[en] An inert gas space is disposed in a wrapper tube. A dropping port for granular or pellet-like neutron absorbers is disposed to the lower end of a neutron absorber-filled portion at an upper level of a reactor core. There are disposed a reactor core constituent element having a plug disposed vertically movably to the dropping port, and a thermally extensible mechanism for opening the dropping port by urging and driving the plug by a thermally expandable medium to drop the neutron absorbers. The neutron absorber-filled portion coaxially comprises, in the wrapper tube, a cylindrical member having a closed upper end and a hopper having the dropping port in the cylindrical member. The thermally extensible mechanism comprises a piston in contact with the upper end surface of a plug-mounting rod, an outer cylinder surrounding the upper portion of the piston with a bellows seal, a sliding guide coaxial with the outer cylinder and a thermally expandable medium at the upper end surface of the piston. Then, neutron absorbers can be dropped by temperature change of the coolants upon failure of scram to conduct passive scram of the reactor by an emergency reactor scram system of a BWR reactor without relaying control rods. (N.H.)