ML20040A958

Leaker Fuel Rod Investigation, Final Rept

ML20040A958
Person / Time
Site:	Arkansas Nuclear
Issue date:	01/15/1982
From:	Arthur J, Husser U, Pyecha T ARKANSAS POWER & LIGHT CO.
To:
Shared Package
ML20040A957	List: ML20040A956 ML20040A958
References
NUDOCS 8201230010
Download: ML20040A958 (51)

Text

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! FINAL REPORT I l' '

j ANO-1 LEAKER FUEL R00 .

,1 INVESTIGATION 1

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i Principal Author: D.L. Husser , ,

i i Contributing Author: T.D. Pyecha 1

1 J.H. Arthur J.T. Mayer l

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I 8201230010 820115 DR ADOCK 05000313 PDR

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! I. INTR 000CTION I

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II. RAD 10 CHEMISTRY Arid SIPPING DATA 2 III. MANUFACTURING DATA 9 IV. PLANT OPERATION ll V. VISUAL EXAMINATIONS 15 VI. DIAMETER DATA 22 VII. CRUD ANALYSIS 31 l VIII. CONCLUSIONS 40 APPENDIX A VISUAL APPEARANCE OF FUEL ASSEMBLIES ANO-1 CYCLE 4

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APPENDIX 8 DEFECTED FUEL RODS - AN0-1 CYCLE 4

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g lI 1. INTRODUCTION I The ANO-1 reactor experienced a sharp rise in the iodine concentration in mid through late September 1979 while operating at 100% power. The initial indica-tion occurred on September 19, 1979 af ter approximately 45 effective full power days (EfPD's) of oper.ition in Cycle 4. The subsequent 15 months of Cycle 4 operations showed no evidence of additional through-wall defect formation. A fuel assembly sipping program at the end of Cycle 4 (January 1981) indicated that a total of 24 fuel assemblies containcd leaking fuel rods. Based on the radiochemistry and sipping results it is estimated that these 24 assemblies

. contained a total of 25 to 40 leaking rods. In order to obtain additional information to assist in determining the cause of the failures, a post irradiation examination (PIE) of selected assemblies in the spent fuel pool was conducted in June - July 1981.

This report provides a summary and discussion of the data obtained concerning the ANO-1 leaking fuel rods. The data includes the results of sipping, manufacturing records search, revicW of plant operations, visual examinations, rod diameter movements, and crud analysis.

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11. RAD 10 CHEMISTRY AND SIPPING DATA Radiochemistry Radiochemistry monitoring of the ANO-1 reactor primary coolant during Cycle 4 indicated that approximately 2 to 4 leaking fuel rods were present in the core I prior to September 19,1979 (approximately 45 EFPD's into Cycle 4). Radiochem-istry indications were that these rods developed defects in the previous cycle (Cycle 3) and had been carried over to Cycle 4. A sharp rise in the coolant I iodine concentration was observed on September 19, 1979 and reached an l equilibrium value approximately two w' eeks later, lodine values increased by a f actor of approximately 10 during this time period. Iodine isotopic ratios indicated the defects were small when initially formed and slowly increased in size over the remaining 15 months of the cyc'e. The increase in defect size results from secondary effects within the defected rod and is consistent with previous experience at other plants.

I lodine spikes were observed du,ing plant shutdowns af ter the event. Such

. iodine spikes are normal occurrences in plants operating with leaking fuel rods l because of the " pumping" action imposed on a leaking rod during power changes.

At no time during the cycle did those spikes result i1 the technical specifica-l tion limit of 3.5pCi/ml being exceeded. The variations in iodine concentra-l tions observed during the remainder of the cycle were consistent with the formation of the defects in the September 1979 time frame and with the defects being initially being very small.

The number of leaker fuel rods is estimated to be between 25 and 40. This estimate is derived from i.'1e iodine concentrations observed during the cycle l and the average power levels # Wich the 24 leaking fuel assemblies operated.

This implies that most of the u king assemblies contain only 1 or 2 leaking '

I fuel rods. This is also consistent with the observation of 3 leaking rods in the 15 assemblies ( 840 rods viewed) in the PIE work. Further substantiation of these estimatei has been possible as a result of the re-insertion of 5 of the Batch 6 (once-burned) assemblies indicated as leakers. Radiochemistry results from Cycle 5 (adjusted for known power levels of these assemblies) indicate 5 to 8 leaking rods in the core, confirming the low number of leaking rods in each assembly. Note: The earlier estimate of 70 leaking rods had been E

I made prior to sipping and assumed the leaking assemblies ran at core average power. As noted below this assumption proved to be incorrect.

Fuel Assembly Sipping I As a result of the iodine levels measured during Cycle 4, a fuel asse:Oly sipping program was conducted at the end of Cycle 4 to determine which assemblies contained leaking fuel rods. All 177 fuel assemblies irradiated during the cycle were sipped. Cesita 134 and 137 were the primary indicators I used for detecting leaking fuel rods. A total of 24 fuel assemblies displayed cesita lev ls indicative of one or more leaking fuel rods. Figure 1 shows the Cycle 4 core locations of these 24 assemblies. Of the 24 assemblies identifief as leakers, 9 were from Batch 4 (3 cycles of operation), 6 were from Batch 5 (2 cycles of operation), and 9 were from Batch 6 (1 cycle of operation).

Based on the sipping results the following observations are made.

l. No batch dependence exists. The percentage of the leakers per batch ranged from 11% to 16% (see Table 1).

2. There is a positive correlation of the leakers to power level. At the time of the iodine increase, eisMy-three percent of the leaking assemblies had a relative power density (RPD) of 0.9 or greater and sixty-two percent had dn RPD of 1.1 or greater (see Table 2). Thirty-ef dt percent of all assemblies with RPD's of 1.30 or greater were identified as leakers. This I compares to 10% or less for all other RPD groupings.

3. Incre is a positive correlation with core location. Seventy-five percent of the leaking assemblies are contained in either the WX or ZW Quadrants

(" west" side), Further, three of the six leaking assemblies on the " east" side of the core are suspected to be " carry-over" leakers from the previous cycle (Batch 4 assemblies on the periphery of the core).

