ML20082A548

Provides Summary of Reevaluations of LOCA Based on Identification of Several Assemblies Containing Leaking Fuel Pins & fuel-related Damage Which Necessitated Rev to Cycle 6 Core Loading Pattern

ML20082A548
Person / Time
Site:	Millstone
Issue date:	11/02/1983
From:	Counsil W NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	John Miller Office of Nuclear Reactor Regulation
References
B10927, TAC-56814, NUDOCS 8311180179
Download: ML20082A548 (4)

Text

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General Offices

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November 2,1983 Docket No. 50-336 B10927 Director of Nuclear Reactor Regulation Attn: Mr. James R. Miller, Chief Operating Reactors Branch #3 U. S. Nuclear Regulatory Commission Washington, D. C. 20555

References:

(1) W. G. Counsil letter to R. A. Clark, dated October 22,1983.

(2) W. G. Counsil letter to R. A. Clark, dated June 22, 1983.

(3) W. G. Counsil letter to J. R. Miller, dated September 12, 1983.

Gentlemen:

Millstone Nuclear Power Station, Unit No. 2 Loss of Coolant Accident Reassessments Northeast Nuclear Energy Company (NNECO) provided to the NRC Staff the results of evaluations of the large and small break loss-of-coolant accidents (LOCA) in Reference (1). Since that time, NNECO has identified several fuel assemblies containing leaking fuel pins as well as other fuel related damage which necessitated a revision of the Cycle 6 core loading pattern. NNECO informed the Staff of these problems in References (2) and (3).

NNECO is also in the process of removing the thermal shield from the core barrel since the support of the shield has been degraded to an unsatisfactory condition.

As was discussed with the Staff in your Bethesda Offices on October 12, 1983, revised reload safety analyses reflecting both the new core loading pattern and the removal of the thermal shield will be submitted in early November. The large arid small break LOCA evaluations have been reevaluated to determine what impacts, if any, the revised core loading pattern or thermal shield rerr. oval have on the previously docketed results. A summary of the reevaluations of these accidents is presented herein. The remaining transients and accidents in the Millstone Unit No. 2 licensing basis will be addressed at the time the revised reload safety analyses are docketed.

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1 f Small Break Loss-of-Coclant- Accident ,

An evaluation of the impact on the Small Break LOCA ECCS performance of removing the ' thermal shield from Millstone Unit No. 2 has been completed.

Removing the thermal shield from the reactor vessel annulus improves the small break LOCA response in that additional liquid coolant will be present in this

region during the transient. Thus, following a small break LOCA, more water

[ inventory is present in the annulus which would delay core uncovery for all of the breaks analyzed in the.small break LOCA spectrum for Millstone Unit No. 2.

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That is, an additional amount of liquid would have to be " boiled off" in the core to reduce the core level to the same elevation as that for the case with the shield present. The additional time to core uncovery will reduce the amount of 4

~ stored energy remaining in' the fuel resulting in lower peak clad temperatures prior to core recovery by the safety injection system.

, The revised loading pattern does not impact the present licensing basis small  ;

break LOCA analysis and the results remain conservative. This is based on the fact that the new loading pattern maintains the Technical Specification limits on linear heat generation rate (15.6 kw/ft) assumed in the licensing basis small break LOCA analysis. - All other Cycle 6 core physical parameters remain bounded by the assumptions utilized in the current small break analysis.

• Large Break Loss-of-Coolant-Accident The Cycle 6 large break LOCA analysis has been evaluated to determine the impact of _the removal of the the rmal shield at Millstone Unit No. 2. The blowdown transient and the reflood transient portions of the accident scenario >

were specifically evaluated.

i Blowdown Transient l

Thermal shield removalimpacts the following portions of the blowdown model.

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1. Reduces downcomer metal heat and heat transfer surface.

l' i- 2. Enlarges downcomer region flow area and volume.

- Note that since the peak clad temperature (PCT) for Millstone Unit No. 2 occurs j during the reflood portion of the transient, the impact of the modification during

the blowdown portion of the transient will generally have a second order effect on the ultimate PCT.  ;

Reducing the downcomer metal heat provides for cooler flows through the core

- resulting in improved core heat transfer prior to core flow removal. After core l flow reversal, the break discharge flow to tbe containment will be at a'slightly i

lower enthalpy, which tends to reduce energy discharged to the containment.

! Enlarging the downcomer flow area (additional 10.5 ft2) and volume (additional

{. 120 f t3 ) by removing the shield has the following ef fects on the large break l LOCA:

1. Less resistance to flow in downcomer allowing reversed core flow velocities to increase which benefits core heat transfer.

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- 2. The additional 120 ft3 of downcomer fluid' is discharged to containment increasing containment pressure in the early porti>n of the transient. -(This effect more than offsets cooler discharge flows due to reduced metal heat).

3. Larger 'downcomer flow area'potentially provides for earlier end of bypass and reduced safety injection tank (SIT) deficit, due_ to easier penetration of SIT water into the downcomer.

The expected net result during blowdown of the effects described above would be a negligible to small decrease in calculated maximum clad temperature at the

.end of the blowdown phase of the accident.

Reflood Transient The thermal shield removal impacts the following reflood mocel assumptions: >

1. Reduces metal heat in downcomer.

2. Enlarges downcomer volume and flow area.

The major effects of these differences on the large break LOCA are as follows:

1. Less metal heat in the downcomer increases the lower plenum fluid subcooling, which improves core reflooding rates and acts to reduce the PCT.

2. Increased downcomer volume ( 120 f t3 additional) requires more SIT i'

' water to fill the downcomer early in the reflood transient. However, an evaluation of the current Millstone Unit No. 2 worst large break LOCA analysis demonstrates that ample additional SIT water is

available,'such that the downcomer will still be filled by the SIT,

providing the same downcomer hydrostatic head for reflooding. This additional SIT cold water, therefore, will - not ' spill to the

containment. The net effect will be an increase in the containment l'

' pressure which impreves the core reflooding rate and reduces PCT.

A slight delay in the filling of the downcomer ( l-2 seconds) early in i

reflood, due to the additional volume, has a negligible effect on the reflooding transient that follows.

The expected net result of the effects during reflood described above is a negligible to small decrease in the ultimate PCT during reflood.

Based on this evaluation, it is conservatively concluded that removal of the thermal shield at Millstone Unit No. 2 can - be expected to have a negligible

effect on the overall calculated PCT for the large break LOCA analysis.

I Furthermore, a PCT benefit from this plant modification could be expected. The

Cycle 6 'large break LOCA analysis predicts a PCT of 20550F demonstrating i

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d significant margin to the acceptance criterion specified in 10CFR50.46. Based on the negligible impacts of the thermal shield removal on the large break LOCA analysis results, it is concluded that the existir.g large break LOCA analysis of Reference (1) remains limiting.

As was the case .for .the small . break LOCA analysis, the. revised Cycle 6 core ,

loading pattern does not impact the Cycle 6 large break LOCA analysis. This is based on the fact the new loading pattern maintains the Technical Specification limits on linear heat generation rate assumed in the large break LOCA analysis.

All other Cycle 6 core physical parameters remain bounded by the assumptions utilized in the current large break LOCA analysis.

NNECO concludes that the evaluations for the small break LOCA and large break LOCA, provided to the Staff in Reference (1), remain appropriate for Cycle 6 operations with the revised loading pattern and thermal shield removal.

Details of the Cycle 6 loading pattern will be provided with the revised reload safety analyses report scheduled to be docketed in early November,1983.

We trust you find this information satisfactory.

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY M/ - , T(4MAI r W. G. Counsil Senior Vice President i

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