ML20246L585
| ML20246L585 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 05/10/1989 |
| From: | Zech L NRC COMMISSION (OCM) |
| To: | Bates J HOUSE OF REP. |
| References | |
| NUDOCS 8905180347 | |
| Download: ML20246L585 (4) | |
Text
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UNITED STATES VStello l
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. NUCLEAR HEGULATORY COMMISSION JTaylor i
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JBlaha-May 10, 1989 JMartin, RV
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' CHAIRMAN JPartlow 1
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MVirgilio DMossburg 4467-i JLee 1
The Honorable Jiin Bates DDVGT File
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United States House of Representatives CTrammell j
Washington, D.C.
20515
Dear Congressman Bates:
I am responding to your letters of May 3-and 4, 1989, in which you l
expressed concern regarding the operation el Southern California Edison's (Edison) San Onofre Unit 1 facility with cracked bolts in the reactor vessel thermal shield.
The Nuclear Regulatory Commission (NRC) staff has been aware of the cracked bolts since January 1989.
The thermal shield (see enclosed diagram) is a 2-1/2 inch thick cylinder that surrounds the reactor core barrel.
Its purpose is to absorb gamma rays emitted by the reactor to reduce heating and
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conseouent thermal stress in the reactor vessel wall.
The thermal shield also absorbs high-energy neutrons, reducing irradiation damage to the vessel wall.
Bolts are used to att)ch the botton of the thermal shield to the core Darrel, ano spring-like devices called flexures are attached at the top.
Four lateral displacement limiters are also located at the top.
On January 3, 1989, Southern California Edison conducted a visual inspection of the TAermal shield with a repott camera; 24-of 30 bolts could be visually inspected.
Three bolts out of the 24 inspleted were found to be broken.
In addition, the locking devices for two bolts and tne locking device for one oowel pin had
- cracks, Five out of six flexures at the top of the thermal shield have been broken since 1978.
The flexures were originally aoded to Frovide radial and tangential restraint to ths top of the thermal shield.
When.these were discovered to be brokent an evaluation was made wh?ch cor.cittdeo that the flexures were un-ne ctis s a ry.
As a result of the January inspection, the lictinsee
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found no evidence of damage to or motion of the thermal shield.
On Jariuary 31, 1989, the NRC sent the licensee a letter requiring that this matter be resolved to our satisfaction before plant startup.
The licensee and its consultant, Westinghouse Electric Corporation, provided an analysis of the thermal shield in its present condition, The analysis concludes that the tharmal shield will remain safely in place, or, in t8e unliRely event it should move, flow to the reactor core would not be significantly 1
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i n affected.
Based on the analysis, the licensee has proposed to return Sen Onofre Unit I to power and monitor the condition of the therm 61 shield during operation by using neutron no~1se analysis -
and acoustical techniques.
Although the sixth flexure is no longer necessary, the licensee's monitoring program will. include monitoring of this flexure, since failure of the sixth flexure could be an indication of motion of the thermal shield beyond what is predicted.
The neutron noise analysis would detect a change in vibration should the sixth flexure fail and this would require the licensee to shut down for repairs.
A mid-cycle inspection would:
also be conducted not later than June 30, 1990, to visually confirm the expected behavior of the thermal shield.
If the inspection proves satisfactory, the licensee would operate until about January 1991, at which time repairs would be conducted.
The staff has reviewed the analysis and concurs with the licensee's evaluation subject to the implementation of the monitoring program and mid-cycle inspection.
The review fully addressed the structural integrity of the thermal shield supports, the licensee's ability to monitor significant changes, and the acceptability of the consequences of a failure of the thermal shield supports.
The staff is in the process of fihalizing a license amendment which would allow operation as proposed.
We have received your letter of May 10 1989, requesting a hearing 3
on this matter.
We are considering your request and will respond to it in a sepbrate letter.
Sincerely, 4-pL+ Ar.
Lando W. Zec Jr.
Enclosure:
AL Stated Oripiinated: NRR:Trammell 4
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CORE BARREL
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8' SPECIMEN TUSC
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NO DAMAGE NOTED
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TLEXtJltt 5 0F 6 BROKEN OR CRACKED SINCE 15 g
FIXTURI. MD CHAtiGE NOTED CTYP G PLACES)
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= LIKTER KEY (TYP 4 PL, ACES) l THEsftAL SHIE1.0 n
e THERMAL SHIELD j
SUPPORT 3LDOK W 6 P M S) 5 BOLTS PER SUPPORT BLC 3 BOLTS (TOTAL) BROKEN AFFECTING 2 0F 6 SUPP0F
The Honorable Jim Bates '
We are reviewing the analysis now and expect to have our evaluation completed I
by next week.
Our rev lew will fully adaress the structural integrity of the therral shisld snpports, thi licensee's ability te monitor significant char.ges and the 6 acceptability of the consequer.ces of an unlikely fc.ilure of the
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thermal shield supports. NRC will not approve the proposal to restart the plant unless we are confident that the plant can operwte safely sc proposed.
Sincerely,
(
Lando W. Zech,
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DISTRIBUTION:
i Docket Files (50 206)
EDO Readir.g VStello JTaylor HThompson
.f JBlaha JMartin, RV i
TMurley/JSniezek JPartlow PD-V Reading (w/ incoming.)
GHolahan MVirgilio OGC l
SECY (CRC.89 0439)
DMossburg (ED0 4457-w/inco. ng)
MKrebs (500 4457)
This correspondence addresses olicy CTrarranell (w/ incoming) issues previousTy resolved,by he q
JLee, LA
/
Commission, transmits factual infor-l PD#V Green Ticket File /(EDO 4457) mation, or restates Commission policy.
FMiraglia FGillpspie 1
i DCrutchfield
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l DOCUMENT WAME: green ticket 4457 7
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