The research and technological development towards compact energy conversion systems for space applications allows the emergence of new mission possibilities, especially those directed for deep space explorations. This work is part of the project for Advanced Fast Reactor Technology (from Portuguese, TERRA), currently under development at the Institute for Advanced Studies (IEAv), which aims to develop a multipurpose reactor having space as one of its applications. Considering the Closed Brayton Cycle (CBC) as energy conversion system, 157 thermal kW from the compact nuclear reactor operating in the fast spectrum are to be converted into electricity for space applications. This work proposes a deepened numerical representation of the conversion cycle's cold side (namely the Heat Pipe-Radiator assembly), which is to be studied alongside a simple thermodynamic model of the CBC in order to propose a lightweight component. The cold side geometric construction and thermal couplings were described considering the heat extraction at the Cold Heat Exchanger's (CHE) duct, its conduction through Heat Pipes (HP) and its rejection to space by the radiator panels. The assembly was separated in operating control volumes, while considering variable geometric parameters for each one, presenting a variable sized trapezoidal space radiator as output. The heat fluxes were limited by the geometry of each element panel and each HP was modeled and verified upon their limitants to fit given geometry and operational parameters. The model considers temperature drops along the HP axial direction, along the panel surface (modeled as radiative fins) and along the CHE duct, providing reasonable global parameters to aid the decision making for the design. An initial evaluation presented optimal temperatures of operation for the CBC's CHE at around 500±20K, which pointed to titanium-water HP with grooved wick configuration. The HP-RAD model set at this range presented tradeoff conditions for the assembly's length and mass while evaluated against the HP spacing and CHE inlet temperature, with minima at about 23m and 600kg respectively. A coupled analysis of both CBC and HP-RAD models presented second law efficiencies up to 45%, with the optimal solution reaching 31% conversion efficiency. In a symmetric configuration it is expected an assembly with 300 titanium-water HP, 540kg and 25m for a reactor temperature of 1200K and a CHE inlet temperature of 513K. This provides roughly 48 kW of useful electricity, while being the solution with the lowest radiator specific mass determined in this study (at 11kg/kWe).