ML19261D010
| ML19261D010 | |
| Person / Time | |
|---|---|
| Issue date: | 04/09/1979 |
| From: | Coffman F Office of Nuclear Reactor Regulation |
| To: | Check P Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 7904230340 | |
| Download: ML19261D010 (5) | |
Text
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S, UNITED STATES y ;f
't NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 2055s s,
APR 0 9 EI9 MEMORANDUM FOR:
Paul S. Check, Chief, Reactor Safety Branch, Division of Operating Reactors FROM:
Franklin D. Coffman, Section Leader, Section B, Reactor Safety Branch, Division of Operating Reactors
SUBJECT:
MEETING
SUMMARY
- OPERATING EXPERIENCE WITH CONTROL -
RODS A meeting was held in Bethesda on March 22, 1979, between General Electric Company representatives and NRC staff members on the subject of recent observations from hot-cell examinations of control rods. An attendance list, the agenda, and a meeting summary are attached. The meeting updated previous information and was subsequently supplemented by infomation given C. Grimes, PSB, during his March 1979 trip to Europe.
The GE representatives reported observed cracking, boron carbide loss, and bonding in control-rod tubes at high exposures. The phenomena have the potential to affect the shutdown margin, the scram reactivity, and the enthalpy deposited during a rod drop accident.
GE concludes that all reactor indicators show no problem at current operating reactors.
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Franklin'D. Coffman, Section Leader Section B Reactor Safety Branch Division of Operating Reactors
Enclosures:
As stated cc:
V. Stello/0. Eisenhut R. Vollmer S. Nowicki V. Rooney C. Grimes R. Lobel C. Berlinger S. Weiss K. Xniel Section B. RS/ DOR NRC Attendees 7so42aosfd
w ATTENDANCE LIST MEETING WITH GE -- MARCH 22, 1979 OPERATING EXPERIENCE WITH C0" TROL RODS Name Orcanization P. Check NRC/ DOR /RS3 F. Co ffman NRC/ DOR /RS3 D. Fisher GE/ Engineering L. Gi f ford GE/ Licensing R. Gridley GE/ Licensing D. Houston NRC/ DSS /CPB T. Ippolito NRC/ DOR /ORf3 B. Morris NRC/ DOR /RSB H. Vander Molen NRC/ DOR /RS3 AGENDA I.
I?!TRODUCTION (R. Gridley)
II.
DESIGN BASES FOR CONTROL 3 LADE LIFE (D. Fisher)
III. OPERATING PLANT PERFORMANCE (D. Fisher)
IV.
TEST PROGRAMS (D. Fisher)
A.
High Exposure Tube Examinations 3.
Recent Experience V.
IMPACT EVALUATIONS (D. Fisher)
A.
Cold Shutdown Margins 3.
Scram Reactivity C.
Rod Drop Accident VI.
SUMMARY
(D. Fisher and R. Gridley)
SUMMARY
MEETI?lG WITH GE -- MARCH 22, 1979 During the repair outage at a German SWR following a 1977 full blowcown to containment, the utility examined 45 tubes from one control rod at a hot-cell. The selected control rod had slightly exceeded the then current design life. The utility observed significant tube cracking, a loss of boron carbide and bonding (both cohesive and adhesive). The utility notified GE by September 1978.
Mr. R. Gridley, GE, requested this meeting both to update information provided D. Eisenhut prior to his Fall 1978 trip to Europe and to present GE's conclusion that the new information does not identify an immediate safety probl em.
Dr. D. Fisher, GE, reviewed the design bases considerations for control rod life which were determined by its mechan-ical and nuclear duty. He also reviewed briefly the operating reactor parameters that might indicate control rod worth problems. -He identified the control rod sequence exchange reactivity as the most sensitive re-actor indicator and concluded that there are no control rod worth problems at currently operating BWRs.
Dr. Fisher described the GE test programs which began in 1962 with four control rod tubes irradiated in fuel bundles. The tubes were examined at 60 percent of the then current design life and no anomalies were observed. Again the tubes were examined in 1953 at 80 percent of their then current design life, and GE observed extensive cracking and
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2 baron carbide loss.
In 1974, GE examined five tubes after 6-1/2 oper-ating cycles (about 50 percent of desigr. life) and discovered cracks in one tube with no loss of baron carbide.
In the Fall of 1978 GE examined thirteen tubes at their hot-cells from a control rod at 80 percert of the then current design life. They observed tube cracking, a loss of baron carbide, and bonding.
Dr. Fisher stated that the tubing diametral increases observed were only about one percent. GE plans additional high exposure examinations on control rod tubes from one domestic and three foreign reactors.
Dr. Fisher presented GE's conclusion that the new infomation does not identify an immediate safety problem. The 1968 cbservations did not create concern at GE since the nuclear life of the four test tubes had exceeded their mechanical design life (internal gas pressure limit).
Dr. Fisher reported that the most significant impact of the new informa-tion was on the cold shutdown margin rather than either the scram reactivity or control rod drop enthalpy deposition.
Dr. Fisher reviewed the development of the new design life for GE con-trol rods. The new design life applies more to the reactor than to control rods. Once any control rod exceeds 81 percent of the original design life during a fuel cycle, then that BWR's shutdown margin must exceed the Technical Specification by a " shutdown margin adder." The shutdown margin adder is reactor specific and is calculated by the BWR SI.'iUt.ATOR Code. The code calculation assumes:
(a) all control
3 rods exceeding 81 percent of the original design li fe are degraded, and (b) all degraded control rods in the core have relative worths equal to that of a control rod at the original design life. The shutdown margin is evaluated throughout the cycle relative to that demonstrated at the beginning of the cycle.
For operating reactors Dr. Fisher stated that the shutdown margin has historically exceeded 1 percent AX which is the design target. The Technical Specifications require less than.4 percent 2K.
The impact of the GE " shutdown margin adder" varies among the BWRs and depends upon the number and location of degraded control rods, the fuel loading pattern, the axial exposure, the operating history, and the location of the highest worth rod.
Mr. Gridley summarized the meeting, their conclusion that there is no immediate safety problem, and the GE procedure to advise the li-censees of the design life reductions.
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