A nuclear fission reactor structure (104, 780, 804) comprising a plurality of layers (102; 200; 500, 720, 740, 802), wherein each layer (102; 200, 500, 720, 740, 802), of the plurality of layers (102, 200, 500, 720, 740, 802) includes: an inner segment body including an inner opening extending axially from a first side of the inner segment body (210, 502) to a second side of the inner segment body (210, 502); an intermediate segment body (240, 504, 704) radially outward of the inner segment body (210, 502); an outer segment body (270, 506, 706) radially outward of the intermediate segment body (240, 504, 704); a first interior interface separating the inner segment body (210, 502) and the intermediate segment body (240, 504, 704); and a second interior interface separating the intermediate segment body (240, 504, 704) and the outer segment body (270, 506, 706), wherein, in a cross-sectional plan view in a plane perpendicular to the axially extending inner opening: the inner segment body (210, 502) includes a plurality of inner cladding arms having a first involute curve shape that spirally radiates outward from a first radially inner end adjacent to the inner opening to a first radially outer end at the first interior interface, the intermediate segment body (240, 504, 704) includes a plurality of intermediate cladding arms (244, 604) having a second involute curve shape that spirally radiates outward from a second radially inner end adjacent to the first interior interface to a second radially outer end at the second interior interface, and the outer segment body (270, 506, 706) includes a plurality of outer cladding arms (272, 608) having a third involute curve shape that spirally radiates outward from a third radially inner end adjacent to the second interior interface to a third radially outer end at a radially outer surface of the outer segment body (270, 506, 706); wherein the inner cladding arms have opposing side surfaces extending from the first side of the inner segment body (210, 502) to the second side of the inner segment body (210, 502), and wherein at least one protrusion projects outwardly from at least one opposing side surface, and wherein the protrusion has a top surface distal from the at least one opposing side surface for which the protrusion projects, and wherein, when assembled in the inner segment body (210, 502) with a first inner cladding arm immediately adjacent a second inner cladding arm, the top surface of a protrusion on the first inner cladding arm contacts an opposing side surface on the second inner cladding arm and forms a channel between the first inner cladding arm and the second inner cladding arm. an inner segment body including an inner opening extending axially from a first side of the inner segment body (210, 502) to a second side of the inner segment body (210, 502); an intermediate segment body (240, 504, 704) radially outward of the inner segment body (210, 502); an outer segment body (270, 506, 706) radially outward of the intermediate segment body (240, 504, 704); a first interior interface separating the inner segment body (210, 502) and the intermediate segment body (240, 504, 704); and a second interior interface separating the intermediate segment body (240, 504, 704) and the outer segment body (270, 506, 706), the inner segment body (210, 502) includes a plurality of inner cladding arms having a first involute curve shape that spirally radiates outward from a first radially inner end adjacent to the inner opening to a first radially outer end at the first interior interface, the intermediate segment body (240, 504, 704) includes a plurality of intermediate cladding arms (244, 604) having a second involute curve shape that spirally radiates outward from a second radially inner end adjacent to the first interior interface to a second radially outer end at the second interior interface, and the outer segment body (270, 506, 706) includes a plurality of outer cladding arms (272, 608) having a third involute curve shape that spirally radiates outward from a third radially inner end adjacent to the second interior interface to a third radially outer end at a radially outer surface of the outer segment body (270, 506, 706); wherein the inner cladding arms have opposing side surfaces extending from the first side of the inner segment body (210, 502) to the second side of the inner segment body (210, 502), and wherein at least one protrusion projects outwardly from at least one opposing side surface, and wherein the protrusion has a top surface distal from the at least one opposing side surface for which the protrusion projects, and wherein, when assembled in the inner segment body (210, 502) with a first inner cladding arm immediately adjacent a second inner cladding arm, the top surface of a protrusion on the first inner cladding arm contacts an opposing side surface on the second inner cladding arm and forms a channel between the first inner cladding arm and the second inner cladding arm.