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4. No significant burnup dependence is evident. Leakers were distributed throughout the burnup range (see Table 3). Twenty-five percent of the leaker assemblies had burnups of 19.8 to 21.4 GWd/mtU at 45 EFPD's. This compares to 14% with burnups of less than 2 GWd/mtU and 17% with burnups

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i i-between 11.4 and 13.7 GWd/mtU. It is believed that 2 to 4 of the leaking 1 j assemblies in the 19.8 to 21.4 GWd/mtU range were " carry-over" leakers from l

the pre.vious cycle. Taking this into consideration, the percentage of "new" leakers for the higher burnup group would be approximately the same l as the remaining groupings. '

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I FIGURE 1 I

NO-1 WRE IIIADING PIAN CYCII 4 FUEL Tk/GiR CANAL i X j 1

FA Face Designadon NXY NXO MIS NXS NXW p A

3 I B WX7 NN 00WU C30 01EL NXT C14 01FG 00M]

C31

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NXR R-03 C A I AUB 01EG B215 00WV CA3 01G3 B214 A118 CS2 01FJ B20V M ID C38 011X B2W 00XB C57 01EN B2LTF 00GZ NXF 01EP OdXS\/01}.V MIW 01EZ M(B O1IN OCllR O1EA GWZ 01DI NX1 D

B216 B21P C13 B21N A05 B20X C16 B20S C19 h2hs A04 AU4 NY3 s01EB\

s MIX '01F3h Ml7 01F1 MIA 011M MLI @ LEU NY0 NIC E x ssN C35 C23 B21M G13 B213 C26 C47 I

CSS B202 J E219s B20W NWX AVP 01G2 AUS .'01Fr , 'A Y 01FA AUC J1FB b0X 01F4 AUA 01G1 00X8 NXD F

C28 B217 M3 h2k)C(20 y B20A C45 BN9 ' 5h-B212 A06 B2N C33 NXA DlIW\ Mil 01FD M13 01N AVO 00WS MfY 01FF Aill 01F9 MIP 011V 00)0 0

\.. C50 B21S C21 B20B C12 C19 B208 C05 B21K C40 MIE NXV 01EC MU 01EX mis \ AWI 1A41 MT AUG 01FJ M16 01IQ GW2 All] y C08 B218 G7 B21R C61 C49 B21D C19 B20G C20 C(1 NXil 01Fl! WU3\ 01FK AUD 01FY OL11F M R. 410 01FR MlV J1FL Ml4 OlFS NXE K

C8 B21T C17 B20C N6 C24 B207 C09 B21F CC42 NXX NWR 01F8 NGX 01)6 \ NU\'01FC \ NGW 01FE ANI\01ET AU9 01EY 00WN 00Y4 l

M2 N N NN 'N\ s \

C27 B21C

(' 21B( C38 (2Ng C37 B20E

\ C B20N A07 B201 C34 I y Ml2

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NXP NX:

C39 OIED 01 (210\

'A MIN C32 01ES JJ(7 (21Eg

\US (41K; C1h J1G5 J1Fp 32111 AlfF (idi fl 01ER 01F5 B20P AUF Nill C29 J119 01F2 B20F MXL 00YS C59 011N

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Cg B211 \ A,(Ig' B21U Q1 ll21J C22 B2(U

'I B21Q A08 320R A112 01EK AIXG 01FZ AUli 01FU AUE 01G4 NY6 31EF ' A}J6\

0 B21L C53 B21G C46 32LN C44 B20L C51 B20K h

p NY7 AU7 NXN JIIT MXU 01FX NX6 NGY NX3 R-04 C25 G52 C36 00XK NXC A1IX 30X2 MM R

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6- 8 9 10 11 12 13 14 15 1 2 3 4 5 7 Fuel I.D. (IA** Batch 1;00G*,00l* 6 OGJa= Batch 4;00h*,00X* 4 00Y*= Batch 5;01E*,01F* f. O1G*= Batch 6) i Source I.D. (R= Secondary) or Control Camp I.D. (C=CRA, A=APSR, B=BPRA) 3 Note: All FA 1.D.'s are prefaced by MJ with the excepticn of Batch 1.

Note: Detected Leaker Fuel Asscmblies are slashed.

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TABLE 1 LEAKING ASSEMBLIES VS. BATCH I Total Number of Number of Percentage of l

I Batch Assemblies Within Batch Leaking Assemblics Leakers Within Batch 1 1 0 --

l 4 56 9 16 I 5 6

56 64 6

9 11 14 Total 177 24 13.5 I

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TABLE 2 LEAKING ASSEMBLIES VS. POWER LEVEL l Total Number of RPD* Number Leaking Percentage ** Percentage of***

Range of Assemblies s of Leakers Total Assemblies I 0.4 - 0.59 40 A_ssemblies 4 17 10 0.60 - 0.89 9 0 0 0 0.90 - 1.09 48 5 21 10 1.10 - 1.29 48 3 12 6

> 1.30 32 12 50 38 TOTALS 177 24

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• Relative Power Density at 45 EFPD.

• • Percentage of Leakers = Number of leakers in range x 100 Total number of leakers

• • • Percentage of Total Asemblies = *ber a numb r s m in range x 100 I

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TABLE 3 LEAKING ASSEMBLIES VS. BURNUP Number of Percentage Burnup at Total Leaker of Leaker 45 EFPD's Number of Assemblies Assemblies (GWD/MTU), Assemblies Within Burnup Range Within Burnup Range

<2.0 64 9 14 7.6 - 9.1 32 2 6 11.4 - 13.7 24 4 17 13.7 - 19.0 29 2 7 19.8 - 21.4 28 7 25 I

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111. MANUFACTURING DATA Tlic as-built characteristics of the critical components, i.e. fuel pellets and

.g 5 fuel rod clad, were reviewed for any indications of a manuf acturing/ material dependency for the leaker rods. The results are discussed below. The discus-sion does not include the one Batch 1 assembly used during Cycle 4 since it was determined not to be leaker.

FUEL PELLET Dimensional Characteristics The only significant variation in design between the three batches of fuel used in ANO-1 Cycle a was a change in the pellet density and a corresponding diameter change. Nominal pellet density / diameter was 93.5% T.D./0.3/00 in. for Batch 4 and 94% T.D./.3695 in, for Batches 5 and 6. As-built dimensions for dll three batches were typical of B&W production. Twenty-two pellet lots were W used in one or more of the leaker assemblies. These lots showed no tendency to group on either side of the nominal or to show greater than normal standard deviations. No single pellet lot was used in more than four of the leaker dssemblies.

I Pellet Moisture Mean pellet moisture for all lots was 3 ppm or less. Standaro ieviations were typically 1 ppm or less.

Fabricator The same pellet and powder vendors were used for all three batches. These vendors have produced all pellets and powdcr for B&W plants since i974.

I FUEL ROD CLAD Dimensional Characteristics I No clad design changes occurred over the three batches of fuel. Batch mean clad I.D. and 0.0. were within +.0001 inch of nominal with sample variances of I

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.0003 inch or less. Lot mean I.D. and 0.D. were within one standard deviation of the batch means. Forty-six clad lots were used in one or more of the leaker assemblies. As with the pellets, no pattern existed for these lots. No single clad lot was contained in more than four of the leaker assemblies.

I Clad Composition / Impurities i

A total of 12 different ingots were used to produce the clad lots contained in the leaker assemblies. No unusual impurity levels were found in any ingot.

Alloying agents were found to be in the normal ranges for all cases.

Fabricator The same clad vendor produced all three batches. The same ingot vendor supplied all ingots for the three batches. These .endors hue provided clad /

ingots for all B&W plants.

