NRC-94-4286, Provides Suppl Info on AP600 PCS Film Flow Coverage Methodology
| ML20072Q522 | |
| Person / Time | |
|---|---|
| Site: | 05200003 |
| Issue date: | 08/31/1994 |
| From: | Liparulo N WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | Borchardt R NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NTD-NRC-94-4286, NUDOCS 9409120094 | |
| Download: ML20072Q522 (13) | |
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Westinghouse Energy Systems kx355 Pms urgh Pennsylvania 15?30 03S5 Electric Corporation NTD-NRC-94-4286 DCP/NRC0198 Docket No.: STN-52-003 August 31,1994 Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555 ATTENTION:
MR. R. W. BORCHARDT
SUBJECT:
SUPPLEMENTAL INFORMATION ON AP600 PCS FILM FLOW COVERAGE METHODOLOGY
Reference:
Westinghouse AP600 I2tter Report, " Method for Determining Film Flow Coverage for the AP600 Passive Containment Cooling System Analyses," transmitted via letter NTD-NRC-94-4247, July 28,1994
Dear Mr. Borchardt:
The reference report presents the film flow coverage model for the AP600 containment shell. This model was discussed with NRC staff during a July 26,1994 meeting. Several questions on the model were raised during the meeting. Westinghouse was requested to provide additional information addressing these questions by the end of August,1994. The attachment to this letter responds to this request.
The Westinghouse Electric Corporathn copyright notice is attached. Please cantact Brian A. McIntyre on (412) 374-4334 if you have any questions concerning this transmittal.
A//7 A
N. J. Liparuto, Manager Nuclear Safety Regulatory And Licensing Activities
/nja Attachment cc:
T. Kenyon, NRC (w/o Enclosures / Attachments)
R. Hasselberg, NRC C. Hoxie, NRC A. Notafrancesco, NRC P. Boehnert, ACRS L Shotkin, NRC (w/o Enclosures / Attachments)
B. A. McIntyre, Westinghouse (w/o Enclosures / Attachments)
~1 m58A 9409120094 940831 PDR ADOCK 05200003 h
A PDR I
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COPYRIGIIT NOTICE The reports transmitted herewith each bear a Westinghouse copyright notice. The NRC is permitted to make the number of copies of the information contained in these reports which are necessary for its internal use in connection with generic and plant-specific reviews and approvals as well as the issuance, denial, amendment, transfer, renewal, modification, suspension, revocation, or violation of a license, permit, order, or regulation subject to the requirements of 10 CFR 2.790 regarding restrictions j
on public disclosure to the extent such information has been identified as proprietary by Westinghouse, copyright protection not withstanding. With respect to the non-proprietary versions of these reports, the NRC is permitted to make the number of copics beyond those necessary for its internal use which are necessary in order to have one copy available for public viewing in the appropriate docket files in the public document room in Washington, D.C. and in local public document rooms as may be required by NRC regulations if the number of copies submitted is insufficient for this purpose. Copies made by the NRC must include the copyright notice in all instances and the proprietary notice if the
{
original was identified as proprietary.
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Attachment to Westinghouse Letter NTD-NRC-94-4286 Supplemental Information Addressing NRC Questions on the AP600 PCS Film Flow Coverage Methodology j
Backaround
)
On July 26,1994, the results of an analysis were presented to the NRC on predicting the water coverage for the AP600 PCS. This analysis was documented in a topical report titled, "A Method for Determining Film Flow Coverage for the AP600 Passive Containment Cooling System"M. The model is based on determining the local film flow per unit l
perimeter, and the minimum film flow needed to sustain a stable film. The stability model M
is based on a paper by Zuber & Staub, and the local film flow model depends on the local surface area that the film is traversing, the mass of water lost to evaporation, and the reduction of the film due to established instability. As a result of this presentation, a number of questions were raised:
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1.
Is a simple 30 year old model adequate for predicting film stability for the l
AP600 1
2.
Does the film exhibit catastrophic breakup or fingering upon reaching an i
unstable state i
3.
What is the sensitivity of the containment response to water coverage 4.
Does the available test data indicate what the film behavior is upon reaching an unstable state 5.
Is it adequate to use a one-dimensional model to predict two-dimensional film flow 6.
What is the effect of aging on the coating surface with respect to water coverage 7.
