ML19309A457
| ML19309A457 | |
| Person / Time | |
|---|---|
| Site: | Point Beach  |
| Issue date: | 12/30/1975 |
| From: | Burstein S WISCONSIN PUBLIC SERVICE CORP. |
| To: | |
| Shared Package | |
| ML19309A455 | List: |
| References | |
| FOIA-80-3 NUDOCS 8003310105 | |
| Download: ML19309A457 (9) | |
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WISCOREln Elactac rosca coueur ecember 30, 1975
?31 WEST MICHIGAN,f/.lLWAUKEE, WISCONSIN 53201 [.p ,
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14r. Benard C. Rusche, Cirector C s.!; , 'l ,-~ .,
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Office of fluclear Reactor Regulation /
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U. S. !!uclear Regulatory Conmission O < ' , '
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Dear fir. Rusche:
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LICE:lSEE EVEllT REPORT T10. 50-266/75-18 $' (
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FAILED FUEL Iti ASSE;;BLY D-03, POSITION K-6, CYCLE 3 .d .19.
POINT BEACH ?mCLEAR PLAflT This letter is to report in detail the discovery and results of sub-sequent investigation of a failed fuel assembly during 17, the1975.
Cycle 3-4 An refuel-initial ing of Unit 1 at Point Beach I!uclear Plant on Decemberto comply with the ten-day report of this event '..Ts filed December 23, 1975 rt. porting requirenants of the Point Beach Technical Specification Section 15.6.6.A.2.
On Decerber 17, 1975, during the refueling of Unit 1, the Core Load-ing Supervisor noted sorething protruding from the side of fuel assembly D-03
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as it was being lowered into the fuel assembly upender. Fuel novement was stcpped innediately while the R.: actor Engineering Group representativeThe moved pre-the underwater TV carara in position to observe the affected area. The liminary inspection indicated that a section of fuel rod was missing.
Reactor Engineer was notified of the finding after thich the subject assembly was coved to the fuel inspection periscope in the spent fuel pit where a more detailed exanination was made.
VISUAL EXA'4It!ATION OF FUEL ASSE"7LY C 03, EAST FACE, 12-17-75 The results of detailed visual examination of the fuel assembly are sunnarized below, starting with the top of the fuel assembly.
Top of the Fuel Asserbly Rods 12 and 13 were observed
- .o be slightly below other rods (approxi-aately 1/16") ar.d esduced spacing betuean rods 12 and 13 was evident.
Top of Grid 1 Rods 12 and 13 shcwed indications of wear adjacent to grid flow vanes.
The vane at rod 13 uas partially worn away.
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!!r. Benard C. Itusche December 30, 1975
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At Grid 1 Rust narks were evident on the grid and rods 12 and 13. Clad of rods 12 and 13 worn away to the extent that fuel springs were visible be-hind slots in the grid.
Below Grid 1 A fuel fra5nent was observed to be lodged between rods 13 and 14.
Gouge marks could be seen in rods 12 and 13 adjacent to grid tab and a hole was visible in rod 12 at grid tab. A tab was touching rods.12 and 13 and a narrow gap exists between rods 12 and 13.
l Between Grids 1 and 2 1
f!o apparent damage to rods was noticeable at,this location, but the spacing between rods 12 and 13 was reduced'.
Top of Grid 2 4
> The cladding of rod 12 was separated near vane with no cladding or fuel behind the upper part of grid. Rod i3 has a cut nark near upper edge of the clad and a hole on the left side. A fuel fragnent was visible between rods 13 and 14. The vane near rod 12 was bent in slightly.
At Grid 2 The spring for rod 12 was missing. Fuel fragnents and open clad
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on rod 13 was visible at this location.
Below Grid 2 A two-inch sect on of clad for rod 12 from behind the grid had slipped down and pinr.ed between the grid and the lower section of rod 12.
This rod has several holes natching the contact points of the grid springs.
"o fuel was visible.
Rod 13 has a hole under the grid tab with an outline similar to the tab.
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"atucen Grids 2 and 3 4 About a four-inch open split in rod 13 with no fuel visible except at the top and of the split.
- 'Tc6 of Grid 3 Open cut in cladding on rod 12 adjacent to -vane. The vane is severely worn down cn the side near rod 12.
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Iir. Benard C. Rusche December 30. 1975
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Complete separation of rod 13 at point just above grid with the section of rod between grids 2 and 3 resting on prid 3 between rod 12 and the second rod in from 12.
The section of clad for rod 13 that was behind the grid springs made contact. Also a lot of gouge marks are present on that section of clad. No fuel fragments are visible..
A fuel fragnent was visible on top of the grid near rod 8.
At Grid 3
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Missing grid spring for rod 13.
No clad for rod 13 visible behind grid.
Below Grid 3 Holes visible in rod 11 near grid tab. Large hole in rod 12 with large fuel fragment sticking out.
Section of cladding for rod 13 missing frcm behind grid. Fresh scrape narks on top edge of the existing rod 13 cladding. No fuel visible at top edge of this section of rod 13.
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! Between Grids 3 and 4 Rub nark on rod 13.
