New Project 1: Development and Validation of models for DNB prediction Departure from Nucleate Boiling (DNB) represents conditions wherein there is a sudden deterioration in heat transfer leading to inordinate temperature rise which may lead to failure of fuel clad. DNB is thus one of the manifestations of the broader term Critical Heat Flux (CHF). Strictly speaking DNB refers to CHF under low quality flow boiling. Correct knowledge of DNB is essential for estimation of thermal margin. Development of computational model for DNB would also, in a broad sense, complement the work already done with regard to high quality flow boiling CHF i.e., dryout which has led to development of models for dryout prediction. For validation, as a start, EPRI DNB database (which includes tubular and rod bundle data) can be used. Additionally, BARC plans to conduct DNB experiments in simulated cluster. These data can also be used when available. Phenomenology of heat transfer deterioration in DNB As an outcome, we would target developing a well validated, robust computational model for prediction of DNB in rod bundles.

New Project 2: Multi-dimensional multiphase flow distribution in fuel assmblies The mixing of flow streams from hot subchannels to relatively cooler subchannels (or vice-versa) plays an important role in the overall heat transfer. Turbulent mixing occurring between the subchannels shows interesting behavior due to the tight arrangement of the fuel pins. In particular, it is seen in literature that the turbulent mixing rate is independent of the gap size. Mixing is often derived empirically from correlations. In this project, it is proposed to use CFD codes for prediction of turbulent mixing in fuel assemblies. BARC is planning to develop facility to study flow and temperature distribution in simulated fuel subchannels. Such data can be used for validation of the CFD model developed. Further, the CFD model will be used to develop simple and effective approximations which can be used in component level subchannel analysis codes. This work will also be beneficial to development of rod bundle DNB prediction model proposed in the previous project. Output: CFD model for turbulent mixing and mixing models for subchannel analysis.

New Project 3: Mathematical modeling and experimental validation of Multi-dimensional boiling two phase flow inside rod bundle Modeling of boiling two phase flows is finds a very important role in nuclear industry. Not only reactor core but almost all the components of nuclear reactor systems deals with two phase flow either it is boiling or adiabatic. Prediction of such phenomena (two phase flow) is still beyond the scope of many commercial CFD codes. However, some models have been developed and incorporated in some codes like Fluent and Star CD, yet, applicability of those models is limited due to empiricism involved in the models. The empiricism in the modeling is due to the characteristic nature and dependency on the geometry of the multi-phase flow. This aspect of the multi-phase flow entails the need of experiments. The measurement of void and flow distribution are important for validation of mathematical model validation. Experimental data will be generated for void and flow distribution inside rod bundle of a BWR. The test data will help for validation of CFD model for boiling two phase flow.

New Project 4: Design of Efficient Passive Air Cooled Condensers Heat rejection by passive air condenser One of the safety goals for advanced reactor designs is to eliminate the emergency planning zone (EPZ). To achieve this, multiple defence-in-depths are built-in in the reactor designs. In case of extreme events such as that happened in Fukushima, the SBO prolonged for several days. The containment got pressurized because of steam and hydrogen. In new reactor designs, a passive means of cooling the containment is essential for prolonged period. Some of the advanced reactors adopt passive containment cooling by air for indefinite period. The design of passive air cooled condensers require efficient compact heat exchanger design which should function with minimum pressure drop. BARC is setting up a test facility wherein different designs of passive air cooled condensers can be tested. The test data will help in determining heat transfer coefficient and pressure drop under natural circulation conditions for different geometry and operating conditions. Schematic, the main phenomena involved in passive post shutdown heat removal during the ‘grace time’ Efficient compact passive air cooled condensers will be designed and its performance will be analyzed by numerical and experimental investigation.

New Project 5: Modeling of helium bubble injection and its stripping from molten salts In a typical molten salt reactor, the fission product gasses (like Xe and Kr) are generated in-situ within the molten salt fuel. Initially they are in dissolved form but soon they will form bubbles once their solubility in the molten salt is exceeded and may cause reactivity issues arising out of voiding. To remove these gasses, fine helium bubbles are injected into the fuel salt so that they form sinks for the xenon and krypton to diffuse into. After sufficient period of time when the Xe and Kr laden helium bubbles are stripped off. Special devices will be used for injecting fine bubbles of helium into the molten salt stream and stripping them from the salt flow. Modeling of these injection and stripping processes involves two phase modeling along with bubble dynamics. Microscopic modeling of the bubble behavior needs to be coupled with the macroscopic flow of the salt so that the flow can be modeled using standard bench top workstations. For validation of the code a water-air based setup may be constructed with provisions to visualize on the flow patterns in the injection and stripping sections and their comparison with the models.

New Project 6: Evaluation of turbulent flow models in liquid coolant through packed pebble bed with heat generation Coolant flow through the interstices of a packed pebble bed is important to analyze to understand the local heat transfer phenomena and to ensure that hot spots are never formed. In order to analyze any significant sized packed beds, detailed CFD mesh of the interstices are practically not feasible given the large number of nodes required. On the other hand conventional modeling of the packed bed using a porosity based approach will not properly take into effect the reduced effective packing fraction near the walls. Thus a CFD code needs to be developed that will analyze the problem in two inter coupled length scales, i.e. the model of the full packed bed with liquid coolant flowing should be analysed using multi-scale models of the flow behavior in bulk of the packed bed as well as near the walls.

New Project 7: Natural circulation studies at high temperature in a molten salt loop Molten salts have been considered as coolant for both advanced nuclear reactors as well as in solar thermal power plants. The systems with natural circulation cooling provide passive safety and simplified design. So, natural circulation will be important phenomena for designing the molten salt heat transfer systems. A Molten nitrate Salt Natural Circulation Loop (MSNCL) is already in operation BARC. Natural circulation experiments can be carried out in different orientations of heater and cooler in the loop. 3D CFD analysis of the loop is required be carried out to simulate the natural circulation transient behaviour in the loop. One dimensional codes have limitations in predicting stability behaviour due to use of closure relationship in heat transfer and fluid flow. CFD analysis will be performed to assess prediction of stability behaviour of the loop. Comparison will also be performed with the analysis performed with one-dimensional codes. The performance of different turbulence models will be assessed in predicting stability behaviour and required modifications can be proposed. Figure 1 shows the schematic of MSNCL. Fig 1: Molten Salt Natural Circulation Loop

New Project 8: Natural circulation studies in multiple loops . The natural circulation in multiple loops in series finds importance in (a) the advanced reactor design where two or more heat transfer circuits are in series, (b) designing passive decay heat removal systems (c) solar thermal systems, where the heat transfer system involves multiple natural circulation circuits in series. Studies on thermal hydraulic behavior of the coolants in the circuits are important in designing such systems. Theoretical modelling is to be carried out to study the natural circulation behaviour of loops in series.