ML20195J955

Proposed Tech Specs Re Turbine Missile Protection

ML20195J955
Person / Time
Site:	San Onofre
Issue date:	11/23/1998
From:	SOUTHERN CALIFORNIA EDISON CO.
To:
Shared Package
ML20195J951	List: ML20195J946 ML20195J955
References
NUDOCS 9811250129
Download: ML20195J955 (3)

Text

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I= l San Onofre 2&3 FSAR

,, Updated 1

! MISSILE PROTECTION hii),

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• Scn Onofra 2&3 FSAR ly . Updstad i

MISSI12 PROTECTION a

• (i) If the electrical load on the turbine generator were lost and if the

' turbine control and protection systems then failed to prevent continuing HP steam admission then the' turbine might be driven to a very high speed

( runaway) . The speed attained would ultimately be limited by the diminishing. relative velocity of the steam and increasing losses but i

could reach a theoretical limit of 190% of normal speed. At this speed

• the level of stress in the rotor could be sufficient, without the i presence of any contributory defects within the rotor, to cause ductile '

failure and the release of rotor fragments. Since the probability of i

I rotor failure in this circumstance is relatively high, the. probability of missile generation is of similar magnitude to the probability of the

} overspeed occurring.

(ii) If cracks were to initiate in a rotor, by fatigue or stress corrosion,

and were to extend over a period of service to a large size, then there would be a risk of brittle fracture. Failure would be expected if the i combination of crack size and applied stress produced a crack tip stress

! intensity equal to the material Fracture Toughness. If the crack size

became sufficient to cause failure at the routinely applied overspeed

!" test condition then failure would inevitably occur and the probability of failure would depend solely on the probability of the initiation and development of a crack of critical size.

f (iii) If crgeks were to initiate by fatigue or stress corrosion but were not to develop to a size that is critical at the routinely applied overspeed test condition then they might still cause failure, but only at a higher overspeed that was not certain to occur. The probability of failure in such cases is the combined probability of both reaching the overspeed, i.e., the loss of electrical load and a control system failure, and of

' developing a crack of the corresponding critical size.

In the assessment of the probability of generating turbine missiles the ,

probabilities of the turbine reaching specific speeds have been calculated on the basis of the probability of loss of electrical load and probabilities of failure of the control and protection system components. The probabilities of ,

failure of the rotor at any specific speed have been calculated on the basis of r the known stress levels and the probable distributions of material parameters ,

contributing to crack initiation, crack growth, and final failure. 6 GEC determined that with a ten year inspection interval and monthly onload testing of the steam valves the total annual probability of failure (P 3 ) at any speed is less than 1 x 10-5 The ten-year inspection interval for the turbine rotors is controlled by the Preventive Maintenance Program. The monthly onload testing of the steam valves is required by the Units 7 and 3 Licensee controlled .

Specifications. l Since P xP is 3 assumed to be 1 x 10-2, the udssile damage probability is less than or equal to 1 x 104 as required per Reg. Guide 1.115. 1 x 10-' is )

considered an acceptable risk rate for an essential system.

3.5.1.3.3.2 Probability of Missile Strike ( P,) and Damace to Missile

]

Taraets ( Pd_

The value of P2 xP is 3 assumed to be 1 x 10-8 in lieu of performing specific calculations for probabilities of missile strike and damage.

3.5.1.3.4 Turbine overspeed Protection A description of the turbine overspeed protection system, in terms of redundancy, diversity, component reliability, and testing procedures, is provided in subsection 10.3.2.

l 3.5-27 l k