PURPOSE: To terminate an accident in advance and safely and to shut down a reactor by arranging preloading fuel assemblies of which power-flow ratio is high, to a region where a density coefficient of coolant of reactor core, is of negative value.

CONSTITUTION: Failure of such preloading fuel assemblies as in this invention, disperses and moves vertically a crashed fuel H which is broken at a periphery G of axial intermediate part of fuel pin 8a. The crashed fuel moving downward, is solidified J at a region I but actually flows out to outside from an outlet 9b at an upper end of opening. This flowing out fuel M tends to be re-solidified by receiving a great resisting force at a fuel pin bundle part 8a, however, since a just above part of the fuel pin bundle 8a is an empty space part 12 and also a hydraulically equivalent diameter is large, the fuel flows out quite easily. Consequently, the fuel is evacuated from a region close to the reactor core center, where axial fuel reactivity value is high, and therefore large negative reactivity (fuel reactivity) is inserted. Fuel burn-out spreading to all the reactor core, can be well prevented, thereby.