| As the only type reactor using liquid fuel in the fourth generation advanced reactor,the liquid fuel molten salt rector can realize online processing and recycling of spent fuel,which make it easy to realize the thorium-uranium fuel cycle.It has great potential advantages in improving the efficiency of resource utilization,reducing the amount of nuclear waste,preventing nuclear proliferation and strengthening national energy security.Small modular molten salt reactor has the characteristics of compact structure,small size and flexible site selection,which makes it become main direction of commercial development of molten salt reactor at present.At present,the graphite isn’t only used as a moderator to slow down the neutrons at most of small modular molten salt reactor,but also forms the molten salt flow channel to support the core.The short graphite assembly lifespan is one of main restricting factor the development of small modular molten salt reactors.The graphite assembly lifespan is usually defined as the time it takes for the graphite volume to first decrease to a minimum and then expand back to the original volume as the neutron fluence increases.The neutron fluence required for graphite assembly to reach the lifespan is different at different temperatures.To remain competitive,the fuel power density should be greater than 100MW/m~3for a typical commercial small modular molten salt reactor.But the graphite assembly lifespan is usually less than 7 years at this power density.At present,most of the conceptual design of molten salt reactors uses traditional central round channel graphite assembly(RCA).In order to prolong the graphite assembly lifespan,this thesis proposes a new method of prolonging graphite assembly lifespan by designing new structure graphite assemblies and designs a new type of solid hexagonal prismatic graphite assembly(HPA)for small modular molten salt reactor.The HPA obtains a lower fast neutron fluence distribution and a more uniform temperature distribution,which increases the graphite lifespan from 7.5 years of RCA to 8.4 years,and it takes 14.9 years to expand before the assemblies are in contact with each other,which greatly prolongs the graphite assembly lifespan.At the same time,considering the effect of the rapid decrease of graphite thermal conductivity with the radiation neutron fluence,this thesis improves the calculation method of graphite dimension change ratio.MCNP5 and Fluent were used to calculate the fine three-dimensional fast neutron flux distribution and three-dimensional temperature distribution in graphite,and finally the three-dimensional dimension change ratio distributions of HPA and RCA were obtained.Graphite assemblies for molten salt reactors(such as RCA)are usually fixed by metal support plates at both ends of the axial direction.Since the thermal expansion coefficient of the metal is much higher than that of graphite,the temperature of the graphite components increases from the room temperature of 20℃ to the operation temperature of 700℃,resulting in the formation of narrow gaps of molten salt flow between closely arranged RCAs(HPA can avoid the formation of narrow gaps).This thesis analyzes the effect of the narrow gaps on the heat transfer characteristics and graphite irradiation-induced dimension change of RCA.It is found that the molten salt flow rate in the narrow gaps of RCA is only about 1/17 of the main channel.All the heat from the molten salt in the narrow gaps cannot be directly taken out of the assemblies through flowing molten salt,and part of the heat is transferred to the main channel molten salt through the graphite,which reduces the cooling effect of the main channel molten salt on the graphite.The narrow gaps doesn’t have a significant effect on the fast neutron flux distribution in the RCA graphite,but they aggravates the graphite radiation dimension change and shortens the graphite assembly lifespan of RCA by 1.25 years by significant increasing the graphite temperature. |
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