05000282/LER-1981-031, Forwards Rept Providing Supplemental Info Re Degradation of Certain Spent Fuel Assemblies First Reported in LER 81-031 on 811230.All Region 4,5 & 6 Fuel Will Be Treated as Susceptible to Bulge Joint Cracking

Forwards Rept Providing Supplemental Info Re Degradation of Certain Spent Fuel Assemblies First Reported in LER 81-031 on 811230.All Region 4,5 & 6 Fuel Will Be Treated as Susceptible to Bulge Joint Cracking

ML20235Z030
Person / Time
Site:	Prairie Island
Issue date:	07/13/1987
From:	Musolf D NORTHERN STATES POWER CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
NUDOCS 8707270159
Download: ML20235Z030 (14)

LER-2081-031, Forwards Rept Providing Supplemental Info Re Degradation of Certain Spent Fuel Assemblies First Reported in LER 81-031 on 811230.All Region 4,5 & 6 Fuel Will Be Treated as Susceptible to Bulge Joint Cracking

Event date:
Report date:
2822081031R00 - NRC Website

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p-Northern States Power Company 414 Nicollet Mall Minneapohs, Minnesota 55401 Telephone (612) 330-5500 July 13, 1987 Director of Nuclear Reactor Regulation US Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT DOCKET NOS, 50-282 LICENSE NOS. DPR 42 50-306 DPR-60 i Supplemental Information - Spent Fuel Assembly Degradation Attached, for the information of the NRC Staff, is a report pro-viding supplemental information related to degradation of certain spent fuel assemblios first reported in Licensee Event Report 81-031 on December 30, 1981.

As noted in the report, it is our intention to treat all region 4, 5, and 6 fuel as susceptible to bulge joint cracking. All handling of this fuel will be suspended until special tooling has been obtained that does not rely on the bulge joints for support.

Routine bulge joint inspections will be suspended.

Please contact us if you have any questions related to the infor-mation we have provided or our plans for resolution of this matter.

& k- .

David Musolf Manager Nuclear Support rvices c: Sr Proj ect Manager, NRC Sr Resident Inspector, NRC Regional Administrator, Region III, NRC G Charnoff Attachment 8707270159 870713 22 I

PDR ADOCK 050 gp,i S

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Director of NRR July 13, 1987 Attachment SUPPLEMENTAL INFORMATION - SPENT FUEL ASSEMBLY DEGRADATION PRAIRIE ISLAND NUCLEAR GENERATING PLANT s

REFERENCES

1. LER 81-031/01T-0; Report Date 12-30-81; " Spent Fuel Assembly D-34 Top Nozzle Event"

2. LER 81-031-01X-1; Report Date May 1982; " Update Report-Spent Fuel Assembly D-34 Top Nozzle Event"

3. " Technical Update Of Investigation Into Separatien Of Top Nozzle From Prairie Island Fuel Assembly D-34";

April 1982; W.R. Smalley & E. Roberts; Westinghouse

4. " Safety Evaluation By The Office Of Nuclear Reactor Regulation, Northern States Power Company Prairie Island Units 1 and 2, Docket NOs 50-282 and 50-306, Spent Fuel Assembly Degradation"; December 1981

5. Letter From K.J.Dzikowski, Westinghouse To M. Klee, NSP; October 2 1986; 86NS*-G-0069; " Bulge Joint Surveillance Assessment"

6. Letter From M. Klee, NSP To Nuclear Engineers, NSP; July 23, 1986; " Handling Of Region 4, S, & 6 Fuel" PURPOSE In the NRC's safety assessment of the D-34 incident (Ref. #4) it.was recommended that NSP " Continue to periodically survey the fuel assemblies in the spent fuel storage pool to assure early detection of any evidence of intergranualar stress corrosion cracking in any other stored fuel assemblies". The purpose of this letter is to update the NRC on the results of those inspections and to inform them of NSP's plans for the future.

BACKGROUND On December 16, 1981 spent fuel assembly D-34 was being moved from pool #2 to pool #1 in preparation for the reracking of pool #2 when the top nozzle separated from the rest of the assembly. The top nozzle remained attached to the tool, l

1 while the remainder of the assembly fell towards the wall and I j

came to rest wedged in a weir gate between pool #2 and the transfer canal. The event was formally reported to the NRC in reference #1. In January of 1982, the assembly was

. uprighted and a plate was anchored to it via the control rod guide tubes. A tool was then attached to the plate and the assembly was properly placed into the storage rack. The top nozzle was shipped to a hot cell facility for examination and investigation into the cause of the failure. The results of this investigation (Ref. #3) were presented to the NRC in reference #2 and at a meeting on February 23,1983 in Bethesda.

