ML19326B353

Final Deficiency Rept Re Makeup Pump Recirculation Orifice Leak.Caused by Orifice Wall Erosion Due to High Exit Flow Velocity.Orifice Design Mod Will Reduce Exit Velocity & Relocate Last Stage Exit

ML19326B353
Person / Time
Site:	Arkansas Nuclear
Issue date:	04/15/1974
From:	ARKANSAS POWER & LIGHT CO.
To:
Shared Package
ML19326B340	List: ML19326B351 ML19326B353 ML19326B357
References
NUDOCS 8004150730
Download: ML19326B353 (2)

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REPORT ON Tile MAKEUP PUMP RECIRCULATION ORIFICE LEAK This report is being made in accordance with 10 CFR 50.55 (e) on signifi-cant deficiencies and relates to the generic investigation made as a result of the erosion of the "B" makeup and purification recirculation orifice on Arkansas Power 6 Light's ANO-Unit 1.

Identification of Incident A pinhole leak in the "B" makeup pump recirculation flow orifice at ANO-1 was first noticed at a location approximately 1 inch below the upper weld where the orifice makes its downstream connection to the 2-inch recircula- c tion line. Examination of the orifice after its removal from the line revealed nearly complete erosion of the last element at the discharge end of the orifice. Because the flow discharge from this last element was near the pipe wall, a portion of the pipe wall had also been eroded to the point o.f generating the pinhole leak observed. This penetration occurred only in the "B" line because of its extended operating time compared to the "A" and "C" lines. The reactor coolant letdown flow orifice is of the same manufacture and was also investigated for possible failures.

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T Analysis of Causes BSN has identified that the leakage resulted from erosion of the orifice wall near the last stage of the orifice. The erosion was caused by high exit flow velocity at the last stage. The last stage exit is located eccentrically in the cylindrical orifice shell and near the wall of the orifice.

Safety Evaluation The recirculation flow orifice is a pressure reducing device between the pump discharge and the makeup tank. The recirculation line is provided to prevent " dead ending" the pump during the low-flow makeup injection required by the core during normal operation. For emergency high-pressure injection, the recirculation line is closed by an automatic valve down-stream of the orifice thereby directing the full makeup pump flow into the core. Communication of the "A" and "C" recirculation lines with the "B" line Icak would have been prevented by a stop-check valve.

The loss of integrity in the "B" recirculation line could, therefore, have resulted in the loss of some core injection flow during normal operation and a significant loss of flow during emergency inj ection. In both modes, however, the remaining "A" and "C" injection lines were each available to deliver their design flow to the core. Only one of the three injection lines operating at design flow is required to provide core protection in the event of a LOCA. liowever, in the event of a singic failure of a diesel

[\mj} generator (thus leaving only one llPI pump with an affected orifice) the design llPI flow could not be maintained.

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Corrective Action 1

The makeup pump recirculation orifice design has been modified to reduce the exit velocity at the last stage from a maximum of 183 fps to 110 fps,

, and to locate the last stage' exit concentrically in the orifice shell. -

B6W is procuring and will replace the existing orifices with those of

the modified design. The reactor coolant letdown flow orifice was exam-

, ined by a UT inspection and showed no erosion. 'Ihe exit velocity from j this orifice was approximately 125 fps in the original design and thus j will not be replaced.

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