Conclusion No evidence has been found to indicate that a manufacturing or materials dependency exists among rods within the leaker assemblies.

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I IV. PLANT OPERATION I AN0-I operated at steady-state 100% power for approximately 30 days prior to the increase in iodine. Figure 2 shows the detailed power histories for the August thru October,1979 time period. A computer analysis was made of the startup imediately prior to this steady-state run. The data indicates no significant increases during the start-up in the local power levels (<lkw/f t max) above those achieved during the previous operation. Based on this dndlysis dnd 30 days of steady state operation prior to the iodine increase, it was Concluded that the startup was unrelated to t.he leakers.

A review of the operating logs (both operator and computer generated) for the time period immediately prior to the iodine increase was conducted. These records provide hourly status of all major plant operating parameters including: core power, imbalance, and tilt by quadrant; control rod location (full and part length); RCS pressure, temperature, and flow by loop; pressurizer level and temperature. Thia review indicates no significant rod I movewnts or major system imbalances.

A core power tilt existed at the time the iodine increase occurred. The tilt resulted in a 4 te 5% higher power on the side of the core in which the majority of the leakers occurred relative to the opposite side of the core.

This magnitude of tilt is not unusual, and based on operating experience f.om other plants, the tilt was not i causative factor. However, its directional correspondence with the leakers indicates that it may represent a secondary f actor associated with the locational dependence of the leakers.

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I Figure 2 - ANO-l Cycle 4 Power History 10 000 ,

f AUGUST 79-S.000 1

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i.000 0.0c0 7.000 11.00C 15 000 13.CCO 23 000 27.000 3t.000 CRTE 11 Inspection of supports and constraints completed. C.'iticality achieved at 1705 on August 12, 1979. Turbine placed on-line at 0034 on August 13, 1979.

12 Reactor '.cip followed a turbine trip at 77% FP. Initieting event was failed capacitor in switchyard resulting in spurious relay signal.

13 Reactor returned to criticalicy at 0300 on August 14, 1979. Turbine placed on-line at 0602. Turbine tripped on vibration at 0618 and reactor was held at 10% FP while vibration was eliminated. At 1855, the generator was brought on-line and power was escalated.

Source BAW-1675, Arkansas Nuclear Unit 1, Cycle 4, Core Operation Report, May, 1981.

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Figure 2 (continued) j.

1 Power History i ANO-1 Cycle 4 i

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) 14 Unit or)erated at full power for entire month, h 15 Iodine increase first identified.

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Power History i

ANO-1 Cycle 4 i 16

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! 16 Unit operated at full power until October 20, 1979 when shutdown for a

! scheduled maintenance outage for replacement of A, C and C reactor coolant

! pumpseals & repair of a OTSG level instrument line isolation valve leak and l investigate leaking primary code safety valve. Work inside the reactor j buildina was delayed due to high airborne activity.

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l V. VISUAL EXAMINATIONS l

Fuel Assemblies Fuel assemblies were visually examined during both the refueling outage at the end of Cycle 4 and the subsequent PIE. .able 4 lists the assemblies examined dnd at which time. The examinations during the outage were made using underwater video equipment. To assure high quality visual examinations, a periscope wa's used during the PIE work. The periscope examinations resulted in the identification of 3 fuel rods naving defects.

Appendix A provides photographs of the appearance of the assemblies. General observations made during the visual examinations of the leaker fuel assemblies dre as follows:

1. The fuel rods exhibited four general types of surf ace condition:

a. A dark brown / black unifonn crud deposit. (Fig. A-1)

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b. Lighter, more reflective areas which represent areas in which the crud had flaked off (loosely adhering flakes of crud were seen in some areas of this type). (Fig. A-1)

I c. White, highly reflective areas which appeared to be white deposits.

(Fig. A-i)

d. Mottled areas which were combinations of (a), (b), and (c). (Fig. A-3)

2. The rods exhibited a " typical" dxial pattern (as observed during the PIE) beginning with brown /diack crud in the lowest span (Span 7), getting pro-gressively lighter / white through the center of the assembly, and becoming highly mottled in Span 3. Span 2 was mottled but somewhat more uniform than Span 3. Span 2 was also darker than Span 3. Span 1 was similar to Span 2. Span 1 also contained one or sometimes two horizontal patterns of l light-on-dark crud (Fig. A-4). These patterns divided the span into three l

roughly equal segments. The higher of the two patterns corresponded to the upper fuel column line. The lower pattern was typically the more

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prominent. Figure 3b shows the conventions used in designating spans on the fuel assembly.

3. The observations made during the refueling outage differed slightly. The white / lighter areas extended upward somewhat f arther, and the lower pattern on Span 1 was not as pronounced as the upper pattern. A darkening of some areas which appeared white in the earlier exam had also occurred. Whether I this darkening is a result of additional crud deposition in the spent fuel pool or dissolving of the white crud deposits could not be determined.

Furthermore, some additional areas of crud flaking were observed in the PIE.

4. The general level of crud appeared to be low and similar in thickness to that observed previously at other reactors af ter 1 or 2 cycles of operation. This is substantiated by the relatively low specific activities and weights of the majority of the crud samples.

5. On a few assemblies, white crud deposits appeared to be thick and mottled in one or two of the top four spans (Fig. A-5). These deposits were localized and seldom occurred on more than one face of an assembly. They had the appearance of dripping or streaming candle wax overlaying a darker, dpparently more uniform crud. Occasional thin streams of light orange t deposits overlayed the thicker white crud. These patterns, however, were I not common.

6. Spacer grid cell damage was observed on two assemblies, OlF7 and 00J5. A broken corner weld (one side only) was observed on grid 2 of OlF7 at the B-C corner of the assembly. Assembly 00J5 had cell 1 on face A damaged at grids 2 and 3 (Fig. A-6). In each case the lower strap and approximately I 60% of the grid spring had sheared off. The spring stop (rod contact point) wa absent in both cells.

In general, the fuel assemblies have a greater variation in crud patterns than previously observed at other reactors. However, their appearance was typical of previous observations with the exception of the extent of the white deposits. White crud deposits have been observed at other plants in associa-tion with known through-wall defects. In those cases the white deposits l

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remained more localized and could be best characterized as a " plume" extending upward f rom the defect. The tendency for the leakers to cluster and the 1 1/4 years of operation af ter the leakers were formed are possible reasons for the extent of the whitish deposits seen at ANO-1. Sound assemblies away from the ciuster of leakers were observed to have little or none of the highly reflective white crud deposit.

Visual Observations of Leaket Rods Three f ael rods with through-wall defects were visually observed. These are Rod A2 in ole 9, Rod C13 in OlF7, and Rod D15 in 00J5. Figures 3a and 3b gives sketches of the areas of the three rods which exhibit through-wall defects and other anomalous areas. Photographs of the defects are included in Appendix B.

The observed defects have several common characteristics. The rods each exhibit a blister type formation (s) on the central part of the rod in spans 3 or 4. Each exhibits a second area of interest somewhat lower on the fuel rod.

In the case of ole 9 and OlF7, the lower defect is a split in the clad. In the Cdse of 00J5, the area appears to be a bubble in the uniform white crud covering span 5. In this latter case it is not clear that this bubble represents a through-wall defect. The lower defects on these rods are believed to represent secondary effects due to subsequent intrusion into the rod. It should be noted that it is possible that all of the observed areas represent secondary defects and that the primary defects either remained too small to detect visually or were in a non-observable location since o y 30% to 40% of the rod tu face area is eadily observable.

l Two other observations of note were made concerning the defects on rod A2 in a:sembly ole 9 and 015 in 00J5. Rod A2 in ole 9 did exhibit rod bow. The rod bow occurred in span 4 and appeared to be maximum at the location of the upper defect. 'ince the defect occurred at approximately center span, the coinci-dence with maximum rod bow is not surprising. Based on the observations made, the direction of the bow is i. 3 plane perpendicular to the "A" f ace, i.e., the bow is inward toward the second row of rods. As a result of the direction of the bow, the amount of bow cannot be estimated from the observations.

The grid damage to 00J5 described previously occurred in the cell restraining rod D15. No relationship between the grid damage and the rod failure is believed to exist.

Burnable Poison Rod Assemblies (BPRAs)

Two BPRAs were inspected during the PIE to determine if failure of individual poison rods could have resulted in local power peaks in the Batch 6 fuel dssemblies. BPRAs B20Y and B21E were examined in the PIE area using a black and white video camera. These assemblies operated in fuel assemblies 01EV 5 (core location 0-5) and OlF7 (core location M-6), respectively, during Cycle 4.

E Both of these fuel assemblies were identified as leakers in the sipping campai gn. Each BPRA was viewed from two sides: sides corresponding to fuel dssembly f aces B and 0 on B20Y and f aces B and A on B21E were viewed.

All of the rods were covered with a dark, sooty, loosely adherent cruc ad many had thin axial scratches which resulted from handling. In addition to the axial scratches, lighter areas were seen. These areas varied in size and shape, ranging from 1/4 to 1/2 inches in diameter and were circular or oval in shape. The oval type areas were occasionally oriented with their majar axis 30* from the vertical giving then a soiral appearance. These area, occurred more frequently and at more periodic intervals (as close as 1/2" apart) on several rods in the central three or four feet of the rods. These areas are probably a result of flow differences in the guide tube / rod annulus resulting from changes in annulus size and local turbulence. No defects or breaches in the clad were fcund.

Based on these examinations, BPRA failure was eliminated as a source of local power upsets.

M M M M M M Figure 3a. FUEL R00 CLADDING DEFECT AREAS. ANO-1. E00-4 IF7. ROD C13 IE9. ROD A2 PROBABLE SMALL

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TABLE 4 i

VISUAL EXAMINATIONS OF l ANO-1 FUEL ASSEMBLIES l

l Assembly No. Cycle 4 Outage PIE Exam Batch 6 i 01E9 (Leaker) Yes Yes I OlF6 OlF3 OlF/

Yes Yes Yes Yes Yes Yes

3 OlEV "

Yes No

.l OlEB Yes No j OlFW Yes N9 OlFP Yes No j

I OlFC "

OlFU (Sound)

OlEQ Yes Yes Yes No No No Batch 5 1 00XS (Leaker)

Yes Yes j- 00XM Yes Yes 00X4 No Yes 00Y1 No Ycs iE i3 00WY 00XJ No No Yes Yes

! 00WZ (Sound) No Yes Batch 4 l

l 00HG (Leaker)

No Yes il 00J1 No Yes 00J3 "

No ' Yes

,' N 00H5 No Yes 00J5 Yes Yes 00JB (Sound) Yes No i

I I .

,I VI. DIAMETER DATA Rod diameter measurements were made on 10 fuel assemblies during the leaking fuel examination. These included three batch 4, five batch 5 and two batch 6 assemblies. A total of 42 rods were measured. Equipment used is described in Reference 1. The measurements consisted of a single orientation line-scan of I the diameter of the fuel rod in .a plane parallel to the face of the assembly.

In most cases, the computer data acquisition cystem (CAS) was used to acquirc and store the data. Table 5 lists all rods that were scanned.

~

Repee c scans of rods were frequently parformed in order to assure the repeatability of the data, in addition, data on randomly selected rods were acquired by both the computer and a X-Y chart recorder to provide added assurdnce of the repeatability of the data.

All diameter data were calibrated to a trace of a three step calibration standard. Each diameter trace was digitized (approximately every 50 mils for DAS data and 200 mils for charts). A distribution mean and standard deviation were calculated for each grid-to-grid segment of each rod.

I Non-Leaking Rode I The average creepdown over the central spans (Spans 2 through 6) for non-leaking fuel rods can be summarized as follows:

Batch 6 creepdown - 1 to 2 mils Batch 5 creepdown - 1 1/2 to 2 mils Batch 4 creepdown - 2 1/2 to 3 mils I These values are somewhat lower, especially for the batch 5 and batch 6 assem-blies, than previously obtained data on Mark B fuel. Previous data indicates that crci.jdown generally ranges upwards from 1.5 mils even at low bu;'nups, and has been as high as 3 mils after one or two cycles and 5 mils af ter three cycles. The AN0-1 cladding creepdown is within the scatter band of previously observed data but is somewhat below the previously observed mean values.

I I

Several possible reasons exist for the " reduced" creepdown. The amount of creepdown is calculated assuming an as-built clad outside diameter of .430; if individual rods have a higher initial diameter, the " measured" creepdown for that rod would be "less". The data for Batch 6 and especially for assembly IF7 indicates that this may be the case for that batch. Measured diameters for dssembly IF7 averaged .330 in Spans 1 and 7 while predicted values are 1-2 mils lower. An adherent crud layer would also result in a higher apparent diameter measurement. The rod appearance cannot rule this out. However, measured flake thickness was 0.04 mils (1 micron) or less while approximately 0.5 miis would be necessary to explain the total apparent differences. A third possibility is j that the creepdown is actually lower. It is highly probable that all three ef fects are playing some role in the apparent lack of creepdown.

Ovality cannot be measured directly from line scans. The method used to provide a measure of the rod cvality is to multiply the standard deviation of the diameter measurements over a span by four. This value is called the dVerdge maximum ovdlity. These ovality values were then averaged over each batch inspected. The values from AN0-1 were compared to similarly obtaineo data from othcr plants. The general trend of the Batch 4 and 5 data is to f all within the scatter of data from other plants. The only notable exception is the Span 7 data which is somewhat greater than that from previous data.

However the trend of the data for Span 7 is consistent with previous data (i.e.

