ML19253C396
| ML19253C396 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 11/20/1979 |
| From: | Grier B NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| To: | Groce R YANKEE ATOMIC ELECTRIC CO. |
| References | |
| NUDOCS 7912050337 | |
| Download: ML19253C396 (1) | |
Text
y(o** " tote,Io, ON UNITED STATES y y'3 s,( [,g NUCLEAR REGULATORY COMMISSION
% JA "'/ ; [E REGION I 0, $5d 631 PARK AVENUE
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,o KING OF PRUSSIA, PENNSYLVANIA 19406 Docket No. 50-29 Yankee Atomic Electric 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, 3-W Sw oyce H. Grier h rector
Enclosures:
1.
List of Recently Issued IE Bulletins CONTACT:
D. L Caphton (215-337-5266) cc w/encls:
H. Autio, Plant Superintendent L. E. Minnick, President 1504 152 1
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7 9120503 3 D
ENCLOSURE 1 UNITED STATES SSINS:
6820 NUCLEAR REGULATORY COMMISSION Accession No.:
OFFICE 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 g nfined 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 compacted g
B C as helium and lithium concentrdtions grow.
Once primary coolant penetrates 4tne cladding (i.e., the cracking has progressed through the cladding wall and thehelium-lithiumpresuresaresufficienttoopenthecrack),gronis leached out of the tube at locations with more than 50 percent B local depletion (local deple'. ion is considered to be twice the average depletion).
It was further found vith similar cracking but with less than 50 percent local y
depletion of B
, that i aching did not occur even though primary coolant had m
penetrated 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 wgthofthebladeisreducedby10 percent,whichcorrespondsto42 percent B
depletionaveragedoverthetopquarterofthecontrolglade.
Because of the leaching mechanism, GE has reduced the allowance for B depletion averaged 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 capability is demonstrated by shutdown margin tests after refueling, the calcu tests are based on the assumption that in scram reactivity due to boron loss c Power Ratio (CPR) reductions during the DUPLICATE DOCUMENT the consequences of control rod drop ac Entire document previously entered into system unde :
1504 153
^"o79/
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