ML19262A327
| ML19262A327 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear, Crane  |
| Issue date: | 04/30/1974 |
| From: | John Miller METROPOLITAN EDISON CO. |
| To: | Knuth D US ATOMIC ENERGY COMMISSION (AEC) |
| Shared Package | |
| ML19262A319 | List: |
| References | |
| NUDOCS 7910260653 | |
| Download: ML19262A327 (2) | |
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'OST OFFICE BOX 542 RE ADING. PENNSYLVANI A 19603 TELEPHONE 215 - 929-3601 April 30, lo75 Dr. D. F. Knuth, Director Directorate of Pegulatcry Operations Office of Regulatien United States Atomic Energy Cc= mission Washington, D. C.
205h5
Subject:
Three Mile Island Unit No. 1 Makeup Pump Bypass Orifices Docket No. 50-269 License No. DPR-50
Dear Dr. Knuth:
You vere notified en April ll, 197h of a situatien which could be considered reportable under 10 CFR 50 55(e), " Conditions of Construe.icn Permits".
This relates to the generic investigatien made as a result of the erosion of the "3" makeup and purification recirculation orifice cn Arkansas Power & Light's ANO-Unit 1.
Backcround The makeup pump bypass crifices are multi-stage plate orifices which are used to control the recirculation flev arcund the makeup pump. Recirculation flow is required to maintain minimum flow thrcugh the pump during normal service.
Com=unication between the recirculation lines is prevented by stopcheck valves.
In accident service, the recirculation flov is valved off by ESAE actuated valves. The orifices were designed and manufactured by 31ngnam Pump Co.
Description of Incident A pinhole leak in the "3" makeup pump recirculation flow orifice at ANO-1 was first noticed at a location approximately 1-inch below the upper veld where the crifice makes its devnstream connection to the 2-inch recirculaticn line. Examination of t' e orifice af ter its rencval '. rom the line revealed nearly complete erosion of the last element at the dischar e end of the orifice.
s Because the flow discharge frcm this last element was naar the pipe vall, a portion of the pipe vall had also been eroded to the point of generating the pinhole leak cbserved. This penetration cecurred only in the "3" line because of its extended operating time compared to the "A" and "C" lines.
Analysis of Incident Investigation has identified that the leakage resulted from erosion of the orifice vall near the last stage of the orifice. The erosion was caused by high exit flov velocity at the last stage.
The last stage is located eccentrically in the cylindrical crifice shall and near the 6all of the orifice.
1485 04 491026o 3
Dr.'D. F. Knuth Page 2 April 30, 107h 5
The loss of integrity in the "B" recirculation line could have resulted in the loss of sc=e core injection flow during normal operation and a significant
._ loss of flev during emergency injecticn.
In bcth =cdes, however, the remaining "A" and "C" injection lines were each available to deliver their design flow to the core.
Only one of the three injectica lines operating at design ficv is required to provide core protection in the event of a LCCA. However, in the event of a single failure of a diesel generator (thus leaving enly one HPI pump with an affteted orifice) the design HPI flow could not be maintained.
I= mediate Corrective Action Since the TMI-1 =akeup pumps and orifices are similar to those at ANC-1, B&W performed an ultrasonic investigation of the TMI-l recirculetien crifices on March 29, 197h cc determine if any wall eresion had cccurred.
...e wall thickness measurements were made using 5 MHz pulse-echo " time of flight" technique.
Sensitivity of these measurements is estimated 5 =113.
Instrument calibratien was obtained frc= an identical, unused orifice and frc= SS calibration discs of known thickness. The entire external surface of the orifice assemblies was examined.
No wall thinning was found in the TMI-l recirculation orifices.
Furthermore, having censidered the operating-heurs-to-failure en the ANO-1 orifices, and the fact that the TMI-l orifices have withstood equivalent hours of operation without any sign of erosion, we have established a regular "T inspecticn interval for the TMI-l orifices until new orifices of modified design are installed. The inspection interval vill be once every two (2) weeks during system operation, which is about half the total-hours-to-failure of the ANO-1 orifices.
Future Corrective Action A new =akeup pump recirculation orifice has been designed to reduce the exit velocity at the last stage and to locate the last stage exit concentrically in the orifice shell. labcock and Wilcox Co. is procuring, and will replace, the existing orifices at IMI-l with those of the new design.
Very truly yours,,
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G. Miller Vice President CC:
Mr. J. P. O'Reilly 1485 041