B12594, Forwards Response to NRC 870427 Question 10 Re Nulap 5

Forwards Response to NRC 870427 Question 10 Re Nulap 5

ML20235N915
Person / Time
Site:	Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png
Issue date:	07/10/1987
From:	Mroczka E, Sears C CONNECTICUT YANKEE ATOMIC POWER CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
B12594, NUDOCS 8707200268
Download: ML20235N915 (16)

Text

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CONNECTICUT YANKEE ATO MIC POWER COMPANY l

B E R L I N, CONNECTICUT. l P.o. Box 270

• HARTFORD, CONNECTICUT 061414270 . l i

Tfil. PHONE en s.som July 10,1987 Docket No. 50-213 B12594 Re: 10CFR50.46 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D. C. 20555

References:

(1) F. M. Akstulewicz letter to E. 3. Mroczka, "Small Break LOCA Code - NULAP3," dated April 27,1987.

(2) E. 3. Mroczka letter to U. S. NRC Doctiment Control Desk,

" Response to Request for Additional Information on NULAP5," dated June 2,1987.

Gentlemen:

Haddam Neck Plant Response to Request for Additional Information on NULAP5 In Reference (1), eleven requests for additional information were made concerning the NULAP5 manual. Reference (2) submitted responses to ten of these questions. The purpose of this letter is to provide a response to the remaining Question 10. Attachment I contains this response.

Connecticut Yankee Atomic Power Company (CYAPCO) trusts this information will enable the NRC Staff to complete their review of the NULAP5 code. Should you have any further questions please contact us.

Very truly yours, CONNECTICUT YANKEE ATOMIC POWER COMPANY h.S. h b E. J. Mroczka l

Senior Vice President 8707DOOP69 B70710 C' DR /spOCK 05000213 gh PDR By: C. F. Sears Vice President cc: W. T. Russell, Region 1 Administrator 0 F. M. Akstulewicz, NRC Project Manager, Haddam Neck Plant [,0 g P. D Swetland, Resident Inspector, Haddam Neck Plant I

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Docket No".' 50' -213 '

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B12594 Attachment 1 Response to Requests for Additional Information on NULAPS 1

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July 1987 1

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0.10..

Northeast Utilities stated in msponse to Omstion 0.X.11 that the f m1 rod temperature incmase terminated at the same time the core two-phase level wnt, to zero because the code calculated a transition to mist flow in the com and increased steam velocities in the com during the loop seal clearing process enhanced the cooling of the rods. While thi's may be what NULAP5 calculatad to occur, it is the staff opinion that these phenomena am not real and that the rod hot spot at the top of the com should be heating up during the. period when the two phase and collapsed levels are at the bottom of the core. The calculated psults for the analyses forming _

the basis of Qmstion 0.Xp 11, the 0.1 ft discharge leg break with the pmps off and the 0.02 ft discharge leg bmak with the pmps off, should be reviewed in detail to check for the possible calculation of nonphysical phenomena and results. Areas myiewed should, at a minimm, include the interphase drag models, flow regime maps, and heat transfer logic. The msults of this study should be provided to the NRC for review.

RESPONSE

The 0.1 ft and 0.02 ft discharge leg breaks with RCPs off wre reviewd to determine whether NULAP5 was calculating nonphysical phenomena during the loop seal clearing period. It was found that in some cases following the calculation of CHF, the NULAPS heat transfer logic would apply a film boiling correlation for all modes of heat transfer. During the short

, duration core uncovery period during the clearing of the loop seals, the l film boiling correlation would he applied in place of the steam cooling l correl ation. The film boiling correlation was found to he slightly non-conservative when used to. calculate steam cooling heat transfer. The NULAPS heat transfer logic was modified to ensure that the steam cooling correlation is always used when soolant void fraction is greater than or equal to 0.99. This modification does not effect the no return to nucleate boiling logic. No modifications mre made to the flow regime maps cr interf acial drag models.

