ML20214A818

Forwards Detailed NSHC Determination Re 860903 Application for Amend to License NPF-37 to Increase Storage Capacity of Spent Fuel Racks.Determination Addresses Changes from floor- Mounted to free-standing Racks

ML20214A818
Person / Time
Site:	Byron
Issue date:	11/07/1986
From:	Ainger K COMMONWEALTH EDISON CO.
To:	Harold Denton Office of Nuclear Reactor Regulation
References
2320K, NUDOCS 8611200246
Download: ML20214A818 (6)

Text

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. [ COmin0nwealth Edison (m . One First National Plaza Chica00, Illinois N"%-

Address Reply to: Post Omce Box 767 Chicago, Illinois 60690 - 0767 November 7, 1986 Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555

Subject:

Byron Station Units 1 and 2 Application for Amendment to Facility Operating License NPF-37 Appendix A, Technical Specifications NRC Docket Nos. 50-454 and 50-455 Reference (a): September 3, 1986 letter from K. A. Ainger to H. R. Denton

Dear Mr. Denton:

Reference (a) requested a license amendment to increase the storage capacity of the spent fuel racks at Byron Station. Attachment C of reference (a) contained an evaluation of significant hazards considerations for the reracking of the spent fuel pool. The evaluation concluded that the proposed amendment presented no significant hazards considerations.

Attached to this letter is a more detailed determination of no significant hazards considerations. This determination addresses the change from floor mounted to free standing racks, as well as other significant changes resulting from installation of high density spent fuel racks.

Please direct any questions regarding this matter to this office.

One signed original and fifteen copies of this letter and attachment are provided for NRC review.

Very truly yours, K. A. Ainger Nuclear Licensing Administrator 1m Attachment

,o cc: Byron Resident Inspector l(

8611200246 861107 2320K PDR ADOCK 05000454 P PDR L

DETERMINATION OF NO SIGNIFICANT HAZARDS CONSIDERATIONS The proposed amendment would authorize the replacement of the currently installed spent fuel racks with free standing, high density racks. This rack replacement will increase the capacity for storing spent fuel at Byron Station from 1060 to 2940 fuel assemblies. Included in the new rack configuration are six locations for storing failed fuel. The details of the proposed reracking are described in Attachment B of reference (a).

Edison has evaluated the proposed reracking in accordance with 10 CFR 50.92(c) and determined that it presents no significant hazards considerations. This determination was based on an evaluation of the significant changes that would result from the proposed reracking. As discussed in detail below, the conclusions of this evaluation were that the proposed amendment would not:

1. Involve a significant increase in the probability or consequences of an accident previously evaluated (Criterion 1);

2. Create the possibility of a new or different kind of accident from any accident previously evaluated (Criterion 2); or

3. Involve a significant reduction in a margin of safety (Criterion 3).

Because the criteria of 10 CFR 50.92(c) are satisfied, this proposed amend-ment should be noticed as involving no significant hazards considerations.

The significant changes that would result from the proposed reracking are:

A - free standing instead of bolted racks; B - increased density of spent fuel storage due to an increased quantity of fuel stored and a decreased spacing between the racks; C - differences in rack design resulting from differences in materials including the incorporation of neutron absorbing materials containing boron; and D - separation into two burnup dependent regions L

For each of these significant changes, the criteria in 10 CFR 50.92(c) are evaluated below. This evaluation included consideration of the following accidents which were evaluated previously:

(a) Radiologic consequences of the drop of a spent iuel assembly onto the floor of the spent fuel pit (accident a);

(b) Radiologic consequences of the drop of a spent fuel cask (accident b);

(c) The potential for criticality from a dropped fuel assembly laying across the top of a fuel rack (accident c);

(d) The potential for criticality from a fuel assembly accidentally droppcd into a position parallel to fuel stored in the most reactive corner of the racks (accident d); and (e) The effect of a reismic event on rack integrity and the potential for criticality (accident e).

BVALUATION A. Free Standing Instead of Bolted Racks criterion 1: This change has no effect on accidents (a) through (d).

As for accident (e), Edison's analysis shows that a seismic event will not significantly increase the probability or consequences of either the potential for criticality or a challenge to rack integrity.

The new racks, like the old ones, are seismic Category I structures and, thus, have been designed to maintain their integrity during a seisaic event. Even though the racks are free standing, that integrity will not be challenged significantly because the safe shutdown earthquake for Byron ?.s not capable of accelerating the racks sufficiently to cause them to experience significant forces, either directly or on collision with each other or with the pool's walls. The safe shutdown earthquake (SSE) for Byron is capable of resulting in a horizontal acceleration of no more than 0.39 The corresponding seismically induced maximum rack movement (including tipping or sliding) is less than 3/16". But the minimum distance between a rack and a wall of the spent fuel pool is 2 5/8". Therefore, an earthquake would not result in a collision between a rack and a wall of the spent fuel pool. Thus, the free standing character of the racks will not increase the probability or consequences of a seismically induced challenge to rack integrity due to a collision between a rack and a wall of the spent fuel pool.

4 Analysis also shows that the free standing character of the racks will not increase the probability or consequences of a seismically induced challenge to rack integrity due to a collision between adjacent racks.

The low acceleration associated with the SSE, the extension of the base plate beyond the last row of cells and the use of framing material (girdle bars) at the top of the racks ensure that rack-to-rack impact loads due to an SSE will not significantly increase a seismically induced challenge to rack integrity.

