ML19282C138
| ML19282C138 | |
| Person / Time | |
|---|---|
| Issue date: | 02/23/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19282C133 | List: |
| References | |
| NUDOCS 7903210059 | |
| Download: ML19282C138 (23) | |
Text
.
OFFICE OF NUCLEAR REACTOR REGULATION TOPICAL REPORT EVALUATION Report No. and
Title:
BN-TOP-4, "Subcompartment Pressure and Temperature Transient Analysis."
Originating Organization:
Bechtel Power Corporation r-Summary of Tonical Report Report BN-TOP-4 describes the COPDA code used by Bechtel to predict pressure and temperature histories in containment subcompartments fol-lowing a postulated pipe failure. The thermodynamic equations used to calculate local temperature and pressure are presented along with the flow equations used to transfer mass and energy between the nodel points. The basic assumption of the code is that the coolant is finely dispersed within the subcompartment so' that the steam-water-air mixture behaves homogeneously.
This assumption is utilized in the temperature pressure calculations as well as the flow rate calculations.
Guidance is provided to the user in selection of proper noding and flow path loss factors.
O 4 e. mm W
7903210059
Methods of calculating break flow into the subcompartment from the reactor system are not included in BN-TOP-4. These will be calculated by the nuclear steam supply system vendor and not by Bechtel.
Summary of Reculatory Evaluation Following a pipe break within a centainment subcompartment, local pressures will be generated that are higher than the average containment pressure.
These pressures are reliev2d as the blowdown proceeds by mass flow through the subcompartment vents that are connected to the main containment. The magnitude of the compartment pressure as well as the spatial gradient is important in determining the force on the subcompartment wall and the force on the external surface of reactor coolant pressure boundary system components located within the subcompartment. These forces are necessary for the calcu-lation of stresses within the reactor vessel supports.
In our evaluation we have applied the acceptance criteria presented in Section 6.2.1.2 of the NRC Standard Review Plan. We have reviewed the applicant's supporting derivations and experimental data and have made audit calculations using the COMPARE Subcompartment Analysis Code developed by LASL (Reference 1).
Our conclusions regarding the use of the COPDA code as described by BN-TOP-4 are stated in the Staff Position ~
section of this report.
O
, Comoartment Thermodynamic Calculations The COPDA code assumes that the mixture (liquid-vapor) leaving the break is homogeneous, and becomes fully mixed with air in the subcompartment.
Although the determination of drop size within a containment subcompartment is currently under staff review, there is some indication that the fluid leaving the break will initially be finely dispersed. An analytical study was performed (Reference 2) which predicts a maximum drop size for PWR LOCA conditions of 7 microns.
In addition, films made at the INEL semiscale tests indicate that the break flow is a plume of finely dispersed droplets.
The assumption of homogeneous flow requires that all liquid ejected from the break must travel through the exit vents instead of being deentrained on the subcompartment floor. The homogeneous assumption (which does not percit deentrainment) reduces vent flow, increases the compartment pressure, and thereby introduces some conservatism in the model.
The assumption is most conservative (over predicts subcompartment pressures) for subcompartments with low pressure and flow when some amount of deentrainment would most likely occur. The homogeneous assumptions are a more accurate prediction fcr subcompartments which have high flow rates and pressures, such as expected in a reactor cavity.
The applicant has compared the steam and water properties used with CDPDA to the detailed equations in the 1967 ASME Steam Tables and determined that the maximum variance is + 0.22 percent.
We find this acceptable.
0
, Heat transfer to the compartment walls is not censidered by COPCA. Although little heat transfer would occur over the time of interest for subcompartment analysis, this effect would reduce the calculated pressure and its omission is conservative.
Flows Exitino the Comoartments The homogeneous assumption of complete mi'xing is also utilized for the calculation of vent flows and the flows between interior nadal points.
Homogeneous compressible flow is approximated using the isentropic gas law equation with the isentropic exponent assumed to be uniform along the length of the vent. The isentropic exponent is selected for the steam air-water mixture using a curve fit that is a function of pressure and steam-air-water fraction. The curve fit is designed to produce the detailed solution of the homogeneous equilibrium model (HEM). As an option, the isentropic constant may be selected based on the conditions in the upstream control vol ume. This is referred to as the " Homogeneous Frozen Model" in BN-TOP-4, Both methods were shown to provide a close approximation of the HEM modt1 at stagnation qualities above 20 percent. At lower qualities, the
" Homogeneous Frozen Model" overpredicts HEM flow and will not be used by Bechtel.
A constant value for inertia is input for each flow patn.
