ML19210E557
| ML19210E557 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 11/20/1979 |
| From: | Grier B NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| To: | Groce R Maine Yankee |
| References | |
| NUDOCS 7912050251 | |
| Download: ML19210E557 (1) | |
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UNITED STATES y ', 3.y.,( [,g NUCLEAR REGULATORY COMMISSION
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Docket No. 50-309 Maine Yankee Atomic Power Company ATTN:
Mr. Robert H. Groce Licensing Engineer 20 Turnpike Road Westborough, Massachusetts 01581 Gentlemen:
The enclosed IE Bulletin 79-26 is forwarded to you for information. No written response is required. If you desire additional information regarding this matter, please contact this office.
Sincerely, cz, NI, s% -
oyce H. Grier Director
Enclosures:
1.
List of Recently Issued IE Bulletins CONTACT:
D. L. Caphton (215-337-5266) cc w/encls:
E. Wood, Plant Superintendent E. W. Thurlow, President 1500 138 7912050
ENCLOSURE 1 UNITED STATES SSINS:
6820 NUCLEAR REGULATORY COMMISSION Accession No.:
0FFICE OF INSPECTION AND ENFORCEMENT 7910250475 WASHINGTON, D.C.
20555 IE Bulletin No. 79-26 Date: November 20, 1979 Page 1 of 4 BORON LOSS FROM BWR CONTROL BLADES Description of Circumstances:
The General Electric Company (GE) has informed us of a failure mode for control blades which can cause a loss of boron poison material.
Hot cell examinations of both foreign and domestic blades have revealed cracks near the upper end of stainless steel tubing and loss of boron from the tubes.
The cracks and boron loss have so far been ggnfined to locations in the poison tubes with more than 50 percent Boron-10 (B ) local depletion.
Observed crack sizes range from a quarter to a half inch in length and from one to two mils in width.
GE has postulated that the cracking is due to stress corrosion induced by solidification of boron carbide (B C) particles and swelling of the comacted g
8 C as helium and lithium concentrHtions grow.
Once primary coolant penetrates 4
the cladding (i.e., the cracking has progressed through the cladding wall and thehelium-lithiumpressuresaresufficienttoopenthecrack),ggronis leached out of the tube at locations with more than 50 percent B local depletion (local depletion is considered to be twice the average depletion).
Itwasfurtherfoundwithsimilarcrackingbutwithlessthan50percentlocal y
depletion of B
, that leaching did not occur even though primary coolant had perstrated the cladding.
The cracking and boron loss shorten the design life of the control blade.
According to the GE criteria the end of design life is reached when the reactivity wggth of the blade is reduced by 10 percent, which corresponds to 42 percent B
depletionaveragedoverthetopquarterofthecontrolglade.
Because of y
the leaching mechanism, GE has reduced the allowance for B depletion cve'. aged over the top quarter of the control blade from the 42 percent value to 34 percent.
The safety significance of boron loss is its impact on shutdown capability and scram reactivity.
Although shutdown ca margin tests after refueling, the calcu tests are based on the assumption that in scram reactivity due to barc: loss c DUPLICATE DOCUMENT Power Ratio (CPR) reductions during the the consequences of control rod drop ac Entire document previously entered into system under:
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1500 139 No. of pages: