ML20002D913
| ML20002D913 | |
| Person / Time | |
|---|---|
| Site: | Big Rock Point File:Consumers Energy icon.png |
| Issue date: | 08/06/1971 |
| From: | Omara J GENERAL ELECTRIC CO. |
| To: | Morris P US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| NUDOCS 8101230394 | |
| Download: ML20002D913 (4) | |
Text
Reguttor,v File Cy, NUCLEAR ENEROY GENERAL
. ELECTRIC DIVISION ATOMIC POWER EQUIPMENT G
AL E ECTR (C COMPANY,175 CURTNER AVE., SAN JOSE, CAllF. 95125 DEPARTMENT e 613 Phone (408) 297-3000. TWX No. 910-338-0116 e-p August 6, 1971 Q ~0-/$5 g}
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. Peter A. Morris, Director 9
- Division of Reactor Licensing AUG 101971 > [,
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Washington, DC 20545 9
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Dear Dr. Morris:
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SUBJECT:
Surveillance of Control Rod Drive Index Tubes Abstract As part of an extensive material development program being conducted by General Electric, four developmental control rod drive index tubes were placed in service at the Big Rock Point reactor in 1964 and removed and examined in 1970.
Three of these index tubes revealed cracks in the nitrided surfaces upon post-service inspection.
This memorandum contains a discussion of the findings to date and a discusssion of why these findings are inconsistent with t'Te expected behavior of the General Electric product li-
' v'ex tube.
Rest ts of Examinations of Big Rock Point Judex Tubes, The material for three of the fcur index tubes was nitrided 304 stainless steel and for the fourth tube, nitrided 309 sta inless steel..
These tubes were manufactured for General Electric by an outside contractor.
These index tubes have bees in service between 1500 and 1900 days in the Big Rock Poi.it reactor.
The first tube, which was of 309 stainless steel material, was removed in February, 1970, and was visually inspected at the Big Rock Point si e.
The inspection indicated the nitrided case to be in good coldition except for a small area at notch #8.
(There are a total of 24 notches with notch #1 at the bottom.)
Subsequent laboratory examination showed a horizontal f]$$N&$$f BE SURE TO INCLUDE MAIL CODE ON RETURN CORRESPONDENCE
i G EN E R A L ([) E LECTRIC Dr. Peter A. Morris August 6, 1971 crack had developed in this area approximately 3/4 inch long and extended through the wall.
The crack was determined to be transgranular with none of the branching which would indicate stress corrosion cracking.
The remaining three tubes, which were of'304 stainless steel material, were removed in June, 1970, and examined at the site by liquid penetrant and ultrasonic techniques.
One of the tubes was free of any detectable indications and was found fit for further reactor service although it has not been reinstalled in the reactor.
The other index tubes showed shallow ultrasonic indications.-
Both tubes showed one indication at notch #8 and one also showed an indication at notch #14 as well.
These two index tubes were subsequently subjected to laboratory examinations which showed nitrided case cracks extending into the base material at the notches.
These extensions were of mixed trancgranular-intergranular nature with little branching tendencies and extended to a maximum depth of 0.020" into the base material.
Nitrided depth is a nominal 0.008".
Other notches examined did not show this condition.
There was no n: iceable general case corrosion, however, a number of areas where the case was cracked or chipped by mechanical impact showed accelerated case corrosion and intergranuler penetrations up to several grains.
The areas however were highly localized.
The results of a detailed metallographic inspection were inconclusive and a specific cause for the cracking could not be determined.
Results of Examinations of other Index Tubes The index tubes in the Big Rock Point reactor were only part of the program being carried out to review the effectiveness of surface nitriding with its resultant sensitization of the stainless steel base netal.
Other index tubes have been and will be hot tested incurSan Jose facility and index tubes from other operating plants are examined from time to time.
Examination of any of the index tubes operated in the test facility at San Jose have never revealed cracks.
Seventy-seven index tubes from the Oyster Creek reactor were ultrasonically examined in the laboratory after approximately one year of service and were free of any indications except in these areas where known mechanical distortion had occurred.
(
GENER AL $ ELECTRIC Dr.-Peter A. Morris August 6, 1971 Five to eight control rod drives are examined each year at KRB during routine maintenance.
Some of the drives have bec in service for five years and the most recent ultrasonic examination (June-July, 1971) has shown them to be free of any indications.
Although none of the other index tubes examined have seen the length of service of those in Big Rock Point, they are more representative of the General Electric BWR product line.
Conclusions The only nitrided index tubes to date which have shown some crack indications are those removed from Big Rock Point.
These cracks appear to be primarily transgranular on mixed mode transgranular-intergranular and not related to stress corrosion cracking.
This theory is consistent with the low stress levels imposed on the index tubes.
During 99% of their life, they support only the driveline weight of less than 150 pounds and do not see any significant stress levels except during scram.
In addition, because the index tubes for the Big Rock Point reactor were developmental and wtre supplied to General Electric,'the possibility exists that excessive mechanical-work could have been done on the tube prior to being placed in the reactor thus leading to adverse observations inconsistent with all other experience.
A review of the detailed stress analysis of the index tube design has been completed and it was again concluded that the stress loadings are within the design requirements even during scram.
Presently available data do not indicate the existence of a generic problem..The only adverse data appears to be from a non-standard situation in which some developmental index tubes have sustained some cracks, the cause of which is not directly determinable.
In addition, the design of the control rod driveline has undergone a change since Big Rock Point such that the index tube on all later plants runs in a guide tube and is shielded from direct exposure to reactor coolant flow.
General Electric plans to carry out additional index tube surveillance wherever possible.
An ultrasonic inspection similar to the one just completed at KRB will be carried out for either the Nine Mile Point or Oyster Creek plant.
4 General Electric will keep the Atomic Energy Commission informed of any significant information which develops from this program.
Very truly yours,
,khf J.
F. O'Mara Acting Manager Design Engineering
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