ML20128E080

Safety Evaluation Supporting Amend to License DPR-22,re Proposed Interim Program for Offsite Shipment of Spent Fuel

ML20128E080
Person / Time
Site:	Monticello
Issue date:	08/26/1976
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20128E078	List: ML20128E073 ML20128E080
References
NUDOCS 9212070466
Download: ML20128E080 (11)

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AUG 2 61976 PLANT SYSTEMS BRANCH INPUT TO THE SAFETY EVALUATION OF MONTICELLO NUCLEAR GENERATING PLANTS PROPOSED INTERIM PROGRAM FOR THE OFFSITE SHIPMEhT OF SPENT FUEL 1.0 Introduction 1

In response to our February 4,1974 request, Northern States Power (NSP) submitted on October 1, 1974 the results of their analyses of the adequacy and consequences of utilizing the 70 ton IF 300, spent fuel shipping cask and the 85 ton bridge crane for the offsite shipment of spent fuel. The analyses indicated a number of areas where damage could result from a cask drop accident. Supplemental information was submitted on February 17, 1975 and May 30, 1975, assuming the use of the 70 ton IF 300 cask, in these reports, the licensee indicated that:

(a) the consequences of a cask drop was unacceptable; (b) it is not feasible to modify the plant's structures to adequately increase their strength; (c) the crane modifications would require roughly three years to complete once the extent of the crane modifications is established; and (d) as an interim solution, to alleviate the present spent fuel storage problem, they proposed to use a 25 ton two element fuel cask until a permanent cask handling program can be established and implemented.

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. l On January 22, 1976, NSP submitte'd its proposed interim offsite fuel shipment program and analyses using'either the NFS-4 or the

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identical NAC-1 25 ton cask. Following a staff review of the interim proposal, NSP submitted supplemental information on June 16, 1976 and July 22, 1976 in response to our requests for certain additional infor-

mation.

This evaluation is limited to the proposed interim fuel handling i

i program. NSP-is expected to submit a final report for NRC review and-i j approvalbnceapermanentcaskhandlingprogramisestablished.

l 2.0 Evaluation

! 2.1 Other Crane Loads l

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In addition to the shipping cask, the overhead handling-crane will i be require? to handle other heavy loads, such as the reactor vessel i

head, reactor-internals and reactor vessel shield plugs, during refueling-

operations. NSP states that the loads associated with refueling will l ,

only be handled when the plant is in a cold shutdown condition. Also, the consequences of dropping one of these refueling loads has the potentici of causing equipment damage but the event _would not pose a

-safety hazard. Further, NSP stated that sufficient diversity exists t

in the plant design to maintain-the reactor in a cold shutdown condi-I tion should one of the refueling loads be dropped.

To reduce the potential for equipment damage and to improve

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the' future load handling capability during offsite shipment ~of fuel, i NSP expects to complete the long term program of upgrading the over-head crane in approximately four years.

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i 5 Providing that handling of the heavy refueling loads is limited

\ to . occasions when the plant is. in a cold shutdown condition, we concur that sufficient diversity exists to maintain the plant in a safe cold J-I shutdown condition in the unlikely event of a load drop. On this basis, f

we find the proposed approach acceptable.

I 2.2 Operational Capabilities of Crane Hoist l'

1 The proposed interim offsite fuel shipment program assumes the

' use of a NFS-4 or NAC-1 25 ton, shipping task and impact limiter.

4 l Also, the maximum cask drop height will be limited to six inches

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when traveling over the prescribed path above the. operating floor.

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I Using these assumptions, NSP performed =an analysis of the adequacy-of the structure.. The lowest factors'of safety at the operating' floor 4

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4 4 level were found to be 1.39'and 1.23 for the floor slab. .(See the i

-discussion in Section by the Engineering. Branch for an

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evaluation of structural adequacy).

In response to staf f concerns regarding "two blocking" of the 1.

' hoist, NSP has indicated 'that they will install;two, directly actuated, redundant hoist upper limit switches on the-upper load block' assembly.

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Further,-to provide additional assurance that the carrying height above 3

the operating- floor will not exceed six' inches, NSP states that it will l

4. 1 incorporate one of the following features in'the program:

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-(a) -disable the "up" circuit of the hoist control when the cask i

! I reaches the proper elevation; or t

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, Ausag og (b) set one of the redundant upper load block limit switches such k

f that the hoist motor will stop automatically with the cas i .he operating floor.

less than six inches above f ,

i In regard to staff concerns on the ability of the hoist to elevate l floor NSP the cask to an acceptable elevation above the operating states:

(a) the minimum clearance required, to avoid having the ca hit the floor due to load swing caused by the application of either l ,

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the bridge or trolley brakes, is 0.8 inches; (b) administrat ve l ill be

! the minimum operating clearance between the floor and cask w held to two inches to provide sufficient assurance that the cask l (c) the NSP operating will not hit the floor during a load sving; experience with the existing hoist, obtained in handling heavy-the crane operator can accurately refueling loads, has demonstrated that t

position the load within one-half inch of the desired elevation, l

Therefore, NSP concludes and we concur that the hoist is capabl i

positioning the cask within the four inch band between the six in l

l maximum and two inch minimum elevation above the floor.

