Optimization Of Flow Rate Distribution Of 100MW Level Small Modular Liquid-fueled Molten Salt Reactor

Molten Salt Reactor(MSR)is one of six generation IV reactor types,which has significant advantages in inherent safety,economic benefits,fuel cycle and online processing,anti-proliferation and other aspects.In 2011,Shanghai institute of applied physics,Chinese academy of sciences undertook “advanced fission nuclear energy--thorium-based molten salt reactor nuclear energy system",one important reactor type in the second stage of which is small modular thorium-based liquid-fueled MSR.100MW level small modular liquid-fueled MSR researched in this paper uses molten salt as coolant and fuel.There is no exchange of mass or momentum between adjacent molten salt channels in the core region,which are called closed circulation channels,therefore the mass flow rate of each channel depends on the distribution function of upper and lower plenum.Reactor core flow distribution is an important issue of thermal hydraulic design of reactors and,together with power density distribution,decides hotspot factor in core region restricting the safety and economy of the reactor.Although the power density varies with the reactor operating state,mass flow distribution could hardly be changed once the core structure is determined.When the mass flow rate of a molten salt channel is too low compared with its power density,hot spots will be generated due to the excessive temperature rise of molten salt,which will weaken the safety margin of transient operation conditions and affect the stability of structural materials.Meanwhile,during the operation of the reactor,eddy may generate in the lower plenum and local eddy may hinder molten salt into several molten salt channels,resulting in extremely uneven mass flow distribution and hotspots.Work done by the viscous stress within the vortex region also causes mechanical energy loss(viscous dissipation).Hence it's necessary to evaluate and optimize the flow distribution performance of this MSR in advance to flatten the flow distribution,suppress vortices,eliminate hot spots and finally design a reasonable flow field distribution device.In this paper,the flow distribution device of this 100 MW level small modular MSR was taken as the research object and 4 indicators for evaluating distribution performance were proposed from two perpectives of “mass flow flattening” and “temperature flattening” respectively.Numerical simulation was inplemented by means of computational fluid dynamics(CFD)analysis software Fluent15.0.Based on the analysis of relevant thermal and hydraulic parameters,the influence of 5 factors,including whether to install a baffle plate,the radius of baffle plate,location of the baffle plate,the height of semi-ellipsoidal upper plenum and aperture array of metal supporting plate on 4 indicators of flow distribution performance was studied.A flow distribution device with good performance was determined for “mass flow flattening” and “temperature flattening” respectively.Research on flow distribution device for “mass flow flattening” prove that the installation of baffle plate in lower plenum can effectively weaken the local eddy in the lower plenum and eliminate flow dead zones at the entrance of salt channels,thus improve the homogeneity of mass flow distribution in the radial direction.Increment of the height of the semi-ellipsoid upper plenum can balance the pressure drop differences of all salt channels in the radial direction,resulting in improvement of mass flow uniformity.Reducing the corresponding aperture on the metal support grid plate of 2~4 rows of channels with higher mass flow can effectively reduce the flow rate at this place and make the overall flow distribution trend more gentle.The final scheme of “mass flow flattening” device is depicted as follows: set a cylindrical baffle with a radius of 240 cm in the central position of the lower plenum.Adjust the height of the semiellipsoid upper plenum from 35 cm to 45 cm.Specifically reduce the corresponding aperture on the metal support grid plate of 2~4 rows of salt channels with higher mass flow rate.Research on flow distribution device for “temperature flattening” prove that the installation of baffle plate in lower plenum can effectively weaken the local eddy and solve the problem of severe overheating at outlet of 4~6 rows of channels caused by radial non-uniformity of mass flow distribution;Reducing the height of the upper plenum can hardly flatten temperature field at outlet of all channels;Reducing the corresponding aperture radius on the metal support grid plate of 9~13 rows of channels with lower power desity can effectively guide molten salt to concentrate to central region channels with higher power density,therefore,makes the overall temperature field more gentle.The final scheme of “temperature flattening” device is depicted as follows: set a cylindrical baffle with a radius of 220 cm in lower plenum 7cm from the metal support grid plate.Remain the height of the semi-ellipsoid upper plenum 35 cm.Specifically reduce the corresponding aperture on the metal support grid plate of 9~13 rows of salt channels with lower outlet temperature.Because of the contradiction between the mass flow uniformity index and eliminating hot spots and guaranteeing thermal margin,the basic task of thermo-hydraulics design of the reactor,the optimization scheme of temperature field flattening was selected as the design scheme of flow field distribution device of 100 MW liquidfueled MSR.The above results have important reference value for the structure optimization design of upper and lower plenum of this liquid-fueled MSR.