Fuel assembly and reactor core

A mass addition member is disposed to the lower portion of a lower tie plate of a BWR fuel assembly for increasing the own weight of the fuel assembly. Namely, in a fuel assembly of a high-conversion reactor, a mass addition member is disposed between support fittings at the lowermost portion of the fuel assembly and a lower tie plate or disposed integrally to the lower tie plate to increase the own weight of the fuel assembly. Even if the flowrate of coolants in the fuel assembly is increased and buoyancy exerted on the fuel assembly is increased as a feature of the high conversion reactor, the buoyancy can be resisted sufficiently to prevent the fuel assembly from rising. The mass addition member is disposed to the lower portion of the lower tie plate, and through holes are formed to the lower tie plate for introducing coolants. Since the coolants below the lower tie late are still in a single phase flow with no boiling, increase of pressure loss can be suppressed compared with a case where the mass addition member is disposed above the upper tie plate where the coolants are two phase flow. (N.H.)