ML19253C136
| ML19253C136 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 11/20/1979 |
| From: | Engelken R NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V) |
| To: | Crane P PACIFIC GAS & ELECTRIC CO. |
| References | |
| IEB-79-26, NUDOCS 7911300032 | |
| Download: ML19253C136 (1) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION y *,,
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E REGION V o,
ga 1990 N. CALIFORNI A BOULEVARD
',%I./,e" SUITE 202, WALNUT CREEK PLAZA
,,4 WALNUT CREEK, CALIFORNIA 94596 flovember 20, 1979 Docket flo. 50-275 50-323 Pacific Gas and Electric Company 77 Beale Street San Francisco, California 94106 Attention: Mr. Philip A. Crane, Jr.
Assistant General Counsel Gentlemen:
The enclosed IE Bulletin fio. 79-26 is forwarded to you for information, flo written response is required.
If you desire additional information regarding this matter, please contact this office.
Sincerely,
/ dw, xN~
R. H. Engelken Director
Enclosures:
1.
IE Bulletin flo. 79-26 2.
List of Recently Issued IE Bulletins cc w/ enclosures:
J. Worthington, PG&E W. Raymond, PG&E R. Ramsay, PG&E, Diablo Canyon 1443 247 O 61 7911800 4;
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o UtlITED STATES SSIf4S:
6820 fiUCLEAR REGULATORY COMMISS10tl Accession flo.:
0FFICE OF IriSPECTI0fl AtlD Er4FORCEMElli 7910250475 WASHIfiGT0tl, D.C.
20555 tiovember 20, 1979 IE Bulletin ilo. 79-26 BOR0tl LOSS FROM BWR C0:4 TROL 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 compacted BCasheliumandlithiumconcentrktionsgrow.
Once primary coolant penetrates tNecladding(i.e.,thecrackinghasprogressedthroughthecladdingwallandthe helium-lithium pressures are sufficient to open the ggack), boron is 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 furtggr found with similar cracking but with less than 50 percent local depletion of B
, 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 reached when the reactigty worth of the blade is reduced by 10 percent, which corresponds to 42 percent B depletionaveragedoverthetopquarterofthecontrolglade.
Because of the y
leaching mechanism, GE has reduced the allowance for B depletion dveraged 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 af ter refueling, the calculated control blade worths used in the tests are based on the assumption that no borun 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 accidents.
Because the locations of limiting Linear Heat Generation Rate (LHGR), CPR, and Average Planar LHGR (APLHGR) ar monitoring is not affected by b DUPLICATE DOCUMENT ld 4} }f]
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