熔盐堆散体石墨组件堆芯结构变化对核反应性的影响

Influence of core structural changes in dispersed-graphite-component molten salt reactor on nuclear reactivity

熔盐堆由于其燃料的流动性,堆芯结构变化会导致堆芯燃料分布及装载量的改变,从而影响堆芯物理特性参数。本文以新型实心六棱柱石墨组件熔盐堆堆芯设计模型为参考,使用MCNP程序分析了热膨胀、流体冲刷及堆本体震动和石墨辐照形变等因素导致的堆芯组件变形、位移等微小几何变化,并研究了这些堆芯结构变化对反应性的影响。结果表明:热膨胀引起的堆芯结构变化引入负反应性;流体冲刷及堆本体震动导致的石墨组件偏移,引入的反应性是波动的,但整体趋势是石墨组件向堆中心偏移,引入正反应性,向堆外围偏移,引入负反应性。为保障反应堆安全运行,需要将石墨组件约束在一定范围内;石墨辐照形变会使反应性先减小后增大,在堆芯寿期末堆芯反应性仍小于寿期初,该反应性变化可由在线加料或移动控制棒补偿,对熔盐堆运行影响有限,但需考虑该批燃料再入新堆后的临界控制问题。本文的研究将为熔盐堆的设计、运行和维护提供重要参考。

[Background]Molten salt reactors(MSRs)feature high temperature,low pressure,high chemical stability,and nuclear non-proliferation,which make them promising for a wide range of applications.However,owing to the fluid nature of the fuel,changes in the core structure of MSRs are bound to affect the fuel distribution and loading,consequently affecting the physical parameters of the core.Currently,the research on the impact of structural changes of the MSR core,composed of dispersed graphite components,on reactivity is insufficient.[Purpose]This study aims to explores the influence of core structure changes on reactivity.[Methods]The core design model of new solid hexagonal graphite module MSR was taken as a reference,the small geometric changes,such as deformation and displacement of the core components,caused by factors including thermal expansion,fluid erosion,core vibration,and graphite irradiation,were analyzed using MCNP code.Additionally,the impact of these structural changes in the core on reactivity was investigated in details.[Results]The results indicate that the structural changes in the core caused by thermal expansion introduce negative reactivity.The reactivity introduced by fluid erosion and core vibration,causing graphite component displacement,fluctuates.However,the overall trend shows that a shift of the graphite components towards the center of the core introduces positive reactivity,whereas shift towards the outer periphery of the core introduces negative reactivity.Graphite irradiation-induced deformation initially decreases reactivity and then increases it.At the end of the core's lifespan,the reactivity is still lower than at the beginning of the lifespan.This reactivity change can be compensated for by online feed or control rod movement,which has a limited impact on the operation of the MSR.However,the critical issue of control should be considered when this batch of fuel is reinserted into a new core.[Conclusions]Results of this study suggest the necessity of constraining the graphite components within a certain range to ensure the safe operation of the reactor,providing an important reference for the design,operation,and maintenance of MSRs.