ML19253C395
| ML19253C395 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 11/20/1979 |
| From: | Grier B NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| To: | Groce R VERMONT YANKEE NUCLEAR POWER CORP. |
| References | |
| NUDOCS 7912050335 | |
| Download: ML19253C395 (1) | |
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Docket No. 50-271 NOV 2 0 is73 Vermont Yankee Nuclear Power Corporation 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 action. A written response is required. If you desire additional information regarding this matter, please contact this office.
Sincerely,
- m. h e oyce H. Crier irector
Enclosures:
1.
List of Recently Issued IE Bulletins CONTACT:
D. L. Caphton (215-337-5266) cc w/encls:
W. F. Conway, Plant Superintendent Ms. J. Abbey, Technical Secretary 1504 150 7912050
ENCLOSURE 1 UNITED STATES SSINS:
6820 NUC $AR REGULATORY COMMISSION Accession No.:
0FFICE OF INSPECTION AND ENFORCEMENT 7910250475 WASHINCTON, 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 losshavesofarbeenggnfinedtolocationsinthepoisontubeswithmorethan 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 soliaification of boron carbide (B C) particles and swelling of the compacted g
B C as helium and lithium concentrdtions grow.
Once primary coolant penetrates g
tne 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).
It was furtherfound with similar cracking but with less than 50 percent local depletion of B
, that leaching did not occur aven though primary coolant had 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 wggth of the blade is reduced by 10 percent, which corresponds to 42 percent B
depletion averaged over the top quarter of the control p ade.
Because of y
the leaching mechanism, GE has redeN d the allowance for 8 depletion averaged over the top quarter of the control blade from the 42 percent value to 34 percent.
The safety significar.ce of boron loss is its impact on shutdown capability and scram reactivity.
Although shutdown capability is demonstrated shutdown margin tests after refueling, the calcul tests are based on the assumption that n in scram reactivity due to boron loss co Power Ratio (CPR) reductions during the DUPLICATE DOCUMENT the consequences of control rod drop acc Entire document previously entered into system under:
1504 iS1 xNO 7%JL6bY7f No. of pages:
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