SUMMARY

OF D-34 FAILURE Figure 1 illustrates the mechanical bulge that attaches the upper end of the RCCA guide tubes to a Stainless Steel sleeve, which in turn is welded to the top nozzle. The hot cell exam (Ref. #3) of the failed top nozzle revealed that all 16 sleeves failed at the apex of the top bulge as shown in figure 1 and:

1. The fractures exhibited little ductility (approximate 10% of thn surface).

2. The grain boundaries showed extensive seperation and pitting.

3. Qualitative analyses of the fracture surface samples showed particles contained high Fe, Si, & Cr and moderately high A1, Cu, & C1. It did not detect any significant quantitias of sulfur.

4. Cracks appeared to originate on the outer surface.

5. Chemical analysis of the sleeve indicated normal constituents for 304 stainless steel.

6. Rust colored, corrosion products were observed on the surface of the bulge joints in the vicinity of the cracks.

The report went on to conclude that:

1. The failure mode was c assic intergranular stress corrosion cracking (IGSCC).

2. The IGSCC developed during storage in the spent fuel pool.

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3. The spent fuel pool water chemistry was maintained within specification except for a few chloride readings which were slightly above the 0.15 ppm limit. However, because of the conservatism in the limits, these deviations are not believed to be sufficiently high to be responsible for the IGSCC.

4. The occurrence of the top nozzle separation does not appear to be correlatable to any data available at the time this report was written.

INSPECTION CAMPAIGNS Since early 1982 a surveillance program has been in place to inspect the mechanical bulge joints for evidence of corrosion and possible cracking. There have been five inspection campaigns resulting in a combined total of 68 exams on 48 different assemblies (table 1). Most of the assemblies in the 1982 inspections had a common tie to D-34 e.g. the assemblies were stored in close proximity to D-34, the sleeves were from the same material lot, or heat treatment batch. The selection of individual assemblies for the '84, '85, & '86 campaigns was based primarily on the desire to increase the number of different assemblies inspected, thus supplying more information as to how wide spread the corrosion / cracking problem was. Unfortunately the process resulted in very few assemblies being inspected more than once. This, along with a change in the inspection technique (discussed below), provided very little information on whether or not the corrosion was still progressing.

INSPECTION TECHNIQUES During the 1982 & 1984 inspections a black & white under water TV camera was used. This camera was mounted in a fixture, which was then lowered onto the top of the storage i racks just in front of the cell containing the assembly to be l inspected. The assembly was then lifted so that the top of the fuel pins were within the viewing angle of the canera.

Figure 2 is a schematic of the bulge joints location in relation to the fuel pins. In order to view the bulge joints the camera had to be moved (via ropes) as to look down between the appropriate row of fuel pins and stay within a few inches of the assembly. An under water drop light was then lowered & positioned such that it was just below the top nozzle and also in direct alignment between the appropriate

e row of pins. This was necessary to prevent shadows from being cast onto the bulge joints. This was a very tricky operation and often resulted in just glimpses of the bulges as the light " swung" past the proper angle. During the 1984 inspections the assemblies would be removed from the cell, rotated, then lowered back in so that another set of guide tubes could be inspected. This procedure was repented until all four faces were inspected. During the other inspection campaigns only 1 face (4 guide tubes) was inspected. It was felt that four guide tubes gave an adequate representation of the assemblies condition while reducing the risk / consequences of top nozzle separation by not removing the assembly from the storage cell.

The 1985 & 1986 exams were done with a color high resolution periscope mounted off the spent fuel pit crane.

The enhanced quality of these exams showed details that would very likely have been missed or misinterpreted using the black & white camera. The quality of most of the black &

white inspection was such that if the examiner knew a crack was present he might have been able to distinguish it as one, but if he didn't know it was there he would have missed it.

The use of the periscope also allowed a color VCR tape and/or colored 35mm pictures to be taken during the exam for documentation and subsequent evaluation. The mounting of the periscope was such that the inspections were done on a different set of guide tubes then those inspected in 1982.

