PURPOSE: To obtain large negative reactivity at a transient phenomenon by making an engaging structure of a coolant inlet part at lower end of control rod guide tube and a reactor core supporting plate, to be a short lengthened and flexible one.

CONSTITUTION: A coolant inlet part 32 of control rod 3 guide tube 31 is shorter than a coolant inlet part 31, and also, in case that an amount of coolant which is engaged solely into a core supporting plate 4 in a manner of a cantilever, get reduced, and coolant temperature rising high, a fuel assembly bends itself with pushing away the control rod guide tube easily. In this way, an extent of radial expansion (deformation) of whole reactor core increases than ordinary one and there fore addable negative reactivity increases. Accordingly, in this example application, a reactor core of which countermeasure plan for coolant flow amount loss and passive safety are much emphasized, can be obtained.