ML20216F050

Forwards Plans for Contingency Fuel Storage Rack for Plant. Rack Designed to Facilitate Emergency Full Core Offload of Reactor Core During Cycle 12 Operation.No Tech Spec Change or Unreviewed Safety Question Involved for Rack

ML20216F050
Person / Time
Site:	Millstone
Issue date:	06/24/1987
From:	Mroczka E NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
B12565, NUDOCS 8706300792
Download: ML20216F050 (9)

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MEAST UTILITIES cenerai Ofrices seiden street. Beriin coanecticut U UU,53 c U P.O. BOX 270

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HARTFORD CONNECTICUT 06141-0270 L t j *]'8],*Q""j j"," (203) 665-5000 June 24,1987 Docket No. 50-245 B12565 Re: 10CFRT0.59 1

s U.S. Nuclear Regulatory Commission Attn Document Control Desk Washington, D.C. 20555 Gentlemen:

Millstone Nuclear Power Station, Unit No.1 Contingency Fuel Storage Rack This letter is to inform the NRC Staff of Northeast Nuclear Energy Company's (NNECO). plans for a contingency fuel storage rack for Millstone Unit No.1.

This rack has been designed and purchased, and will be installed if necessary to facilitate an emergency full core offload of the reactor core during Cycle 12 operation. NNECO has completed an analysis under 10 CFR 50.59, and has concluded the installation would involve no change to technical specifications and no unreviewed safety question. No NRC Staff action is therefore necessary.

The present capacity of the Millstone Unit No. I spent fuel pool is 2,184 spent fuel assemb!!es. The ongoing refueling outage (Cycle 11) discharged 196 assemblies increasing the inventory of spent fuel assemblies in the spent fuel pool to 1,732. The existing spent fuel racks are short of a full core reserve by 128 assemblies.

Full core reserve capacity is not re (NWPA) or the NRC's regulations.(l) quiredNNECO However, by the National Waste Policy Act follows a corporate policy to maintain such capacity. Unless alternative storage plans are implemented for Cycle 12 operation, extensive shutdown delays would occur until additional storage is made available if the need to offload the core ever arises.  !

(1) NWPA's provisions encourage the maintenance of full core reserve l capability and establish it as an objective to be served in determinations as to eligibility for Federal assistance with waste disposal. Specifically,  !

under 42 U.S.C.10155(b) and (g), the absence initially of full core reserve  !

capability could jeopardize eligibility for Federal assistance. However, NWPA does not require provisions for full core service capability for a plant to continue to operate. See also 10 CFR Part 43; 30 Fed. Reg. 555 at col.1 (February 11, 1985). g ,

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4 e-U.S. Nuclear Regulatory Commission B12565/Page 2 June 24,1987.

NNECO plans to address this situation by maintaining the contingency spent fuel rack to provide full core reserve following the 1987 (Cycle 11) refueling outage through the 1989 (Cycle 12) refueling outage. The rack is designed to be placed in the cask lay-down area of the spent fuel pool should the need for an emergency core offload arise. NNECO currently plans to complete a project prior to the 1989 outage to permanently expand the spent fuel pool capacity, thereby obviating the need for the contingency rack after that time.

The contingency rack is designed for installation in the designated spent fuel pool cask lay-down area. The rack would not modify, alter, or otherwise change the existing rack design. Engineering analyses have been performed to assure that this rack will not jeopardize the design and function of the existing storage racks. The contingency rack is designed to be temporary and freestanding. It would not be attached to any equipment or structures in the spent fuel pool and would require no lateral restraints.(2) The rack is also designed to store spent fuel assemblies in a coolable subcritical configuration, with design conditions meeting or exceeding those of the current racks. A more detailed design  !

description is provided in Attachment 1.

As noted, the contingency fuel rack will be a temporary rack and its use will be dependent upon the need to offload the reactor core during Cycle 12 operation. -l Until required for use the rack will be stored and covered on site in a controlled '

area. The storage area has been selected such that the rack will not jeopardize

'any safety related plant equipment, structures, or components. The reserve rack is defined as a heavy load by NUREG-0612 and, if ever used, will be handled i according to appropriate Millstone Unit No. I heavy load procedures. Also, af ter its use (i.e., af ter the core is reloaded and the rack emptied), the rack will be removed from the pool, decontaminated, and again stored and covered for I possible future use.