I the ovality increases with increasing burnup and residence time). Since the data for this span is consistent with the trends of previous data, its differences in magnitude from data from other plants is judged to be insignificant.

The ovality for the batch 6 assemblies is somewhat higher than data from other plants at similar burnups for all spans. One possible cause is the variation I in crud patterns discussed in the visuel examination section. Such variations (e.g. areas of peeled crud vs areas of non-peeled crud) will tend to increase the standard deviation on the diameter measurements. (Note: This variation would not effect the Batch 4 and 5 data as significantly due to the higher ovalities of these batches.) The Batch 6 data does exhibit h1gher ovalities in the upper spans where these variations were greatest; however, the lower spans I 4-7 also exhibit increased ovalities relative to the previous data and had a much more uniform crud pattern. The as built ovalities of the Batch 6 rods were reviewed; there is a trend for this batch to have slightly larger as-built ovalities relative to Batches 4 and 5. A combination of the slightly higher

I as-built ovalities and the variations due to crud patterns are probable causes f or the differences between the Batch 6 data and that found in other plants at similiar bu nups.

In conclusion, the diameter data for non-leaker rods ir.dicates that these batches behaved in a manner cunsistent with other fuel rods from other plants.

The differences noted above all appear to be explained by assignable cadses unrelated to any known f ailure mechanism.

Leaker Rods I Diameter profiles of two of the three vi.,ually confirmed leaker rods are : hors in Figure 4. Rod A2 in ole 9 (Batch 6) exhibited two visible defects. The first was midway between grids 3 and 4 and is blister-like in appearance.

Figure 3 shows that this defect caused the diameter to undergo a small I increase, then a sudden decrease and a sharp increase (scanning down from the top of the rod). The " trough-to-crest" height of this peak is approximately 2:.

mils. The second defect was a vertical crack about 0.60 inch in length occurring below grid 6. This produced a very wide and tall peak approximately 20 mils in height and about 2.5 inches long. Another sharp peak of about 13 mils occurred midway between grid 6 and the lower skirt. In this latter case, I no defect was seen in this area during the visual examination.

There are other anomalies associated with this cod. Spans 4 and 7 (which contain the confirmed defects), as well as Spans 1 and 2, exhibit an above average degree of diameter variation when compared to the relatively flat traces for Spans 3, 5, and 6. Span J, however, includes a 4 mil diameter decrease just below grid 2, which extends at least 2 inches axially. All of these areas were closely re-examined folluwing the diameter scans, but nothing unusual was seen except for the two defects and some peeling crud in span 4.

It should also be noted that all of these features repeated well on all runs, including the OAS and X-Y recorder traces. The lack of creepdown and ovalization in the mid-region of the rod indicates that this rod became defective in the early part of Cycle 4. This is consistent with the radiochemistry data discussed earlier.

Rod C13 in OlF7 (Batch 6) exhibited three visible defects; two blister-like defects between grids 2 and 3, arid a hairline crack just above grid 6. The two blisters produced sharp peaks, 8 and 10 mils in magnitude (Figure 3). Similar

I peaks occurred just above grid 2 and just below grids 5 and 6. No defects were seen in these areas. The hairline crack just above grid 6 produced no significant change in the diameter trace. Like ole 9/AE, this rod showed a large amount of diameter variation. The only flat span, between grids 4 and 5, I shows little ovalization or creepdown. Based on mean diameter data, this rod also apparently became defective early V cycle 4.

The third rod with a visible defect was rod 015 in 00J5 (Batch 4), (diameter trace not shown), which has a small blister like hole just below grid 2. This defect was not detectable with the diameter scan due to its proximity to the grid. The diameter downscan, when it passed grid 3, indicated a gradual increase in diameter over several inches, and then a voltage outside the range of the computer /LST interface. This resulted in an indicated tall plateau along the entire span between grids 3 and 4. It is not known whether this is a true diameter or whether the fingers were cocked or " rode up" on the rod. The rest of the scan showed an abnormally high diameter, except for Span 3. The rod mean diameter neglecting Span 4, was over 0.434 inch. Altnough the general shape of the trace showed reasonably good repeatability on a repeat scan, the values must ce considered suspect. In addition, the orthogond trace, on rod Al, has a relatively flat shape with a mean diameter of 0.430 in,-h. Visual examinations did not indicate any identifiable diameter increase in Span 4.

I a result of these facts, the DAS data on this rod is not considered to be As reliable.

The diameter traces for rods A2 of ole 9 and Cl3 of OlF7 were reviewed for indications of pellet clad contact. Both the clad diameter and the distance between diameter " spikes" are inconsistent with that which would be expected in It is, therefore, concluded that pellet clad-I the case of pellet-clad contact.

contact did not occur in these rods.

Other Anomalous Diameter Traces in addition to the three visually confirmed failed rods, there were several diameter profiles that showed unusual behavior. These are discussed below.

I Rod D15 in ole 9 had a sharp,12 mil downward spike in Span 2. The spike corre-sponded to a region of high ovality (about 10 mils) in this region. The rod was scanned twice, and showed good repeatability. Also, the orthogonal trace for Al correlated well. Thus, the spike and ovality are not thought to be 1

i

I I l artif acts, but are probably real. Rod 01/C15 in OlF7 had a similar-looking occurrence, also in Span 2, although the downward spike was not as great. Rod l B15/C1 in 00HG has larger than normal diameter variations in the bottom three spans. The traces repeated well and.the orthogonal scans show good correla-tion. These variations appear to be greater than expected from rod ovaliza-I tion. In each of these cases, no defects or anomalous areas were visually observed in these areas.

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i l REFERENCE 1: T.A. Coleman, et al., Development of an Extended Burnup Mark B Design, Second Semi-Annual Progress Report, January - June 1979, BAW 1932 - 2,

, 00E/ET/34213-2, Babcock & Wilcox, December 1979.

1

I 450. FLEL ASSEMBLY i4J01E9 FACE A ROD 2 l _

$ 3 4 35. __

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~-- _ _ - - =-_ _- :  % n-- -

- ^ ,

420.

8 0 11 0.1 G6 GS G4 G3 G2 G1 TOP 450. FUEL ASSEMBLY t!J01F7 FACE C R0013 U3 435' h5 s _- =nN &r _ - _ --w Mk Q )%. J e

m 420.

DOTIDM GB GS G4 G3 G2 G1 TOP AXIAL POSill0f4 Figure 4 - Expanded-scale Diameter Traces on Failed Fods

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.i

. TABLE 5

ANO-1 DIAMETER SCANS Assembly Face /Rodl Scan Dir.

