ML20056H644
| ML20056H644 | |
| Person / Time | |
|---|---|
| Site: | Vogtle  |
| Issue date: | 08/26/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20056H640 | List: |
| References | |
| NUDOCS 9309100247 | |
| Download: ML20056H644 (3) | |
Text
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UNITED STATES j
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION 1
CONCERNING A TECHNICAL SPECIFICATION CHANGE FOR THE SURVEILLAN2 0F THE EMERGENCY CORE COOLING SYSTEM GEORGIA POWER COMPANY. ET AL.
i V0GTLE ELECTRIC GENERATING PLANT. UNITS 1 AND 2 f
DOCKET NOS. 50-424 AND 50-425 i
INTRODUCTION i
By [[letter::ELV-03832, Application for Amends to Licenses NPF-68 & NPF-81,revising TS SR 4.5.2.b by Changing Frequency for Verifying That ECCS Piping Is Full of Water from Once Per 31 Days to Once Per 6 Months|letter dated September 17, 1992]], Georgia Power Company (GPC or the licensee) requested an amendment to change the Technical. Specifications (TS)
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for the Vogtle Electric Generating Plant, Units. I and 2.
The change would revise TS 4.5.2.b to relax surveillance requirements on the Emergency Core l
Cooling System (ECCS).
The new specification would require the licensee to check ECCS piping for the accumulation of gas by vent %g it every 6 months.
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The current specification requires the ECCS piping to be vented every 31 days.
l BACKGROUND Venting the ECCS piping is performed to remove any gas accumulated in the
-f system. Hydrogen is present in the water because it is used as the cover gas i
for the volume control tank (VCT). The water flowing through the VCT becomes l
l saturated with hydrogen.
The gas will readily come out of solution if the liquid undergoes a decrease in pressure.
Nitrogen and air can also accumulate l
in ECCS piping. Gas in sufficient volumes can damage the ECCS or degrade the performance of the ECCS in mitigating an accident. Gas in the ECCS can lead to waterhammer that can damage piping. The gas can directly bind the ECCS l
pumps.
Pump damage can come quickly in different mechanisms primarily in the high head or intermediate head pumps, because of the very small clearances and very high revolution rates at which the impellers normally run.
Complete and common cause failure of the ECCS pumps, as a result of gas accumulation, is a l
potential concern.
l The licensee states in the amendment request that the monthly surveillance is l
excessive and the plant workers are being exposed to radiation needlessly. Of the 36 locations for venting gas from the ECCS, 13 are in high radiation areas and the rest are in radiation areas. Reducing the number of times the labor intensive surveillance must be done would reduce the amount of radiation dose to workers.
In 1991, the personnel responsible for the ECCS surveillance l
received a collective radiation exposure of 0.5 person-rem. - This value is small when compared to the average collective radiation exposure.at nuclear-l power plants in the United States in 1991; 256 person-rem per plant.
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4 9 In its letter of September 17, 1992, the licensee contends that the monthly surveillance is unnecessary because no gas accumulation has been found in the ECCS system during previous surveillances. The licensee's review of surveillance records and survey of personnel responsible for the surveillance have indicated that the ECCS has always been full of water.
The current procedural requirements at Vogtle require the licensee to fill and vent the entire ECCS piping after any maintenance on the associated system, assuring the system, while operating, is always full of water. The licensee has reviewed the ECCS design and contends that the design of the ECCS system has no high points where gas can accumulate and that the aligned water source is above the associated ECCS piping, allowing no gas to accumulate.
EVALUATION Based on NRC inspection reports since the September 17, 1992, license amendment request, the licen;ee has discovered a small amount of gas in the ECCS piping. On October 2, 1992, 20 cubic centimeters of gas was discovered in the ECCS piping at Vogtle Unit 2 during a regular monthly surveillance.
This small amount of gas would probably not have damaged any of the ECCS pumps and, therefore, had little or no safety significance. The volume of gas, however, was present and its source is currently unproven. Again, on April 15, 1993, a small volume of gas was vented dt. "?g surveillance from the return line to the refueling water storage tank (RWST).
It was concluded by the inspector that there would be no impact on the operability of the ECCS from such a small volume of air. The monthly surveillance is intended to detect and eliminate such volumes of gas.
Operating experience of the nuclear industry shows that gases can build up in a piping system from many sources. Gases can also accumulate in many different places.
In other Westinghouse designed nuclear power plants, volumes of gas have been found in the suction of the high head safety injection pumps, in piping from the refueling water storage tank, and in the crossover line between the residual heat removal pumps and the high head pumps. Very large quantities of gases have been discovered in the ECCS piping of many plants. Some of these occurrences are discussed in Information Notice 88-23 and its supplements.
North Anna, Surry, and Sequoyah have all found large quantities of gases in their ECCS systems, even after reviewing the designs of their ECCS piping and determining that they had a sufficient program for detecting and venting the accumulated gases. North Anna determined that they could not produce hydrogen if the system was fully vented before operation and subsequently did produce large volumes of hydrogen.
Surry had reviewed all sources of hydrogen and initiated a gas mitigation plan.
It was subsequently discovered that they were accumulating hydrogen and other gases from a leak into the ECCS from the primary system. This evidence shows that hydrogen and other gases can accumulate in unexpected places from unexpected sources.
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r CONCLUSION Although reducing the radiation exposure to workers to levels as low as is reasonably achievable is important, it does not justify a relaxation of surveillance requirements for the Engineered Safety Features of the plant.
The ECCS is essential in the mitigation of many accident scenarios, and the l
safety of the plant depends on the system functioning as designed. The staff concludes that monthly venting of ECCS piping as recommended in NUREG-1366,
" Improvements to Technical Specifications Surveillar.ce Requirements,"
l published in December 1992, and required in accordance with the current Standard Technical Specifications should be maintained until the various mechanisms for introducing gases into ECCS lines in nuclear power plants are better understood, and appropriate procedures have been implemented and proven to preclude such occurrences. Accordingly, the proposed changes are deemed premature and, therefore, are denied.
I Date: August 26, 1993 i
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