ML19253C178
| ML19253C178 | |
| Person / Time | |
|---|---|
| Site: | Hatch  |
| Issue date: | 11/20/1979 |
| From: | James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| To: | John Miller GEORGIA POWER CO. |
| References | |
| IEB-79-26, NUDOCS 7911300105 | |
| Download: ML19253C178 (2) | |
Text
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UNITED STATES 3*, -(g NUCLEAR RF.GULATORY COMMISSION 8
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REGION ll g
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101 MARIETTA ST N.W.. SUITE 3100
%,**U ATLANTA, GEORGIA 30303 In Reply Refer To:
NOV 2 01979 RII: O
-321, 5 Georgia Power Company Attn:
J. H. Miller, Jr.
Executive Vice President 270 Peachtree Street, N.W.
Atlanta, Georgia 30303 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, a
~'%
James P. O'Reilly Director
Enclosures:
1.
List of IE Bulletins Issued In The Last Six Months 79113 00 /d 1443 281
NOV 2 01979 Georgia Power Company.
cc w/ enc 1:
M. Manry, Plant Manager Post Office Box 442 Baxley, Georgia 31513 C. E. Belflower Site QA Supervisor Post Office Box 442 Baxley, Georgia 31513 1443 282
k UNITED STATES SSINS:
6820 NUCLEAR REGULATORY COMM1SSION 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 cracks near the upper end of stainless steel tubing and loss of borcn 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 (BIO) 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 t.arrosion 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 cla(. ding wall and the helium-lithium pressures are Lifficient to open the crack), boron is leached out of the tube at locations with more than 50 percent BIO local depletion (local depletion is considered to be twice the average depletion).
It was further 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 contral blade.
According to the GE criteria the end of design life is reached when the reactivity worth of the blade is reduced by 10 percent, which corresponds to 42 percent B IO depletion averaged over the top quarter of the control blade.
Because of the leaching mechanism, GE has reducci the allowance for BIO 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 calculated control blade worths used in the tests are based on the assumption that no boron loss has occurred.
Reduction in scram reactivity due to boron loss could increase the severity of Critical Power Ratio (CPR) reductions during the plant transients and could increase the consequences of control rod drop ace' Because the locations of limiting Li Average Planar LHGR (APLHGR) are not monitoring is not affected by boron DUPLICATE DOCUMENT } kk3 2rbJ 7
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