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t Top of Grid 4 Uide gap between rods 12 and 13.
At Grid 4 '
Center of grid spring for rod 12 separated. Many small fuel frag-r.2nts visible behind grid at rods 12 and 13.
L' ear in grid spring at rod 13.
Felow Grid 4 ,
Gouge narks and hole in rod 12. Fuel visible in hole. Rod 13 miss-ing completely from grid to midway between grids 4 and 5. Gouge marks on se i rod in from rod 13.
,Catween Grids 4 and 5 1
Top 11 inches of rod 13 missing. End of rod 13 bent to a horizon-tal condition about two inches 1cng.
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, December 30, 1975
- Too of Grid 5
!! car in rod 11 adjacent to vane. Rod 12 clad worn through 'at vane with vane worn down also.
Rod 13 has an unusual ragged hole next to but not parallel to the edge of the grid vane. The vane is severely worn and bent.
A piece of clad about one-half inch long is lodged between rod 14 crd the grid.
Eelow Grid 5 .
Rod 12 has a hole with gouge marks around it. A fuel fragment is visible in the hole.
Eelo.t Grid 6 Hole 'in rod 12. Couge marks and hole in rod 13 with fuel showing.
otton Grid and I;czzle Rod 12 alr.ast bottcmed out and resting on bottom. The normal rods are about 3/4 inch off the bottom.
SL""'ARY OF FUEL ASSE",ELY D-03 OPERATlilG HISTORY A brief suntary of the operating history of fuel assembly D-03 is as
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follows:
Fuel Assenbly Identification:
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D-03
?rspressurized low parasitic design with fuel rods off the bottom nozzle.
Fericated at !!estinghouse !!uclear Fuel Division, Cheswick, Pennsylvania.
Da:e received on site: August 22, 1972
- nitial inspection revealed no abnornalities.
Leided into Unit 1, Cycle 2 at core location H-1. Insert was a thiable plug device. Burnup during Cycle 2 was 7,071 M'!D/liTU. ~
Leidad into Unit -1, Cycle 3 at core location Ve6. Insert was a thimble plug davice. Burnup during Cycle 3 was 16,306 !C!D/MTU.
Tc:a1 buraup: 23,377 :".!D/MTU.
'bre as no difficulty in the fuel handling operation during Refueling 2, which irc olved 0-03 and/or adjacent assemblies.
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/ Hr. Benard C. Ruscha December 30, 1975
- DETAILED FUEL HAflDLIliG STEPS LEADING TO DISCOVERY OF OCCURRE!1CE
. Anonalous behavior was first noted during the fuel reloading se-quence when fuel assembly E-06 was being lowered into adjacent core position K-5. The operator noted some deflections of the order of 120 pounds in the load cell '..hich monitors the weight of the fuel assembly and mast. This occurrence is .10t particularly unusual as the assembly slides into its opening between four other assenblies. He also noted that this assembly was slightly higher in elevation (1/8" to 1/4") when it was positioned on the lower core support plate. Uhen it was observed that there was a section of fuel rod approximately 11 inches long missing from D-03, its forner location (K-6) was inspected and nothing abnormal was noted, assembly E-33 was then loaded into core position K-6. Fuel assenbly E-06 was then removed froi, core position K-5 to facilitate an inspection of the core support plate. A portion of fuel rod, approximately 11 inches long, was observed to be lying diagonally across the K-5 location.
Assemblies in J-5 and L-5 (E-29 and G-24, respectively) were removed from the core to give clear access to the broken piece of rod. An air-operated vise grip was utilized to retrieve the rod. Several small piles of debris were also noted and reno'v ed by using an underwater vacuun.
Faces and corners of assemblies which could have been affected either during Cycle 3 operation of D-03 or during the fuel movements were inspected.
This effort included the southwest ccrner of F-07; the southeast corner of E-33; the south face of G-24; the north face of E-29; the west face of F-25; and t!- .:est face of D-34. A small plug-like object was noted on top of the botton nozzle of F-07 and this was removed to the bottom of the refueling cavity where it was vacuumed. On D-34's west face several fuel fragments were observed uedged between the rods and grids. Scratches were also noted
. on D-34 in the area of the gross failure on adjacent fuel assenbly D-03. There was no apparent failure to D-34. D-34 was later renoved permanently from the core per the preplanned refueling procedure.
PR03ABLE CAUSE OF FUEL CLAD FAILURE The nuclear steam supply system vendor, Westinghouse, has stated that a nighly prcbable initiating Tactor for the fuel failure was water im-pingenent through the baffle plate while the failed assembly was in its origi-nal Cycle 2 position H-1. This problem was experienced previously at two for-eign nuclear plants.
Experience at the foreign plants has indicated that flow through the stitch weld joining in the baffle plate, or bolted baffle plates as may be the case at Point Beach, has occurred at locations similar to H-1. This can result in a water inpincrent on the corner or near corner fuel rods of the assembly at this position, thus leading to vibration and fretting wear of one or more fuel rods.