RESULTS Table 2 shows the tabulated results of all exams in terms of the number of guide tubes that had any evidence of corrosion vs the numb 9r of guide tubes inspected. This approach differs slightly from that used in Westinghouse's i evaluation which reported the number of bulge joints showing '

signs of corrosion vs the nuaber of bulge joints inspected (Ref #5). NSP prefers the approach in table 2 because; 1.) it includes and accounts for the few instances were the corrosion occurred on the undeformed portion of the sleeve, j

and 2.) it's a better measure of a particular bulge susceptibility to the corrosion attack (ie. if one bulge on a sleeve has corroded the rest will be susceptible to corrosion). Table 2 also shows the number of assemblies from each region of fuel that were inspected and the number that were corroded.

The results show that the corrosion attack spans all of regions 4, 5, & 6 fuel from both unit 1 and unit 2. This virtually eliminates the root cause of the attack being traced back to a sleeving material lot or heat treatment l

I 1

i batch. The results also showed that none of the 18 assemblies from region 1, 2, or 3 had any evidence of corrosion. These assemblies have been in the spent fuel pool the longest and (all other things being equal) should have the highest percentage of corroded guide tubes. There is

,however, a difference in the design between the region.1, 2,

& 3 and regions 4, 5, & 6 fuel. The initial core loading had i

only two mechanical bulge joints attaching the stainless "

steel : sleeve to the RCCA guide tubes, while the reload designs used three bulge joints as shown in figure 1.

Table 3 lists the fuel assemblies where the IGSCC has f progressed to the point where a visible crack could be seen on at least 1 bulge joint. On assembly D-54 the examiners were able to identify a crack on each of the four guide sleeves inspected. This assembly had been inspected in 1982 however, the inspection was done on a different face of the assembly so that only 1/2 of one cracked guide tube was actually inspected twice. The resolution on the video tape from the 82 inspection was not good enough to determine if the crack existed at that time. The suspected crack on assembly F-06 was found during the review of the video tapes after the 1986 inspections and unfortunately the tapes of the 1985 inspection did not provide sufficient detail of the suspected bulge to verify one way or the other the presence of the crack.

CORROSION VS TIME.

To determine if the corrosion attack was still progressing the inspection results were reviewed to determine if any trends could be found. Unfortunately the selection process for the the assemblies in the 84, 85, & 86 campaigns was based on a desire to. expand the data base thus limiting the number of repeated inspections to just three assemblies i available for direct comparison. Of those three assemblies (F-06, D-08, & D-15) two showed no change in the number of corroded bulge joints, while the third (D-15) was reported as having two bulge joints that were corroded in 1985 that were not corroded in 1984. It should be noted however that this change could be attributed to the improved inspection technique used in 1985.

The data in table 2 was reviewed to see if the percentage of corroded guide tubes for a given unit & region of fuel showed signs of increasing with the amount of time the assembly spent cooling in the spent fuel pool. From the table, the frequency of corrosion for the region 4 fuel does not appear to be increasing. For regions 5 & 6 fuel there isn't a sufficient data base within a given region & unit

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7 (only 1 to 3 assemblies inspected during a given campaign) to base any trends on, so the data was combined into 3 groups based on the time spent in the spent fuel pool. The first group in table 4 shows the combine results of fuel inspected with less than 2 years cooling, the second with between 2.5 &

3.9 years cooling, and the third with between 4.2 & 5.2 years. With the data on the region 5 & 6 fuel grouped this way, there does appear to be an increase in the corrosio.n frequency with time. However; it must be stressed that while this grouping does enlarge the data base it is still very small with only 3 out of 160 region'5 & 6 fuel assemblies

.being inspected in 1986. Although it has not been verified, it is the evaluators opinion that the magnitude of the corrosion has increased from inspection to inspection.

Another observation of the data is that it took the region 5

& 6 fuel 4 to 5 years of cooling to experience the same corrosion frequency that region 4 fuel experienced with only 2 to 3 years of cooling.

CONCLUSIONS / FUTURE PLANS Based on the evaluation of the bulge joint inspections, NSP has arrived at the following conclusions:

1. All the fuel assemblies in regions 4, 5, & 6 are susceptible to the corrosion attack.

2. All the fuel assemblies in regions 4, 5, & 6 are susceptible to bulge joint cracking and thus top nozzle separation.

3. The bulge joints are still corroding but that the rate of corrosion has slowed down.

4. The corrosion / cracking can not be attributed to a contaminate in the spent fuel pool.

5. The corrosion / cracking is attributed to the 3-bulge joint design used in regions 4, 5, & 6.

NSP believes that sufficient information has been obtain from the bulge joint inspections to take the following i actions:

)

1. Stop periodic bulge joint inspections at Prairie Island.  !

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2. Suspend all normal handling of all Region 4, 5, &6 k fuel (this action was taken on July 23, 1986 Ref #6).

