| The lead cooled fast reactor is one of the generation Ⅳ advanced reactor that using liquid lead or liquid lead-bismuth eutectic(LBE)as coolant,is expected to achieve the goal of improving safety and sustainability compared to previous generations of reactors.There are two main ways to position the core fuel assembly in lead cooled fast reactor:the wire spacer and the grid spacer.The grid spacer layout is usually layered and the wire spacer layout is usually in the fuel assembly of total length.Because it is arranged in the narrow space of the core,the flow and heat transfer of liquid metal coolant will be affected to some extent.Therefore,it is necessary to conduct study on the effect of fuel assembly positioning mode on thermal and hydraulic characteristics of lead cooled fast reactor.At present,domestic and foreign scholars mainly focus on the thermal and hydraulic characteristics of the fuel assembly with a single positioning mode.For the same fuel assembly,however,there is a lack of comparative study on the effect of the fuel assembly positioned by the wire spacer and the grid spacer on thermal and hydraulic characteristics.First of all,in this paper,compared with the flow and heat transfer experiment of LBE in annular space channel,the theoretical calculation model combination standard kε/Cheng model suitable for numerical calculation of liquid metal is determined.Secondly,for the grid spacer positioning mode,the thermal and hydraulic characteristics of the fuel assemblies with three basic grid spacers,namely,plate-airfoil spacer,double airfoil spacer and single plate spacer,were compared by using the standard k-ε/Cheng model.The single plate spacer assembly has the advantages of more uniform temperature distribution of LBE,relatively large mainstream deflection coefficient(K)of LBE,and the smallest spacer resistance coefficient.However,there are still some problems such as low temperature in the edge subchannel of single plate spacer assembly,and the thermal and hydraulic characteristics can be improved by improving the spacer structure.Therefore,I-shaped spacer is designed on the basis of single plate spacer.The results show that the I-shaped spacer makes the relative temperature of the LBE edge subchannel increase by about 4%and the internal subchannel decrease by about 3%.The I-shaped spacer has some improvements compared with the single plate spacer in improving the transverse velocity,reducing the surface temperature of fuel elements and improving the temperature uniformity of the different subchannels.Thirdly,for the wire spacer positioning mode,numerical simulations were carried out on the thermal-hydraulic simulation of the LBE wire spacer assembly with different inlet flow velocities and different wire pitches.The results show that increasing the inlet velocity has a certain threshold for increasing the mainstream deflection coefficient K of LBE.When the inlet velocity is too large,the mainstream deflection coefficient K will decrease instead;with the increase of the wire pitch,the mainstream deflection coefficient K is smaller,the faster you reach stability.Finally,the thermal and hydraulic characteristics of the two positioning modes were compared.The NuL of the grid spacer assembly LBE and the fL of the assembly are larger than that of the wire spacer assembly,and the flow through the grid spacer makes the NuL improve to a certain extent.The NuL of the I-shaped spacer assembly LBE oscillates more obviously than that of the single plate spacer assembly.The effects of increasing the wire pitch on the NuL and fL of the assembly have both positive and negative correlations,which are nonlinear.For the Thermal-hydraulic Comprehensive Evaluation Criterion(TCEC),the TCEC of grid spacer assembly is larger than that of wire spacer assembly,and for the wire spacer assembly,the increase of wire pitch makes TCEC first decrease and then increase;For the grid spacer assembly,the TCEC value of I-shaped spacer assembly is 0.353 larger than that of single plate spacer assembly,which shows that the designed new I-shaped spacer assembly can improve the thermal and hydraulic characteristics to some extent.The study results of this paper can provide reference for the design of fuel assembly positioning mode of lead cooled fast reactor and have certain academic and engineering value. |
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