In the event of a cask drop accident. the analysis of-the l

structural adequacy of the operating floor assumes the cask wil follow a prescribed path of travel. Bright colored floor markings will define the path and serve as a guide to the crane operator NSP states there are no critical, drop locations during cask handling.

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i within about plus or minus 2 feet of this prescribed travel pat f Further, NSP will install bridge and trolley limit switches -to pre-(

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i AUG 2 61976 i

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' clude cask movement outside the north and west limits of travel required by the spent fuel shipping cask. _

j Contingent upon (a) the= adequacy of the structural review (see Section ); (b) the installation of two upper load block limit i switches; (c) implementation of one of the two above described means 4

.to provide assurance-that the cask will not be elevated more than six 1

1 i inches above the operating floor; (d) the installation of bridge and trolley limit switches limiting the north and west movements of the ,

cask; and (e) the bright colored floor markings to define the cask's-path of travel, we conclude that adequate interim measures have been i

i taken, when handling a NFS-4 or NAC-1 cask above the operating floor i

j i (elevation 1027'-8") to preclude unacceptable consequences following 4-i~ a cask drop accident on the operating floor,.and therefore, in this

! respect the proposed approach is acceptable. Furthermore, we expect k

that the interim offsite fuel shipment program will be replaced with j a permanent cask handling program in approximately four years.

l 2.3 Precautions Taken To Preclude The Crane From Experiencing A Hard Stop i

During the lifting of the shipping cask, several potential points i-l- exist where the load or load carrying members _could encounter a hard I

I stop (i.e., a rigid structure causing a sudden'stop).that would create

! excessive dynamic ' loads which -incturn could lead to the . failure of a i

j load carrying member and the loss of ' the load. One example of such l_ a. possibility.is for'the hoist lower load block to contact the upper-4 1

load block while lif ting the cask.- The licensee indicated that the

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! two upper limit switches, directly coupled to the drum rotation would

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'  ! i I preclude the above described event from occurring. Considering the  :

i relatively short distance between the limit switch settings and where i

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'.'two blocking" could occur, the staff expressed concern regarding this 1

indirect method of establishing the upper limit height of the load and

'its accuracy. The potential for erroneous-information by the present

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system exists if the cable should jump the cable drum grooves or if a f ailure should occur in the gear train that transmits load travel 1

information to the limit switch actuator.

1 NSP, in response to staff concerns, has agreed to install two i

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limit switches on the upper load block and thereby remove these t

uncertainties (also discussed in Section 2.2 above).

In addition, a hard stop may occur when the cask is lifted from i

the transporter. Should the bridge, trolley or cask transporter be f

improperly positioned during this lift, it is possible for the lower l load block assembly or cask to be stopped by the underside of the floor

-at elevation 962'-6". To preclude such an occurrence, the following i

steps will be taken

(a) the bridge and trolley will be properly positioned L over the equipment hatch prior to hoisting the.caskt.and (b) power to 3

the bridge and trolley drive motors will be locked.out.to prevent f

horizontal movement while the cask'is being raised clear of'the'trans-

-porter. Further, while hoisting the cask'in the e'quipment hatch,_th'e maximum lift speed will be limited to five feet per minute'and_thereby

1. reducing the kinetic energy and the developed dynamic loads in the o

j- unlikely . event that a hard stop should be encountered.

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- 7-We conclude that the proposed interim measures are adequate to t assure that a hard stop will not occur while lifting the cask in the equipaent hatch and to ameliorate the effects in the unifkely event that a hard stop is experienced. Therefore, in this respect the proposed appra4ch is acceptable.

2.4 Spent ruel Shipping Cask The NFS-4 shipping cask and lif ting yoke will be transported to the plant site by a transporter. Impact limiters are attached to the e

cask to protect it from axial and radial accident impact loads that may be experienced during transport. In the NSp analysis of structural adequacy of the reactor butiding operating floor, it was assumed that the cask bottom head impact limiting device remains attached to the [

cask. The analysis takes credit for the resulting reduction in impact load due to its action.

The staff expressed its concerns regarding the potential effects j of variations in crushing strength of the stainless steel encased balsa wood impact limiting device-if it should become water logged.

