ML20204H832
| ML20204H832 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 03/23/1999 |
| From: | Clifford J NRC (Affiliation Not Assigned) |
| To: | Ray H SOUTHERN CALIFORNIA EDISON CO. |
| References | |
| TAC-M97727, TAC-M97728, NUDOCS 9903290172 | |
| Download: ML20204H832 (4) | |
Text
~_ _ _ _
4 l
March 23, 1999 l~
+ -.
i Mr. Harold B. Ray Executive Vice President Southern California Edison Company.
San Onofre Nuclear Generating Station P. O. Box 128 -
San Clemente, CaliforniaL 92674-0128,
?
SUBJECT:
CLCSEOUT OF REVIEW OF SPENT FUEL POOL GATE DROP CALCULATIONS (TAC NOS. M97727 AND M97728)
Dear Mr. Ray:
i.
In a letter dated October 10,1996, Southern California Edison (SCE, or the licensee) submitted a request for NRC review and approval of a calculation for the drop of a spent fuel pool gate in j
the spent fuel pool (SFP). The review was a followup to a concern raiseo during the review of Amendment Application numbers 153 and 137. In the interim period, the licensee committed to not move a SFP gate except in a safe load path, with no fuel assemblies in the drop zone.
During its review, the staff identified several concerns regarding the methodology that the licensee used in its calculations. These concerno are discussed in the enclosure. The staff and the licensee have not been able to reach resolution on these concems. The staff is i
therefore closing its review of this issue to provide the licensee the cpportunity to evaluate the concerns discussed in the enclosure. The staff understands that the licensee is likely to submit a revised request for this issue at some future time. Until this issue is resolved, the staff expects the licensee to continue to conform to its commitment to not move a SFP gate except i
in a safe load path.
If you have any questions related to this letter, please contact me at (301) 415-1352.
j Sincerely,
)
Original sianed by
)
James W. Clifford, Senior Project Manager 1
Project Directorate IV-2 1
Division of Reactor Projects til/IV Office of Nuclear Reactor Regulation j
l Docket Nos. 50-361 DISTRIBUTION:
j and 50-362 4Dochet*%
Enclosure:
Calculational Methodology Concerns PDIV-2 Reading KBrockman, RIV JZwolinski/SBlack LSmith, RIV cc w/ encl: See next page WBateman Skim, SGSB I
JClifford g
Document Name: SO97727.WPD EPeyton h'
v
(
OFC PDIV-2 PDIV-2 NAME-JCli$Dd EN DATE 3/N/99 13 EQ/99
.~w.--..
-~---v OFFICIAL PECORD COPY 9903290172 990323 i
PDR ADOCK 05000361 j
P PDR 4."JUUUU a,
l Mr. Harold B. Ray '
cc w/ encl:
Mr. R. W. Krieger, Vice President Resident inspector / San Onofre NPS Southern California Edison Company clo U.S. Nuclear Regulatory Commission San Onofre Nuclear Generating Station Post Office Box 4329 P. O. Box 128 San Clemente, California 92674 San Clemente, California 92674-0128 Mayor Chairman, Board of Supervisors City of San Clemente j
County of San Diego 100 Avenida Presidio i
1600 Pacific Highway, Room 335 San Clemente, California 92672 Gan Diego, California 92101 Mr. Dwight E. Nunn, Vice President Alan R. Watts, Esq.
Southern California Edison Company Woodruff, Spradlin & Smart San Onofre Nuclear Generating Station 701 S. Parker St. No. 7000 P.O. Box 128 Orange, California 92668-4702 San Clemente, California 92674-0128 Mr. Sherwin Harris Resource Project Manager Public Utilities Department City of Riverside 3900 Main Street Riverside, California 92522 Regional Administrator, Region IV U.S. Nuclear Regulatory Commission Harris Tower & Pavilion 611 Ryan Plaza Drive, Suite 400 Arlington, Texas 76011-8064 Mr. Michael Olson San Onofre Liaison San Diego Gas & Electric Company P.O. Box 1831 San Diego, California 92112-4150
' Mr. Steve Hsu Radiologic Hestth Branch State Department of Health Services Post Office Box 942732 Sacramento, Califomia 94234
NRC STAFF CONCERNS REGARDING SPENT FUEL POOL GATE DROP CALCULATIONAL METHODOLOGY SAN ONOFRE NUCLEAR GENERATING STATION (SONGSt UNITS 2 AND 3 DOCKET NOS. 50-361 AND 50-362 1.
