ML19290E861
| ML19290E861 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 03/10/1980 |
| From: | Groce R Maine Yankee |
| To: | Reid R Office of Nuclear Reactor Regulation |
| References | |
| WMY-80-48, NUDOCS 8003170184 | |
| Download: ML19290E861 (4) | |
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ENGINEERING OFFICE WESTBoRo, MASSACHU3ETTS 01581 7
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March 10, 1980 B.3.2.1 WMY 80-48 United States Nuclear Regulatory Commission Washington, D.C. 20555 Attention:
Office of Nuclear Reactor Regulation Mr. Robert W. Reid, Chief
References:
(1)
License No. DPR-36 (Docket No. 50-309)
(2)
LER No. 80-004-01T-1
Dear Sir:
Sub jec t:
Maine Yankee Fuel Performance As previously reported in Reference 2 and discussed with mc=bers of your staff in recent telephone conversations, a small number of perforated fuel rods were found in Maine Yankee's fuel at the end of Cycle 4.
This letter is in response to a verbal commitment to provide more information regarding the perforated fuel rods.
Based on primary coolant activity levels observed during Cycle 4 operation, a decision was made to sip the core during the January - February 1980 refueling outage to identify failed fuel assemblies.
The sipping program was conducted by Combustion Engineering and nine (9) failed fuel assemblies were identified.
These assemblics, as well as others, were examined visually using the site periscope.
Certain of the fuel rods were eddy current tested and profilometry measurements were made on a small number of rods.
Five (5) assemblies were repaired by replacing failed fuel rom; with fresh fuel rods or solid Zircaloy rods and returned to the core (. Table 1).
Based on past experience, it is believed that c.111 failed fuel assemblies were identified and essentialls all failed rods were removed from assemblies returned to the core.
Analyses performed by YAEC demonstrated that the repaired assemblies satisfy physics and safety analysis criteria for Cycle 5 operation.
It is believed that in the nine failed fuel assemblies, a total of fifteen to twenty five fuel rods failed of which eleven were positively identified.
Most failed rods exhibited a crud pattern, unique to Cycle 4 operation, which appeared between grids 6 and 7 on peripheral fuel rods.
The crud pattern was similar in appearance to a water mark and in general was oval in shape with rather well defined perimeters which at times ran together to produce a chain-like pattern.
At least one of the observed cladding penetrations occurred in one of these areas.
Eddy current examination produced OD signals in the vicinity of some of these crud patterns.
The visual appearance of these areas indicated external corrosion.
8 0 03170 J t 4
U.S. Nuclear Regulatory Commission Attention:
Mr. Robert W. Reid, Chief Page 2 Profilometry measurements showed an increase in ovality at the same location as the crud pattern. The fuel rod diameter was not appreciably different in the area of the crud pattern from adjacent locations.
Peak cladding temperatures were calculated to occur in the region between grids 6 and 7 The abovo would suggest that the principal failure mechanism was external Corrosion.
Based on coolant activity, it would appear that the majority of the fuel, rod failures occurred in the February - March 1979 time period.
A change in core pressure drop trend occurred about December 27 concurrent with a decrease in the primary cool
- ant conductivity.
The conductivity change occurred when the primary system resin beds uere changed.
The core pressure drop increased during the period between December 27 and the March 15 shutdown.
Similiar changes in core Ap have been noted in previous cycles.
Core pressure drop data is not availaole after March 1979; however, based on core flow data which is available, it would appear that the core pressure drop did not continue to increase at the rate observed in the January - March 1979 period.
An increase in the activity level and concentration of the crud in the primary coolant following the March - June outage is indicative of crud removal from the core.
Steady state, at power, physics analysis performed by YAEC has shown tha.t the peripheral rods are not the peak heat flux rods in any fuel assembly and no large heat flux gradient is evident across these rods.
Combustion Engineering has performed scoping analyses which support these results.
CONCLUSIONS:
The Batch H fuel rod failures observed at the end of Cycle 4 were most likely caused by external corrosion associated with crud deposition during the January - March 1979 time period.
The inspection repair program which has been completed was adequate to identify all failed fuel assemblies and minimize the return of failed fuel rods to the core.
Further degradation of the fuel during Cycle 5 operation is not expected.
Combustion Engineering is preparing a report based on their inspection work.
This report is expected to be available by March 31, 1980.
No additional inspections are presently planned.
However, we are still assessing the potential value of further inspection and will advice you should any additional programs be developed or undertaken.
l 6
U.S. !!uclear Regulatory Commission Attention:
Mr. Robert W. Reid, Chief Page 3 We trust this information is satisfactory; however, should you desire additional information, please feel free to contact us.
Very truly yours, MAINE YANKEE ATOMIC POWER COMPANY tw R. 11. Groce Senior Engineer, Licensing Jdil/sec O
9
.1 TABLE 1 Cycles Cycles No. Failed Rods S/N Type Scheduled Achieved Rods (3)
Replaced EF00SE E-16 2
2
>1 EF007W E-16 2
2
>1 Discharged EF0072 E-16 2
2
>1 EF009E F-0 3
3
>1
- u
- xx
- n*****************
G123 H-0 3
2 2
4 (2)
G132 H-0 3
2 1
5 (2)
Returned to H208 I-4 3
1 5
19 (3)
Core H214 I-4 3
1 1
9 (3)
H218 I-4 3
1 1
7 (4)..
O
^
(1)
Minimal inspection was performed on the E & F, fuel.
(2)
Solid Zircaloy rods.
(3)
Fresh 1.95 w/o UO2 rods.
(4)
] solid Zircaloy rod plus 6 fresh 1.95 w/o UO2 rods.
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