International Conference
Nuclear Energy for New Europe 2006 Portorož, Slovenia, September 18-21, 2006
http://www.djs.si/port2006
Numerical Study of Interaction Between NPP Containment Atmosphere and Passive Autocatalytic Recombiners Miroslav Babić, Ivo Kljenak, Borut Mavko “Jožef Stefan” Institute Reactor Engineering Division Jamova 39, SI-1000 Ljubljana, Slovenia
[email protected]
ABSTRACT The interaction of a nuclear power plant containment atmosphere with a Passive Autocatalytic Recombiner (PAR) was simulated using the Computational Fluid Dynamics code CFX4.4. The main purpose of the simulation was to observe containment atmosphere mixing phenomena, influenced by the PAR, during a postulated severe-accident scenario. A two-dimensional geometrical model of the simulation domain was developed. The containment was represented by an adiabatic rectangular box with a PAR situated at an intermediate elevation near a wall. The flow in the simulation domain was modelled as singlephase and the turbulence was modelled with a Menter modified low-Reynolds k-ε model. The results of the simulation are presented and analysed. 1
INTRODUCTION
During a severe accident in a Light Water Reactor nuclear power plant, large amounts of hydrogen would presumably be generated due to metal oxidation during core degradation and released into the containment. The integrity of the containment could be threatened due to hydrogen combustion. One of the possible ways to remove hydrogen is to use Passive Autocatalytic Recombiners (PAR) for hydrogen combustion. During the SARNET (Severe Accident Research Network of Excellence, which is part of the 6th EU Framework Programme) CAM (Containment Atmosphere Mixing) workshop session dedicated to “Interaction of containment atmosphere with recombiners”, which was held in held in Paris on June 23-24, 2005, participants decided to define a numerical benchmark on Passive Autocatalytic Recombiner Interaction Studies (PARIS) [1]. The longterm aims of the benchmark project are to: • check the PAR elevation influence on the hydrogen distribution, • study natural convection loop interactions when several PARs are present, • compare CFD (Computational Fluid Dynamics) and lumped-parameter codes results in such situations. The main purpose of the first benchmark simulation was to observe the containment atmosphere mixing phenomena during a postulated severe-accident scenario. The containment had to be represented by an adiabatic rectangular box with two PARs situated at intermediate elevations near the left and right wall, respectively. To limit computational resources, it was 807.1
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decided to consider a simple two-dimensional (2D) geometry with no heat transfer to walls and no steam condensation on the wall. In the present paper, the results of the proposed benchmark simulation, obtained at the Reactor Engineering Division of the Jozef Stefan Institute (Ljubljana, Slovenia), using the CFD code CFX4.4 [2], are presented. A two-dimensional model of the simulation domain was developed, according to the benchmark specifications. However, as the domain, including the position ob both PARs, is symetrical with respect to a vertical axis in the middle, only half of the simulation domain was considered. The flow in the simulation domain was modelled as single-phase and the turbulence was modelled with a Menter modified low-Reynolds k-ε model. The first 2000 s of the simulated transient are presented. The time-dependent pressure and average atmosphere temperature are shown, as well as vertical profiles of temperature and hydrogen mole fraction at different times at two representative locations (distances from the wall). The structure of the atmosphere resulting from the interaction with the PAR is inferred. 2 2.1
DEFINITION OF THE BENCHMARK PROBLEM PAR description
The PAR considered is a SIEMENS FR90/1-150 [1]. For the simulation, the following characteristics are defined: • PAR height h = 1 m, width and depth w = 0.2 m. • PAR entry and exit section widths are also equal to w = 0.2 m. • Each PAR has 15 autocatalytic plates with dimensions of 0.15 m x 0.15 m x 0.0001 m (height x depth x width), and an inter-space of 0.01 m. The hydrogen consumption rate is given by a Siemens-like law: m0H2 = min (XH2, 2XO2, 0.08) . (A . p + B) ,
(1)
where X is the molar fraction of the mixture component, p is the pressure and the two parameters A and B have the following values: A = 0.48 . 10-8, B = 0.58 . 10-3 [1]. 2.2
Geometrical data
The containment is represented by a H x W rectangular box, where height H = 5 x h and the width W of the box is equal to the height. There are two PARs located in the containment, symmetrically with respect to the vertical axis at x = 2.5 m, as shown on Figure 1.
