ML20127H155

Provides Answers to LOCA Analysis Questions in

ML20127H155
Person / Time
Site:	Monticello
Issue date:	11/01/1977
From:	Mayer L NORTHERN STATES POWER CO.
To:	Stello V Office of Nuclear Reactor Regulation
References
NUDOCS 9211180413
Download: ML20127H155 (3)

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y,y NOMTHERH STATES POWER COMPANY M I N N E A PO L,18. M I N N E S OT A S t oot

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Mr Victor Stello Director l,gg 2)

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Division of Operating Reactors

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@[h c/o Distribution Services Branch, DDC, Alti

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U S Nuclear Regulatory Conniesion r

Washington, DC 20555

Dear Mr Stello:

MONTICELLO NUCLEAR GENDIATING PLANT Docket No. 50-263 License No. DPR-22 Answers _to LOCA Analysis Questions An October 12, 1977 letter from Mr Don K Davis of your organization requested answers to two questions concerning our September 15, 1977 submittal.

The questions are repeated below along with their respective answers.

The responses make reference to the following document:

Reference $

General Electric Company " Analytical Model for Loss-of-Coolant Analysis in Accordance with 10CFR50 Appendi.x K" NEDO-20566, Vol. II Questien 1 From the August 1975 analysis and from the present analysis, provide tabular MAPulGR (Maximum Average Planar Linear lleat Generation Rate) values, without rounding off or truncation, to two significant figures to the right of the decimal point (XX.XX) for the following:

SD219 fuel at 30,000 MWD /MTU exposure 8D250 fuel at 15,0001MD/tfrU exposure 8D250 fuel at 20,000 }MD/MTU exposure If any of the requested MAPulGR values correspond to a PCT (Peak Clad Temperature) below 2196 F, provide the corresponding PCT.

Response

The requested information is given below:

1975 Analysis 1977 Analysis Puel Type Exposure MAPUlGR PCT MAPUlGR PCT MWD /T*

_kw/ft O F kv/ft_

0F 8D219 30000 10.51 2200 10.28 2138 8D250 15000 10,80 2197 10,78 2197 8D250 20000 No value was cal-10.68 2196 culated for this 773110153 exposure 9211180413 771101 PDR ADOCK 05000263 M

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NOR RN GTATES POWER COMFANY Mr Victor Stello Page 2 November 1, 1977 Please note that General Electric provides MAPLHCR's to operating plants which a re truncated after the first significant figure following the decimal point.

Further, it is recomended that the operating limit be established on the basis of the truncated MAPIEGR.

• The exposure units used here are consistent with both analyses being MWD /STU.

Question 2 Provide a qualitative explanation of hot mode uncovery vs.

break size (Figure 6).

The explanation should clarify how depressurization rate (in the new model as modified from the previous model) and the current flow limiting phenomenon as applied to the Monticello geometry (by-pass area, vessel size, flow paths through the tower core region into the lower plenum, etc.) combine to cause the most limiting break to be 407, of the DBA (Design Basis Accident). Also, explain why this com-1.

bination of effects causeo a greater increase in PCT for smaller breaks for Monticello than for the Icad plant (Quad Cities), where PCT for smaller breaks remains slightly below the DBA PCf.

Response

The most limiting break for Monticello was detemined to be 407, of the DBA for the following two reasons:

1) The depressurization rate, in the new model as modified from the previous model generally has a greater impact on the smaller breaks than on the larger breaks as the new method results in longer periods of steam generation due to flashing. The increased steam generation calculated then affects the amount of core spray flow to the lower plenum as detemined by the counter current flow limiting characteristics of the core or by bypass regions.

2) At some break size smaller than the DBA and generally for all breaks smaller than that, the REFLOOD code uses the small break model (SBM) instead of the large break model (LBM).

(The differences in the two models are discussed below.) As there are some differences in the two models, there appears to be an apparent discontinuity in the break spectrum analysis of these breaks.

For Monticello, arourid 407. of the DBA is the break at which SBM is used. This difference in models, combined with the reason disc.issed in (1) above, combine to make the 407. of the DBA the most limiting break for Monticello.

NOR RN CTATED POWER CO NY

' Mr Victor Stello Page 3 November 1, 1977 he reason the results for Monticello are different from those for the lead plant is the difference in the sensitivity of the results to changes discussed in (1) above, i.e., as the 1 cad plant has more bundles, it has more leakage paths; hence the results are less sensitive to the CCFL phenomena in the core _ or bypass region and hence the shape of the break spectrum (PCT or time during which hot mode remains uncovered versus break size) is slightly different.

Difference Between REFLOOD Small and Large Break Models The REFLOOD code automatically uses the small break model for any transient for which there is a water level in the active core region, when the calculation switches from the SAFE code to the REFLOOD Code.

The two most significant differences between the inna11 and large break models are:

a) Use of th1 Vaporization correlation The vaporization of spray water in the core during the period when core sprays are operating is calculated using a bounding correlation. The correlation, as discussed in Reference 1, requires the PCT at time of spray initiation.

The LBM correctly uses a constant value where as the SBM y~

censcrvatively u.ies a continuously increasing value.

This difference generally results in a more conservative calculation of the reflooding time using the small break model, s

b) Level and Vaporization Following Bottom Reflooding: The LBM uses an empirically based. void fraction of 0.50 for calculating the level and the vaporization below the level. W e SBM uses the conservative fuel rod heatup model with a reflooding heat transfer coefficient to calculate the level and the vaporization below the level.

This difference generally results in a mor'e conservative calculation of the reflooding' time using the SBM,

Yours very truly, p

s o,

L 0 Mayer, PE Manager of Nuclear Support Services LOM/MHV/deh cc:- J G Keppler G Charnoff

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-J W Ferman

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