In addition, the use of base plates and girdle bars also ensures that an SSE will not reduce the spacing between cells in adjacent, either in Region 1 or in Regions 1 and 2, racks to a distance less than the minimum spacing assumed in the criticality analysis. Intercellular spacing is not a concern in Region 2 because all cells in that region are separated by the neutron absorber sheet. Thus, the free standing character of the racks will not cause a significant increase in the probability or consequences of a criticality event.

For the foregoing reasons, Criterion 1 is satisfied in light of the change to free standing racks.

Criterion 2: The circumstance that the new racks are free standing does not create the possibility of a new or different kind of accident. The only new consequence of the free standing nature of the racks is their ability to move in the event of a seismically induced acceleration.

However, as discussed above, the maximum accelerations associated with the SSE for Byron are not capable of inducing significant rack motions.

Moreover, rack design precludes any impact batween a rack and a wall of the spent fuel pool, and limits the effectu of rack-to-rack collisions to forces well below the force necessary to challenge rack integrity.

Therefore, the new free standing racks satisfy Criterion 2.

Criterion 3: For the reasons discussed above, the fact that the racks are free standing does not result in a significant reduction in a margin of safety. The BSE for Byron is simply not powerful enough to cause the unbolted racks to experience stresses which would challenge their structural integrity to any significantly greater extent than stresses would have challenged the integrity of the old ranks.

B. Increased Density of Stored Spent Puel criterion 1: The increased density of stored spent fuel does not increase the probability of any of the accidents previously considered.

The probabilities of those accidents are controlled by the probabilities of the initiating events and these are independent of the density of spent fuel stored. Increased storage density also does not increase significantly the consequences of the previously analyzed accidents.

The new racks have been designed to ensure that none of the accidents will result in an exceedance of the criticality acceptance criterion that K,gg will not exceed 0.95. Thus, criterion 1 is satisfied.

b

c Criterion 2: No new or different kinds of accidents are presented by the circumstances that the new racks are closer together and hold more spent fuel. Neither the decrease in rack spacing nor the increase in quantity of spent fuel has led to the identification of any type of accident not already considered in the initial safety analysis for the spent fuel pool. Thus, Criterion 2 is satisfied.

Criterion 3: Installation of the new spent fuel racks will not result in a significant reduction in a margin of safety. A small increase in the spent fuel pool heat load is expected due to the storage capacity expansion. However, the spent fuel pool cooling system design can handle the increased heat load and maintain the temperature peaks of the pool below design values. Installation of the new spent fuel racks will also result in a small increase in the pool reactivity as measured by the neutron multiplication factor (K,gg). However, the maximum neutron multiplication factor will be maintained less than or equal to 0.95.

The radiological consequences also were evaluated to determine the impact on off-site and on-site doses previously determined. The increase in the storage capacity of the spent fuel pool will neither significantly alter the operating characteristics of the current spent fuel pool nor result in a measurable change in impact on the environment. The design basis fuel handling accidents, described in FSAR Section 15.7.4, were reviewed for possible effects on radiological dose consequences. The review determined that the conclusions in the PSAR will remain valid and that off-site radiological dose consequences will remain within 10 CFR 100 limits.

For these reasons, increasing the spent fuel pool storage capacity will not significantly reduce a margin of safety. Therefore, Criterion 3 is satisfied.

C. Differences in Rack Design criterion 1: The new racks are made of the same materials as the old ones except that they also include a neutron-absorbing borated material. The inclusion of this material has no effects on accidents (a) through (d) and is relevant to accident (e) only to the extent that the potential for criticality is affected. By its nature, the borated neutron absorbing material cannot significantly increase the probability or consequences of a criticality accident. Therefore, the inclusion of this material satisfies criterion 1.

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Criterion 2: Nothing about the materials in the new racks can create the possibility of a new or different kind of accident. Only the borated neutron absorber is a new or different kind of material in the racks. That material is passive and will perform its function as long as it remains in place. And the construction and testing of racks render insignificant the possibility that enough borated material could shift to result in a significant increase in criticality. Accordingly, this criterion is satisfied.

Criterion 3: Because shifting of the borated material is the only kind of safety-related occurrence associated with the design of the new racks, and the probability of that occurrence is very low, these racks do not present a significant reduction in the margin of safety.

Accordingly, this criterion is satisfied.

D. Separation into Two Burnup Qependent Regions criterion 1: Of the accidents previously analyzed only accident (d),

the accidental drop of a fuel assembly parallel to fuel stored in the most reactive corner of the racks, requires a re-evaluation of its consequences. The probability of occurrence of this accident is not affected by the presence of two burnup dependent regions for storing spent fuel. As for the consequences of this accident, calculations show that it will not result in an exceedance of Keff above the acceptance criterion of 0.95. Thus, Criterion 1 is satisfied by the separate locations.

Criterion 2: The use of separate locations for storing different burnup fuels will not create the possibility of a new or different kind of accident. The two region design does create the possibility that a fuel assembly may be placed in the wrong region. Itowever, the probability of such a misplacement is very low due to the obvious differences in design of the two regions, design differences which are readily discernable from the position above the pool which would be taken by an operator loading fuel into the pool. In any event, the misplacement of a fuel assembly would not lead to a new or different kind of accident because it is in essence just a version of accident (d). Therefore, the steps which have been taken to maintain Keff below the acceptance criteria for the present racks will also maintain Keff below the acceptance criteria for the new racks. Thus, Criterion 2 is satisfied by the separate locations.

Criterion 3: Nothing about the use of separate locations for storing fuel with differing burnup can affect the margins of safety.

Accordingly, Criterion 3 is satisfied.

2320K