Thermal-hydraulics Characteristics Research Of Molten Salt Cooled Pebble Bed Reactor

The molten salt cooled pebble bed reactor inherited and integrated coolant and fueltechnologies from liquid fuel molten salt reactor and conventional gas cooled high temperaturepebble bed reactors. Benefits of the high heat capacity, excellent heat transport characteristic, andneutron slowing down characteristic of fluorine-lithium-beryllium (FLiBe), could allow toincrease+reactor operational power density, to improve thermal electricity efficiency, to enhancepassive safety, and to reduce core dimensions. However, due to the special chemical and physicalcharacteristics of Flibe and the rigorous high temperature experiment environment, the flow andheat transfer performance of Flibe has not been systemically researched. The pressure drop andheat transfer characteristic of molten salt flow in the pebble bed directly influence the design,safety analysis and optimization of the reactor system. Limited to the performance andComputational Fluid Dynamics (CFD) method and codes，the conventional Pebble bed Reactorthermal hydraulics codes were based on the macroscopical porous media model, which failure topredict detailed distribution of temperature of pebbles in various work conditions. In the pastdecades, the improvments of computational fluid dynamics and computer capability underpinneda solid foundation to develop the thermal-hydraulics analysis method and codes.There are two episodes in this dissertation: the fist is pore scale thermal-hydraulis studiesof molten salt cooled packed bed; the second is molten salt coold reactor thermal-hydraulicsanalysis based on macroscopic porous media local thermal non-equilibrium model.In order to analysis pore scale thermal-hydraulics of molten salt coold pebbe bed, two orderpacked pebble beds (BCC and FCC model) were constructed in the third chapter, pressure dropand Nusselt number were calculated. The CFD results of pressure drop were in accordance withexperimental data fitting curves, the model and computational scheme are reliable. Bycomparing the results of BCC and FCC model, it was indicates that the configuration of packedbed has an important influence on the pressure drop. The results of convective heat transfercomputations indicated that the Nusselt number is dependent on the structure of the order packedbed and specified Reynolds number range, it was need to develop new pressure drop and Nusseltnumber correlations in order to study and construct new type ordered packed pebble bed reactors.In the second part of episode one, a random packing of pebble bed was generated by usingdiscrete element method, by comparing CFD results of pressure drop and Nusselt number to experimental data, it was indicated that the model is reliable for thermal-hydralics studies of porescale random bed. It was found that, the KTA pressure drop coefficient is applicable to moltensalt cooled pebble bed, and the Wakao equation is credible only in the case of low Reynoldsnumber, thus a new Nusselt number correlation is proposed in the case of Re>500:Nu=2.91Re0.415Pr1/3.Based on the CFD model of ordered and random bed, a general method was developed toconfirm the design could ensure the safety of pebbles under full power nominal operation, andpredicte the tempture distribution of pebble surface under the condition of local power promotionand loos of partial coolant. The results show that in the random packed bed the pebble surfacetemperature is very sensitive to local region pebbles dentisification and local power promotion,but lack of sensitivity to loss of partial coolant.The second episode focuses on the macroscopic characteristic analysis of molten saltreactor. Based on advanced porous media theorey and CFD codes, a local thermalnon-equilibrium model was developed, and a modifid k εturbulence model was implied. Theresults of PBMR-400full load nominal condition thermal-hydraulics studied show a goodconsistency with well-established thermal-hydraulic code THERMIX and TINTE. It was foundthat the turbulence equations source term has a significant influence on convective heat transferperformance between fluid and solid phase, there is a more uniform distribution of temperatureof solid and fluid phase due to turbulence equations source term. More importantly, the methodin this dissertation based on profouned theoretical foundation, which stand a chance to developmore accurate pebble bed thermal-hydraulics analysis codes.Based on the results of pore scale thermal-hydraulis studieds and porous media ocalthermal non-equilibrium model, the steady-state thermal-hydraulics of annular core PB-AHTRwas studied. By comparing the results under two coolant inlet/outlet designs, a better plan wassuggested. It was found that, there exists a high temperature region of pebble bed in the reactorcore, which needs optimaization of core structure.