Effect of NonCondensables on Natural Circulation Passive Safety Systems: Modelling and Experimentati

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2 RCM of the IAEAs CRP Oregon State
University, Corvallis, USA Aug-Sept 2005
Effect of Non-Condensables on Natural
Circulation Passive Safety Systems Modelling and
Experimentation in the AP-600 Reactor Luis
Herranz, José Luis Muñoz-Cobo, Juan Carlos de
la Rosa, Alberto Escrivà Presented by Juan
Carlos de la Rosa Instituto de Ingeniería
Energética Universidad Politécnica de
Valencia Edificio I4 Camino de Vera s/n 46022
Valencia - SPAIN delarosablul_at_yahoo.es
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Main Contents

• General Presentation of the UPV
• Main aspects of the developed work
• Brief Introduction and Overview to the
  Condensation on Containment Structures
• Study of the effects of the main variables on the
  Condensation on Containment Structures
• Analytical Condensation Model
• Validation of the model
• Conclusions

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General Presentation of the UPV

• The UPV is one of the leading Technical
  Universities in Spain with 35000 Students and
  2500 Faculty lecturers, Professors and
  Researchers
• The Institute of Energy Engineering belongs to
  the UPV and has 60 researchers that works in the
  following areas
• Thermal Engineering
• Nuclear Engineering and Thermalhydraulics
• Electric Engineering
• Renewable Energy Sources

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General Presentation of the UPV

• Thermalhydraulics and Nuclear Engineering Group
  Participation in Projects related with Natural
  Circulation Passive Safety Systems
• CEE-TEPSS Technology Enhancement of Passive
  Safety Systems 1996-1999, European Fourth
  Framework.
• CONGA (Subcontract with CIEMAT). Containment
  Behaviour in the Event of Core Melt with Gaseous
  and Aerosol Releases 1996-1999.
• CEE-NACUSP NaturalCirculation and Stability
  Performance of BWR 1999-2004. Fifth European
  Framework Project.
• EPP-1000, Small and Large Break LOCA Analysis in
  collaboration with ANSALDO and DTN. 1999-2000.

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Main Aspects of the developed work
By means of the proposed PIRT -which has to be
finished and consolidated in this 2 RCM-, the
UPV and Ciemat have already made the following
issues

• Related with the 2 Phenomena of the proposed
  PIRT -Tracking of Non-Condensables-, we have
  studied the main different models which simulate
  the condensation inside vertical tubes in the
  presence of NC gases. Also, we have made a model
  for the condensation on finned tubes heat
  exchangers in presence of NC gases, which gives a
  good agreement. The main prototypes of passive
  safety reactors which can incorporate this models
  in its simulations are the ESBWR and SBWR for the
  Passive Containment Cooling Condensers (PCCC),
  and the SWR-1000 for the Finned Tubes Heat
  Exchanger.
• Related with the 3 Phenomena of the Proposed
  PIRT -Condensation on the containment
  structures-, we have developed an analytical
  model for the condensation on the containment
  structures, explicitly done for the AP600 passive
  safety reactor.

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Main Aspects of the developed work

• Finally, related with the 9 Phenomena of the
  proposed PIRT -Liquid Temperature
  stratification-, we have studied the
  Stratification that occurs in the hot legs of a
  Pressurized Water Reactor (PWR) using commercial
  codes (like CFX) and developing new codes (like
  TUBO3D).

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Brief Introduction and Overview to the
Condensation on Containment Structures

• AP600 passive containment cooling system relies
  heavily on the internal and external natural
  convection, where condensation takes place on
  externally cooled metallic surfaces.
• In the event of a postulated accident
  where high pressure cooling water escapes into
  the containment, the pressure and temperature
  will increase as water flashes to steam. The
  steam will in turn start to condense on the steel
  containment vessel which is initially at ambient
  temperature. This results in an increase in the
  surface temperature of the steel wall.
• The heating of the steel containment
  wall causes air from outside, due to buoyancy
  forces, to be drawn in through an air baffle
  between the concrete containment and the steel
  inner wall (not present in current reactors).
• This process along with the release of
  cooling water, by gravity from reservoirs
  situated above the containment, hold the wall
  temperature well below that of the internal bulk
  atmosphere.

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Brief Introduction and Overview to the
Condensation on Containment Structures
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Brief Introduction and Overview to the
Condensation on Containment Structures

• Even though there has been extensive
  experimental and theoretical research in the area
  of condensation (Collier, 1994), much less work
  specifically addressed the condensation process
  at large scales, such as reactor containments.
• Uchida et al. (1965) and Tagami et al.
  (1965) provided some of the pioneering work in
  this area, which has recentIy been corroborated
  by other investigators (Kataoka et al.,1995).
  This work has led to the development of
  correlations used in containment safety analysis
  (Corradini,1984 Kataoka et al.,1995) that
  estimate the Heat Transfer Coefficient HTC.
• Green et. al. (1996) carried out a peer
  review and analysis of both small and large scale
  experiments. They concluded that the above
  mentioned correlations provide a too simplistic
  method of estimating the Heat Transfer
  Coefficient (HTC), missing variables such as
  pressure, temperature, and bulk velocity which
  are of primary importance in condensing
  scenarios.