3. Obtain teoling that will allow handling of region 4, 5, & 6 fuel without relying on the bulge joints for support. <

Sincerely: -.-- .

Y- Q H. O. Nelson Nuclear Engineer, Prairie Island l

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TABLE 1 LIST OF RESEMBLIES INSPECTED FEB '82 iTUL ' 8 2 MAR '84 NOV '85 MAY '86 A0,1 A09 A15 B04 B04 B05 k B22* I CO3 CO3 Cll

{

C32 C39 D02 D02 D03 D03 DOS DOS D06 D08 D08 D08 D09 D12 D12 D12 D14 D15 D15 D15 D16 D21 D21 D21 D25 D30 D34 '

D36 D40 E26 E36 E36 F03 F03 F05 F06 F06 F17 F20 F35 A59 A71 B90 C54 C54 D54 D54 D70 D73 D75 D75 E62 E65 E65 E85 E89 F70

• WAS INSPECTED IN DEC 1983 .

]

TABLE 2 INSEPCTION RESULTS UNIT U1 U2 U1 U2 U1 U2 U1 U2 U1 U2 U1 U2 REGION 1 1 2 2 3 3 4 4 5 5 6 6 FEB '82

.# FA INSP 1 5

1. FA CORR 0 3

1. GT INSP 1 10

1. GT CORR 0 5

% GT CORR 0 50 JUL 82

1. FA INSP 3 3 2 1 10 -3 1 1 2

1. FA CORR 0 0 0 0 7 3 0 1 0

1. GT,INSP 12 16 8 4 41 12 4 4 8

1. GT CORR 0 0 0 0 21 8 0 2 0

.% CT CORR 0 0 0 0 51 67 0 50 0 NAR 84

1. FA ZNSP 2 2 1 2 3

1. FA CORR 0 2 1 2 3

1. GT ZRSP 24 24 12 23 34

1. GT CORR 0 10 1 6 7

% GT CORR 0 42 8 26 20 NOV 85

1. FA INSP 1 1 1 1 1 6 3 1 1 2 O FA CORR 0 0 0 0 0 4 2 1 1 1

1. GT INSP 4 4 4 4 4 25 12 4 5 7 0 GT CORR 0 0 0 0 0 10 6 1 4 1

% GT CORR 0 0 0 0 0 40 50 0 80 14 MAY 86

1. FA INSP 1 3 1 1 1

, # FA CORR 0 3 1 1 1

1. .GT INSP 4 13 4 3 4

1. GT CORR 0 9 3 2 1 1

% GT CORR 0 69 75 67 25 )

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4 TABLE 3 LIST OF ASSEMBLIES WITH CRACKED BULGE JOINTS Fuel Assembly. .Date of Time in # GT Cracked ID # Insp. SFP (yrs) # GT Inspect

+ D-34. Dec 81, 2.6 16/16

• D-73' Jul 82 2.5 1/4 D-54 .Nov 85 3.8 4/4

++ D-70 Nov 85 5.8 1/4

'D-03 May 86 7.1 1/4 SUSPECTED CRACKS

++* F-06 Mar-84. 4.7 1/12 E-85 May 86 5.2 1/4

+ ' Failed'(top nozzel seperated)

- ++ Crack found on 2nd bluge joint

• Found during review of video tape TABLE 4 GROUPING OF REGIONS 5 & 6 FUEL COOLING UNIT REGIOT YEAR # ASS # ASS # GT # GT % GT YEARS INSP INSP CORR INSP CORR CORR GROUP #1 I

. 0.8 1 6 82 2 0 8 0- 0 if 1.4 2 5 82 1 1 4 2 50 1.9 1 5 82 1 0 4 0 0 TOTAL 4 1 16 2 12 GROUP #2 2.5 1 6 84 3 3 34 7 20 3.1 2 5 84 2 2 23 6 26 3.6 1 5 84 1 1 12 1 8 -

3.9 2 6 86 1 1 4 1 25 f TOTAL 7 7 73 15 20 i

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GROUP #3 .,

4.2 1 6 85 2 1 7 1 14  !

4.7 1 6 86 1 1 3 2 67 l 4.8 2 5 85 1 1 5 4 80 5.2 1 5 85 1 1 4 1 25 5.2 2 6 86 1 1 4 3 75 TOTAL 6 5 23 11 47