ML13330B409
| ML13330B409 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 10/18/1988 |
| From: | Medford M SOUTHERN CALIFORNIA EDISON CO. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| NUDOCS 8810240343 | |
| Download: ML13330B409 (8) | |
Text
Southern California Edison Company P. O. BOX 800 2244 WALNUT GROVE AVENUE ROSEMEAD, CALIFORNIA 91770 M. 0. MEDFORD TELEPHONE MANAGER OF NUCLEAR ENGINEERING (818) 302-1749 AND LICENSING October 18, 1988 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555 Gentlemen:
Subject:
Docket No. 50-206 Spent Fuel Transshipment San Onofre Nuclear Generating Station Unit 1 On August 12, 1988, SCE met with the NRC staff reviewers to discuss their concerns regarding the transshipment of spent fuel from San Onofre Unit 1 to Units 2 and 3. Responses to the NRC concerns were documented in a submittal dated September 23, 1988 to the NRC. An additional concern which came out of the meeting was the effect of the cask drop in the spent fuel pool on the basemat. In a followup telephone discussion on September 8, 1988, the NRC staff requested a detailed response on the drop of the spent fuel cask in the spent fuel pool.
Provided as an enclosure to this letter is a detailed discussion of that drop.
If you have any questions regarding this information, please let me know.
Very truly burs, Enclosure cc: 3. B. Martin, Regional Administrator, NRC Region V F. R. Huey, NRC Senior Resident Inspector, San Onofre Units 1, 2 and 3 001 S810240343 88101:
PDR ADOCK 05000206 P-PDC:
Enclosure POSTULATED CASK DROP IN THE SPENT FUEL POOL The NRC staff has expressed concern over the drop of the spent fuel cask in the spent fuel pool. A detailed discussion of the drop considered was requested to include the following items:
- 1. Possibility of a tear of the liner.
- 2. Discussion on why the liner is not affected.
- 3. Cracking, strains, and stresses.
- 4. Local rebar yielding.
- 5. Repair of the liner in the event it is penetrated.
- 6. Spent fuel pool makeup capabilities.
The following discussion is provided to describe the effect of a postulated 70 ton cask drop on the pool liner and concrete basemat of the Fuel Storage Building. The cask laydown area is the region of concern and the plan is shown in Figure 1. The primary criteria were to prevent water leakage out of the pool and to maintain the structural integrity of the Fuel Storage Building.
The evaluation of the cask drop used an impact force of 16,380 kips.
The impact force is based on analytical results given in the "GE IF-300 Shipping Cask Consolidated Safety Analysis Report," dated September 1984 and the cask's interaction with the basemat.
Pool Liner In order to prevent water leakage out of the pool due to cask impact, the pool liner must not be perforated or torn by stretching when the basemat deflects.
Liner perforation will be precluded by a 2 1/4 inch thick stainless steel plate which has been installed at the bottom of the cask laydown area. The plate thickness to preclude perforation was calculated by the following the Ballistic Research Laboratory equation for perforation of steel plates with material constant taken as unity.
C)2/3 62/3 T
(E k (4.207 x 10 6 p
672D
=
672 (24.7)
= 1.57 in.
where:
m s_
4348 (43.99)2 6
E k =
2
=
2
-4.207 x 106 ft-lbs
-2 and:
Tp = steel plate thickness for threshold of perforation (in)
Ek = missile kinetic energy (ft-lbs)
Mm = mass of the missile (1b-sec 2lft) = 140,000 = 4348 32.2 Vs = missile striking velocity (fps) = 43.99 D = missile diameter (in) = 24.7 (tilted drop)
The design thickness to prevent perforation, tp, must be 15% greater than the predicted threshold value:
tp = 1.15Tp = 1.15 (1.57) = 1.81 in < 2.25 inch provided where:
tp = design thickness to preclude perforation (in)
The result of the calculation is that the 2 1/4 inch plate will protect the pool liner from perforation.
Liner tearing was evaluated by determining its strain and bearing stress.
Assuming a one inch vertical displacement of the basemat (actual deflection is
.44 inch), the strain will be.0002 in/in which is much less than the minimum
.40 in/in strain required by ASTM for stainless steel material.