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y w
2xh
H 2xh
W 0
x Figure 1. Containment model geometry with 2 PARs (arrows indicate the presumed flow directions)
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Initial conditions
The initial conditions for the PARIS benchmark were defined as: • Pressure: 3.36 bar • Temperature: 393 K • Oxygen mass fraction: 0.1203 • Hydrogen mass fraction: 0.0018 • Steam mass fraction: 0.4817 • Flow velocity: no fluid motion The mass fractions were calculated from the benchmark specification [1]. Initial conditions were assumed to be uniform over the simulation domain. 3
SIMULATION
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Input model for CFX4.4 code
The CFX4.4 code is a general-purpose Computational Fluid Dynamics (CFD) code, which has been first developed by AEA Technology (UK) [2] and is now being developed by ANSYS Inc. The code solves the conservation equations for mass, momentum and energy together with their initial and boundary conditions. The software uses the finite volume method for the numerical solution of these equations. For the simulation of the PARIS benchmark, a two-dimensional grid was developed, covering only one half of the containment domain, due to the vertical symmetry axis assumed at x = 2.5 m. Namely, in our opinion, an assymetric flow pattern would be the consequence of Proceedings of the International Conference Nuclear Energy for New Europe, 2006
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some numerical instability, so that the resulting interaction of natural convection loops would not necessarily realistically represent actual physical phenomena. The numerical grid consists of 150 cells in the vertical direction and 120 cells in the horizontal direction. The average computational cell dimension is about 0.7 mm (Figure 2). The nitrogen-oxygen-hydrogen-steam atmosphere was treated as a homogenous mixture with nitrogen as the “carrier fluid” [2]. The calculation of the mixture properties is described in the CFX documentation [2]. The following options were prescribed in the CFX “command file”: • compressible flow, • low-Re turbulent flow model (Menter modified k-ω model), • buoyant flow (C3 constant of k-ε model set to 1.0), • no-slip condition at the walls. The default options of the CFX4.4 code that correspond to these physical models were applied.
Figure 2. Geometry of the simulation domain (left) and computational grid (right) 3.2
Hydrogen recombination modelling
The hydrogen recombination rate is calculated from Eq. (1). The heat is released when chemical reaction occurs between hydrogen and oxygen: 2 H2 + O2
→ 2 H20
(2)
The released heat was calculated from the following equation: Q = HP – HR, Proceedings of the International Conference Nuclear Energy for New Europe, 2006
(3)
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where HP is the enthalpy of products and HR is the enthalpy of reactants. This energy is considered in cells, which represent the PAR plates. The enthalpy of reactants was calculated as: HR = mH2 . (cp,H2 T – cp,N2 Tref) + mO2 . (cp,O2 T – cp,N2 Tref ),
(4)
as nitrogen is defined as the carrier fluid. The enthalpy of products was calculated as: HP = mH2O . (cp,H2O_liquid T – cp,N2 Tref) + Hfg + Hformation
(5)
where Hfg denotes the enthalpy of phase change from liquid water to steam (< 0) and Hformation the enthalpy of formation of liquid water, which is equal to - 286 MJ / kmol H2O . 3.3
Modelling of flow resistance of PAR plates
Instead of modelling the flow around PAR catalytic plates in detail, it was decided to use engineering correlations instead. The pressure drop due to fluid friction on PAR plates is given by: dP / dx = Cf . 0.5 . ρave . (Uave)2
(6)
where the average velocity is calculated as Uave = UIN . Tave / TIN
(7)
and the average temperature is calculated as Tave = (TIN + TOUT) / 2.
(8)
Subscripts IN and OUT denote averages over PAR plates inlet and outlet area, respectively. The friction coefficient Cf was calculated by applying a correlation from Idelchik ([3], pp. 397, 398) Cf = λ . Lplates / Dhydr + 2.0 . (TOUT - TIN ) / Tave
(9)
where the last term accounts for the expansion of gas due to heating. Lplates and Dhydr denote the length of plates in vertical direction and the hydraulic diameter of the plates assembly. The friction factor was calculated from standard engineering correlations for tubes together with shape correction factors for an infinite rectangular slot:
4
λ = 64 / Re . 1.5,
for Re < 2000
(10)
λ = 0.31 / (Log (0.143 . Re) )2 . 1.1,
for Re ≥ 2000.
