ML20205B883
| ML20205B883 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood, 05000000 |
| Issue date: | 03/17/1987 |
| From: | Ainger K COMMONWEALTH EDISON CO. |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| 2865K, NUDOCS 8703300062 | |
| Download: ML20205B883 (4) | |
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$O 4((g, One First Nabonal Plaza Chica0o Illinois
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\\ " j Address Reply to: Post Office Box 767 Ll $ 7 Nd Chicago, minois 60690 0767
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Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555
Subject:
Byron Station Units 1 and 2 Braidwood Station Units 1 and 2 Emergency Diesel Generator Missile Effects NRC Docket Nos. 50-454/455 and 50-456/457
Dear Mr. Denton:
This is to inform you of the results of our recent re-evaluation of the effects of missiles generated from a potential failure of an emergency diesel generator engine at Byron and Braidwood Stations. This review was undertaken as a result of the recent diesel engine failures at Zion Station i
and Palo Verde Station which indicate the crankcase of Cooper-Bessemer diesel engines may not contain all possible missiles. Attachment A documents our analysis which acknowledges missiles are possible but, due to the arrengement of systems at Byron and Braidwood, will not result in loss of safety function beyond the failed diesel. The results of this re-evaluation were discussed with NRC Region III personnel at a meeting on February 20, 1987.
Please direct any questions regarding this matter to this office.
Very truly yours, K. A. Ainger Nuclear Licensing Administrator 1m 8703300062 87031L, PDR ADOCK 05000W57 PDR Attachment p
cc: Region III Office Byron Resident Inspector Braidwood Resident Inspector OS 2865K
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ATTACHMENT A
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Byron /Braidwood Diesel Generator Missile Effects The Byron and Braidwood Stations utilice 20 cylinder ~ Cooper-Bessemer diesel.
generators for Emergency Power requirements. Each Unit is provided with two 100% capacity diesels, each of which powers a redundant electrical division. In the initial' design of the plant, effects of missiles caused by internal failures of the engines were evaluated. At that time it was judged by the manufacturer that no credible missiles could penetrate the crankcase and cause damage external to the engine. As a result, no specific design features were added to protect against internally generated missiles.
Recent failures of similar diesels at the Zion and Palo Verde stations resulted in damage to the crankcase and, at Zion, generation of relatively large missiles which exited from the engine.
In light of these events, the potential for missiles and the possible effects have been re-evaluated in detail.
The analysis indicates that missiles are possible but, due to the arrangement of systems at Byron and Braidwood, will not result in loss of safety function' beyond the failed diesel.
Description of Byron /Braidwood Design Although specific design features were not added to accommodate diesel generator missiles, the design of the station eliminates most potential adverse effects because of the separation included in the layout of the safety systems.
Each diesel is independent of support systems to the extent possible to eliminate common mode failures. The diesels are in separate rooms at the end of the Auxiliary Building. These rooms are protected by fire walls. A review of the potential effects of missiles in either the "A"
or "B" diesel room demonstrates that only one case exists where missiles in ene diesel room could potentially affect functions of the other diesel or any other systems in the redundant division.
This case occurs only in the "A" diesel room where the Essential Service (SX) water Itnes which supply cooling for the "B" diesel pass through the room.
This occurs in both the Unit I and Unit 2 "A" diesel rooms although the routing of the SX piping is somewhat different on each unit. The effects of fire in one diesel room on SX supply to the other was previously evaluated and found to be insignificant but effects of missiles were not reviewed since missiles were not postulated.
If failure of the "A" diesel resulted in damage to the SX lines to the "B"
diesel, the safe shutdown capability of the plant could be impacted.
Missile Generation Criteria Internal failure of a diesel generator, although it has occurred recently, is a relatively rare event.
Only three major potential causes of diesel failure have been identified and steps have been taken to minimize the probability of these events, as described below:
-Overspeed Protection Failure Overspeed is prevented by automatic systems which sense the overspeed condition and shut down the engine as well as alerting the operators.
Overspeed protection is initiated both mechanically and electrically and functions to shut off both air and fuel to the engine.
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-Improper Component Assembly The diesel is a safety related component and is tested, monitored, and maintained under the Quality Assurance Program.
-Internal Parts Failure Quality replacement parts are used in maintaining the diesels.
When defects are identified (e.g. connecting rods at Palo Verde) inspections or upgrades are performed as required.
Nevertheless, the potential effects of missiles have been assessed. In order to evaluate the effects of missiles on the SX piping, it was first necessary to define the potential missile size, energy, and path. In discussion with Cooper-Bessemer, it was determined that the maximum size of a part which could become a missile was 200 lbs which corresponds to a counterweight or a broken section of connecting rod. The maximum initial velocity of the missile was calculated to be 69 fps in a direction perpendicular to the centerline of the crankshaft. This value corresponds to the tangential velocity of a point 1 foot from the crankshaf t centerline (crankpin is at a radius of 8 inches).
In order to exit the crankcase, a missile must strike the engine covers or explosion doors which are located at about the elevation of the crankshaft.
This is the only point on the engine block which is weak enough to fail upon missile impact.
Other areas are heavily reinforced and would withstand the impact. The crankcase failure was assumed to result from cover bolt failure and local fracture of the opening flange.
The minimum energy necessary to cause this failure was calculated to be 36% of the initial energy resulting in a horizontal missile exiting at 48 fps. It was assumed that the missile could be
-deflected up to 25o away from perpendicular but no energy loss was assumed in this deflection. The angle was taken from Regulatory Guide 1.115 on Turbine Missiles.
Effects of Missiles The SX piping in the Unit 1 "A" diesel room is located high in the room, almost 8 feet above the top of the openings in the crankcase, and is protected by structure and other piping. The vertical angle is high (about 60") and there is no clear line of sight between the crankcase doors and the piping. Therefore, there will be no impact on the Unit 1 piping.
The Unit 2 piping is routed lower and there is a clear line of sight between some of the doors and the piping but the lines in question are off the end of t' e engine and the minimum angle with the perpendicular is 32".
Also, the pipe tu high enough that gravity will prevent a hori::ontal missile from striking the pipe.
Therefore we conclude that the pipe will not be impacted.
A more limiting assessment was performed assuming the missiles could leave the crankcase with any vertical and horizontal angle. Again no energy loss was assumed to occur in the deflection.
The Unit 1 piping was found to not be impacted because of intervening structure.
Under these more conservative assumptions the Unit 2 piping could theoretically be impacted. An analysis of
Page 3 the effects of this impact on the piping resulted in a worst case prediction of a 3.5' deflection in the piping at the anchor and damage to some of the supports.
The functional capability of the piping is maintained and plant safety is not adversely impacted.
Conclusion Even under very conservative arbitrary assumptions, diesel failure, will not result in missiles which could cause a common mode failure, and extend the event beyond the loss of one emergency diesel generator.
The analytical techniques used in this assessment result in a postulated event which is more severe than that experienced at Zion. The size of the missiles assumed are somewhat larger than those at Zion and the energy is sufficient to result in a predicted travel of about 30 feet prior striking the floor while the actual missile travel at Zion, was less than 15 feet.
We conclude that the current design is adequate.
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