ML20084F729

Supplemental AO 251-75-4:on 750312,broken Lateral Restraints for Spent Fuel Racks Discovered.Caused by Stress Corrosion. Original Springs Replaced.On 750321,broken Restraint Found on Unit 3.Insp Underway

ML20084F729
Person / Time
Site:	Turkey Point
Issue date:	05/01/1975
From:	Schmidt A FLORIDA POWER & LIGHT CO.
To:	Rusche B Office of Nuclear Reactor Regulation
Shared Package
ML20084F705	List: ML20084F703 ML20084F716 ML20084F729 ML20084F745
References
AO-251-75-4, NUDOCS 8304210259
Download: ML20084F729 (3)

Text

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FLOR;DA POWER f. LIGHT COMPANY May 1, 1975

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Mr. Benard C. Rusche, Director '

Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Co:smission ,Vg//

Uashington, DC 20555

Dear Mr. Rusche:

ABNORMAL OCCURRENCE NO. 251-75-4 OCCURRENCE DATE: MARCH 12, 1975 SUPPLEMENTARY REPORT TURKEY POINT UNIT NO. 4 LATERAL RESTRAINTS FOR SPENT FUEL RACKS BROKEN I INTRODUCTION Our abnormal occurrence report 253-75-4 on the broken spent fuel rack lateral restraints 0.eaf springs) dated March 21, 1975, indicated a supplementary report would be issued if additional corrective action was required. . This supplemen- ~

tary report provides information relative to the results of our investigations which were incomplete at the time of-the abnormal occurrence letter dated March 21, 1975 and also provides information relative t'o the additional corrective action taken.

Subsequent to our March 12, 1975 inspection,an additi.onal seven springs were discovered broken on March 27, 1975.

This information was verbally given to the NRC-I&E inspectors on site at that time and is discussed in their inspection report Nos. 50-250/75-4 and 50-251/75-4,. transm'itted to;FPL.

by letter dated April ~17, 1975.

3I . ADDITIONAL INVESTIGATION-AND RESULTS As mentioned in the previous report one of the-broken springs was sent to a laboratory for metallurgical analysis. This analysis indicates that the spring failed in a stress corrosion mode. There was-no evidence of hydrogen embrittlenent or.

qE iich cisicW ng. The analysis further revealed a high inclusion content but the inclusion was determined to be; insufficient to have caused the spring failure. Chemical analysis showed that the material was as specified.

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. 'Mr. Benard C. Rusche, Director May 1, 1975 l Page 2 l

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l Purther investigation included a load test of an unbroken spring. An unbroken spring was removed fron the spent fuel l The spring functioned l

racks and loaded to design capacity.

l satisfactorily.

l In addition to the load test, the restraint design was re-l viewed by the original designer - the plant engineer constructor.

As a result of the design review they concluded that the restraint design was satisfactory. The design review also included an investigation into the possibility of utilizing a rigid restraint in place of the spring type of design. As a result of this investigation they concluded that use of rigid restraints was not feasible with the existing spent

! fuel rack design and continued use of the spring type of design was recommended.

l III ADDITIONAL CORRECTIVE ACTION Because of the additional failed springs that were found on March 27, 1975, it was decided to replace all of the original springs installed on the Unit No. 4 spent fuel racks. Based on the above investigations it was concluded that the springs should be of the same basic design as the original springs exce'pt that the improved fabrication procedures should be utilized.

In procuring the material for fabrication of the replacement springs, it became necessary to obtain a substitute for the originally used 420 stainless steel which was not available.

Based on metallurgical properties and availability, 440 Cad stainless steel was selected as a suitable substitute material.

The replacement springs were then fabricated, utilizing the following special quality control measures:

a) The hole in the spring was drilled rather than punched.

b) Sharp edges that could possibly result in " stress risers" were removed.

c) The bonds in the spring were hot formed.

l d) The springs were de-scaled by sandblasting after heat treatment, c) A liquid penetrant examination was conducted after heat treatment.

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Mr. Benard C. Rusche, Director May 1, 1975 Page 3 7

f) A hardness test was performed after heat treatment.  ;

g) A replacement spring was load tested to verify design adequacy. ,

The springs were installed on the Unit No. 4 spent fuel racks utilizing an approved procedure. The installation was com-pleted on April 5, 1975.

IV UNIT No. 3 As stated in our March 21, 1975 Abnormal Occurrence letter, a broken restraint has been found by visual inspection in 3

the Unit No. 3 spent fuel pool. We are presently conducting an underwater television inspection of the Unit No. 3 spent fuel racks to determine if more restraints are broken. Our review of the spent fuel rack design with the plant engineer constructor revealed that the racks will satisfactorily maintain their function undqr the Turkey Point design seismic conditions with one lateral restraint missing on each of the four spent fuel pool walls. Thus, the single broken restraint that exists on Unit No. 3 will not render the restraint system inoperable.

He will advise you of the results of the Unit No. 3 inspection as soon as they are available. We will also advise you regarding plans for any additional corrective action at that time.

Very truly yours,

@[g/d..4 b A.D. Schmidt Vice President l Power Resources IINP/dd cc: Mr. Norman C. Moseley Jack R. Newman, Esquire i

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