According to a first aspect, there is provided a nuclear fission reactor. The nuclear fission reactor comprises a core, a tank surrounding the core, and a cooling system located outside the tank. The cooling system comprises one or more structures configured to absorb thermal radiation emitted from an outer wall of the tank. The structures are not substantially thermally coupled to the tank except by radiation. The cooling system further comprises a cold air inlet and a hot air outlet, positioned such that air flows from the cold air inlet to the hot air outlet over, around and/or through the one or more structures.