ML20028C003
| ML20028C003 | |
| Person / Time | |
|---|---|
| Issue date: | 12/30/1982 |
| From: | Boyle M, Hermann R Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| NUDOCS 8301050207 | |
| Download: ML20028C003 (4) | |
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ALTERNATIVE PIPE BREAK CRITERIA AND
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EVALUATIONS FOR SEP PLANTS INTRODUCTORY STATEMENT The application of current high energy pipe break criteria to those operating plants under review in the NRC's Systematic Evaluation Program (SEP) has iden-tified the impracticality of the plant modifications that would be necessary to to mitigate the consequences of certain postulated pipe breaks.
In response to this situation, the NRC staff has developed an integrated approach that will provide a means of adequate alternative protection.
This approach utilizes the defense-in-depth philosophy and consists of augmented inservice inspection (ISI),
leak detection and fracture mechanics evaluations.
DISCUSSION The current licensing criteria against which the SEP plants are evaluated are presented in Standard Review Plan Section 3.6.2, " Determination of Break Locations and Dynamic Effects Assoc.iated with the Postulated Rupture of Piping," NUREG-0800.
When deviations from the review criteria are identified engineering judgment is used to evaluate the consequences of the postulated pipe break to assure that the break would not cause loss of required function of safety-related structures, systems and components, and to assure that the plant can be safely shutdown.
The NRC staff has developed alternative criteria to be used in those cases where it is 1
impractical to modify a plant to protect against the effects of pipe break.
The alternative criteria are based on an integrated application of augmented ISI, leak detection and fracture mechanics evaluations.
These criteria rely upori the ability to identify (1) incipient degradation in piping by nondestructive volu-metric examination, (2) leakage in small through-wall cracks; and (3) the inherent stability of the piping material.
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1 Augmented ISI would be performed with the goal of detection and limiting service induced flaws to sizes allowable in the requirements of Section XI of the ASME Boiler and Pressure Vessel Code (ASME Code).
Piping for which this augmented in-spection is to be conducted should be inspected to ASME Code Class 1 requirements regardless of the actual classification of the piping.
Should flaws go undetected, a leakage detection system should be provided with l
the requisite sensitivity to identify leakage from a small crack for the minimum flow rates associated with normal (ASME Code Level A) operating conditions.
The existing plant leakage detection system should be evaluated against the requisite sensitivity and either be accepted as adequate, or modified or augmented by a local leakage detection system.
Fracture mechanics evaluations should be performed to detemine, for a through crack of length four times the wall thickness (4t) subjected to maximum design loads (ASME Code Level D), that (1) substantial monotonic crack growth does not occur, (2) local or general plastic instability does not occur; and (3) flow l
througn the crack does not impair the function of necessary safety-related systems.
Also, to provide assurance that a pipe break would not occur due to unanticipatei loads being applied to a large undetected crack, a fracture mechanics evaluation should be performed to demonstrate that a 4t through crack (or 90* circumferential crack, whichever is greater) would remain stable for local fully plastic large defomation bending conditions.
1 As an aid in the evaluation procedure, the NRC staff has refined existing analy-tical methods and written a leakage rate estimation program named LKRATE*.
This program calculates flow rates out of through cracks taking into consideration l
crack opening areas, crack surface roughness, and system geometry and operating conditions.
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Currently greater than half of the SEP plants have identified the need to utilize the alternative criteria for pipe break evaluations.
The findings of the staff review of these plants are documented in the respective plant Integrated Safety Assessment Reports.
These reports have been subjected to peer review by consultants and by the NRC Advisory Committee on Reactor Safe-guards.
Final Commission approval of the alternative criteria and the report findings has not, to date, been received.
CLOSING STATEMENT The criteria described above has been proposed and utilized by the NRC staff for the resolution of high energy pipe break issues for the SEP plants.
The adequacy of the alternative criteria is based upon the margin of safety that exists between incipient cracks detected by ISI or small through cracks sub-jected to ASME Code Level A loading detected by leakaged detection systems, and the point of instability in piping associated with large cracks and ASME Code Level D loading.
Also, the criteria adequately protect against the ad-(
verse effects of jet spray on safety-related equipment.
l The results of the SEP reviews have been documented and subject to peer review.
Commission approval of the review criteria and evaluations is still pending.
However, even upon Commission approval, these alternative criteria should not be construed as a staff position that is generically applicable to routine licensing applications.
4 Copies of the LKRATE program can be obtained upon request from the U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation, Systematic Evaluation Program Branch, Washington, D.C. 20555.
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