Plurality of layers form a nuclear fission reactor structure, each layer having an inner segment body, an intermediate segment body, and an outer segment body (each segment body separated by an interface). The layers include a plurality of cladding arms having involute curve shapes that spirally radiate outward from a radially inner end to a radially outer end. Chambers in the involute curve shaped cladding arms contain fuel compositions (and/or other materials such as moderators and poisons). The design of the involute curve shaped cladding arms and the composition of the materials conform to neutronic and thermal management requirements for the nuclear fission reactor and are of sufficiently common design and/or have sufficiently few variations as to reduce manufacturing complexity and manufacturing variability.

A high-quality tomographic image acquisition device for a densely arranged spent nuclear fuel according to an embodiment of the present inventive concept relates to an gamma emission tomography device for detecting radiation of a fuel assembly which receives a plurality of fuel rods and may comprise: a first detection part disposed on one side of the fuel assembly and detecting radiation emitted from the fuel assembly; and a second detection part including a rod-shaped sealing part disposed inside the fuel assembly and a detection member disposed inside the sealing part and detecting radiation emitted from the fuel assembly.