ML19319A660
| ML19319A660 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 01/12/1978 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19319A658 | List: |
| References | |
| NUDOCS 7911190647 | |
| Download: ML19319A660 (4) | |
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SAFETY EVALUATI0'1 BY THE OFFICE OF flUCLEAR REACTOR REGULATIC:1 SUPPORTING NP.C REVIE!l 0F FUIL R0D REliOVAL FR0!LASSEICLY lM0 DUKE P0MER COMPA'iY OCO: LEE ;[UCLEAR STATIC:6 (!!ITS 1, 2 A!!D 3 DOCKET tt05. 5 1269, 50-270 A*!D 50-2P.7 Introduction By letter dated October 14, 1977, Duke Pcwer Conpany (the licensee) rem ::si.ed amendments to Facility Operating Licenses CPR-38, DPR-47 and DPR-55, for tv Oconee Nuclear Station, Units 1, 2 and 3 uhich would alloa for the dismit:c-of one fuel rod froa Unit 1 feel assembly 1D40.
licr.rever, as in6ic.v. -
.-l'r, our review indicates that amend:aents to the licenses are not ne r.ssary.
Discussion During the Unit I refueling outage follcuing Cycle 3 operation, fuel ane 11y 1040 sustained damage to a fuel rod during handling in the Spent Fuel %01.
The incident resulted in the upper ten inches of a corner rod in cn outer row of rods being bent outward frca the assembly at about a 450 cngle.
- e radioactive releases were detected.
Because of the rod protrusion, i.t was impossible to set the assembly fully down on the Spent Fuel Pool floor without interference with the storage rack grillage. Also, any attempt to return the rod end to the spacer grid cell could result in breakage.
Thus it was decided that, pending further disposition, the assembly would be temporarily left suspended from the port-irradiation examination (PIE) crane approximately 2-1/2 feet above the bottom of the pool.
With the protrusion of the damag'd rod the assembly cannot be fitted into a e
shipping cask and removed "as is" because of the limited clearances withir.
the cask.
Since the prime consideration is that of maintaining the intecrity of the cladding and minimizing the chance of radioactive release, attem.cir.c
- to regain the original assembly envelope by bending the rod back is not considered to be a viable alternative.
Therefore, the licensee has concirdad that the most reasonable option is the removal of the bent fuel red fro?. th:
assembly within the Spcnt Fuel Paal.
The damaged rod would then be transferred to a shipping cask pending its ultimate dispositicn.
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i Evaluation Pulling fuel rods from fuel assenblies has been performed in several instances at reactor sites.
Although this would be the first time at Oconee, it.has been derconstrated at Surry 1, Zion 1, Paine Yankee end
.St. Liicie, that a fuel or poison rod can be removed safely from a fuel asserrbly at a reactor site under similar circumstances.
In addition, the B&W PIE program has demonstrated that this type of operatica can bc accomplished in a safe manner.
The procedures developed by the licensee are such that the pulling of the bent fuel rod would be virtually the same as pulling cn unbent feel rod.
In addition, the entire operation will be viewed with underaater televi;im cameras.
The cameras will also monitor the pulling force continucusly registered on a meter so that should any problems develop during the operation they will be detected before a safe force is exceeded.
Although the red pulling equipment and the precedurcs have been develced to mininize the risk of damage, a small but finite prchability of aa accident remains.
Potential accidents that could occur durina the operation a.e described below including discussion of precautions the licensee intends to take to prevent them.
All of these accidents iraclve the potential breckage of a single fuel rod which is much 1 css severe than the breakage of 56 fuel rods already analyzed on the FSAR and evaluated by the HRC prior to licensing Unit 1.
Rod Break Due to D_umning - Certain operations will result in objects bcing
_ brought close to or in contact with the upper oart of the fuel rod.
There is the possibility that the strain-hardened region of the bend may be scr.e-what brittle.
However, the region has already shown itself to be capable of withstanding significant strain and impact during the bending and sub-sequent removal from the assembly examination frame.
Thus, minor contact with the upper portion of the rod is not considered to be a problea.
Nevertheless, to assure little or no chance of adverse ccntact, all critical operations (such as shield installation, gripDer installation, tightening and entracticn) will be closely followed through direct obscrvz-tion or TV monitoring.
All operations will be performed uith caution using lightweight tools and safety lines where applicable.
