ML20211A009

Nonproprietary Sser of WCAP-8822(P) & WCAP-8860(NP), Mass & Energy Releases Following Steam Line Rupture

ML20211A009
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Issue date:	05/27/1986
From:	Office of Nuclear Reactor Regulation
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ML19292F393	List: ML20211A003 ML20211A009
References
NUDOCS 8606110022
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.- Enclosure 2 Supplement to Safety Evaluation Report WCAP-8822(P) and WCAP-8860(NP)

Nonproprietary Version I. Introduction In the staff's evaluation (Reference 1) of Westinghouse Topical Report

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WCAP-8822 " Mass and Energy Releases Following a Steamline Rupture," the staff noted that the effect of heat transfer to steam from the uncovered portion of the steam generator tube bundle may be significant especially for ice condenser containments. Based on a simplified scoping analysis of a large dry containment, Westinghouse viewed this heat transfer mechanism as having only a minor effect on containment response.

Westinghouse submitted additional steamline break analyses in Supplements 1 and 2 to WCAP-8822, by letters dated February 27, and October 7, 1985.

In Supplement 1, a modified version of the LOFTRAN code, accounting for heat transfer to the steam during steam generator tube bundle uncovery, was used to calculate the mass and energy blowd'okn for ice condenser con-tainment analysis. In Supplement 2, the same modified LOFTRAN code was used.to confirm the earlier scoping assessment for large dry containments and to support a similar finding for subatmospheric containments. The staff's current evaluation includes the above two supplements to WCAP-8822.

1 8606110022 860527 PDR TOPRP EMVWEST C PDR

II Review and Evaluation A. LOFTRAN/ MARVEL Codes The MARVEL code was used in WCAP-8822 for calculating mass and energy release rates following a MSLB. In Supplement I to WCAP-8822, the LOFTRAN code was used for the same purpose. These two codes are similar in their modeling. The MARVEL code was developed for asym-metric transients such as steamline breaks, while the LOFTRAN Code employed a lumped model for symmetric loop transients. The subse-quent development of the LOFTRAN code to include multiloop capabil-ity, and an entrainment model, ultimately enabled the LOFTRAN code to be used for non-LOCA system transient analysis.

In the topical report evaluation (Reference 2) for the LOFTRAN (WCAP-7907) and MARVEL (WCAP-7909) codes, the staff found both codes acceptable for the analysis of transients based on extensive verifi-i l cation and comparison of the two codes. Furthermore, in Supplement 1 to WCAP-8822, Westinghouse compared the blowdown calculations of both n

codes, and the results showed good agreement. Based on the above '

studies, the staff concluded that the LOFTRAN code (prior to incor-poration of the heat transfer modifications discussed in the following l section) and the MARVEL code are equally acceptable.

, B. Heat Transfer Calculation Method 1

The LOFTRAN code has been modified to incorporate the effect of heat transfer from the uncovered region of the tube bundle into the basic l 2

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LOFTRAN model (WCAP-7907). The heat transfer correlation used is also used in the Westinghouse TRANFLO and NOTRUMP codes to calculate forced convection heat transfer under superheat conditions. Based on the results of experiments and comparison studies, Westinghouse con-cluded 'that the modified correlation is satisfactory for use in pre-dicting the heat transfer to steam in the uncovered tube region of a steam generator.- The staff has reviewed this information and concurs

• with the Westinghouse conclusion, and, therefore, finds the heat transfer modifications in the LOFTRAN code for calculating the blowdown for containment analysis to,be acceptable. ,

C. Mass and Energy Release Calculational Model The calculational model for predicting mass and energy releases, in-cluding steam superheat, is presented in Supplement I to WCAP-8822.

The model is a modified version of LOFTRAN to account for superheat-ing of the steam in the uncovered tube bundle region. Other important assumptions used in the model are as follows:

(1) A uniform primary sie v i ebs emperature is assumed throughout the uncovered tube regior..

(2) Secondary side heat transfer to the steam is bar,ed on the tube surface temperature and the steam bulk temperature. No credit is taken for primary side film heat transfer resistance or tube 3'

r wall heat transfer resistance. Therefore, the secondary side tube surface temperature is assumed to be equal to the primary side fluid temperature.

(3) All energy transferred to the steam in the uncovered tube region is assumed to exit the steam generator with the calculated steam flow and not to mix with the remaining steam volume in the steam generator.

- The staff finds these assumptions conservative in that they maximize the blowdown energy release, and are, therefore, acceptable.

Westinghouse has also performed sensitivity ' studies of various parame-ters which could potentially affect tube bundle uncovery, and, there-fore, the time of occurrence and degree of superheating. The parane-ters include: steam generator level, power level, break size, aux-111ary feedwater flowrate, failure of a feedwater isolation valve, failure of a main steam isolation valve, and protection system re-sponse time. The results show that minimum auxiliary feedwater flow, early steamline isolation, and delayed auxiliary feedwater actuation all serve to increase the enthalpy reached during the blowdown. The initial power level and steam generator inhentory do not have a significant impact on the enthalpy.

