Radial inflow turbines, characterised by a low specific speed are a candidate architecture for the supercritical CO2 Brayton cycle at small scale, i.e. less than 5MW. Prior cycle studies have identified the importance of turbine efficiency to cycle performance, hence well designed turbines are key in realising this new cycle. With operation at high Reynolds numbers, and small scales, it is uncertain as to the relative importance o f loss mechanisms in supercritical CO2 turbines. This paper presents a numerical loss breakdown study of a low specific speed radial inflow turbine operating on supercritical CO2. A combination of steadyeng-state and transient calculations are used to determine the source of losses within the turbine stage . Losses are compared with preliminary approaches. Geometric variations to address high loss regions of stator and rotor are trialled. Analysis shows stage losses to be dominated by endwall viscous losses in the stator when utilising stator geometry definitions derived from gas turbines. These losses are more significant than predicted using preliminary methods. A reduction in stator -rotor interspace and use of a foiled blade showed a significant improvement in stage efficiency, without detriment to stator -rotor interaction. An investigation into rotor blading shows favourable performance gains through the inclusion of splitter blades. Through these modifications, a stage performance improvement of 7.5 points is possible over the baseline design ., {"references":["[1] Angelino G. (1968). Carbon Dioxide Condensation Cycles for Power Production. Journal of Engineering for Power 90 (3) 287- 295.","[2] ANSYS (2017). CFX solver theory guide 18.1. Retrieved January 2018. support.anays.com","[3] ASTRI (2012). Australian Solar Thermal Research Initiative. www.astri.org.au","[4] Balje O.E. (1962). 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