[en] In a fuel assembly, the length of a fuel region at the downstream of a coolant flow is increased than that of the fuel region at the upstream. A neutron reflecting region is set to the downstream of the coolant flow, and the cross sectional area of the coolant flow channel is increased than the cross sectional area of the flow channel in fuel element bundles. Further, a second fuel assembly having enriched fissionable materials is set in the radial outer region of the reactor core. Then, a neutron flux level near the downstream end where coolant temperature reaches maximum can be kept at a high level. When the temperature of coolants is elevated and the density of the coolants is reduced due to thermal expansion upon occurrence of a transient phenomenon, since neutron leakage passing through the neutron reflecting region at the downstream where a volume ratio of coolants is great is promoted, increase of reactivity is moderated. As a result, inherent high safety relative to the transient phenomenon can be attained, increase of the entire length of the fuel assembly is minimized, to increase economical merits due to saving for the amount of materials. (N.H.)