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g a Based on a review of the assuiaptions used in developing the blowdown l methodology, and the results of sensitivity studies, the staff finds  ;

the' methodology acceptable for calculating mass and energy releases l -

. following a MSLB.

D. The Impact of Steam Superheating for Large Dry and Subatmospheric  ;

l Containments i

In Supplement 2 to WCAP-8822(WCAP-8822-S2), Westinghouse performed a parametric study of the effects of steam superheating on mass and l energy releases and on containment pressure and temperature responses for large dry and subatmospheric containments. Westinghouse used the modified LOFTRAN code to calculate mass and energy release rates and l

the C0C0 code to calculate containment pressure and temperature .

, responses.

i The containment response for two sets of calculations were compared by Westinghouse. The first set was performed assuming no steam superheating; the second set was perfomed in a similar manner except that steam superheating was accounted for using the models discussed above in Sections II.A, II.B. and II.C. For each set of calculations,

! a spectrum of break sizes and power levels was studied. The resulting break mass flow rates, flow enthalpies, and break energy release rates were compared. Th'e break mass flow rate was found to be unaffected and although a significant amount of steam superheating occurred, as evidenced by the blowdown enthalpy, it had only a minor impact on the energy release rates.

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1 Based on a review of the assumptions used in developing the blowdown metnodology, and the results of sensitivity studies, the staff finds the' methodology acceptable for calculating mass and energy releases

. following a MSLB.

D. The Impact of Steam Superheating for large Dry and Subatmospheric Containments In Supplement 2 to WCAP-8822(WCAP-8822-S2), Westinghouse performed a

- parametric study of the effects of steam superheating on mass and energy releases and on containment pressure and temperature responses for large dry and subatmospheric containments. Westinghouse used the modified LOFTRAN code to calculate mass and energy release rates and the C0C0 code to calculate containment pressure and temperature responses.

The containment response for two sets of calculations were compared by Westinghouse. The first set was performed assuming no steam superheating; the second set was performed in a similar manner except that steam superheating was accounted for using the models discussed above in Sections II.A, II.B, and II.C. For each set of calculations, a spectrum of break sizes and power levels was studied. The resulting

! break mass flow rates, flow enthalpies, and break energy release rates were compared. The break mass flow rate was found to be unaffected i and although a significant amount of steam superheating occurred, as evidenced by the blowdown enthalpy, it had only a minor impact on the energy release rates.

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. The mass and energy release rates, along with other containment data for a typical large dry or a subatmospheric containment, were then input to the C0C0 code to calculate the containment pressure and -

temperature response. The most pronounced effect of steam superheat-ing was found for a 4.6-ft2 double-ended rupture case. Although this case has the most pronounced effect of steam superheating, it is not the limiting case for calculated peak pressure or temperature. The peak pressure and temperature occur for small breaks which do not result in tube bundle uncovery and, therefore, steam superheating does

. not occur. Based on these results, Westinghouse concluded that the steam superheating effect has very little impact on the peak temperature and pressure response of large dry and subatmospheric containments.

After reviewing the Westinghouse parametric study presented in WCAP-8822-S2, the staff agreed with Westinghouse's conclusion, with one exception.

Since containment spray initiation can insnediately subdue the super-heating effect inside containment, the staff felt that any delay in spray initiation could affect the conclusions drawn in WCAP-8822-S2.

By letter from E. P. Rahe (Westinghouse) to H. R. Denton (NRC), dated December 13, 1985, Westinghouse stated that. delaying the initiation of containment spray would not adversely impact the results or change the conclusions of WCAP-8822-S2. The Westinghouse statement was based 6

on the analysis results of peak containment temperature. Although some increase in peak temperature was observed for cases without containment spray, the increase was found to be small. Therefore, the conclusions of WCAP-8822-52 remain valid regardless of the time of spray initiation.

III Sumary Based on the above discussion, the staff's conclusions are summarized as follows:

1. The LOFTRAN code is acceptable for calculating the mass and energy re-lease rates from postulated main steam line breaks, for use in contain-ment analysis.

2. The LOFTRAN code modifications, as described in Supplement 1 of WCAP-8822, for calculating steam superheating effects, are acceptable for containment analysis. The LOFTRAN code methodology is applicable to ice condenser, large dry, and subatmospheric containments.

3. The impact of steam superheating is not significant for large dry and subatmospheric containments. ,

4. Supplements 1 and 2 of WCAP-8822 are found acceptable.

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References:

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1. Letter from C. O. Thomas (NRC) to E. P. Rahe (Westinghouse), dated -

August 22, 1983,

Subject:

Acceptance for Referencing of Licensing Topical Report WCAP-8821 (P)/8859(NP), "TRANFLO Steam Generator Code Description,"

and WCAP-8822(P)/8860(NP), " Mass and Energy Release Following a Steam Line Rupture."

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2. Memorandum from R. W. Houston to G. C. Lainas, dated June 27, 1983,

Subject:

Topical Report Reviews (1) LOFTRAN (2) MARVEL.

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