When the flow rate is subtritical, the flows are corrected at each time step for the effect of
. inertia by solution of the one-dimensional momentum equation. Momentum flux is not included in the flow model so that stagnation pressures are used to compute the pressure differentials.
The flow model will thus overpredict pressure for compartments having the flow parallel to the subcompartment surface. For compartments with multi-dimensional flow fields, the subcompartment pressure would approach the stagnation pressures predicted by COPDA, when the flow impinges on the subcompartment wall.
Both subcritical and critical flow rates are determined assuming no losses in the momentum equations but are multiplied by a discharge coefficient to account for nonrecoverable momentum changes.
The discharge coefficients are determined from one over the square root of the nonrecoverable loss coefficient which is determined using values available in engineering 3 or Idel Chik.4 This method was algebraically handbooks such as Crane shown by Bechtel to be equivalent to the direct application of loss coefficients in the canpressible flow equations.
When the inter-compartment flow reaches sonic velocity, reductions in the downstream pressure have little effect on the flow rates and the flow is referred to as critical. There are no available experimental data for critical flow of steam air-water mixtures through large vents.
Justification for the critical flow model is derived from small scal,e experiments without air. Typical examples of such experiments are:
O
.s-1.
The Moody slip flow model with a 0.6 correction factor was found to give a reasonable prediction of pressure decay in the 700 Series Semiscale Tests when used in the RELAP-3 code (.
(s. 5 & 6).
These comparisons are presented in Figures 1 and 2.
2.
The HEM =odel was found to underpredict critical flows over a wide range of qualities and pressures when compared to the data from small steady flow experiments (Refs. 7, 8 and 9).
See Figures 3 through 6.
~
3.
The HEM model was found by F. Moody to provide a best fit through low quality experimental data when based-on upstream stagnation properties (Ref. 10).
See Figure 7.
4.
For two phase flow conditions, the HEM model was found to overpredict flow rates by approximately 20% in comparisons with semiscale test S-02-4 (Ref.11, see Figure 8). Similar results were obtained in blowdown experiments by M. N. Hutcherson as discussed in Reference 7.
The NRC Standard Review Plan (Section 6.2.1. 2) states that both the 0.6 Moody and the Hemogeneous Equilibrium Model are acceptable. The HEM model is more conservative for flow at qualities less than 35 percent and the 0.6 Moody model is more conservative at high cualities.
8
The 0.6 Moody model is most conservative at a quality of 1.0 where the Moody model (without correction) and HEM produce approximately the same flow rate. The qualities predicted for most subcompartment analyses fall within a range where both models produce approximately the same results.
We have, therefore, concluded that either model provides a best estimate calculation of flows over the range of interest for subcompartment analysis.
Code Verification The predictions of the COPDA code were compared to experimental data from the Battelle-Frankfurt containment test facility (Test C2).
The Battelle
~
facility is a large compartmented structure designed to simulate a German PWR containment building. The flow paths are orifices in large rooms-through which the flow in most cases was one dimensional.
The test was initiated by the blowdown from a simulated reactor vessel.
Temperatures and pressures within the various compartments of the containment were measured. Although the flow rates between the compartments were not measured, pressure difference readings across the compartment vents indicate that the flow in all but one or two cases including test C2 was
' subcritical. The COPDA code was found to overpredict the measured pressures by 10 to 30 percent and to overpredict the measured temperatures by 9 to 40 percent. These results approximate those obtained at the-Los Alamos Scientific Laboratory using the COMPARE code. The COMPARE code was developed for the NRC for use in subcompartment analysis.
There are no available experimental data for critical ficw through full scale subcompartment vents. There are also no experimental data to verify the conservatism of the one-dimensional ficw assumption for subcompartments producing multi-dimensional flows. Only in one location would two-dimensional flew have occurred in the test facility.
The one-dimen-sional model was found in the COMPARE analyses to be conservative for the Battelle-Frankfurt results in all locations except in the region of two-dimensional flow. These pressure differentials were under-predicted by COMPARE.
The COPOA code results were also compared to the results from the COMPARE code for a series of 12 subcompartment standard problems. These problems consist of a series of two node problems designed to evaluate code perfor-mance for a variety of flow conditions. When the flow was subcritical, the COMPARE code and the C0F? \\ code predicted essentially the same compart-ment pressur es. When the flow was critical, COPDA predicted higher pressures than the COMPARE code. The COMPARE code used 0.6 times the Moody slip flow correlation to predict critical flow. The COPDA code used the HEM model with a discharge coefficient to account fo.r losses in the vent.