NJ01E9 D15 Up i A2 Down i A2 Down i 01 Up

.; A15 Up Al Up i D15 Up l NJ01F7 Al Up i A15 Up j B1 Up

BIS Up ig Cl Up jg Cl3 CIS Up

Up i CIS Up l DI Up l D15 Up NJ00J5 Al Down A15 Down 1 81 Down l 815 Up i Cl Down j CIS Up D1 Down D15 Down D15 Down NJ00J1 Al Dwn

, A15 Up BI Down BIS Up Cl Down 01 Up j D15 Up

!l NJ00XJ Al A15 Dowa Up Bl Down

'I BIS Cl Cl Up Down Down C15 Up

,l i

D1 Down 015 Up I  :

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E l TABLE 5 (Cont'd. )

I Assembly Face / Rod Scan Dir.

NJ00HG Al Down A15 Up A15 Down Bl Down B15 Up B15 Down Cl Down C15 Up D1 Down D15 Up NJ00WZ Al Up Al5 llp B1 Up I BIS DI D1 Up Up Up I D1 D8 D8 Up Up Up D15 I

Up D15 Up D15 Up I NJ00WY Al A15 81 Down Up Down P'c Up Up

'6 Up L: Down I D15 015 D15 Up Down Up NJ00XS Al Down A15 Up Bl Down I B15 Cl CIS Up Down Down I D1 D15 Down Up I

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I f

l TABLE 5 (Cont'd.)

Assembly Face / Rod Scan Dir.

! NJ00XM Al Down i AIS Down

! B1 Down

! BIS Up t

Cl Down

i. Cl Down Cl2 Down CIS Up l CIS Down i D1 Down

. D15 Up

Note 1: Based on the conventions used, Rod 1 of any given face represents an

! orthogonal trace of Rod 15 of the counter clockwise face, i.e., traces of l, Rods Al-015, B1-A15, Cl-BIS, and Dl-C15 are orthogonal traces of the corner 3 rods.

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I Vll. CRUD ANALYSIS Crud samples were obtained from selected fuel assemblies during the poolside ex amination. The crud sampling tool is comprised of a steel knife-edge scraper with a curved blade machined to conform to about one-third of the rod' circum-ference. This blade is attached to a handling pole, manually positioned against a fuel rod, and moved up and down about 4 inches to remove the crud.

The removed crud flakes were drawn into a suction hose positioned near the tip l of the blade and drawn up to poolside (along with pool water) into a collection flask. Each water and crud mixture was then passed through a 0.45 micron filter to collect the crud flakes.

Thirty-faur crud samples were taken from 11 fuel assemblies. No failed rods were sampled as it was felt that the presence of material leached from the fuel pellets would mask any other material of interest. The crud samples were dnalyZed using two methods. The first method was gamma spectroscopy which was used to provide isotopic data for the major isotopes present. The second was scanning electron microscopy (SEM), which includes energy-dispersive X-ray (EDX) analysis for elemental composition. All samples underwent gamma spectroscopy and 19 samples were subjected to SEM and EDX analysis. Table 6 provides a listing by sample of assembly, location on assembly, and tests performed on the sample.

Results I Sample Weights For each sample, the weight of crud collected was determined. A background correction f actor was determined from the background samples and applied to each sample to determine the estimated crud weight. A +0.3 mg uncertainty was determined for the samples. The estimated crud weights were small. Only two samples (15 and 24) had estimated weights in excess of 1.7 mg. Samples 15 and 24 had estimated weights of 22.3 and 3.9 mg. respectively. The overall average weigot was 1.3 mg.; however, if Sample 15 is excluded, the average is reduced to 0.7 mg. Several trends were noted in the data:

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1. For Spans 2 thru 5 , Span 2 had the highest average weight (even with i Sample 15 excluded).

2. For Batches 4, 5, and 6, the Batch 5 assemblies exhibited generally higher weights and Bat h 4 exhibited gentrally lower weights.

I 3. No apparent trend by core location was identified; however only one assembly wds sampled in each of the XY and XZ quadrants.

I Visual Appearance of Sample Areas The appearance of the sampled areas varied from marked changes in appearance to essentially no change in appearance af ter testing. Some of these differ-ences result from the manual nature of the sampling method and to real differ-ences in amount of crud present in specific areas. The differences however I dppear to be greater than these two f actors can explain. The conclusion reached is that the crud itself has various degrecs of adherence to the rods.

In general, change in appearance was more pronounced as one moved up the fuel dssembly. This is consistent with the earlier observations relative to sample weights; although on an individual sample basis, the relationship between change in appearance 3.J sample weight was not strong.

Gamma Spectroscopy Each sample was gamma counted and decay corrected to the end of Cycle 4. Tha rod samples were background corrected using the pool background samples. Table 7 provides a listing of the decay and background corrected values. Because of the small sample weights and resulting large relative uncertainties in those weights, Table 7 is given in terms of total activities. Samples whose weights dre sufficiently different to justify correcticas for sample weight have been identified in the table and a correction factor provided.

Table 8 lists the isotopic data for each sample normalized M its 58 Co activity. Since the source of the 58C o is nickel (as is CuS7) and the source of 54Mn is iron, the normalized 54Mn values on Table 8 should represent a measure of the relative proportion of Ni Fe20 4 to Ni0 within l

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l the semple. Similarly the 51 Cr values would provide similar information l relative to the Cr2 03 to Ni0 within the sample.

l The following observations were made from the data.

1. No unusual or abnormal isotopes were found in the samples.

I

2. Sample to sample and assembly to assembly variances were much more dominant than batch or core location dependencies.

4

3. No core location trends were identified.

4. No consistent axial trends were identified. One assembly, IF6, does

.I exhibit an increase in activity mcVing up the assembly which is not a direct function of sample weight. In the case of OXM the apparent axial trend is explained by the sample weights.

t

5. All three samples from assembly OXM exhibit higher activity levels than the remaining samples, even af ter accounting for sample weight. These samples I also exhibit significantly higher iron to nickel ratios (See Table 8, 54Mn values for this assembly).

While particular samples or assemblies may show some interesting behavior, there is no information which would indicate any specific failure mechanism for the leakers.

SEM/EDX As noted earlier, a tott! cf 19 samples (including two background) were subjected to SEM/EDX analysis. The results of the EDX analysis are given in Table 9. This data is in the form of peak heights of the primary peak for the elements detected.

The EDX results are consistent with the gamma data above. The nickel and iron contents for the high activity samples (15,16,17 and 18A) are higher +han the remaining samples. The source of the Cu, A1, and Si found in many of the samples appear to be the spent fuel pool water since similar levels of these three elements (along with Fe) were also found in the background samples.

The SEM photographs show numerous foil-like flakes which of ten had a coiled or curved shape. Figure 5 shows a typical SEM photograph. None of the flakes exceeded 1 micron in thickness. Sample 15 which exhibited higher sample weight and activity showed no significant difference in crud flake thickness when compared to the other samples.

I Conclusions The crud data provided no additional information as to the cause of the leaking rods at AN0-1. The data supports the visual observation that the average quantity of crud was similar to that previously observed in other plants.