What would occur if the PCS water fai!ed to initiate, and the containment was dry 8.
Explain the basis for applying full PCS water flow after a 660 second delay These questions are addressed in the following sections.
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l Attachment to Westinghouse Letter NTD-NRC-94-4286 l
l Observations of Film Behavior in the AP600 PCS Tests 1
\\
l Observations by test engineers and review of video recordings of two AP600 PCS tests, I
the Large Scale Tests (LST)W and the Water Distribution Tests (WDT)M, show that the film becomes unstable due to flow maldistributions which are caused by the uneven application of the film from the distributors in the case of the LST (see Figure 1), and from the weirs in the case of the WDT (see Figure 2). These instabilities were found to initiate and split the water flow into alternating wet and dry stripes which remain constant with i
j time. The width of the dry stripes increased very slowly due to the thinning film caused l
by increasing surface area along the dome, and evaporation along the dome and l
cylindrical wall.
At no time was a sudden change from a smooth film to several small rivulets or
" fingering" observed. This is believed to be due to the low surface heat flux values which are common to tests and the AP600 containment. This is further supported by the Film R
Evaporation Tests conducted for AP600 on a flat heated plate which showed for high heat flux, low flow tests, the film formed alternating wet and dry stripes which did not fragment. The stripe pattern corresponded to the discrete water applicators which indicates that the observed dry patches resulted from instability aue to flow maldistribution, similar to that observed for both the LST and the WDT.
While in the context of a cylindrical coordinate system, the film flow has components in all three dimensions (r,0, and z). However, in a curvilinear sense, the flow is largely one dimensional from the center of the dome to the bottom of the cylinder wall. Thus, the model, which tracks the film along this path while accounting for thinning of the film due l
to area divergence is adequate to predict the film behavior.
The Zuber-Staub Film Stability Model The Zuber-Staub stability model consists of a force balance on the film surface at the point of dry patch formation. The model accounts for all the major forces acting on the interface, including the momentum of the spreading film, the surface tension, the thermocapillary effects which arise from the change of fluid properties across the film thickness, and the vapor thrust, which is significant only at surface heat flux values much greater than those for AP600. The model provides a simple method for determining the local film stability for a film which is spreading across a predom'inantly flat surface (dome),
and for a film which is falling down a predominantly vertical surface (cylindrical wall). It is for these reasons that the Zuber-Staub stability model was chosen to model the tests and to predict the AP600 water coverage. Comparisons with the test results indicate that the stability model, coupled with a mechanistic coverage reduction model, predicts the coverage values observed in both full and reduced scale tests, and matches the observed 2
i Attachment to Westinghouse Letter NTD-NRC-94-4286 behavior of the films in both tests.
The fact that this model includes the pertinent, physically observed phenomena, and closely matches and predicts observed coverage values indicates that the model is applicable for evaporative film flow over containment shell geometries with relatively low surface heat fluxes. While more recent models have addressed specialized geometries, surface conditions, heat fluxes, etc., most refer to this earlier work as the basis for their model. Thus, the Zuber-Staub model, in conjunction with the film spreading model described in Reference 1, is appropriate for analysis of the AP600 PCS.
Surface Aaino and Weathering
)
The effects of aging and weathering of the Carbo-Zinc coating on surface wetting and film stability has been addressed to the degree that both weathered and unweathered samples were tested to determine the contact wetting angle between the coated surface and the water film. It was observed that the weathered surface exhibited marginally better wetting characteristics than the unweathered surface.
l Conversations with the coating manufacturer indicate that the surface stabilizes after "a few years", and, although no formal tests of wetting have been conducted, good wetting characteristics were observed for these stabilized surfaces.
RAI 252.28 provides additional information on the coating.
l-WGOTHIC Delay in Applyina Water Film l
Built into the WGOTHIC AP600 model is a 660 second delay in applying the PCS water i
to the outside of the containment shell. The actuation of the system occurs after a 5 psi containment overpressure. At this point, water is applied to a large receptacle at the top l
of the dome. After a few seconds, the receptacle is filled and the water flows onto the horizontal surface and spreads radially outward toward the weirs. After collecting at the first weir, the water is distributed through grooves cut into the weir wall. The water then collects above the second weir, which is located down on the sides of the dome. The water is uniformly distributed by the second weir onto the remaining shell surface. The total time to actuate the system, including signal delays, filling the lines leading to the receptacle, filling the receptacle, filling the first and second weirs, and establishing a steady-state water coverage pattern was found to be approximately 660 seconds. Up to that time, the containment shell is covered by varying amounts of water, especially in the dome region. However, it has been cmservatively assumed that no water coverage is applied until 660 seconds at which time the steady-state coverage is assumed.
