[en] This paper systematically develops the scaling laws that have to be satisfied between a model and the prototype for their identical non-dimensional steady state and transient behaviour. From the rules derived, the dimensions of a scaled model for an assumed reactor configuration are worked out. (author)

[en] Experimental studies have shown that there is heat transfer enhancement (HTE) for supercritical fluid near the pseudocritical temperature at relatively low heat flux to mass flux ratios. At very high values of heat flux, a peak in wall temperature appears due heat transfer deterioration (HTD). In the present research work R22 has been selected as working fluid as the simulant fluid for water. A supercritical Freon test facility is then designed and built. Two vertical tubular test sections of ID equal to 6 mm and 13.5 mm are employed. Experiments with vertically upward flow at 55 bar system pressure were carried out. Thermal camera is used to obtain wall temperatures at distances about 1-1.5 mm apart. Experiments were conducted with water before using R22 to validate experimental procedure. Initial experiments with R22 were conducted to demonstrate the reduction in peak heat transfer enhancement with increase in heat flux. Experiments are then conducted at several heat and mass flux values and inlet temperature. It is observed from experimental results that onset of HTD occurs when q/G more than 0.056 to 0.072 (kJ/kg). When the inlet temperature is lowered, the onset of HTD appears at relatively high q/G. The bulk fluid enthalpy and temperature at which onset of HTD appears also reduces when the inlet temperature was decreased. At the lower inlet temperature, two peaks in wall temperature were observed in the experimental results

[en] Full text: Hydrogen mixing/distribution in nuclear reactor containment atmosphere is one of the important safety issues under accident conditions. It is therefore necessary to predict the hydrogen distribution accurately. Computational Fluid Dynamics (CFD) codes are increasingly used for this purpose. However, the commercially available CFD codes do not have the condensation models and one has to incorporate these models before applying these codes for this purpose as condensation of steam affects the distribution of the hydrogen. Further, during accident conditions, release of hydrogen into the containment may form combustible mixtures and which can pose a threat to the containment integrity. Re-combiners are placed in the containment to mitigate such risk by removing hydrogen from the containment atmosphere by catalytic reaction. Hence, re-combiner models also need to be implemented in the CFD codes to predict the hydrogen distribution. Finally, the turbulence models play a key role in dictating the convective diffusive behavior of hydrogen transport. The present paper systematically describes the selection of the condensation and re-combiner models to be implemented based on the model capabilities, applicability and their validation aspects. In addition, it also brings out the most competitive turbulence model in terms of accuracy and computational speed

[en] The heat transfer behaviour of a supercritical fluid is an important input for the performance evaluation of SuperCritical Water-cooled Reactors (SCWR) of Generation IV nuclear power plants. SCWRs have very high overall thermal efficiency of about 45-50% because their operating pressures and temperatures are high. Accident analyses for licensing are carried out using system thermal hydraulics codes in which well established and validated correlations for Heat Transfer Coefficient (HTC) have to be built in. Experimental studies have shown that there is Heat Transfer Enhancement (HTE) for supercritical fluid near the pseudocritical temperature at relatively low heat flux to mass flux ratios. The peak HTC decreases as this ratio increases. At very high values of heat flux, a peak in wall temperature appears due to Heat Transfer Deterioration (HTD). This phenomenon is important as the nuclear fuel clad may fail at high temperatures induced by HTD and result in release of radioactive nuclides into the coolant streams. It is also important for the sizing of core. Several investigators have carried out experiments using water, CO2 and R22 etc. due to similarity in their thermophysical properties, and it was observed that the bulk fluid enthalpy at which peak wall temperature appears is different at different test specifications (heat flux, mass flux and different inlet temperature). Effects of inlet temperature are found to be significant for HTD but results reported in literature have not reported this effect in detail. Experimental wall temperatures available in open literature were measured by thermocouples positioned at regular intervals. However, since the HTD is shown to result in very steep temperature changes, highly local temperature measurements are desirable. HTC correlations are available which are able to predict HTE satisfactorily but HTD predictions from available correlations are poor and therefore, better correlations are required to predict HTD.

[en] In this paper, a study on role of expansion tank in single-phase natural circulation loop dynamics is presented. The interaction between the main loop fluid and expansion tank fluid is systematically investigated. The paper brings out the energy exchange between the expansion tank and the main loop due to natural convection currents. The results presented challenge the hypothesis that the expansion tank can be simulated as a time dependent volume having constant fluid temperature and constant pressure. The investigations carried out in this study also provide a possible explanation for hysteresis observed in these loops. (author)

