Pavan K. Sharma

CFD analysis of passive autocatalytic recombiner interaction with atmosphere

Abstract In water cooled power reactors, significant quantities of hydrogen could be produced following a severe accident (loss-of-coolant-accident along with non availability of Emergency Core Cooling System) from the reaction between steam and zirconium at high fuel clad temperature. In order to prevent the containment and other safety relevant components from incurring serious damage caused by a detonation of the hydrogen/air-mixture generated during a severe accident in water cooled power reactors, passive autocatalytic recombiners (PAR) are used for hydrogen removal in an increasing number of French, German and Russian plants. These devices make use of the fact that hydrogen and oxygen react exothermally on catalytic surfaces generating steam and heat. Numerous tests and simulations have been conducted in the past to investigate passive autocatalytic recombiners behaviour in situations representative of severe accidents. Numerical models were developed from the experimental data for codes like COCOSYS or ASTEC in order to optimise the passive autocatalytic recombiners location and to assess the efficiency of passive autocatalytic recombiners implementation in different scenarios. However, these models are usually simple (black-box type) and based on the manufacturers correlation to calculate the hydrogen depletion rate. Recently, uses of enhanced CFD models have shown significant improvements towards modeling such phenomenon in complex geometry. The work presents CFD analysis of interaction of a representative nuclear power plant containment atmosphere with passive autocatalytic recombiners simulated using the commercial Computational Fluid Dynamics code for PAR Interaction Studies (PARIS benchmarks) exercise. A two-dimensional geometrical model of the simulation domain was used. The containment was represented by an adiabatic rectangular box with two PAR located at intermediate elevations near opposite walls. The flow in the simulation domain was modelled as single-phase. The results of the simulations are presented and analysed.

Simulation of Hydrogen Distribution in Containment Studies Facility: A Parametric Study Using the CFD Code FDS

Hydrogen transport behavior in the containment of NPPs is an extremely complex phenomenon and must be properly understood for the assessment of the overall plant safety under accident conditions. A case study has been carried out to predict the hydrogen concentration profile in the multi-floor multi-compartment geometry of Containment Studies Facility (CSF), to understand the complex hydrogen transport behavior in general and the influence of the location and direction of injection of hydrogen, in particular. A computer code FDS has been used for conducting these studies. The results indicate a strong possibility of stratification of the lighter gas within as well as across the compartments during both the injection and post injection phases. After the momentum effects due to the convective currents die down, stratification was noted to be more or less stabilized. Molecular diffusion was seen to have insignificant diffusion.Copyright

Numerical simulation of passive catalytic recombiner

Abstract Resolving hydrogen related safety issues, pertaining to nuclear reactor safety have been an important area of research world over for the past decade. Studies on hydrogen transport behavior and development of hydrogen mitigation systems are still being pursued actively in various research labs, including Bhabha Atomic Research Centre (BARC), in India. The Passive Catalytic Recombiner is one of such hydrogen mitigating devices consisting of catalyst surfaces arranged in an open-ended enclosure. In the presence of hydrogen with available oxygen, a catalytic reaction occurs spontaneously at the catalyst surfaces and the heat of the reaction produces a natural convection flow through the enclosure. The present study aims for performing a CFD simulation to obtain the temperature distribution inside the recombiner. A 3D CFD model has been developed to study the mechanism of catalytic recombination and has been tested for literature quoted experiments. A parametric study has been performed for a particular recombiner geometry for various inlet conditions. Salient features of the simplified CFD model developed at BARC and results of the present model calculations are presented in this paper.

CFD Analysis of Passive Autocatalytic Recombiner

In water-cooled nuclear power reactors, significant quantities of hydrogen could be produced following a postulated loss-of-coolant accident (LOCA) along with nonavailability of emergency core cooling system (ECCS). Passive autocatalytic recombiners (PAR) are implemented in the containment of water-cooled power reactors to mitigate the risk of hydrogen combustion. In the presence of hydrogen with available oxygen, a catalytic reaction occurs spontaneously at the catalyst surfaces below conventional ignition concentration limits and temperature and even in presence of steam. Heat of reaction produces natural convection flow through the enclosure and promotes mixing in the containment. For the assessment of the PAR performance in terms of maximum temperature of catalyst surface and outlet hydrogen concentration an in-house 3D CFD model has been developed. The code has been used to study the mechanism of catalytic recombination and has been tested for two literature-quoted experiments.

Fire Testing and Study of Liquid Pool Fire in Multiple Compartments

Many experimental/numerical studies on single compartment or single room fires have been reported in the literature but few studies are available for multiple compartments. There is a need for fire studies in multiple compartments as it may help in predicting fire impact in buildup environments with multiple compartments. In the present work, fire experiments were conducted in a real life two-storey compartmentalized concrete building. Aviation Turbine Fuel (ATF) was used as the liquid fuel to generate pool fire. The work, focused on the thermal hydraulic aspects like heat distribution to the corresponding walls, temperature near walls, vertical distribution of temperature close to fire and comparison of heat-flux and temperature at specified locations in the interconnected rooms. The study elucidates the impact of ventilation on various thermal and hydraulic aspects of fire.