Introduction in a letter dated October 10,1996, Southem Califomia Edison (SCE, or the licensee) submitted a request for NRC review and approval of a calculation for the drop of a spent fuel pool gate in the spent fuel pool (SFP). The review was a followup to a concern raised during the review of Amendment Application numbers 153 and 137. The licensee performed an analysis to demonstrate that a dropped pool gate would not damage more than one fuel assembly.
2.
Discussion The licensee evaluated several modes of a potential gate drop, and calculated the penetration depth (or magnitude of deformation) of the rack's upper structure by equating work done by the descending gate to the work done by the structural member of the rack. The rack structure consists of open square enannels which maintain the fuel assemblies in a vertical position. The channel walls cre 0.11 inch thick stainless steel. The gate weighs about 4000 pounds and is 3/4 inen thick. The gate and rack with fuel assemblies are submerged in the spent fuel pool water. The gate is assumed to drop 30 inches. In the first analysis, the edge of the 3/4 inch thick gate impacts the 0.11 inch tNek rack walls. An assumption was made that the wall would deform downward uniformly without tearing or buckling. After penetration, the gate was assumcd to tilt sideways and damage only one fuel assembly.
3.
Staff Technical Concerns 3.1 One of the licensee's major assumptions is to postulate failure of the rack wall when shear stress in the wall, calculated by a formula provided in the reference, Formulas for Stress and Strain (Young and Roark), Fifth Edition, reaches the yield stress.
The staff has several concems with this approach. First, it is not clear that the true shear stress in the wall can be represer.ted by the formula provided by the licensee.
The formula in the reference is derived from another reference, Theory of Elasticity (Timoshenko and Goodier), Second Edition. There are several formulas in the second l
reference, and it is not clear which formula is used for the calculation from the first reference, or how the formula was modified for the licensee's calculation. In any event, the formulas ' 'he second reference deal with the assumption of plane stress. For plane stres.nalysis, an infinite thickness of the body is assumed. This is not appropriate for the rac,k walls.
p
2 Secondly, even if the formula represented the true state of shear stress in the walls, the staff does not consider the calculation conservative since there is also a compressive i
stress at the point where the shear is calculated. Comparison with other commonly used failure criteria would show that the material will fail at a much lower shear stress and not at the yield stress when a compressive stress is also present (see Advanced Mechanics of Materials, Sealy and Smith, Second Edition).
In addition, once failure is initia*.ed, further progression of the gate's downward l
movement might require much less stress, since the failure of the wall will introduce stress concentrations. Therefore, assuming a constant load for the work done by the rack wall could result in a significant underestimate of the amount of damage to the walls.
3.2 The licensee used another basic assumption that the wall does not buckle while the gate deforms it. A knife edge type rack wall (thickness of 0.11 inch) impacted by a 3/4 inch thick, approximately 4000 pound, gate will most likely fail by local instability (buckling of the wall), not by shear stress as assumed by the licensea. In this case, the work done by the rack wall will be considerably lower than the licensee's value resulting in larger deformation and larger penetration than those that the licensee calculated.
3.3 The sudden change of the velocity of the falling gate introduces impact forces to the i
rrck wall. These impact forces cause amplification os bad. This has not been j
- ansidered in the licensee's analysis. The licensea's evaluation is based on the assumption that the load of the gate on the rack can be treated as a static load. This does not appear appropriate, since the impact load will be higher than the static load.
ii
\\
\\
f
.