[en] Indian Pressure Heavy Water Reactors generally consists of 300-400 horizontal fuel channels depending on the rating of the plant. It consists of a hot Pressure Tube (PT) and a concentric Calandria tube (CT) with a 9 mm annulus containing CO₂ gas as annulus gas monitoring system. Inside each pressure tube there are 10-12, 0.5 m long fuel bundles, each consisting of 19 or 37 fuel pins with fuel elements containing natural UO₂ pellets and Zircraloy-4 clading. During a postulated loss of coolant accident (LOCA), the coolant supply is lost and designed Emergency Core cooling system (ECCS) will actuate and removes decay heat from fuel. Aim of the present research is to find means of enhancing the heat transfer efficiency between the CT and moderator, such that in the case of any accident, film boiling period is reduced and channel integrity is ensured. One of the ways to enhance heat transfer from CT to moderator is by increasing the critical heat flux (CHF) on the outer surface of the calandria tube. Increasing the CHF for a given subcooling on the outer surface of the calandria tube can be achieved by surface modifications. The ability of the surface modification to increase the CHF on the outside surface of the calandria tube is demonstrated in a series of small scale experiments on a 15.2 mm OD and 0.4 mm thick zircaloy tube. Analysis of the test results indicated that increasing the tube's CHF using a glass-peening or grounded or grit blasted process is a promising option for reducing moderator subcooling requirements

[en] Research highlights: → Hydrogen transport in containment with recombiners is a multi-scale problem. → A novel methodology worked out to lump the recombiner characteristics. → Results obtained using commercial code FLUENT are cast in the form of correlations. → Hence, coarse grids can obtain accurate distribution of H₂ in containment. → Satisfactory working of the methodology is clearly demonstrated. - Abstract: This paper aims at formulation of a model compatible with CFD code to simulate hydrogen distribution and mitigation using a Passive Catalytic Recombiner in the Nuclear power plant containments. The catalytic recombiner is much smaller in size compared to the containment compartments. In order to fully resolve the recombination processes during the containment simulations, it requires the geometric details of the recombiner to be modelled and a very fine mesh size inside the recombiner channels. This component when integrated with containment mixing calculations would result in a large number of mesh elements which may take large computational times to solve the problem. This paper describes a method to resolve this simulation difficulty. In this exercise, the catalytic recombiner alone was first modelled in detail using the best suited option to describe the reaction rate. A detailed parametric study was conducted, from which correlations for the heat of reaction (hence the rate of reaction) and the heat transfer coefficient were obtained. These correlations were then used to model the recombiner channels as single computational cells providing necessary volumetric sources/sinks to the energy and species transport equations. This avoids full resolution of these channels, thereby allowing larger mesh size in the recombiners. The above mentioned method was successfully validated using both steady state and transient test problems and the results indicate very satisfactory modelling of the component.

[en] Highlights: • Start-up of single-phase natural circulation systems from the state of rest. • A model has been presented for enhanced thermal diffusion in the fluid. • Numerical simulations for different heater and cooler orientation are presented. • The model captures the different modes of oscillation. - Abstract: Single-phase natural circulation loops are used in many industrial systems like nuclear reactors, geothermal systems, solar water heating systems and process industry. The present paper presents a 1-D model for simulating the startup from rest of water cooled single-phase natural circulation loops. A pseudo-conductivity model was developed previously by the authors to account for heat diffusion caused by local convection currents and the results of numerical simulation for loops having horizontal heater and horizontal cooler were presented. In the present study, the model has been used to simulate the start-up of loops having different heater and cooler orientations and the applicability of the model as a general tool for simulating start-up of rectangular natural circulation loops is assessed. The paper presents more insight into the behavior of pseudo-conductivity models developed to simulate start-up of these loops from state of zero flow.

[en] Highlights: • Steady state performance of a single-phase NCL has been investigated experimentally. • Numerical investigations on role of friction factor correlation are reported. • A correlation is proposed for friction factor flow through a horizontal pipe. • Effect of wall friction on loop behavior has been investigated. - Abstract: Unlike forced circulation systems, natural circulation systems need to be started from the state of zero flow. Literature study shows that under low flow conditions, the velocity field near the wall is significantly modified by secondary convection currents particularly during diabatic conditions. In view of this, the applicability of conventional forced convection wall constitutive laws to these systems has been investigated both theoretically and experimentally. First the applicability of conventional wall constitutive laws derived from steady state forced convection experiments is examined. Next, the results of experimental investigations carried out in a rectangular natural circulation loop are presented. Finally, a new correlation for wall friction factor is proposed for flow in horizontal pipes under diabatic conditions.

[en] The phenomenon of carryover, i.e. entrainment of liquid along with separated steam is observed in all the steam separators. Due to the risks, such as turbine blade erosion and radioactivity leakage, associated with it, it is desired to have an estimate of the carryover value. This is all the more important when the separation is only under the influence of gravity as proposed in some of the new generation natural circulation reactors. Experiments were carried out in an air-water facility at atmospheric conditions to characterize the entrainment in drums with ratio of the drum diameter to riser diameter varying from 1 to 6. Various parameters influencing the liquid entrainment were identified. The vapour superficial velocity and the drum diameter to riser diameter ratio were found to be the most influencing parameters. A dimensionless prediction correlation was evolved for the liquid entrainment and it was found to agree with previous works in the literature for drum to riser diameter ratio equal to 1.