During the Horizon 2020 sCO2-4-NPP project, several key components for the sCO₂ power cycle as an option for the innovative decay heat removal system for nuclear power plants were developed. One of them was the diverse ultimate heat sink (DUHS), which is an air/sCO₂ plate and fin heat exchanger with straight fins. A representative DUHS mock-up was manufactured and its thermal–hydraulic performance was tested using the sCO2 loop at Research Centre Rez (CVR) at parameters of 8 MPa of pressure and temperatures up to 170°C on the sCO2 side. The main findings were the acquisition of the heat transfer correlation on the air side of the heat exchanger and the fanning friction factor in the tiny channels. The collected data were used to verify the heat exchanger design and, moreover, a mathematical model was developed and validated. Furthermore an optimisation study was done using the validated model to find the best channel geometry with the trade-off between high heat transfer coefficient and low pressure losses.