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Brief Introduction and Overview to the
Condensation on Containment Structures

• Westinghouse (Kennedy et al., 1994) performed
  some large scale experiments designed to look at
  the entire cooling system to evaluate its
  performance and provide test data for license
  approval by the NRC, but at the same time lack
  some of the insight given by more closely
  controlled facilities on the effects of both
  primary and lower order variables. Some past
  experiments have been designed to look
  specifically at these effects at smaller scale,
  as those of Debhi et al., and Huhtiniemi et al.,
  which led to the development of some correlations
  relied on the variables controlled in the
  experiments.
• To bridge the gap between the large
  time varying simulation of an accident
  (Westinghouse) and the smaller separate effects
  studies (Debhi and Huhtiniemi), an experimental
  program was conducted by the UW-Madison (Anderson
  et al., 1998).
• Specifically this experimental program
  was aimed at achieving a thorough understanding
  of the role of both major and minor variables on
  the heat transfer rate, along with providing a
  valuable data base to validate heat transfer
  models.

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Study of the effects of the main variables on the
Condensation on Containment Structures

• Effects of surface orientation
• Little variations of 20 -higher in the
  horizontal plate- between the HTC measured at
  horizontal and vertical locations was found.
• This slight dependence with the film structure
  was attributed to the fact the presence of
  noncondensables decreased the importance of the
  film structure, that is, the relatively high
  resistance of the noncondensable boundary layer
  dominates heat transfer.
• The slight increase observed was thought to be a
  consequence of the disruption of this gaseous
  boundary layer due to the departure of the
  droplets from the horizontal plate. Such an
  effect would increase the turbulence within the
  boundary layer and reduce the local thickness,
  decreasing the gas resistance to heat transfer.

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Study of the effects of the main variables on the
Condensation on Containment Structures

• Effects of bulk temperature variations
• The figure shows the effects of changing the bulk
  temperature between 60C and 90C, while
  maintaining a constant wall temperature of 30C.
• Such an increase of the bulk temperature is
  achieved by injecting steam in the vessel, so the
  HTC will be higher due to the increase of the
  steam concentration difference between bulk and
  interface, along with the enhancement of natural
  convection by increasing the pressure difference
  due to the condensation in the interface.

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Study of the effects of the main variables on the
Condensation on Containment Structures

• Effects of wall temperature variations
• The figure shows the effects of changing the wall
  temperature between 30C and 80C, while
  maintaining a constant wall temperature of 90C.
• As the wall temperature is decreased, the steam
  concentration difference will be higher -which
  increases the natural convection movement towards
  the interface-, but at the same time the NC gases
  accumulated at the interface will also increase,
  which finally will result in a lower HTC.

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Study of the effects of the main variables on the
Condensation on Containment Structures

• Effects of pressure
• As we can see in the figure, HTC increases with
  pressure, showing a quasi-linear trend for
  pressure higher than 1.5 bar.
• This increase can be attributed to the rise in
  steam concentration that produces the increase in
  the system pressure.

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Study of the effects of the main variables on the
Condensation on Containment Structures

• Effects of helium in the noncondensable mixture
• For concentrations of Helium below 40 molar of
  the noncondensables, the helium effects can be
  neglected.
• But when the molar concentration is higher, we
  can find two main effects
• The less important is related with the natural
  convection forces as we are decreasing the NC
  gases density, the force created by the pressure
  difference due to the condensation of the steam
  is being overwhelmed by a larger NC gases layer
  in the interface (due to its lower density),
  which finally results in a lower HTC. We can see
  this analytically comparing the HTC by
  condensation with 0 helium and an specific
  helium concentration as the only variables
  depending on the gas composition are related with
  the density difference and the Diffusion
  coefficient, we can see by means of two
  normalized numbers, how the product of them
  causes a lower HTC starting from 20 total helium
  concentration.

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Study of the effects of the main variables on the
Condensation on Containment Structures

• Effects of helium in the noncondensable mixture

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Study of the effects of the main variables on the
Condensation on Containment Structures

• Effects of helium in the noncondensable mixture
• The second effect is that when we have an helium
  molar concentration of the NC gases more than
  40, there exists an stratification caused by the
  lower density of the NC gases, which accumulate
  in the higher part of the wall, giving also a
  steam rich layer in the lower sections. This
  stratification can be break down when the helium
  concentration is about 45 and the steam
  concentration is increasing in a way that its
  temperature is getting higher.
• This stratification, when it is stable,
  can result in a reduction of 50 in the Heat
  Transfer. However, the minimum helium
  concentrations are above that which can exist in
  the bulk of the containment since they will be
  above the detonation limit, although there might
  be sub-compartments where these concentrations
  might exist.

