[en] In reactor safety research, it is important to know transient fuel behavior under accidental conditions. Transient histories such as temperature and axial expansion of fuel and cladding and internal pressure of fuel rod are thus measured in experiments simulating accidents. If fuel motion could then be observed during and after fuel failure, this would greatly make for fuel behavior research. The present status is reviewed of fuel motion detections by radiations such as neutron, γ-ray and X-ray, including the principle and system. A neutron hodoscope among them is used already with practical results in in-reactor safety experiments of sodium-cooled fast breeder reactor. So, this is described in detail and its conceptual design as applied to the NSRR is presented. (auth.)