ML19340D195
| ML19340D195 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 12/23/1980 |
| From: | Sylvia B VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | Clark R, Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8012290280 | |
| Download: ML19340D195 (2) | |
Text
.
g-
}
Vruoxx1A Ex.ncruzc Axn Powna COMPANY U
a l
Racuwown,VamorazA 20261 hg
)
3, I
~
'E
=,', y b
dj D
E:a C
December 23, 1980 2
y g
Mr. Harold R. Denton, Director Serial No. 1015 Office of Nuclear Reactor Regulat.)n N0/SGE:smv Attention:
Mr. Robert A. Clark, Chief Docket No. 50-338 Operating Reactors Branch No. 3 License No. NPF-4 Division of Licensing U. S. Nuclear Regulatory Commission Washington, D. C. 20555
Dear Mr. Denton:
TURBINE MISSILE CONCERN NORTH ANNA UNIT NO. 1 Confirming our dicussions with Mr. V. S. Noonan on December 22, 1980, we believe that there is sufficient conservatism in the calculations to permit the continued operation of the North Anna 1 turbines until January 1,1981 without undue risk of failure.
Specifically, the major areas of conservatism are:
1.
The fracture mechanics analysis used to determine the critical crack size assumes a sharp straight crack, whereas, stress corrosion cracks are branched.
Limited analytical and experimental evidence indicates that the critical size of stress corrosion cracks is at least 1.7 to 2 times greater than that which the fracture mechanics model predicts.
2.
The fracture mechanics model utilized a hypothetical elliptical crack with an a:2c ratio of 1:4. This model adequately decribes the early stages v..en several cracks may grow together to form long, shallow cracks. However, the deeper cracks which have been observed in the Cooper, Zion, and Yankee Rowe turbines all exhibited a:2c ratios approaching 1:2.
The ultrasonic inspection at Farley gave results which indicate that the ratio is not larger than 1:3.5.
l These changes in the a:2c ratio will result in increases in the critical crack size by a factor of 1.1 to 1.8.
3.
The stress used in the analysis contains contributions from rotation, the shrink fit, steam bending, and thermal gradients.
The model used to calculate the thermal gradient stress conservatively assumed l
a severe startup condition.
Under steady state conditions for gradually decreasing loads, which will be representative of the North Anna operation until January 1, 1981, the disk stresses are expected to be at least 10% lower. This will result in an increase in the critical crack size by a factor of approximately 1.2.
)d O h 8012290zy p 5
/*
j
homa ELzcTaic awn Powsm Compawv to Mr. Harold R. Denton, Director Considering the above three factors, it is reasonable to expect that the critical crack sizes used in the analysis are conservative by at least 2.25 (1.7 x 1.1 x 1.2).
The ratio a/aCR can similarly be reduced.
In addition, several other factors need be considered.
In evaluating the risk of failure, it must be remembered that not all disks are observed to crack, and not all cracks propogate at the maximum observed rate.
It should also be noted that the crack growth rate observed in the Farley turbine and used for these analyses greatly exceeds the rates previously observed in many other turbines. Therefore, it is likely that the cracking that occurs at North Anna, if any, will be considerably less than that which occurred at Farley.
To further assure the safe operation of the unit until Jaauary 1,1981, the unit will not be subjected to cyclic operation. We further intend that if the unit is tripped, for any reason, it will remain shutdown until after the turbines are inspected.
I Very truly yours,,
h 9
/
B. R. Sylvia f'V' Manager - Nuclear Operations and Maintenance cc:
Mr. James P. O'Reilly, Director Of fice of Inspection and Enforcement Region II Atlanta, Georgia 30303 Mr. B. Joe Youngblood, Chief Licensing Branch No. 1 Division of Licensing e