ML19253C129
| ML19253C129 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 11/20/1979 |
| From: | Engelken R NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V) |
| To: | Van Brunt E ARIZONA PUBLIC SERVICE CO. (FORMERLY ARIZONA NUCLEAR |
| References | |
| IEB-79-26, NUDOCS 7911300023 | |
| Download: ML19253C129 (1) | |
Text
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'o UNITED STATES l
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'b NUCLEAR REGULATORY COMMISSION 8' -
'E REGION V o, i
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1990 N. CAllFORNIA BOULEVARD O
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SUITE 202, WALNUT CREEK PLAZA
- ,,,a WALNUT CREEK, CALIFORNIA 94596 flovember 20, 1979 Docket flos. 50-528 50-529 50-530 Arizona Public Service Ccmpany P. O. Box 21666 Phoenix, Arizona 83036 Attention: Mr. E. E. Van Brunt, Jr.
Vice President, Construction Projects Gentlemen:
The enclosed IE Bulletin tio. 79-26 is forwarded to you for information.
rio written response is required.
If you desire additional information regarding this matter, please contact this office.
Sincerely,
,mSL R. H. Engelken Director
Enclosures:
1.
IE Bulletin rio. 79-26 2.
List of Recently Issued IE Bulletins cc w/ enclosures:
F. W. Hartley, APS 1443 243 DN*
7911800
s UNITED STATES SSINS:
6820 NUCLEAR REGULATORY COMMISSION Accession No.:
0FFICE OF INSPECTIGN AND ENFORCEMENT 7910250475 WASHINGT0il, D.C.
20555 November 20, 1979 IE Bulletin No. 79-26 BORON LOSS FROM BWR C04 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 Baron-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 thecladding(i.e.,thecrackinghasprogressedthroughthecladdingwallandthe helium-lithiumpressuresaresufficienttoopentheggack),baronisleachedout 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 furtgr -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 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 accidents.
Because the locations of limiting Linear Heat Generation Rate (LHGR), CPR, and Average Planar LHGR (APLHGR) monitoring is not affected by DUPLICATE DOCUMENT Entire document previously entered into system under:
ANO No. of pa es: