PURPOSE: To eliminate the mutual fusion of neutron absorbers so as to improve the reliability of a back-up reactor stop device by disposing boron metal at the core part of the neutron absorber, and enveloping its periphery with graphite and also forming it into the granular state.

CONSTITUTION: A neutron absorber 20 is mounted into the hopper 15 of a back-up reactor stop device 13 earthed on the upper part of the reactor core 2 of a graphite-moderated helium gas cooling reactor. During the operation of the reactor, the absorber 20 is placed for many hours under the temperature close to 500°C, but the mutual fusion of the absorbers 20 is not generated since the absorber 20 does not contain noncombustible boron oxide. Accordingly, when the back-up reactor stop device 13 is actuated to burst a rupture disc 16 and thereby to open the lower end of the hopper 15, the absorber 20 falls by its own weight into an insertion hole 19 in the reactor core 2 without delay to stop the reactor.