Rod Break During Pulling - The two areas of potential concern durino this operation are:
(a) tensile or bending failure of the rod below the clar.o on the fuel rod due to the pulling. force, and (b) sliopage of the cla=
upward to the bend area, thus impacting stress at the bend.
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3 It is very unlikely that the rod will fail in tension due to the pulling forces. To date, 38 1rradiated rods have been withdrawn at the BCJ Lynchburg P.esearch Center (LRC) without incident (relatively lcw pullirc 4
l forces were sheran to be required).
Outer row corner rods irradicted through two reactor cycles have been pulled, and the maxinum (brenk n y) l force required of any of the four rods was 91 lbs. Tliis was for the case of any assembly with double end grids top and bottom (total of 10 prid:).
Assembly 1040, on the other hand, is a single end grid design, and cnly one end grid (the bottom) is still gripping the rod under consideration.
Thus, only 7 grids are holding the rod, so the maximum expected pulling force should be no more than 70% of 91 lbs, or about 64 lbs.
Even less may be required because the end grids have the stiffest surings and IL40 now has only one intact, as compared with the four present-during the two-cycle tests mentioned above.
A force of 65 lbs will produce a cicdding axial tensile stress of about 2000 psi.
This is very small L
when compared with a value for the 0.23 yield strength cf unirrediated BU! 7.ircaloy-4 cladding of about 54,000 psi (irradiated yield streny n through two cycles is evcn higher).
Dua to the slight angle of pull planned (chout P.0 frem th: vertice.1),
small amount of rod bending will prebably occur at the top intermdic:._
spacer grid. However, since the rod has already been shoan to take a 450 bend without breaking, no problem is expected during the nulling operation.
Tests on irradiated cladding have shown that it retains abe d 75% of its as-built ductility (based on decrease in total elongation).
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The possibility of clamp slippage during the pulling oceration is remo'.e.
The licensee has stated that no sign of slippage has been seen during the
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dry runs conducted. Additional clamping checks have shown tha+ "ith a l
pulley rope force of approximately 7 lbs, the clamp will hoh nout i
slipping to a force of 150 lbs.
The pulling force will be lim.
d to 1E0
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lbs.
Since the expected force is less than half of this, the margin to l
prevent slipping is quite high.
Furthermore, the tests to date have been on unirradiated (relatively smooth) cladding.
The friction coefficient on irradiated, oxidized, crudded cladding is expected to be higher.
As cn i
extra safety precaution, any sliopage would be detected by monitcring before the clamp reached the bend area.
Since the load is continuously monitored by watching the spring scale, sicnificent slippage would be i
immediately noted as a sharp drop in pulling force, and the ou11ing operation would be stopped. The cicap area will be folluaed with the underwater TV camera to determine whether slippage has occurred.
Corrective action (such as repositioning or tightening) would then be taken' as nece!,sary.
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. Rod Break During Subsequent Handling - After removal from the assembly, the rod (with clamp still attached) would need to be moved only a few feet to
'j the storage container. flothing will be between the two locations that the rod could strike.
Chances of dropping the rod are negligible since the entire assembly is being doubly held by the pulling cabic and the pull y.
- ' rope'until the rod is in the container Transport of the container to the
. shipping cask should also present no problems.
To ensure that the cc.d.
cover will not interfere with the top of the rod, underwater TV will be used to confirm adequate clearance.
Once the cover is installed, no
- adverse effects would arise from a rod break (e.g., during shipping).
Although the probability of fuel rod rupture is very low, the licensec has calculated the consequences of such a rupture.
The results show the off-site two-hour doses would be well below 10 CFR 100 guidelines. He hava compared the rupture during handling of this one fuel rod with the fuel i
handling accident in the FSAR.
The FSAR case assumes 56 fuel rods (which have cookd and decayed for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />) fail.
Based en our review, we conclude thdt the FSAR evaluation bounds this case and, in f;:ct, the consequences of the rupture of 1 fuel rod v ould he signi fictntly lez.
Based on our evaluation and review of (1) the experience to date in removing fuel rod at reactor sites, (2) the procedures and precautier.s to be taken by the licensee, (3) the strength of the fuel rod cleddino, ;r.d (4) the consequences of an unlikely fuel rod cladding break, we cor. clue that the proposed action to remove the ' fuel rod from fuel assembly 1MO is acceptable.
Conclusions We conclude that there is no significant increase in the probability or consequences of accidents previously considered, and (2) there is reascn-able assurance that the health and safety of the public will not be endangered by this action.
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