Equilibrium based analytical model for estimation of pressure magnification during deflagration of hydrogen air mixtures

Abstract During postulated accident sequences in nuclear reactors, hydrogen may get released from the core and form a flammable mixture in the surrounding containment structure. Ignition of such mixtures and the subsequent pressure rise are an imminent threat for safe and sustainable operation of nuclear reactors. Methods for evaluating post ignition characteristics are important for determining the design safety margins in such scenarios. This study presents two thermo-chemical models for determining the post ignition state. The first model is based on internal energy balance while the second model uses the concept of element potentials to minimize the free energy of the system with internal energy imposed as a constraint. Predictions from both the models have been compared against published data over a wide range of mixture compositions. Important differences in the regions close to flammability limits and for stoichiometric mixtures have been identified and explained. The equilibrium model has been validated for varied temperatures and pressures representative of initial conditions that may be present in the containment during accidents. Special emphasis has been given to the understanding of the role of dissociation and its effect on equilibrium pressure, temperature and species concentrations.

Thermal plume behaviour in the Kadra reservoir at Kaiga atomic power station - Part 2: studies for the case of four and six units in operation

Abstract A computational model was developed earlier for 2 units of Nuclear Power Plants (NPPs) operational at Kaiga Atomic Power Station (KAPS) to understand the thermal plume behaviour in the Kadra reservoir wherein the hot water from the plant condensers is discharged. The model was successfully validated against the site data. The same model has now been extended for analyzing the thermal plume bahaviour in case of 4 NPP units as well as 6 NPP units operational at the same site. The present paper briefly describes details of the studies along with the results of earlier study to understand the overall behavior of thermal plume in Kadra reservoir.

CFD simulation of thermal discharge behaviour in the Kadra reservoir at the Kaiga atomic power station: Part 1: Validation for 2 power plant units in operation

Abstract The thermal pollution arising out of discharge of hot water from the power plant condensers into the natural water bodies such as rivers, lakes, reservoirs, oceans etc. has been a serious concern to environmentalists ever since the plants started operating world over. In the past forty to fifty years, the methods of calculations for predicting the velocity and temperature fields in the affected regions of the stagnant/flowing water bodies have undergone a significant improvement. Currently, use of Computational Fluid Dynamics (CFD) codes for performing these calculations is gaining popularity. However, several factors such as the assumed computational domain and its discretisation, the boundary conditions used, representation of hydrodynamic characteristics (laminar/turbulent, buoyant/non-buoyant), etc. have a strong influence on the accuracy of predictions by such a model. A CFD code STAR-CD has been used for analyzing the thermal plume behaviour in the Kadra reservoir at Kaiga Atomic Power Station (KAPS). The predictions from these calculations of two units in operation have been found to be in good agreement with the site data made available from earlier studies. The present paper briefly describes the model developed using STAR-CD and results obtained for the Kadra reservoir at KAPS.

Inverse problems using Artificial Neural Networks in long range atmospheric dispersion

Abstract Scalar dispersion in the atmosphere is an important area wherein different approaches are followed in development of good analytical models. The analyses based on Computational Fluid Dynamics (CFD) codes offer an opportunity of model development based on first principles of physics and hence such models have an edge over the existing models. Both forward and backward calculation methods are being developed for atmospheric dispersion around NPPs at BARC. Forward modeling methods, which describe the atmospheric transport from sources to receptors, use forward-running transport and dispersion models or computational fluid dynamics models which are run many times, and the resulting dispersion field is compared to observations from multiple sensors. Backward or inverse modeling methods use only one model run in the reverse direction from the receptors to estimate the upwind sources. Inverse modeling methods include adjoint and tangent linear models, Kalman filters, and variational data assimilation, and neural network. The present paper is aimed at developing a new approach where the identified specific signatures at receptor points form the basis for source estimation or inversions. This approach is expected to reduce the large transient data sets to reduced and meaningful data sets. In fact this reduces the inherently transient data set into a time independent mean data set. Forward computations were carried out with CFD code for various cases to generate a large set of data to train the Artificial Neural Network (ANN). Specific signature analysis was carried out to find the parameters of interest for ANN training like peak concentration, time to reach peak concentration and time to fall. The ANN was trained with data and source strength and locations were predicted from ANN. The inverse problem was performed using the ANN approach in long range atmospheric dispersion. An illustration of application of CFD code for atmospheric dispersion studies for a hypothetical case is also included in the paper.

Review and investigations of oscillatory flow behaviour of a horizontal ceiling opening for nuclear containment and fire safety analysis

Abstract In the thermal hydraulics codes developed for fire safety analysis and for containment thermal hydraulic analysis, junctions in the multi-compartment geometries is often modeled as uni-directional junctions. However, ceiling junctions are known to depict unstable/oscillatory bi-directional flow behavior. Detailed investigations have been carried out to understand the unstable flow behaviour of a junction by analyzing an earlier reported experiment and its subsequent two dimensional numerical RANS based study of fire in an enclosure. The authors attempt more realistic and desired three dimensional and inherently transient large eddy simulations using a computer code Fire Dynamics Simulator (FDS). The paper presents the details of the analysis, the results obtained and further studies required to be conducted so that the findings can be applied to the fire/containment thermal hydraulics analysis codes successfully.