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Attachment to Westinghouse Letter NTD-NRC-94-4286 The water coverage assumed in the SSAR analysis,40% coverage on the dome, and 70% coverage on the side walls, was based on an early weir design. The latest Phase 3 Water Distribution Tests showed that, for the nominal flow case, the cov'erage in the dome region was greater than 40% out to the second weir, and 100% on the wall from the second weir to the bottom of the PCS. For the AP600 containment, the stability q
model is applied from the second weir downward to determine the coverage based on the water flow and the wall heat flux. Since both of these quantities vary with time, a time-weighted average PCS water flow rate, and a time-weighted average heat load are assumed to determine the nominal water coverage for AP600. The coverage values for j
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the nominal, three-quarters nominal, one-half nominal, and dry cases are shown in Table i
1.
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AP600 Sensit:vity to Film Coverage From Reference 1, the AP600 containment peak pressure resulting from a design basis
=
LOCA is relatively insensitive to the coverage. This sensitivity study was performed by 1
arbitrarily reducing the coverage fraction to approximately one-half of the SSAR values.
it was requested that a sensitivity study be performed using a more mechanistic determination of coverage fraction based on the stability model.
For this analysis, the WGOTHIC model of the AP600 is used with the prescribed PCS water flow from the SSAR. The extent of water coverage is determined from the film coverage model described in Reference 1.
Contained in this model is the stability parameter, R,,,, which, when set to 1.75, was found to conservatively predict the 1/8*
scale LST and the full scale WDT test results. To adjust the water coverage for this sensitivity study, R,,, is increased, and the model predicts film instabilities occurring closer to the top of the dome. This causes a reduction in the coverage fraction, which allows more of the water to reach the bottom of the PCS without being evaporated, resuiting in a decrease in the overall heat removed from the containment shell, and a corresponding j
increase in the internal containment pressure and temperature. In addition, the cooldown rate for the containment is impacted by the assumed coverage.
4 The limits on design basis pressure for containment response following a design basis j
LOCA are outlined in Reference 6:
1.
The peak pressure must remain below the design pressure limit 2.
To assure rapid reduction in the containment pressure, the pressure should be reduced to less than 50% of the design limit within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
)
The resut.s of a sensitivity analysis during which the water coverage fraction is varied is 4
l Attachment to Westinghouse Letter NTD-NRC-94-4286 shown in Figure 3. The nominal coverage case assumes that the dome region is 40%
covered until the film reaches the second weir. From the second weir to the bottom of the cylindrical wall, the stability model described in Reference 1 (with a nominal value of R,,,=1.75) is use to determine the coverage. The peak pressure for the nominal coverage case is 52 psia, and the containment pressure is reduced by the PCS to a value of 28 psia in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
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Also shown in Figure 3 are the containment responses for the reduced coverage cases, assuming one-half nominal coverage, and three-quarters nominal coverage. These coverage values were determine by assuming 40% coverage out to the second weir, and allowing the stability model to calculate the coverage for the remainder of the film flow path. The fractional coverage was determined by adjusting the stability parameter, R,,,,
until the total coverage over the entire surface was reduced by the decired amount. It should be noted that the values of R,,, needed to achieve the fractional coverage is far i
more restrictive than what was correlated for the tests. In fact, attempts to predict the test results by using these values in the stability model, resulted in grossly underpredicted coverage.
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The peak pressure for the three-quarters coverage case is 53 psia, and the containment pressure is reduced by the PCS to 30 psia in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. For the one-half coverage case, the peak pressure is 54 psia, and the pressure is reduced by the PCS to 34 psia after 24 l
hours. For both cases, the pressure response is well within the design limits, and indicates that significant margin exists in the AP600 containment design.
Also shown in Figure 3 is the case of a dry containment with no water film. This case is presented for comparison purposes and is not a plausible sequence for design basis accidents. For this case, the system exceeds the design pressure of 59.7 psia after 2300
- seconds, it is apparent from Figure 3 that the peak containment pressure is relatively insensitive to the film coverage for any credible reduction in coverage fraction. The net effect of reducing the coverage is to increase the heat storage inside the containment heat sinks, resulting in a small increase in the peak pressure, and reducing the rate at which the pressure is reduced after the peak is reached.
