ML19253C190
| ML19253C190 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 11/20/1979 |
| From: | James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| To: | Parris H TENNESSEE VALLEY AUTHORITY |
| References | |
| IEB-79-26, NUDOCS 7911300125 | |
| Download: ML19253C190 (2) | |
Text
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UNITED STATES 8
NUCLEAR REGULATORY COMMISSION o
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E
.E REGION 11 o
101 MARIETTA ST., N.W., SUITE 3100 o
ATLANTA, GEORGIA 30303 NOV 2 01979 In Reply Refer To:
RII: PO
-259, 50-260 0-296 s
Tennessee Valley Authority Attn:
H. G. Parris Manager of Power 500A Chestnut Street Tower II Chattanooga, Tennessee 37401 Gentlemen:
The enclosed IE Bulletin No. 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, n
x -htD (,. s -
3 g
James P. O'Reilly Director
Enclosures:
1.
List of IE Bulletins Issued In The Last Six Month:;
1443 s07 i26 7911300 4
NOV 2 g ggyg Tennes.,ee Valley Authority,
cc w/ encl:
H. L. Abercrombie Plant Superintendent Post Office Box 2000 Decatur, Alabama 35602 J. F. Cox 400 Commerce Street W10C131 C-K Knoxville, Tennessee 37902 D. E. McCloud, Project Engineer 400 Chestnut Street Tower II Chattanooga, Tennessee 37401 E. G. Beasley Tennessee Valley Authority 309 Grant's Building Knoxville, Tennessee 37902 I
UNITED STATES SSINS:
6820 NUCLEAR REGULATORY COMMISSION Accession No.:
OFFICE OF INSPECTION AND ENFORCEMENT 7910250475 WASHINGTON, D.C.
20555 November 20, 1979 IE Bulletin No. 79-26 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 cratks near the upper end of stainless steel tubing and loss of boron from the tubes.
The cracks and boron loss have so far been confined to locations in the poison tubes with more than 50 percent Boron-10 (B10) local depletion. Obrerved 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 4
B C as helium and lithium concentrations grow. Once primary coolant penetrates 4
the cladding (i.e., the cracking has progressed through the cladding wall and the helium-lithium pressures are sufficient to open the crack), Lo..n is leached out of the tube at locations with more than 50 percent B10 local deplet:i.a (local depletion is considered to be twice the average depletior ).
It was farther found with similar cracking but with less than 50 percent local depletion of BIO, that leaching did not occur even 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 r eached when the reactivity worth of the blade is reduced by 10 percent, which corre sponds to 42 percent B10 depletion averaged over the top quarter of the control blade. Because of the leaching mechanism, GE has reduced the allowance for B10 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 t.hutdown capalllity and scram reactivity. Although shutdown capability is demoistrated by nhatdown margin tests after re'ueling, the calculated control blade worths used in the tests are based on the assumptio.i that no boron loss has occurred.
Reduction in scram reactivity due to boron loss could increase the s. verity of Critical Power Ratio (CPR) reductions during the plant transients and :ould increase the consequences of control rod drop acci Because the locations of limiting Lin Average Planar LHGR (APLHGR) are not DUPLICATE DOCUhEFEP
-09 7'
monitoring is not affected by boron 1 Entire document previously entered into system under:
h ANO No. of pages:
9