ML19263F284
| ML19263F284 | |
| Person / Time | |
|---|---|
| Issue date: | 06/04/1979 |
| From: | Hazelton W, Rachel Johnson, Snaider R Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| NUDOCS 8001110403 | |
| Download: ML19263F284 (10) | |
Text
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SAFETY EVALUATION FOR THE GENERAL ELECTRIC TOPICAL REPORT a
o BWR FEEDWATER N0ZZLE/SPARGER FINAL REPORT (NEDE-21821 AND NEDE-21821-01)
PREPARED BY OFFICE OF NUCLEAR REACTOR REGULATION U. S. NUCLEAR REGULATORY COMMISSION REVIEWERS:
R. P. Snaider, Systematic Evaluation Program Branch, Division of Operating Reactors (Task Manager)
R. E. Johnson, Engineering Branch, Division of Operating Reactors W. S. Hazelton, Engineering Branch, Division of Operating Reactors R. W. Klecker, Engineering Branch, Division of Operating Reactors J. J. Zudans, Engineerin~g Branch, Division of Operating Reactors R. K. Mattu, Mechanical Engineering Branch, Division of Systems Safety M. R. Hum, Materials Engineering Branch, Division of Systems Safety S. D. MacKay, Plant Systems Branch, Division of Operating Reactors
[t t*IU s
P. N. Randall, Structures and Components Standards Ec.ica, Office of Standards Development 2213 185 800o10V03
BWR FEEDWATER N0ZZLE/SPARGER FINAL REPORT SAFETY EVALUATION TABLE OF CONTENTS
1.0 INTRODUCTION
AND CONCLUSIONS
2.0 BACKGROUND
3.0 DESCRIPTION
OF SOLUTIONS 4.0 VERIFICATION OF SOLU' IONS 5.0 OTHER SPARGER DESIGNS 6.0 ULTRASONIC TESTING AND RECOMMENDLu.dSPECTIONS 7.0 IMPLEMENTATION 8.0 SAFETY CONSIDERATIONS
1.0 INTRODUCTION
AND CONCLUSIONS By' letter dated May 2, 1978, the General Electric Company (GE) submitted for staff review a topical report entitled " Boiling Water Reactor Feedwater Nozzle /Sparger Final Rpport".
By letter dated February 20, 1979, GE sub-mitted a related report, NEDE-21821-01, entitled " Boil'ng Water Reactor Feedwater Nozzle /Sparger Final Report - Supplement 1".
These reports, including changes forwarded by GE letters dated March 1, 1979, March 20, 1979, and
, provide generic information relative to (1) the design of a highly modified feedwater sparger and thermal sleeve assembly; (2) testing and analysis of this design; (3) analysis of nozzle cracking and its safety implications, (4) analysis of other modifications, such as nozzle clad removal and system changes, which would further serve to prevent cracking or decrease the rate of crack propagation; and (5) discussion of new, enhanced Non-Destructive Examination methods and their recommended application to the inspection of BWR nozzles.
The NRC review of the topical reports addressed the generic design and analyses of the modified sparger/ sleeve assembly, the descriptions and analyses vf the other available solutions, the verification of solution reasonableness and ability to withstand BWR environmental conditions during design lifetimes, and the capability and limitations of the proposed inservice inspection program and proposed frequency of inspection.
Based on our review and evaluation of the information provided by GE, the staff finds the GE topical reports, NEDE-21821 and NEDE-21821-01, as supplemented and with specific exceptions as noted herein, to be acceptable for reference in BWR licensing actions, either for the modification of licensed plants or for the licensing of future plants. Additional plant-specific information may be required during the course of these licensing 2213 187
O ETf*it)CD actions and will be reg:c;ted at the time of request submittal from the licensee or applicant. Also, this portion of the NRC's generic review did not include, and therefore will not serve as, documentation of the effort applied to review of the control rod drive return line nozzle cracking issue or the removal of the control rod drive return line. These matters will be covered in separate NRC correspondence.
We have concluded that the proposed GE thermal sleeve /sparger modification, srl rrrt,wmL :xtrk. <>mscs owrn tJEccufh whencoupledwiththeremovaloftheexistingstainlesssteelcladdly is a substantial improvement over the original GE designs and will enable a reactor vessel thus modified to operate an extended period of time between internal nozzle surface examinations, thus reducing substantially the radiation burden upon-the plant staff and contractor personnel. However, we cannot state that this specific sparger and thermal sleeve configuration is the only design which will prove to be acceptable to the NRC.
In fact, we have already approved the installation and use of a similar, non-GE design at two operating plants. Any design approved for use will of course receive in-service evaluation to determine itScontinued acceptability.
One area which must be highlighted here, as it.is in the specific evaluation which follows, is the matter of inservice inspection interval and the use of certain non-destructive examination (NDE) techniques, particularly ultrasonic testing. The NRC staff is not ready to accept the industry claims regarding the improved sensitivity of the various new ultrasonic testing techniques and will, in its forthcoming NUREG report (to be published at the completion of the ongoing generic program) specify acceptable NDE 2213 188
metiods and inspection intervale Changes to the NRC program may be forthcoming as the result of industry-sponsored NDE studies being undertaken by the Electric Power Research Institute.