The bearing stress at impact is 5.09 ksi which is less than the allowable of 27 ksi.
Therefore, the stretching of the liner will not cause any tearing failures in the liner for the postulated cask drop.
Since the pool liner integrity is not adversely affected by the cask drop, water leakage will not result.
-4 Concrete Basemat The reinforced concrete basemat is 4'-9" thick and has an average span of 9'-8" at the point of cask laydown (see Figure 2).
The ability of the basemat to withstand the cask impact force was evaluated by considering the basemat as a beam spanning between the pool walls and also as a footing.
In conservatively assuming one-way beam action and no soil resistance under the basemat, the maximum moments are 9667 k-ft at the center and -16,971 k-ft at the support ends. Since the cracking moment capacity of 4358 k-ft is exceeded, cracking can be presumed to occur in areas of exceedence as shown in Figure 2.
Due to the aspect ratio of shear span to concrete depth, the basemat will behave as a deep beam after some flexural yielding has occurred. Thus, the cask impact force must be resisted in shear by the basemat. When considering the basemat as a footing, the calculated impact shear stress is.267 ksi which is less than the allowable shear stress of.295 ksi for impactive loads. The allowable shear stress was determined by 4 f'c times 1.10 which is the Dynamic Increase Factor for shear.
Concrete crushing under the cask will not happen because the computed bearing stress is 5092 psi which is less than the allowable impactive compressive stress of 5625 psi (f'c x 1.25). After the impact, the moments for the dead load of the cask are 83 k-ft at the center and -146 k-ft at the support ends.
The corresponding moment capacities when only considering the rebars for tension are 1710 k-ft and -2311 k-ft.
The impact soil bearing pressure is 65 ksf which is less than the allowable of 100 ksf. A basemat deflection of.44 inch due to soil settlement at impact will cause the rebars in tension to yield. The strain is conservatively calculated assuming the yielding only occurs over a 12 inch length. The strain is.036 in/in which is less than the ultimate strain of.200 in/in for Grade 40 rebar.
Therefore, some flexural cracking may result from the cask impact force, but the basemat has the shear capacity and the soil has the bearing capacity to withstand the cask drop without adversely affecting the structural integrity of the Fuel Storage Building.
The cracking in the concrete will not allow water leakage into the soil because there is a waterproof membrane encompassing the entire basemat up to Elevation 12 feet. In addition, any water leakage will follow the path of least resistance which is the leak chase system.
-6 Liner Repair The method of repairing the liner will be dependent on the extent of damage that occurs from the drop of the cask.
The first action will be to determine if water leakage is occurring or increasing by monitoring the leak chase system. Compensatory measures are then enacted to maintain the proper water levels in the pool.
The cask and the liner protector plate are removed from the cask laydown area so that an underwater inspection by remote TV camera or diver can be accomplished.
Liner defects can be repaired underwater by welding or welding new plates over the defective areas.
Another option is to dewater the cask laydown area in order to implement the repairs. A dam would be installed in the canal between the spent fuel area and the cask laydown area in order to dewater the damaged area. The pool liner would then be repaired by welding a new liner over the existing liner.
Upon the completion of repairs, the cask laydown area would be refilled and the dam removed.
-7 Spent Fuel Pool Makeup Capabilities The water makeup capabilities for the spent fuel pool consist of the following:
- 1. Boric Acid Storage Tank 7,000 gal.
(Two 45 gpm pumps)
- 2. Primary Makeup Water Tank 150,000 gal.
(Two 100 gpm pumps)
- 3. Refueling Water Storage Tank 240,000 gal.
(One 80 gpm pump)
Approximately 9425 gallons of water is required to fill a one-foot height area in the spent fuel pool.
0419n
ClASK I PENT -FUEL POOLo UNIT 1.
CSLYWw wAREA Figure 1
CASK LAYDOWN AREA OF SPENT FUEL POOL 54!w LIlk CASK I PI
-24 ?LCXsA SECTION LOOKING SOUTH Figure 2 I
- b