In the Point Coach case, assembly D-03 occupied position H-1 during Cycle 2 before being moved to position Ps-6 for the ensuing Cycle 3. A small
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hole in the affected fuel rods due to Tretting from fuel rod vibration is postu-
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lated to have taken place prior to relocaticn of the assenbly into position K-6.
The Cycle 3 position subjected the assably to.a masurably higher power rat-ing than in its Cycle .2 position. ,
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11r. Benard C. Ruscha December 30, 1975
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The ' initial escalation to power at the beginning of Cycle 3 was at a rate higher than the present operating guidelines and a sharp increase in reactor coolant activity was noted between power levels of 40 to 50%. It is presumed that the fuel rods containing holes in the clad became water-logged during the shutdciin and burst due to steam pressure and/or the lack of suffi-cient time for the Zircaloy to plastically deform during the power escala-tion phase.
SAFETY EVALUATIO;i A!!D CORRECTIVE ACTION Two potential safety concerns have been evaluated: (1) The poten-tial for and consequences of addi t ional fuel failures and '(2) the effect of loose fuel pellets in the coolan; system on the health and safety of plant operating personnel and the public.
The first item requiring a safety evaluat[ ion is the potential for and the consequences of additional fuel failures of a'similar nature. If baffle inpingement at core location H-1 is the problem, a potential problem exists in two additional assenblies. Fuel assembly F-36 had been located in position H-1 during Cycle 3 and has been .ovad to position J-3 for Cycle 4 operation. Iew fuel asscmbly G-07 is presently located in position H-1 for Cycle 4 operation. Both assemblies will be carefully inspected at the end of Cycle 4 oper-ation.
In reviewing the safety aspects of locating assembly F-36 at position
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J-3 during Cycle 4, it was confirmed that the assembly does not contain a con-trol rod in this position; therefore no possibility exists of a danaged fuel rod
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interferrinq with a control rod dropping action. A notable factor mitigating the possibility of cladding damage to this assembly during Cycle 4 is that some months ago Uestinghouse instigated guidelines governing the rate of power escala-tion following a cold shutdown of the reactor. These guidelines will be fol-lowed rigidly during future startups from the cold condition for Unit 1, Cycle 4.
The purpose of centrolled power escalation is to allow fuel rods which might have taken in water via pinhole leaks to expel the water, thus preventing the bursting of cladding due to steam pressure buildup within the fuel rod as appear nay have happened in this instance.
It is considered that careful nonitoring of the reactor coolant activ-ity during reactor startup and later operation will provide the necessary informa-tion for nonitoring the condition of fuel assembly F-36, as well as the other fuel assemblies in the reactor. Reactor coolant system activity monitoring and allowable limits will be continued as prescribed by the Technical Specification.
The second item requiring safety evaluation is that of loose fuel pel-lets in the reactor ccolant system during subsequent plant operation. As de-scribcd previously, all reasonable attccpts to locate and renove loose pellets
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December 30, 1975 lir. Cenard C. Rusche
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from adjacent fuel assemblics and the loner core support plate were performed.
flo known loose pellets. remained in the reactor vessel or on fuel remaining in the core. If there are any pellets or parts of pellets remaining in the reactor vessel they would be small fragments that traveled upward between the adja-cent fuel assenblies during Cycle 3 operation (assembly to assembly nominal clearance is .040 inches) or larger fragrants that dropped. to and through the lower core support plate when fuel was noved .during Cycle 4 reloading. itater-ial on the lower core support plate was removed.
It is not known if pellets below the lower core support plate would stay in place or be lifted by the coolant flow. If the pellets are lifted by the coolant flow, they are small enough to pas's through the botton nozzle but they would be caught by the fuel assenbly bottom grid. This might cause some local flow blockage, but it is tolerable at this axial core location.
If pellets remaining in the system retain' their integrity, there should be no significant reactor coolant activity increase because these pel-lets will not be exposed to core neutron flux and hence cannot generate fission products. Should the pellets disintagrate, it is expected that temporary and ninor increases in reactor coolant activity will be detected. The chemical and volume control system purification dcminerali7.crs and filters will remove par-ticulate and dissolved material from this source. It is not' expected that opera-tion will result in reactor coolant activity levels higher than those experi-cnced during Cycle 3.
Therefore, based on the safety evaluation given above, operation of the Unit 1, Cycle 4 core is not considered to pose a hazard to the health and
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safety of the public. Investigation as to the exact cause of the fuel failure will continue. Present plans are to remove all fuel and the lower core barrel of the Unit 1 reactor vessel during its Cycle 4-5 refueling as part of the in-service inspection program. This will permit a thorough fuel inspection and investigation of baffle plate joints to determine if this is the cause of D-03 failed fuel rods.
Very truly yours,
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Executive Vice President Sol Burstein .
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.- ~COP.E !.0ADitlG Para rR:1
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CORE LOADING PATlERN,
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POINT BEAcil UNIT 1- cycw __ _ ' 4 Tl(N TEST ASSDiBLY
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Insert ID : PD - Pluq Device R - RT,CControl Rod
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270 RS - RCC Partial Length Control Rod 0 29 G 0 G2G A- 7 7 7 12P - 12 Burneble Poison A
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(Revision 1 12/17/75)