NSp states that the potential for water-logging of the balsa wood, ,

when it is submerged in the storage pool, does not appear probable for the following reasonst (a) the past operating experience with two 25 ton . casks shows that in 2,500 setdown conditions, no observable damage or leakage has occurred to the_ impact limiter; 4

I (b) in addition to the preoperational tests on the cask impact h '

limiters, i.e., dye penetrat.t'and leak tests, the impact

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limiter is leak tested annually using the bubble check method at Nuclear Fuel Services (NTS). Furthermore, the cask is subjected to a thorough visual inspection at both the ITFS facility and the reactor sitel and (c) during f abrication of the impact limiting device, each individual piece of balsa wood is coated with epoxy and once the pieces have been assembled, the entire assembly is recoated with epoxy. These epoxy coatings provide additional assurance that water-logging of the balsa wood is unlikely if the encasing stafuless steel water barrier were to develop a leak.

We conclude that adequate measures have been taken to preclude a change in the impact limiting characteristics due to water-logging.

Therefore, in this respect, the proposed interim program for offsite shipment of spent fuel, using the NFS-4 or the identical NAC-1 cask, is acceptable.

2.5 Adequacy Of Shipying Cask Trunnions And Handling Yoke NSp states that the shipping cask's lifting trunnions and handling yoke, respectively, have safety factors at rated load of 5.7 and 3.0.

Further, it is stated the handling yoke is proof tested to 130,000

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pounds (65,000 pounds on each aru) based on a 250 percent load test t!terion.

I In response to a staff concern on the magnitude of the maximum possible dynamic loads impo.med on the shipping cask trunnions and yoke by the hoists two spring loaded solenoid operated brakes,

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I NSP states that adequate allowance has been taken in the design for the [

r dynamic loads incurred during cask handling.

We find that the information submitted in response to item 11 does ,

t not adequately address the staff's concern and does not support the ,

licensee's conclusion cited above when the following assumptions are made:

(a) the load is being-lowered at its maximum speed ,

(b) the hoist experiences a power failure thereby causing each of the two hoist brakes (each rated at 150 percent of the hoists rated load of 85 tons) to automatically set when handling the 25 ton shipping caskt and -

the least amount of distance separates the upper and lower (c) hoist load blocks when the loss of electrical power is experienced.

We require that NSP perform a conservative dynamic analysis which demonstrates that the cask trunnions and handling yoke will not fail due to the dynamic loads created under the above listed assumptions..

2. '6 Cask Drop In Equiptnent Hatch Area Analyses have been conducted by NSP to predict the effects of

. a cask. drop at two locations in the equipment hatch of-the reactor butiding. - The results are presented in Table 3-2 of ;the licensee's-

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January 22, 1976 submittal. The two 1ocations are designated as 4

position numbers-7 and 8. Position nuinber 8 is the location of. the

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-cask when it is being lifted from or lowered onto the transporter.  ;

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i The cask will be elevated slightly to clear the transporter and

! Once i yet not exceed 10 feet above the floor slab at this location.

the cask has been raised clear of the transporter, the crane bridge and trolley will move the cask horizontally to a location denoted as j

Position number 7 in the equipment hatch area from which the lif t will r be continued until the cask is above the operating floor (93'-2" above the equipment hatch floor slab).

1 A three foot thick diagonal wall is located beneath the equipment hatch floor slab. This wall rests on the reactor building base slab.

f This diagonal wall and the reactor building walls form one of the four 1.

triangular corner compartments housing essential engineered safety feature a

equipment, i.e., two RHR pumps, one RER heat exchanger and one core

spray pump. The equipment hatch floor slab thickness between the diagonal vall and torus is a 68-inch thick concrete slab. In the portion of ' the triangular corner -compartment beneath lifting location Position number 7 the equipment hatch slab is a 24-inch thick concrete i slab.

At Position number 8 the impact area of the cask would be

' bisected by the junction of the 66-inch thick slab and the 36-inch thick concrete diagonal vall that rests on the base mat. At t

Position number 7 the impact area of the cask would be above the 24-inch thick concrete slab forming the ceiling of the corner

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compartment.

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I MG 46 c/ci The NSP analysis shows that at Position number 8, the safety factor is 1.26 when a 25 ton cask is used and a drop height of 10 feet is assumed. At Position number 7, the potential cask drop height is 93'-2". At this location, the calculated safety f actor is 0.14 indicating the slab cannot sustain a cask drop from a height of 93'-2".

The licensee has made the following statements in its submittalt (a) "The crane system is not designed to be single failure proof and may be subjected to either single random mechar.ical or or control system failures" (b) "The analyses results show that the 24 inch thick slab belov elevation 935-0" at drop location 7 in the equipment hatch opening vill not sustain a 93'-2" free drop of the cask."

We find that sufficient information has not been furnished to support the licensce's conclusiont "A drop down the equipment hatch would not impair the ability to complete a normal reactor shutdown or the ability to maintain cold shutdown conditions".

We have requested and vill require NSP to furnish additional information relative to item 12 and to supply any other additional information which vill support the conclusion quoted above.