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O CR III Decay Heat Pump Introduction and Summary I

Babcock 6 Wilcox has evaluated the second failure of the 1A Decay Heat Pump (DHP-1A) at Crystal River Unit 3 and the results are presented in this report.

Various investigations of the failure were performed and 1.npeller/ casing interference

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was identified as the most probable cause of the shaf t failures.

'" Corrective actions were taken to increase the clearances between the casing and the impeller, as well as actions: to improve other possible contributing effects.

Test instrumentation has been used to monitor pump performance and to verify proper pump operation.

Our conclusion is that DHP-1A will now operate correctly and reliably.

II.

Operating History Two identical Decay Heat Pumps (DHP-1A and DHP-1B) were installed at CR-3 during plant construction.

These pumps are designed for cwo functions:

A.

To remove decay heat from the core and sensible heat from

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the reactor coolant system during the latter stages of plant cooldown and to maintain reactor coolant temperature s

during refueling.

~B.

To inject borated water into the reactor vessel for long-term emergency cooling in the event of a loss-of-coolant accident.

On December 2, 1976 the initial shaft of DHP-II failed.

The plant l

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'wasLin initial; startup testing and DHP-1A had been running for approximately nine days in a recirculation mode & 80gpm).

Examinations showed that the' shaft _ cracked, starting in the corner of the. impeller 8003130 D M

. keyway.and propogating by cyclic fatigue stresses.

A new shaft was manufactured and installed, operating restrictions were imposed to prevent extended use of the pump in the recirculation mode, and the pump was returned to service.

This failure was reported in References 1 and 2.

The second shaft on DHP-1A failed, at the care point as the previous failure, in April, 1978 while the pump was operating in the decay heat removal mode.

The second shaft had operated for 1356 hours0.0157 days <br />0.377 hours <br />0.00224 weeks <br />5.15958e-4 months <br /> in this mode prior to its failure.

There have been no problems or failures of DHP-1B.

III. Areas Examined A.

Visual - A visual inspection of the failed pump showed that the shaft had broken near the end of the keyway on the inboard side of the impeller.

There are score marks near the outer edge of the suction side of the pump impeller with related score marks in the upper half of the pump casing.

In addition the shaft sleeve and the throttle bushing were scored, apparently as a result of the failure.

B.

Dimensional - Readings taken upon disassembly of the pump revealed that the pump casing casting was skewed relative to the norma?. centerline of the shaft (Figure 16 2).

Minimum clearances, which occurred in the vicinity of the score marks on the casing, were less than acceptable.

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C.

Operational - Plant records were reviewed and there was no indication of any low flow transients or operational problems with pump DHP-1A.

D.

Metallurgical - The shaft was manufactured from SA 564, Grade 630-17-4 pH material, heat treated to the H 1150 condition.

A

,_s metallurgical examination showed a transverse orientation of the f at -fatigue crack surface, indicating that bending stresses are

3 predominant in this failure mode.

The shaft contained no apparent me'allurgical' flaws that could have initiated a fatigue 1

t CraCE..

E.

Stress Analysis - Following the initial shaft failure a stress analysis (Reference 3) was performed, and confirmed that normal operating stresses are well below the design limits.

-IV.

Most Probable Cause All of the items listed above were considered in arriving at

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the most probable cause of the pump shaft failure.

It was duter-mined that the interference-of the casing with the rotating impeller i

resulted in cyclic bending stresses which, combined with the normal operating stresses on the shaft, were sufficient to cause the failure.

4 V.

Corrective Actions The following actions have been taken to eliminate not only the most probable cause but also possible contributing causes.

A.

DHP-1A casing was-blended out to remove the upset metal, thereby increasing-the original clearance (See Figure 3).

B.'

1/8" was machined from the backof the impeller hub, shifting the Il a

impeller up the shaft anc increasing its clearance to the

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casing.. (See Figure 4) l C.

The inpeller was dynamically balanced, in two planes,- to-eliminate any possible imbalance in the final i

' rotating assembly.

D.

A "Belleville" washer was added under the impeller nut to relieve strerses from the differential thermal expansion of the impeller and the shaft.

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4 VI. Operational Surveillance To. insure the adequacy of the repair, a pump signature will be obtained and analysed for DHP-1A upon its return to service.

The data will be obtained by a series of accelerometers mounted on the pump casing.

A pump signature of DHP-1B has aircady been obtained to use for a baseline comparison.

Data will be recorded

,as soon as the pump is restored to operation with more data to be obtained during the firs, scheduled plant cooldowr.

VII. Conclusion DHP-1A has been throughly examined and all observable problems have been corrected; pump performance will be monitored to confirm the adequacy of the repair.

It is our conclusion that DHP 1A will operate satisfactorily in the future.

References 1.

Florida Power Corp. (Docket NO. 50-302-License NO. DPR.-72)

Licensee Event Report 102 (Sept. 9, 1977 -

3-0-3-a-1-CS-77-192) W. P. Stewart (Fla. Power Corp.)

to J.P. O'Reilly (NRC).

j 2.

Florida Power Corp.,(Docket No. 50-302) Report (3 June, 1977 3-0-3-a-1-CS-77-97) W. P. Stewart (Fla. Power Corp.) to i

Norman C. Moseler (NRC).

'l 3.

Metropolitan Edison Co. (Docket No. 50-289 - License No. DPR-50) TMI-1 Response - GQL 1496 - November 2, 1977 J. G. Herbein (Met. Ed.) to R.W. Reid (NRC).

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