2 The 0.1 ft and 0.02 ft discharge leg breaks were rerun with the modified model using a maximum time step of 0.01 seconds. The results are shown on 2

Fiqums2 1 through 3 for the 0.1 ft break and Figures 4 through 6 for the 0.02 ft bmak. Figum I shows that the clad surface temperature continms to increase during the uncovery period and reaches a maximm vaim of 11380 F at 749 seconds which correspond to the tire of core level recovery.

Figure 4 shows that for the 0.02 ft 2 reat, the clad temperature increase terminated at a maximm valm of 1366 F prior to level recovery. This was ',

dm to the establishment of mjst flow in the core resulting in lomr heat sink temperatures.The 0.02 ft break was rerun using maxima time steps of 0.005 seconds and 0.002 seconds to assess the effect of very small time step sizes on the clad temperature msponse during the loop seal clearing '

pe riod. Figums 7 through 9 show the results for case using 0.005 seconds time step. De to mist flow in the com channel, D the clad temperature increase terminated at a maximm val m of 1356 F prior to level recovery.

Figums 10 through 12 show the results for the case using 0.002 second time f

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,c ste p. Figure 10 showg that the clad temperature continu>s to increase to 'a maximun valu> of 1455 F at which time the temperatum increase is

. terminated dm to level recovery.

There results. indicate that. during loop seal clearing, the com thermal hydraulics and clad temperatum response may be sensitive .to the .maximun allowed time step. Not all break sizes, however, are effected by the mist flow that can occur .in the the upper portion of. the com during the loop seal clearing uncove ry pe riod.. To ensure a conservative clad temperature response ~ in all f uture runs,: tha. clad temperature and core the rmal hydraulics will be checked to ensure that the clad temperature increase 'is not artificially terminated by mist flow in the uppar core during the loop seal ~ clearing uncovery period.

It. should be noted that the RCS thermal bydraulics are characterized by a highly transient condition .iuring.the' loop seal clearing process. The large amounts of water held in the steam. generator tubes (upside) also contribute heavily to the highly transient conditions of this process. The effect of water from the steam generator tubes attempting to drain into the hot legs and steam flow retaining water in. the tubes cause presstre variations in the upper plenun maion which may cause core level and clad tempe rat ure fl uct uations. ~Be c a use of t he se e f fe ct s , i t .i s ne ce s s a ry t o ensure that the time steps are appropriately chosen to maximize ppak clad temperature during the 1 pap seal clearing . period.

It is also appropriate to identify other conservatism inherent in the Northeast Utilities small break LOCA modelling. In order to conservatively predict the clad temperature response during the loop seal clearing process, the following; asstaptions have been made:

1. All- of the bundles in the hot bundle mgion in the NULAP5 blowdown and heatup'model am assuned to be at the power level of the hottest assembly in the core. 36 assemblies represent the hot hundle region.

In fact, there are only 4 hot bundles in the core.

2. The hot red model assunes a. pnak linear best generation rate of 17.0 kw/ft compared to-the Tech. Spec. val m of 14.3 kw/ft.

3.- Loop seal noding was chosen to maximize core uncovery during tne loop seal clearing process.

4. The leakage paths between the hot leg and annul us were not modelled to maximize thn depth of core uncovery during loop seal blowout. This precl(des a direct steam vent path to the break during loop seal bl owo ut .

5. Since NULAp5 conservatively treats water hangup in the steam generator, tha duration and depth of com uncovery during loop seal blowout is maximized since the steam must pass through the water held in the steam ge ne rators. This is a major conservatism in the NULAp5 small bmak model.

Based on the above, the overall model can he considered to be very conservative mgarding the blowdown hydraulics and associated core incove ry/he at up analyse s. As stated above. based on the tine step studies, all new analyses using the NU small bmak LOCA ECCS eval uation model will be caref ully reviewed to assure the time steps have been properly chosen to maximize peak clad temperatums during the loop seal clearing period.

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