ML19253D052
| ML19253D052 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 11/20/1979 |
| From: | Seyfrit K NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| To: | Pilant J NEBRASKA PUBLIC POWER DISTRICT |
| References | |
| NUDOCS 7912120553 | |
| Download: ML19253D052 (1) | |
Text
CEfiTRAL FILES UNITED STATES PDR:HQ
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NUCLEAR REGULATORY COMMISEJON LPDR
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REGloN IV q
2 II9 d,. no 611 RYAN PLAZA DRIVE, SUITE 1000 y
ARLINGTON, TEX AS 76012 131C 0,'
\\lmaj Noveraber 20, 1979 Docket No. 50-298 Nebraska Public Power District ATTN:
J. M. Pilant, Director Licensing & Quality Assurance Post Office Box 499 Columbus, Nebraska 68601 Centlemen:
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,,
vb Karl V.lSeyfrit Directord
Enclosures:
1.
List of Recently Issued IE Bulletins cc:
L. C. Lessor, Superintendent Cooper Nuclear Station Post Office Box 98 Brownville, Nebraska 68321 1540 338 Y912120
t UNITED STATES SSINS:
6820 NUCLEAR REGULATORY COMMISSION Accession No.:
OFFICE OF INSPECTION AND ENFORCEMENT 7910250475 WASilINGTON, 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. Ilot 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 4
B G as helium and lithium concentrations grow.
Once primary coolant penetrates g
trie cladding (i.e., the cracking has progressed through the cladding wall and the helium-lithium pressures are sufficient to open the f5 ck), 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).
Itwasfurtggrfound 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 re-activity ygrth of the blade is reduced by 10 percent, which corresponds to 42 percent B depletionaveragedoverthetopquarterofthecontrolblge Because of the 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 Lin Average Planar LilGR (APLIIGR) are not DUPLICATE DOCUMENT monitoring is not affected by boron Ic Entire document previously enteredintosystemMnder:
xNo 77/og6h T 1540 339 No. of pages:
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