ML20128D888
| ML20128D888 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 05/30/1972 |
| From: | James Shea US ATOMIC ENERGY COMMISSION (AEC) |
| To: | Ziemann D US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| NUDOCS 9212070413 | |
| Download: ML20128D888 (4) | |
Text
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UNITED STATES
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ATOMIC ENERGY COMMISSION
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MAY 3 01972 Files (Docket No. 50-263)
THRU:
emann, Chief, ORB #2, DRL mW lhW STEAM FLOW NALS - ADEQUACY OF STEAM FLOW RESTRICTORS AT MONTICELLO (NORTHERN STATES POWER COMPANY)
Northern States Power Company, NSPCo, has reported (references 1 - 7) steam flow measurements at the four steam line flow limiting Venturi nozzles significantly lower than expected.
'Ihe measured dif f erential pressure was 5 psi compared with the expected value of 12.5 psid at steam flows corresponding to 50% power.
Instead of a 122 paid high steam flow sensor trip to initiate primary containment isolation in accordance with the Technical Specifications limit, it was necessary to reduce the flow pressure differential set point to 40 psid.
Shortly after reaching 100% steam flow for the first time,;a main steam line high flow trip occurred resulting in main steam isolation and reactor scram.
According to NSPCo, the trip was due to hydraulic noise in the differential pressure signals.
Following this incident, snubbers were installed in the sensing lines to dampen the noise, and set points of the high flow sensors were readjusted to 43 psid maintaining the trip below the 140% of rated flow limit, but not so low that uninten-tional scrams are likely.
Evaluation The flow restrictors in each of the four steam lines from the reactot vessel perform two safety functions:
1.
limit steam flow f rom the reactor vessel to 200% of rated flow in the event of a main steam line break outside of primary containment.
2.
provide a differential pressure signal to initiate main steam isolation in accordance with the lechnical Specifi-cation limit :G.140% of rated steam flow.
Information provided by NSPCo (reference 2) confirms that the flow restrictors were properly designed and were properly installed to ful-fill the steam flow limiting function.
It also confirms that the changes to the dif ferential pressure vs steam flow calibration and the consequent adjustment of pressure dif ferential trip setting provided reliable initiation of main steam line isolation.
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"i'les MAY 3 01972 Tests perf ormed by the General Electric Company (references 3 and 5) confirmed that the low pressure drop across the main steam line venturi elements, in contrast to design characteristics, was due to a leakage path to the throat tap. More specifically, it was determined by tests carried out in air at ambient temperatures and pressures that the cause of lower than expected steam flow pressure drops was due to leakage into the throat pressure sensing piezometer ring from the high pressure regions surrounding the ring.
Leakage occurred through the eight 1/4-inch holes that form the sensing elements of the piezometer ring chamber.
Figures C and G of reference 5 show the leakage path that resulted in higher than normal throat pressures and the consequent lower than normal differential pressures that were observed prior to the modification.
The solution to the low indicated inlet to throat differential pressure problem was to bypass the piezometer zing. A 1-inch tube, that required drilling a 1-inch hole radially through the original 1/4-inch pressure sensing hole, was inserted with the end flush with the venturi throat.
Tests showed that the dif ferential pressure measurements with the new 1-inch tube bypassing the piezometer ring were in accordance with the original predictions, and that the ef fects of the 90* elbow upstream of the venturi, circumferential location of the pressure taps with respect to the plane of the elbow, precise positioning of the end
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of the tube at the venturi throat, and the jet action due to leakage from the 1/4-inch piezometer ring taps are negligible.
Each of the four venturis was modified (reference 3) by boring a 1-inch hole f rom the point at which the original instrument pressure tap entered the steam line into the nozzle throat and inserting a tight fitting sleeve. The modifications were reviewed and approved by DRL representatives at a meeting on November 16, 1971 (references 3 and 9).
NSPCo has presented the results (reference 9) of plant steam flow differential pressure measurements which show that the steam flow vs venturi pressure differential measurements are now in accord with design expectations.
Conclusion Based on information provided by NSPCo, we have concluded that:
1.
The venturis will limit steam flow to 200% of normal following
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a main steam line break outside the primary containment because it has been verified radiographically that the venturi throat diameter is correct.
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Fi'les MAY 3 01972 2,
The leakage at the piezometer ring which resulted in low differential pressures across the venturi throat did not violate the safety requirement that steam isolation valve closure be initiated before attaining it'% steam flow because the trip set points were lowered to be consistent with the plant steam flow calibration.
3.
Leakage at the venturi throat pressure tap has been eliminated by inserting a 1-inch tube into the venturi throat.
Cali-brations verify that pressure differential across the venturi throats of all four flow restrictors are within design expectations. Performance with the modified throat pressure tap connections is acceptable.
4.
The modification to the venturi throat pressure tap does 4
not represent an unreviewed safety problem and the health and saf ety of the public was not af fected by the low pres-sure signals prior to the fix or the modification to achieve the design differential pressure.
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c James J.
e a Operating Reactors Branch #2 l
Directorate of Licensing cc: DJSkovholt, DL TJCarter, DL DLZiemann, DL JJShea, DL RMDiggs, DL WHaass, DL i
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4 REFERENCES 1.
"Inoperability of Main Steam Line High Flow Sensors", NSPCo letter to DRL dated May 12, 1971.
2.
" Main Steam Flow Res trictor Anomaly", NSPCo letter to DRL dated June 2, 1971.
3.
" Modifications to Main Steam Flow Element Throat Pressure Taps",
NSPCo letter to DRL dated December 28, 1971.
4.
" Main Steam Line Flow Element Response", NSPCo letter to DRL dated January 10, 1972.
a 5.
" Modifications to Main Steam Flow Element Throat Pressure Taps",
ESPCo letter dated February 2,1972, transmitting GE Steam Line Flow Limiting Venturi Test Program Results (November 1971).
6.
" Follow-Up Report on Main S team Isolation Valve Problem", NSPCo letter to DRL dated February 18, 1972.
7.
" Performance Report for Main Steam Line Flow Elements", NSPCo
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letter to DRL dated April 25, 1972.
8.
DRL letter to NSPCo dated July 26, 1971, expressing concern that the steam flow restriction following a complete rupture of the steam line might be less than design, and that the high flow set point may not be valid, and directing that tests be performed as soon as possible to determine the cause of the low flow signals.
9.
Meeting Notice to discuss the flow restrictor anomaly with NSPCo on November 16, 1971.
10.
Compliance Inquiry Memorandum No. 263/71-10 dated May 6, 1971,
" Inoperable High Steam Flow Sensors".
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