Thermal performance analysis of a nuclear heated gas turbine with nitrogen coolant

Abstract

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.