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l ABNORMAL OCCURRENCE REPORT j
Report No.: EFAO-749W, Amendment 1 i
Beport Date: W rch 21, 1974 i
Occurrence Date: February 12, 1974 Facility: Browns' Ferry Nuclear Plant unit 1 3
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Designation of Anperent Cause of Occurrence i
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j On Jaumany 16, 1974, relief valve PCV l-kl was disassembled and inspected with the manufacturer's representative present to detemine the cause of the valve's failure to reclose. The second-stage valve piston nut keeper i
was found to have slipped over the nut-keeper retaining wire because of excessive clearance between the second-stage valve stem and the nut keeper.
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(See sketch 1 for locking arrangement.) This allowed the nut keeper to i
become vedged between the air operator valve stem and the second-stage j
valve stem holding the second-stage disc off of its seat. This prevented t
i the main disc from reclosing since pressure equalization across the main j
piston is necessary for main disc reclosure.
Corrective Action j
To prevent the possibility of a similar recurrence, the decision was made j
to shut down unit 1 on February 28, 1974, and modify all valves.
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A modification to the second-stage piston nut-keeper arrangement was made to all unit 1 Target Rock safety / relief valves to prevent recurrence of I
j this event. The modification consisted of eliminating the second-stage piston nut keeper and retaining vire. The new nut-keeper arrangement consists of inserting a 308 stainless-steel pin into a 1/16-inch hole ' drilled into l
the nut second-stage disc stem thread interface.
(See sketch 2 for new locking j
arrangement.) The pin was secured by peening the edge of the hole on the nut.
The second-stage disc was leak-tested subsequent to the modification. A l
similar modification vill be made to the units 2 and 3 Target acek safety /
j relief valves to prevent the occurrence of a similar event on those units.,
The unit was returned to service on March 8, 197L. Thesafety/reliefvalves vere functionally tested at 250 and 920 psig. The test results were satis-factory.
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INFORMAT10'] 0FFICE WA$HINGTON
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New Sprenkle Building Woodward Buildi..,
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20444 AUTHORITY mone sarciai we. code sisi mene 3424s27
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Contact:
H. J. Green, Browns Ferry Nuclear Plant, Athens, Alabama For in: mediate release The Tennessee Valley Authority has temporarily discontinued test operation of Unit 1 at Browns Ferry Nuclear Plant near Athens, Alabama, to permit inspection of small valves used on sampling lines and elsewhere on the primary cooling system.
TVA said the one-inch stainless steel valves have been operating The normally, but may have been purchased under incorrect specifications.
oT y*1fternoon after it was discovered that the pressure unit was shut down rating of the valves is somewhat less than the rating specified for them.
Valves of this type are designed to withetand higher pressures than their nominal rating would indicate, TVA said, and it is likely that these valves were operating well within their capability. However, temperature measurements, stress analysis, and metallurgical investigations are under way to determine the capability of these valves.
No hazard to public safety was involved,.TVA said, since plant safety systems would have operated automatically if any of these valves had failed.
If studies show the valves are fully adequate for their purpose the unit probably will resume operation within a few days, TVA said. However, a longer unit outage may be necessary if the valves must be replaced.
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INF0ntdATION OFFICE WASHING *.
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Knonvisie Tennesse. 37902 Washington D. C. 20444 T
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Contact:
H. J. Green Browns Ferry Nuclear Plant, Athens, Alabama (205) 729-6202 For immediate release Unit 1 at Browns Ferry Nuclear Power Plant remains shut down, the Tennessee Valley Authority said today, as analysis continues on the adequacy of the one-inch stainless steel valves for meeting pressures to which they are subjected in the primary cooling system.
TVA al.so is replacing a large relief valve which malfunctioned during recent operations. This type of valve vents steam from the reactor to a vessel that absorbs heat and pressure.
The nuclear generating unit had automatically shut itself down on Sunday and again on Monday. On both occasions, those relief valves which should open did so to relieve primary system pressure. During each of the shutdowns, however, one of the relief valves was slow to reclose and this caused a faster than normal drop in the primary system pressure.
No hazard to public safety was involved. TVA said, and the relief valve problem is not considered serious. The valve being replaced will be inspected to determine the cause of its malfunction.
Test operation of Unit I will begin again when the relief valve has been replaced and the small stainless steel valves prove adequate or are -
i replaced. TVA said.
The plant is located near Athens Alabama.
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