This paper investigates the comparative performance of nitrogen and helium as primary coolants for prismatic High Temperature Gas-cooled Reactors (HTGRs) operating with direct cycles coupled to a gas turbine. The attraction of using a nitrogen coolant is to enable the coupling of two proven technologies; prismatic HTGRs, and commercially-available combustion gas turbines with a working fluid that is compatible with both components. The investigation was performed using a bespoke model incorporating the turbine performance at design point, core pressure drop and core heat transport mechanisms enabling an interrogation of the comparative effects of different coolants on the whole system performance. Where inlet core temperatures are maintained consistent between a comparable helium and nitrogen cooled design with an atmospheric compressor inlet pressure, the overall cycle performance was shown to be comparable. Although the cycle performance was comparable, coolant to fuel temperature differences were shown to be larger in the nitrogen cooled design, although this design was also shown to be less sensitive to coolant pressure drops. The larger temperature differential was shown to be due to significantly poorer heat transport in the fuel to fuel hole gap and lower convective heat transport. The differences in the convective heat transport were shown to less significant that might be implied from the differences in thermal conductivity as with nitrogen the higher mass flow rates improve the Nusselt number largely offsetting the differences in thermal conduction. Furthermore, through optimisation of fuel block design or incorporation of multi-pass core concepts comparable performance was shown to be possible with nitrogen cooled reactor design. Overall, no fundamental core thermal hydraulic impediment was discovered which is expected to inhibit development of the nitrogen cooled direct cycle concept.

本文研究了氮和氦作为棱柱形高温气冷堆（HTGR）的主要冷却剂的比较性能，该反应堆与燃气轮机直接循环运行。使用氮气冷却剂的吸引力在于能够实现两种成熟技术的结合。棱柱形HTGR，以及可与两种组件兼容的工作流体的商用燃气轮机。使用定制模型进行了研究，该模型结合了设计点的涡轮性能，堆芯压降和堆芯传热机制，从而可以询问不同冷却剂对整个系统性能的比较效果。在大气压压缩机入口压力下，可比氦气和氮气冷却设计中入口核心温度保持一致的情况下，整体循环性能表现可比。尽管循环性能相当，但在氮气冷却的设计中，冷却剂与燃料的温差显示出更大，尽管该设计对冷却剂压降的敏感性也较低。结果表明，较大的温差是由于燃料与燃料孔之间的热传递明显较差，而对流热传递较低。对流传热的差异显示为不那么显着，这可能是由于导热系数的差异所暗示的，因为使用氮气​​时，较高的质量流量会改善Nusselt数，从而在很大程度上抵消了导热系数的差异。此外，通过优化燃料块设计或引入多程堆芯概念，氮冷却反应堆设计具有可比的性能。总体而言，未发现根本的核心热液阻滞现象，这有望抑制氮气冷却直接循环概念的发展。