The predictions of COPDA and COMPARE were evaluated for a typical reactor cavity (Figure 9). This prcblem contained 56 control volumes and 122 ~
flow paths. The COMPARE code in this study used the HE4 medel to predict 6
9 critical flows. The COPDA predictions were about 5 percent higher than the pressures calculated by the COMPARE code. These results are presented in Figure 10.
Although COPDA provides results that are conservative to both pressure data and to COMPARE, this is not fully definitive since the tests cited above do not embrace the postulated flow rate conditions for subcompartments.
Since the flow equations in COPDA are one-dimensional, multi-dimensional ficw fields that would occur following a pipe break in a reactor cavity are approximated by a network of vertical and parallel flow paths connected by control volumes. The proper selection of the flow path and volume network should be based on noding studies in which the number of nodes is increased until a convergent solution is obtained. Since subcompartment designs differ between plants, the adequacy of the noding should be addressed with each plant application.
O m
Staff Position We have reviewed the methods and assumptions described in BN-TOP-4 and have concluded, subject to the following conditions, that the COPDA code is an acceptable method for calculating subcompartment pressures following a piping rupture within a containment subcompartment.
Methods of calculating break flow into the subcompartment from the reactor system are not included in BN-TOP-4. We require that the break flow be
~
calculated using methods that have been approved by the NRC.
The approximation to the HEM flow model referred to in BN-TOP-4 as the
" Homogeneous Frozen Model" should not be used for qualities below 20 percent. We require that the critical flow option used by the COP,DA code and the minimum calculated subcompartment quality be specified for all applications.
The selection of flow losses between compartments is dependent on the subcompartment geometry and requires judgments to be made based on the experience of the user.
Since subcompartment designs vary between plants, we require that the selection of flow loss coefficients be justified fo,r each application.
e emMm hD The noding arrangement must also be determined individually for each a; plication. We require that the adequacy of the noding be addressed with each application.
We require that all plant dependent input information such as volumes, inertia coefficients, flow loss coefficients, and flow areas be provided and justified with each application.
Based on our evaluation as detailed above, we conclude that the COPDA code is acceptable for use in predicting:
(a) nominal values of sub-compartnent pressures when sonic vent flow conditions exist; and (b) conservative values of subcompartment pressures when subsonic vent ficw conditions exist. An uncertainty analysis should be provided using relevant available data, whenever the COPD/s code is used for subcompartment analyses involving sonic vent flow conditions.
6 m
O REFERENCES 1.
R. G. Gido, "CCMPARE-Mod 1: A Code for the Transien: analysis of Volumes with Heat Sinks, Flowing Vents and Doors, LA-7199-MS, Los Alamos Scientific Laboratory, March 1978.
2.
R. G. Gido and A. Koestel, "LOCA-Generatcd Drop Size Prediction - A Thermal Fragmentation Model, ANS Transactions, Vol. 30, pp. 371 and _
372, November 1978.
3.
Crane Co., " Flow of Fluids," Technical Paper No. 410, 1969, Engineering Division, New York, N.Y.
4.
I. E. Idel Chik, " Handbook of Hydraulic ' Resistance Coefficients of Local Resistance and Frictions," USAEC-TR-6630,1966.
5.
F. J. Moody, " Maximum Flow Rate of a Single Component, Two-Phase Mixture,"
Journc1 of Heat Transfer, Vol. 87, Trans, ASME, pp. 134-142, February 1965.
6.
C. E. Slater, " Comparison of Predictions from the Reactor Primary System
~ Decompression Code (RELAP 3) with Decompression Data from the Semiscale Blowdown and Emergency Core Cooling (ECC) Project;" USAEC Rep" ort, IN-144, 1970.
e
_ _ _ 7.
M. N. Hutcherson, " Contribution to the Theory of the Two Phase 31:wdown Phenomenon, "ANL/ RAS 75-42, November 1975.
8.
H. K. Fauske, " Contribution to the Theory of Two-Phase, One-Com onent Critical Flow," AEC Report, ANL-6633, October 1962.
9.
K. H. Ardron and R. A. Furness, "A Study of the Critical Flow Models Used in Reactor Blowdown Analysis," Nuclear Engineering rnd design' 39, 1976,'257-266.
10.
F.J. Moody, " Maximum Discharge Rate of Liquid-Vapor Mixtures frem Vessels," NEDO-21052, General Electric, September 1975.
11.
D. G. Hall, "A Study of Critical Flow Prediction for Semiscale Mod-l Loss-of-Coolant Accident Experiments," TREE-NUREG-1006, Idaho National Engineering Laboratory, December 1976.
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