Nuclear fuel element, fuel assembly and reactor core thereof

In a nuclear fuel element, a spring chamber is disposed above a fuel pellet-stuck portion, and a gas plenum is disposed to the lower end of a cladding tube. A volute spring is disposed in the spring chamber. Then, the length of the spring chamber can be shortened to less than 10cm, and frictional pressure loss due to axial flow of coolants in the fuel assembly is reduced. The lower surface of a spacer which bundles the fuel elements at a gap in the lateral direction and is disposed to the uppermost portion of the fuel effective portion is positioned about 15cm lower from the upper end of the fuel effective portion. Spacers therebelow are disposed each at 30 to 51cm distances. This can prevent lowering of limit power. Further, under the condition that whole control rods are withdrawn, the upper end of the effective portion of a control rod is disposed at the same position or at a position less than 5cm lower than the lower end of the effective portion of the fuel assembly. With such a constitution, reduction of reactor shutdown margin and degradation of scram characteristics can be prevented. (I.N.)