However, this work does not eliminate the possibility of direct contribution by crud or an active agent to the failure mechanism since the testing occurred approximately tWo years af ter the leakers were formed. Changes in amount of crud and composition of crud is known to occur through the life of an assembly.

For example, the visual inspection section of this report noted that the residence time in the spent fuel pool between examinations had resulted in changes in appearance of the crud on the assemblies. As a result, lack of any direct evidence from these samples is not conclusive with regard to any waterside affects which may have occurred in the September 1979 time period.

I I

I TACLE 6 CRUD SAMPLE Sample Fuel Leaker / Core Quadrant Gamma No. Assembly Span Non-Leaker (Core Location) Spectra SEM/EDX Comments 1 Background X X 2 lE9 7 L WX (F-5) X 3 6 X 4 5 X 5 4 X X 6 3 X 7 2 X X 8 1 X 9 OX4 4 L XY (F-10) X X 10 5 X 11 - 1A41 4 NL Center Ass'y (H-8) X In reactor during Cycles 1 & 4 only 12 3 X X 13 0J3 4 L ZW(K-3) X 13A 4 X Crud flake from Sample 13 g 14 3 X

, 15 0XM 2 L ZW (N-4) X X Tan Crud of a "hewier" ap,nearance 16 3 X X 17 4 X X l 18 0XS 2 L WX (D-4) X X l 19 'l X X 20 4 X X 21 Background X 22 OY1 4 L YZ (L-10) X X 23 3 X X 24 2 X X 25 OHG 2 L ZW (M-7) X 25A 3 X 24A 4 X 23A 0J5 5 ZW (N-6) X 22A 4 X 21A 3 X X 20A 1F6 4 L ZW (L-5) X X 19A 3 X 18A 2 X X 17A lA42 4 NL ---

X Assembly was in core during Cycle 1 only 16A 3 X X 15A 2 X 14A Background X X

M M M M M M TABLE 7 CRUD SAMPLE ACTIVITIES (aci)

BACK-GROUND CORRECTED Sample Correctior.**

Number FA/ Span Factor Cr* Mn Fe* Co Co Co Zr 15A 1A42/2 -- --

.275 --

6.03(-3) .315 6.13 0.58 16A*** 3 -- --

.066 --

2.67(-3) 1.27 2.64 0.77 17A*** 4 -- --

.149 --

5.98(-3) 4.62 2.14 4.98 12 1A41/3 --

12.86 .0579t --

.0355 27.2 .563t 6.31 11 4 --

5.14 .0415t --

.0353 26.6 .392t 3.50 14 0J3/3 --

21.96 .1644t --

.0208 17.9 1.ll2t 7.19 13 4 --

15.26 .1195t --

.0287 18.5 .924t 5.61 25 OHG/2 --

11.6 .0466 --

.0172 13.6 .492 2.59 25A 3 --

6.68 .196 --

.0236 11.0 1.39 5.90 24A 4 --

4.06 .0802 --

.0037 4.02 .532 3.81 21A 0J5/3 --

14.5 .142 --

.0239 20.5 1.05 7.28 22A 4 --

9.19 .147 --

.0059 10.3 1.02 7.38 23A 5 --

10.46 .160 --

.0380 19.4 1.18 5.69 9 OX4/4 --

5.95 .0926t --

5.72(-3)t 6.61t .739t 5.96 10 5 -- --

.0574t --

6.38(-3)t 3.46t .498t 6.61

, if OXM/2 34.8 --

236. 135. 6.35 4500. 1260. 76.1 u, 16 3 2.5 --

11.8 --

.256 262. 61.6 9.19 o' 17 4 -- --

6.95 --

.130 137. 37.5 7.10

' 18 0XS/2 2.5 17.18 .789 .253 .0347 30.1 4.32 5.78 19 3 --

39.1 .279 --

.0199 24.6 2.38 3.44 20 4 --

31.9 .302 .347 .0322 21.6 2.46 6.26 24 OYl/2 6.1 20.1 .328 .415 .0284 31.1 2.59 11.60 23 3 --

19.5 .180 --

.0246 22.5 1.62 7.42 22 4 --

23.0 .276 --

.0203 14.0 2.12 9.19 18A !F6/2 2.7 168. 7.34 4.17 .632 421. 45.0 33.5 -

19A 3 --

32.6 .681 --

.0703 60.9 4.33 63.7 20A 4 --

15.7 .151 .261 .0132 11.0 1.06 9.78 8 lE9/1 2.3 10.18 .242 .323 .0121* 9.47 1.33 56.46 7 2 --

5.65 .0795 --

.00760* 7.77 .534 5.03 6 3 --

7.44 .0753 --

.00950* 8.37 .537 4.74 5 4 -- --

.0702 --

.0229 12.9 .440 4.23 4 5 --

.0594 .0137* 10.3

.3P4 3.13 3 6 -- 7.13 .0948 --

.0248* 16.6 .665 5.93 2 7 2.3 8.48 .0697 --

.0619* 42.9 .713 1.66 tuncertainty in background correction of +.02 for 54Mn, +.0016 for 57C0, + 7 for 58Co, +.13 for 60Co.

• Not corrected for background; SIC r correction may be as high as 2-3pCi,-$9F e as high as 0.lp Ci, and 57C0 as high as 0.001,4Ci .

• • Activity divided by correction factor will provide comparable activity valves. Unless indicated correction facter equals 1.

• • • Data from these two samples indicate a carry-over from sample 18A and are not considered reliable.

l TABLE 8 NORMALIZED ACTIVITIES (CoS8 1) 54 59 59 60 95 Nu r FA/ Span Cr Mn Fe Co Co Zr 15A 1A42/2 -- -- -- -- -- --

16A 3 -- -- -- -- -- --

17A 4 -- -- -- -- -- --

12 1A41/3 .473 t2.13(-3) --

1.31(-3) t2.07(-2) .232 tl.56(-3) ll 4 .193 --

1.33(-3) tl.47(-2) .132 14 0J3/3 1.227 t9.30(-3) 1.16(-3) t6.21(-2) .402 I

13 4 .825 ts.46(-3) --

1.55(-3) t5.00(-2) .303 25 OHG/2 .853 3.43(-3) --

1.27(-3) 3.62(-2 .190 25A 3 .607 17.82(-3) 2.15(-3) .536 12.64(-2 24A 4 1.010 19.95(-3) .948 21A 0.92(-3) 13.23(-2)

OHS /3 .707 6.93(-3) --

1.17(-3) 5.12(-2) .355 22A 4 .892 14.27(-3) .717

.57(-3) 9.90(-2) 23A 5 .539 8.25(-3) --

1.96(-3) 6.08(-2) .293 9 OX4/4 t.900 tl4.01(-3) --

t.87(-3) til.18(-2) t.902 10 5 tl6.59(-3) tl.910 15 11.84(-3) tl4.39(-2)