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Study of the effects of the main variables on the
Condensation on Containment Structures

• Effects of helium in the noncondensable mixture

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Analytical Condensation Model

• The overall heat flux from the
  containment bulk gas mixture at temperature Tb,
  to the coolant circulating inside the finned tube
  at temperature Tc, is given by Newtons law of
  cooling
• And the total HTC is

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Analytical Condensation Model

• where the film heat transfer coefficient
  is calculated by the Nusselt equation
• where hfg accounts for the condensate
  subcooling across the film, and ? takes into
  account the enhancement in heat transfer caused
  by the rippled structure of the film, waviness
  effect, described by Kutateladze et al. (1979)
  as

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Analytical Condensation Model

• The gas thermal resistance, Rg,
  consists of two parallel components, related to
  the actual heat transfer mechanisms convection
  and condensation
• The non-dimensional version is
• keff may be written as

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Analytical Condensation Model

• Herranz et al. (1998) modified the
  theoretical approach due to Peterson et al.
  (1993) and proposed an approximation for kcond
  where the importance of noncondensable gases in
  the scenarios is explicitly stated as follows
• The Sherwood number is definied as the
  one for single phase mass convection without mass
  suction and a suction factor ? that accounts for
  the thinning of the boundary layer

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Analytical Condensation Model

• Some authors such as Mickley (1954) and
  Herranz (1996) have suggested an alternative
  expression for the quantification of ?, namely
• The natural convection correlation used
  to estimate the Sherwood number (according to
  heat-mass transfer analogy) was
• The Grashof number is dependent on both
  temperature and composition differences and it
  should be estimated as

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Validation of the model

• Now we compare the model described above with
  the data obtained by Anderson et al. in the
  UW-Madison experimental program. We analyze the
  overall model performance as well as its
  capability to reproduce specific variable
  effects.
• Empirical correlations derived from
  other databases have also been included to extend
  this validation (Dehbi et al., 1991 Kataoka et
  al., 1992)

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Validation of the model

• Overall Performance
• A total of 75 tests have been simulated
  and the results obtained have been plotted versus
  the experimental measurements

Average error with our model 12
Average error with Dehbis correlation 43
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Validation of the model

• Noncondensable Concentration
• The total heat transfer coefficient has
  been plotted versus the ratio of air and steam
  mass fractions. Experimental data recorded by
  both HFM (Heat Flux Meters) and CEB (Coolant
  Energy Balance) have been included along with the
  model and Uchida predictions

The standard deviations of the model with respect
to HFM and CEB are 8.0 and 12.3
respectively. We find a similarity between
Uchidas and our model. However, Uchida
correlation tends to overpredict the HTC at Pnc lt
1.0, whereas at Pnc gt 1.0 bar the opposite
behavior should be expected.
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Validation of the model

• Noncondensable Composition
• The figure shows the variation of HTC
  with the total molar fraction of helium (XHe) at
  atmospheric pressure. The quantitative agreement
  of measurements and estimates is outstanding with
  respect to both HFM (10.2) and CEB (8.2).

Anderson et al. (1998) performed experiments at
3 bar. The average error being 17.1 (HFM) and
20.1 (CEB). Another set of estimates obtained
using Dehbi's correlation for helium (Dehbi et
al.,1991) has been also included.
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Validation of the model

• Subcooling
• The figure shows the experimental data
  along with the model results for atmospheric
  pressure. The standard deviation of the model is
  about 13 and 17.5 with respect to HFM and CEB,
  respectively

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Validation of the model

• Subcooling
• The figure presents the variation of
  HTC with subcooling at 3.0 bar. Along with the
  data and the model predictions, other
  correlations have been included. The model
  standard deviations with respect to HFM and CEB
  are 9.5 and 12.2, respectively. Unlike Dehbi's,
  the rest of correlations do not show any
  sensitivity to the subcooling (the light
  variations observed are precisely due to Tb
  fluctuations)

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Validation of the model

• Pressure
• The figure shows the experimental
  variation of the HTC's as a function of pressures
  along with model's predictions. The model showed
  a trend similar to data's and a good quantitative
  agreement, with an average error of approximately
  20 with respect to both HFM and CEB
  measurements

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Validation of the model

• Pressure
• In order to extend the model validation
  to other databases, Dehbi's data (Dehbi,1991)
  were also considered.
• The model average error has been found
  to be 17.3, very similar to the one from
  Dehbis.
• Uchida's, Tagami's and Kataoka's correlations
  show average errors higher than 35.

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Conclusions

• The use of a more sophisticated equations than
  simple correlations is required to achieve
  accurate estimates of heat transfer under the
  natural convection prevailing in future AP600
  containments.
• Stratification of light noncondensable gases in
  the containment could affect dramatically heat
  transfer by condensation as their concentration
  exceeds approximately 40 molar with respect to
  the air. Under these conditions, stratification
  prohibits condensation and can result in a 50
  reduction in the heat transfer. However, the
  minimum needed helium concentrations are above
  that which can exist in the bulk of the
  containment.
• The analytical model presented in this paper
  presents a good capability to predict the heat
  transfer process in the AP600 Containment.
• In this way, we can recommend, as we did with the
  PCCC and the FTHX, the use of analytical models
  instead of empirical correlations, which its
  dependence on the variables controlled in the
  facilities, as well as with the facility itself,
  makes them not appropriated for a wide range of
  scenarios.