Fuel assembly

In a cluster type fuel assembly for a pressure tube type reactor, a great number of fuel rods are disposed coaxially, in which several fuel rods among those situated at an intermediate layer and an inner layer are incorporated with poisons. With such a constitution, since the fuel rods containing the burnable poisons show great neutron absorption, change of the reactivity due to the presence or the absence of light water (void ratio) is reduced. Accordingly, compared with the fuels not containing the burnable poisons, the void reactivity of the coolants is shifted to the negative side. Although the reactivity is increased along with burning due to increase of the enrichment degree for the fuel rods in the inner layer and the intermediate layer, the reactivity is improved since the burnable poisons are contained also in the inner layer fuel rods. (I.S.)