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l Prior to installation of the rack, a structural pedestal and structural plate must l be installed in the cask lay-down area. The pedestal and structural steel plate are required to transfer the rack foot loads to the concrete pool floor structure. )

1 Installation of the pedestal and plate will not affect any functional aspects of pool use nor degrade or invalidate any pool design bases or analyses.

NNECO has performed an analysis of installation of the contingency spent fuel rack pursuant to 10CFR50.59 and concluded that no technical specification change or unreviewed safety. question is involved. The rack therefore can be installed pursuant to 10CFR50.59 without prior NRC approval. A summary of NNECO's unreviewed safety question analysis is provided in Attachment 2.

(2) The current spent fuel racks are installed in a fixed location, locked in I place by a restraint system integrated between the existing spent fuel rack ,

base and spent fuel pool walls. l

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U.S. Nuclear Regulatory Commission

. B12565/Page 3 June 24,1987 Based upon the above, NNECO concludes that no NRC Staff action on this matter is presently required. We trust you will find this information satisfactory and we remain available to answer any questions you may have.

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY E. 3r1%f4czka O Senio(Vice President cc: W. T. Russell, Region I Administrator M. L. Boyle, NRC Project Manager, Millstone Unit No.1 T. Rebelowski, Resident Inspector, Millstone Unit Nos. I and 2 I

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Docket No. 50-245 1 B12565 4 i

i Attachment I  ;

l Summary of Design of Contingency Spent Fuel Rack l

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l Summary of Design <

Of Contingency Spent Fuel Rack i

A. The' contingency spent fuel rack is a large metal structure, fabricated of stainless steel tubes (304 stainless), all welded to a common base-plate.

The rack is freestanding and has adjustable feet which rest in contact with the built up floor system. There are no lateral braces for seismic rigidity.

The rack is designed such that seismic sliding and tipping is negligible. The rack has a capacity for storing 152 fuel assemblies. Subcriticality is maintained by the cell pitch (6.28 inches), and by utilization of a neutron absorption material (poison) called Boraflex. The Boraflex is contained in the cell walls of the rack such that it comes between any two adjacent cell walls. The new rack has been designed as a seismic Category I structure.

' The rack is designed to accommodate GE 7 x 7,8 x 8, and 8 x 8R fuel types (with or without fuel channels) at a design enrichment of 3.4 maximum weight percent U235-B. A structural analysis was performed on this rack for deadweight, liveload, thermal and seismic load conditions. The seismic load condition produces the' governing loads and stresses on the rack. Westinghouse Electric Corporation has performed this analysis using non-linear and linear finite element computer codes. The rack was mathematically modeled, and ,

dynamically analyzed for seismic conditions specific for Millstone Unit '

No.1. The analysis accounts for submerged water effects, impact of fuel to rack, and rack sliding and tipping. The results of this analysis demonstrated that all stresses for deadweight, liveload, thermal, and seismic loadings are below the code allowables.

Additionally, structural analyses were performed to simulate a stuck fuel assembly. This would impose an uplif t to the rack. The results showed that the resultant stresses are below the code allowables. An analysis was performed to simulate a fuel assembly drop on the fuel rack. The results of this analysis showed that such an occurrence will not appreciably deform the rack cavity, and hence will not degrade the ability of the rack to store the fuel in a coolable, subcritical configuration.

C. A thermal hydraulic analysis was performed to determine maximum fuel clad temperatures, and to verify that the new rack design allows coolant flow for the removal of spent fuel decay heat. The results of this analysis demonstrated that the pool coolant remains subcooled at all points in the pool when the cooling system is operational, and that temperature of the fuel cladding does not lead to structural failures nor safety concerns when the cooling system is postulated inoperable.

Another analysis was parformed to simulate a flow blockage accident. The results demonstrated that up to 90% flow blockage would not produce local or bulk boiling inside the cells. Loss of pool cooling conditions were also sirnulated for coolant levels of 24 feet,10 feet, and cavity seal failure scenarios. The results of these analyses showed that due to the effects of natural circulation, the fuel cladding temperatures do not lead to structural failurcs of the cladding, i

-D.. A criticality analysis was performed to assure that the rack can store fuel assemblies in a subcritical configuration. An extensive computer analysis was performed, simulating normal fuel storage conditions, accounting for rack fabrication geometric tolerances. This analysis determined that there is a. 95% probability, at a 95% confidence level, that Keff of the fuel j assembly storage pattern is less than or equal to 0.90. T l

Criticality ' accident analyses were performed to evaluate the accident f scenarios given in Appendix A of Regulatory Guide 1.13. The results of these analyses demonstrated that there is a 95% probability, at a 95% i confidence level, that Keff of the fuel assembly storage pattern is less I than or equal to 0.90.