(11)
RESULTS
Figure 3 shows the pressure variation during the simulation of the PARIS benchmark. During the first 500 s, there is a relatively steep increase of pressure, which corresponds to a relatively high hydrogen recombination rate, as shown on Figure 4. In the later stages of the simulation, there is little hydrogen recombination, which corresponds to low pressure increase Proceedings of the International Conference Nuclear Energy for New Europe, 2006
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Pressure [Pa]
from 1000 s to 2000 s. Figure 5 shows the variations of the average temperature in the containment vessel, average inlet and outlet temperature of the PAR, and the average temperature of the PAR plates during the simulation. 4.6x10
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Figure 4. Hydrogen recombination rate
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Figure 5. Average, PAR inlet, PAR outlet and PAR plates temperatures As can be seen from Figures 6 and 7 (which show vertical temperature profiles at different times), after the initial homogeneous temperature field, a steep temperature front occurs between elevations 1.5 m and 1.8 m, which divides the containment into two distinctly different regions with different atmosphere temperature: the upper part with higher temperatures and the lower part with lower temperatures. On Figure 6, which shows vertical temperature profiles near the PAR, the influence of the hotter gas flow from the PAR exit can be clearly observed. 600 580
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Figure 6. Vertical temperature profiles at 0.6 m from left wall at different times
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Figure 7. Vertical temperature profiles at 2.5 m from left wall at different times Figures 8 and 9 show vertical profiles of dry hydrogen mole fraction. The figures show that hydrogen stratification occurs in the containment atmosphere. The dry hydrogen mole fraction evolves in a similar way as the temperature: first a uniform distribution, then a steep front and finally a gradual transition zone. The upper part of the containment atmosphere (approximately above elevation 1.5 m) intensively interacts with the PAR, resulting in hydrogen consumption, whereas below elevation 1.0 m, the hydrogen concentration remains more or less constant. On Figure 8, which shows vertical dry hydrogen moral fraction profiles near the PAR, the influence of the hydrogen depleted gas flow from PAR exit can be clearly observed. For the time being, the only assessment of the validity of the obtained results is the comparison with results from other research institutions that participated in the first phase of the PAR benchmark (the comparison report is being prepared at CEA, France). A necessary condition for a thorough assessment of the validity of the used approach would be a comparison with experimental results. However, no adequate experimental data are available at present.
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Dry hydrogen molar fraction [-]
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Figure 8. Dry hydrogen mole fraction vertical profiles at 0.6 m from left wall at different times 0.05 0.045
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Figure 9. Dry hydrogen mole fraction vertical profiles at 2.5 m from left wall at different times 5
CONCLUSIONS
A simplified simulation of interaction of a passive autocatalytic recombiner with the containment atmosphere in a nuclear power plant was performed with the CFD code CFX4.4. Proceedings of the International Conference Nuclear Energy for New Europe, 2006
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A two-dimensional model was developed, and simple physical and geometrical conditions were assumed. A low-Re turbulence model was used. As symmetric flow was assumed, only one half of the simulation domain was considered. The beginning of the transient is characterised by a high hydrogen recombination rate and a steep increase of pressure and average atmosphere temperature. As hydrogen is consumed, the hydrogen recombination rate decreases and the pressure and average temperature stabilise. However, the simulation reveals that the interaction with PARs causes a partitioning of the atmosphere in two different regions: a lower region, with lower temperature and higher hydrogen concentration, and an upper region, with higher temperature and lower hydrogen concentration. The regions are separated by a “buffer” zone. In further work, simulations with more PARs should be performed to study the interaction of natural convection loops. However, PARs should not be placed symmetrically, so that numerical instabilities would play a minor role in the resulting asymmetric flow. ACKNOWLEDGMENTS The research presented in the present work was performed within the research project J2-6614 (contract no. 3311-04-8226614) financed by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia. The Jožef Stefan Institute is a member of the Severe Accident Research Network of Excellence (SARNET) within the 6th EU Framework Programme. The benefit from European Commission RTD Programme for research is acknowledged. REFERENCES [1] F.Dabbene, H.Paillère, Data for a Numerical PAR Benchmark (Brief specification), Commissariat à l’ Energie Atomique, Saclay, France, August 26, 2005. [2] AEA Technology plc., CFX-4.4: Solver Manual, Harwell, United Kingdom, 2001. [3] I.E. Idelchik, Handbook of Hydraulics Resistance, 2nd edition, Springer-Verlag, 1986.
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