OXM/2 --

52.44(-3) 3.00(-2) 1.41(-3) 28.00(-2) .0169 16 3 45.04(-3)

.98(-3) 23.51(-2) .0351 17 4 50.73(-3)

.95(-3) 27.37(-2) .0518 18 0XS/2 .571 26.21(-3) .84(-2) 1.15(-3) 14.35(-2) .192 19 3 1.589 11.34(-3) --

.81(-3) 9.68(-2) .140 20 4 1.477 13.98(-3) 1.61(-2) 1.49(-3) 11.39(-2) .290 24 0Yl/2 .646 10.55(-3) 1.33(-2) .91(-3) 8.33(-2) .373 23 3 .867 8.00(-3) --

1.09(-3) 7.20(-2) .330

'I 22 18A 19A IF6/2 4

3 1.643

.399

.535 19.71(-3) 17.43(-3) 11.18(-3) 0.99(-2) 1.45(-3) 1.50(-3) 15.14(-2) 10.69(-2)

.656

.080 1.15(-3) 7.11(-2) 1.046 20A 4 1.427 13.73(-3)

I 8 7

1E9/1 2

1.075

.727 25.56(-3) 10.23(-3) 2.37(-2) 1.20(-3) 3.41(-2) t1.28(-3) t.98(-3) 9.64(-2) 14.04(-2) 6.87(-2)

.889 5.96

.647 6 .889 3 9.00(-3) --

tl.14(-3) 6.42(-2) .566 I 5 4

3 4

5 6

.430 5.44(-3) 5.77(-3) 5.71(-3) tl.78(-3) ti.33(-3) 3.41(-2) 3.73(-2)

.328

.304 tl.49(-3) 4.01(-2) .357 I 2 7 .198 1.63(-3) --

tl.44(-3) 1.66(-2) .039 I tThese values are less reliable.

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TABLE 9 j EDX RESULTS Sample j Number Fe Cu Mn Cr Ni Al Si j 1 (Bk'd) 1.9 1.2* - - -

3.3 4.0 5 3.6 1.3 2.8 2.0 1.5 4.0 4.5 7 2.7 1.4* - - - - -

9 4.4 1.3 2.5 2.0 1.6 3.8 4.0

12 6.1 1.4 3.5 2.3 1.9 3.2* 4.3 15** 18.4 -

2.0 1.6 12.0 -

1.4 16 6.0 - -

1.7 5.0 3.1 3.0

17 5.0 1.3 2.6 1.9 2.9 3.3 4.0 l

18 6.5 1.4 -

1.6 1.6 -

3.0 19 2.7 1.2 - -

2.3 - -

! 20 3.4 1.4 -

1.6 2.2 3 3 j 22 2.5 1.4 -

1.5 1.6 3 3 i 23 2.1 1.5 -

1.6 2.1 - -

i 24 2.5 1.6 -

1.6 2.1 - -

! 21A 2.2 1.5 - -

1.8 - -

l 20A 3.0 1.5 -

1.8 2.1 - -

) 18A** 4.2 - -

2.2 8.2 - -

16A 3.0 1.3 -

1.6 1.9 2.5 3.6 14 (Bk'd) 2.5 1.5 - - -

2.3 3.5

• Weak; difficult to distinguish.

• • 1000 vertical scale; others are 512.

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Figure 5. Crud Flakes From Sample No. 19 t

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Vill. CONCLUSION No definite cause for the leakers at ANO-1 has been established. However, based on data obtained, the following statements can be made. A brief discussion is given with each.

1. There are no indications that the leaking rods resulted from manufacturing related sources.

Manuf acturing related rod failures, i.e., primary hydride failures, weld defects, etc., are typically seen early in the rod lifetime. After dccounting for sarryover leakers, it is estimated that at least 50% of the leaking rods had successfully operated either one or two full cycles prior to developing through-wall defects. The lack of any correlation between manuf acturing related attributes and the leaker assemblies provides further evidence of the absence of a relationship. In addition, the " simultaneous" nature of these defects which occurred after 45 EFPD's of Cycle 4, is not consistent with past occurrences related to manufacturing related failure modes. These types of failures have more commonly been observed either "immediately" af ter startup or show up as a gradual increase in activity over the early part of the cycle.

I 2. There are no indications that the leaking rods resulted from he pellet-clad interaction induced stress corrosion cracking mechanism (PCI/ SCC).

PCI/ SCC is associated with rapid power changes (ramps) resulting in high local powers of the fuel rods. No operational event occurred around the time of the iodine increase which would have resulted in a significant power ramp to the leaker assemblies. As noted in the Operational Data section, the startup 35 days l-ior to the iodine i'ncrease did not result in any rods experiencing significantly higher oower levels than they had experienced previously. No significant control rod '(full or part length) movement occurred around the time of f ailure which could have induced a local power ramp. Rod diameter traces of the two batch 6 known leaker rods show no evidence of pellet clad contact (i.e., claJ ridging). Such contact would be expected to be present if PCI/ SCC had in fact occurred. The low burnups (<2000 mwd /mtV) of the batch 6 leakers at the time the defects occurred would have required much higher power levels to induce PCl/ SCC.

These f actors and past experience indicate that PCl/ SCC was not the defect formation mechanism.

3. Some form of external clad attack represents the most probable cause of the

) __1eaking fuel rods.

The lack of any evidence of internal clad attack (i.e., hydride and/or PCI), along with the simultaneous nature of the formation of the defects in three different batches of fuel point to some form of external clad attack.

4. Further work to establish the exact nature of the attack appears unwarranted for the following reasons:

a. The attack, regardless of its actual form, was limited to a short period of time early in Cycle 4. The subsequent 15 months of operation following the defect formation showed no evidence of additional defect formation. Thus, it is probable that the agent and/or conditions for the attack were removed and/or modified at that tiene.

b. The attack was limited to a small number of fuel rods. Only 0.1% of the rods in-core at the time developed through-wall defects.

c. The lack of additional rod failures indicates that, if rods did not experience through-wall defects during the September 1979 time period, subsequent formation of additional through-wall defects was not enhanced. The final 15 months of Cycle 4 operation and the additional 8 months of operation in Cycle 5 show no evidence of additional through-wall defect formation.

In conclusion, the cause of the through-wall defects has not been established.

Some form of external clad attack is the most probable mechanism. The limited nature of the event in terms of number of rods, time period of occurrence, and the lack of any apparent effect on non-leaker rods does not appear to warrant the expenditure of additional resources in an attempt to explicitly determine the f ailure mechanism. Further, the isolated nature of the event makes the probability J determining the " exact" nature of the attack doubtful regardless of the extent of additional investigation.

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1 4

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i APPENDIX A VISUAL APPEARANCE OF cIJEL ASSEMBLIES

! ANO-1 CYCLE 4 I.

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APPENDIX B DEFECTED FUEL RODS ANO-l CYCLE 4 1

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