MECHANICALLY SPECTRUM-SHIFTED NUCLEAR REACTOR

PURPOSE:To simplify an upper core structure, by placing control rod guide tubes and water remover rod guide tubes with a comparatively large spacing between them, corresponding to a space above each one of every two fuel assemblies. CONSTITUTION:A control rod guide tubes 11a' having an almost octagonal outer shape of their cross-section are placed above the fuel assemblies 6 corresponding to a space above each one of the every two fuel assemblies, on the other hand, water remover rod guide tubes 11b' having an almost square outer shape of their cross-section are placed above the fuel assemblies 6 corresponding to a space above each remaining one of the every two fuel assemblies as in the same manner. Moreover, at appropriate positions of enclosure plates 20a and 20b, a plurality of openings 24a and 24b are formed to allow flowing- through of a coolant and to change a flow direction crosswisely. In this case, the coolant heated by the fuel assemblies 6 in a reactor core 7 flows upwardly along the control rod guide tubes 11a' and the water remover rod guide tubes 11b', however as there exists the openings 24a' and 24b' which provide flow pass for the coolant, it is allowed for the coolant to flow crosswisely at a guide tube region and therewith a calandria part is not required.