2.0 BACKCROUND Of the 23 operating BWRs with feedwater nozzle /sparger systems (normally 4 nozzles /spargers per BWR, nominal nozzle diameter being 10" - 12"), 22 have been inspected to date resulting in the discovery of blend radius or bore cracking in all but four vessels. Although most cracks have been in t.a range of 1/2" to 3/4" total depth (including cladding), one crack pene-trated the cladding into the base metal for a total depth of approximately 1.50 inches. The initiation of cracking is due to high cycle fatigue caused by fluctuations in water temperature within the vessel in the sparger-nozzle region during periods of low feedwater temperature when the flow may be unsteady and intermittent. Once initiated, the cracks are driven deeper by the larger pressure and thermal cycles associated with startup and shutdown.
Fracture analyses indicated that the cracks found to date in the feedwater nozzle constitute a potential safety problem because the observed rate of crack growth with time in service is such that the margin of safety against fracture will be reduced below acceptable values unless the cracks are detected and ground out every few years. Obviously, repair by grindout can be repeated only a few times before ASME Code limits for nozzle reinforce-ment are exceeded. However, repair by welding buildup of the grindout has not been demonstrated to be acceptable.
In addition, the inspection and removal of cracks by grinding has caused enough radiation exposure to 2213 189
personnel to be deemed unacceptable as a long-term solution.
Extensive study of the problem has been undertaken by the General Electric Company (GE) with review by the NRC staff. The culmination of the various studies has been the publicaticn of the two documents evaluated herein.
These documents represent only a summation of the engineering design, test, and development effort undertaken and accomplished by GE.
A safety objective of this significant undertaking was to produce a effi-cacious sparger/ thermal sleeve design which would not only assure long-term reactor ve:,sel integrity but would also lengthen the time between in-vessel inspections of the nozzle. The NRC staff believes this objective to have been met.
5.0 OTHER SPARGER DESIGNS The GE report briefly discusses three alternative sparger designs, each of which is seeing service use today. The first of these designs, the welded sparger, is in use at three operating reactors (Duane Arnold, Brunswick Unit No. 1, and Hatch Unit No. 2) and has been installed in one other reactor (Zimmer) for which licensing is still in progress.
The staff generally agrees with the GE assessment that this design, with the thermal sleeve welded at the nozzle safe end, provides the most assurance of protection against crack initiation. However, as GE has noted in the report, there are several drawbacks to this particular design.
Not noted, and foremost in the mir.ds of the NRC staff, is the lack of suitable 220 M0
inspectability of the thermal sleeve-to-nozzle weld. Our concern is that weld failure could result in substantial bore cracking prior to the appearance of cracking on the accessible areas of the blend radius. The staff is still devoting effort to the resolution of the inservice inspection issue, as noted in the introduction and sections 6.0 and 7.0 of this SER.
However, dye penetrant inspections of accessible nozzle areas performed already at Duane Arnold and Brunswick Unit No. I have proven the success of the welded design early in the plant life, in that no indications of cracking were found.
In addition, a limited visual inspection of the sleeve-to-nozzle weld was performed at Duane Arnold, where sparger design allowed such inspection. The weld was noted to be intact.
The second design noted by General Electric is the single piston ring design, which is simply an augmentation of the interference fit sleeve design and would serve, in the opinion of the staff, as an interim "fix" only. GE ccknowledges this in their statement that the "... interference fit will not be lost during the limited design life of this component."
The only o rating plant with an installed sparger/ sleeve similar to this is the Monticello Nuclear Generating Plant. The cladding was removed from Monticello in 1977 and this interim design installed with a 13 mil
(.013") diametral interference fit. The staff will continue to monitor nozzle inspections at this plant in order to determine the efficacy of this particular design.
The final design discussed by GE is the interference fit thermal sleeve design, which evolved after the discovery of cracking in the original loose-fit sparger designs. This design has been proven to be an effective 2213 191
method for the prevention of crack initiation, but its longevity is limited by the degradation of the interference fit with time. Therefore, although it has proved to be a satisfactory stopgap method, the staff will not normally accept it as a long-term replacement without increasing the fre-quency of inservice inspection.
Although of course not mentioned in the GE report, there are other designs in use in operating reactors today, notably at Nine Mile Point and Oyster Creek. The staff, while accepting the GE final design as an efficacious long-term solution, will of course review other proposed designs for acceptability.
8.0 IMPLEMENTATION The staff concurs with the General Electric Company position on implementation, especially in the statement that changes for operating plants should be implemented as soon as possible. We also concur, based on the experience of plants which have already removed nozzle cladding and installed advanced design sparger/ thermal sleeves, that the work may be deferred until a lengthy outage is required for other reasons, such as work on the pressure-suppression pool (torus). We will maintain our position as stated in the interim guidance document)NUREG-0312 (July 1977), that credit will be Such actions, given for actions taken to prevent crack initiation and growth.
to[includesystemmodificationsandproceduralchanges,canservetolengthen the time between in-vessel inspections.
2213 192
7 We must also emphasize here that the GE final design sparger/ thermal sleeve is not the only unit to have gained NRC approval, and our further review of system changes may lead us to conclude that the combination of system changes and a sparger/ thermal sleeve other than the GE final design is an acceptable long-term solution. Any such determination would be stated in the NUREG document which serves to complete the NRC generic program.
While the staff also concurs that plants with welded thermal sleeves (see Section 5.0) need only implement system changes at this time, we will reserve comment on the ability of the weld to remain intact for design plant life until further inspections are performed on these relatively new plants.
2213 193
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