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Attachment to Westinghouse Letter NTD-NRC-94-4286 Conclusions Several conclusions can be drawn from these additional analyses.
l 1.
The Zuber-Staub model, while 30 years old, adequately models the important physical phenomena, and provides an excellent match to the observed experimental results. Agreement with this model was observed for both the overall coverages that were measured, and for the way the film instabilities resulted in these reduced coverages (i.e. alternating, steady, wet and dry stripes with the dry portions slowly increased as the film flow per unit perimeter decreased due to surface area change and evaporation).
The one-dimensional model accurately tracks film flow from the top of the dome to the bottom of the cylinder wall.
2.
The effects of surface aging and weathering actually aids in maintaining a stable film by decreasing the contact wetting angle between the surface and the film. Observation by the coating manufacturer indicate that the surface stabilizes and maintains these excellent wetting characteristics over the li.'e of the structure.
3.
The time delay of 660 seconds is based on the time it takes to reach a steady-state film flow pattern in the Water Distribution Tests. By applying this delay, WGOTHIC conservatively ignores any benefit from water applied after the PCS is actuated a few seconds into the event, until the steady--
state coverage is reached.
4.
Sensitivity studies were conducted by decreasing the coverage by forcing the R,, stability factor to be artificially high. These studies show that the coverage can be reduced to one-half of the nominal coverage without reaching the design pressure limit of 45 psig. In addition, the pressure is reduced below the rapid reduction limit of 22.5 psig after 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. To achieve this degree of film coverage, a value of R,,,=6.6 is needed compared to a value of R,,,=1.75 which conservatively fits the test data. In addition, for the case where no water is applied to the containment shell, the containment pressure remains below the design limit for 2300 seconds after the initiation of the postulated event.
These studies show that significant margin exists in the AP600 PCS design.
Furthermore, the ccverage values assumed in the SSAR, which were based on earlier observations from the Water Distribution Tests are adequate to use to predict the heat removal from the containment and the containment pressure and temperature response to a design basis event.
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4 Attachment to Westinghouse Letter NTD-NRC-94-4286 References 1.
"A Method of Determining Film Flow Coverage for the AP600 Passive Containment Cooling System", AP600 Doc. # PCS-GSR-003,1994.
2.
Zuber, N. and Staub, F.W., " Stability of Dry Patches Forming in Uquid Films Flowing Over Heated Surfaces", Int. J. Heat Mass Transfer, Vol. 9, pp 897-906, 1966.
3.
"AP6001/8* Large Scale Passive Containment Cooling System Heat Transfer Test Baseline Data Report", AP600 Doc. # PCS-T2R-003, Rev.1,1992.
i 4.
" Phase 3 Passive Containment Cooling Syctem Water Distribution Tests", WCAP-i 13817,1994.
5.
" Tests of Heat Transfer and Water Film Evaporation on a Heated Plate Simulating Cooling of the AP600 Reactor Containment", AP600 Doc. # PCS-T2R-011,1992.
6.
U.S. NRC Standard Review Plan, NUREG-0800, Sec. 6.2.1.1.A,1981.
4 2
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Attachment to Westinghouse Letter NTD-NRC-94-4286 Table 1: AP600 Water Coverage Sensitivity Study - Coverage Fractions Dome Cylindrical Wall Case R,,,
Top Mid Bot Top Mid1 Mid2 Bot Average Nominal 1.75 40%
40%
66%
55%
43%
34 %
30%
48%
i 0.75 Nominal 3.2 40%
40%
48%
36%
31 %
28 %
26%
36%
0.5 Nominal 6.5 40%
40%
37%
19%
18%
17%
16%
24 %
Dy 0%
0%
0%
0%
0%
0%
0%
0%
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S Attachment to Westinghouse Letter NTD-NRC-94-4286 WGOTHIC Water Coverage Sensitivity Based on Film Stability Model 8k 75-l
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,i umit at 2300 sec.
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50 Design Limit 5 40-E
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as E 30-8 25-l Application of PCS Water 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> Reduction Limit 20-15
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Dry Figure 3: Results of Sensitivity Study 11
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