ML19253D058
| ML19253D058 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 11/20/1979 |
| From: | Seyfrit K NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| To: | Millen C PUBLIC SERVICE CO. OF COLORADO |
| References | |
| NUDOCS 7912120559 | |
| Download: ML19253D058 (1) | |
Text
CENTRAL FILES en a'c UNITED STATES
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NUCLEAR REGULATORY COMMISSION LE0'l J~fC U ' '.e.
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R EGION IV 7 i ' - :idi S G11 RYAN PLAZA ORtVE,SulTE 1000 N$1C 5,, ' I, ' # '
o, ;%u- (:j#' lj ARLING TON, TEX AS 76012 November 20, 1979 Docket No. 50-267 Public Service Company of Colorado ATIN:
Mr. C. K. Millen Senior Vice President P. O. Box 840 Denver, Colorado 80201 Centlemen:
The enclosed IE Bulletin No. 79-26 is forwarded to you for information.
No written response is required.
If you desire additional information re-garding this catter, please contact this office.
Sincerely, t
4,, Karl V. Spyf rit Director l'
Enclosures:
1.
List of Recently Issued IE Bulletins t
D. W. Warembourg, Nuclear Production cc:
Manager Fort St. Vrain Nuclear Station P. O. Box 368 Platteville, Colorado 80651 L. Becy, Manager, Quality Assurance 1540 356 N
7912120
9 UNITED STATES SSINS:
6820 NUCLEAR RPGULATORY COMMISSION Accession No.:
OFFICE OF INSPECTION AND ENFORCEMENT 7910250475 WASHINGTON, 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. 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 gnfined to locations in the poison tubes with more than 50 percent Boron-10 (B ) local depletion.
Observed crack sizes range from a quatter 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 gthe cladding (i.e., the cracking has progressed through the cladding wall and the helium-lithium pressures are sufficient to open the gack), 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.
1CAq 7C7 Because the locations of limiting Lin Average Planar LHGR (AFLHGR) are not monitoring is not affected by boron 1 DUPLICATE DOCUMENT Entire document previously entered into system under:
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