E. A building structural evaluation was performed for the new rack. This evaluation accounts for the deadweight,liveload, and seismic loadings from i the new rack. An analysis was performed to demonstrate the effect that the rack loads would have on existing structural steel and concrete. The results of this evaluation demonstrated that a structural pedestal and plate are required to provide an adequate load transfer mechanism to the i

existing concrete floor. With these items, the resultant stresses of the pool support structure are below the code allowables. j F. A pool cooling sv-tem evaluation was performed for the increased pool fuel inventory. This evaluation considered the additional heat load generated from the' increased fuel inventory. This evaluation considered the appropriate single active failures, and determined that under all analyzed conditions the existing cooling system is capable of handling all expected heat loads.- l 1

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Docket No. 50-245 B12565 l

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- Attachment 2 l l

Summary of Unreviewed Safety Question Analysis 1

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Summar/ of Unreviewed Safety (>uestion Analysis  !

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A. Installation and use of the coritingency spent fuel rack would not increase the probability of, occurrence or the consequences of an accident or malfunction of equipment important ' to safety (i.e., safety-related) 3 i

previously analyzed. j The change does not impact any of the Millstone Unit No. I design basis l events. The rack is a freestanding, passive structure with no moving parts.

There are no credible failure modes of the rack itself. The failure modes associated with the criticality calculation do not initiate any design basis events.

During installation, the rack could be dropped. However, the rack drop will' l not initiate any design basis event. The rack is considered a heavy load and {

therefore 'will be handled using heavy load procedures which prohibit '

movement over fuel assemblies. If the rack were dropped there would be 3 no affect on the reactor coolant system (RCS) or fuel assemblies in the l spent fuel pool (SFP).

The change willimpact the heat load and structural forces in the spent fuel pool. An increase in heat load is generated from the increased spent fuel 1 storage capacity. The 152 new storage locations will allow an additional 152 fuel assemblies to be stored in the pool. However, the fuel pool drain- l down analyses assumed a' full core offload in calculating the heat load in the SFP. Because the SFP heat load already accounts for the heat load of 152 freshly off loaded assemblies, the additional heat load from decayed spent fuel will be insignificant. The increased heat load also will not ,

change the probability of failure of the SFP cooling system.

The impact on the response of the SFP to a seismic event has been evaluated. The results show that the additional loads on the existing SFP lines,' slab, and wall structures do not exceed the original design limits or current criteria. In addition, the potential for rack movement due to a  !

seismic event was reviewed. It was determined that such rack movement would be negligible and much less than the gap between the temporary rack and the cask anti-tip frame. The installation and use of the contingency rack also will not impact the probability of a seismic event.

B. Installation and use of the contingency spent fuel rack would not create the potential for an accident or malfunction of a different type than previously analyzed.

As stated above, the installation and use and/or failure modes of the contingency rack do not significantly affect the function of any system and therefore do not impact the plant response. The contingency rc.ck is similar in design and identicalin function to the existing racks.

The failure modes associated with the criticality calculations were reviewed and found to not represent a new unanalyzes accident.

Installation and removal of the rack will be treated under the i..avy loads procedure and a dropped rack does not, therefore, represent a new event.

The probability of accidents associated with moving the rack will be the same as that for the existing racks.

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i C. Installation and'use .of the contingency spent fuel rack would not reduce l any_ margin of safety. This change does not impact the safety limits of any I' protective boundary. Moreover, the. contingency rack is designed such that the SFP Keff si less than or equal to 0.90 and thus SFP suberiticality is maintained.

NNECO also reviewed the impact on Technical Specification (TS) bases, specifically the impact on TS 4.10.E, which is meant to ensure that a spent i fuel cask cannot be lif ted over irradiated fuel. Crane interlocks are

.provided to prevent cask movement over all SFP areas except the cask lay-down area. When the temporary rack is in the cask lay-down area, the .

ability to raise a cask over the rack would not be prevented by the existing interlocks. However, in order to lif t a cask, the crane selector switch must be placed in Mode 2 per operating procedures. This enables the crane interlocks. To prevent cask motion over the temporary rack the Mode 2 position of the crane selector switch will be red tagged out of service.

Thus, no cask maneuvering will be permitted and no cask / fuel interactions will occur. Therefore, installation and use of .the contingency racks will  ;

not impact the basis of any Technical Specifications.

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