Abstract: The reactor shut-down margin and the thermal margin, as well as the output per unit volume of the core is increased. When A is the total cross sectional area of the non-boiling water areas in the channel boxes each surrounding a fuel assembly and non-boiling water areas outside the channel boxes and B is the cross sectional areas of boiling water areas in the channel boxes, the relation of 1>B/(A+B).gtoreq.0.76 is set. A control unit adjusts a rotational speed of a recirculation internal pump which controls the cooling water flow rate per unit area in the core section to 3.0.times.10.sup.3 t/h/m.sup.2 at the beginning of the operation cycle and controls the cooling water flow rate per the unit area in the core cross section to 3.3.times.10.sup.3 t/h/m.sup.2 at the end of an operation cycle.

Abstract: A fuel assembly comprises fuel rods arrayed in a square lattice pattern of 10 rows and 10 columns, and three large-diameter water rods arranged along a diagonal line of the fuel assembly in such a region as able to accommodate 10 fuel rods. Partial length fuel rods are arranged in an outermost layer of the fuel rod array at fuel rod setting positions other than corners of the outermost layer. Ordinary fuel rods are arranged in a layer inside the outermost layer and adjacent to the outermost layer at positions adjacent to the partial length fuel rods in the outermost layer.The struction of the fuel assembly enables a reduction in the void coefficient and an improvement in the reactivity control capability. Also, the void coefficient can be reduced without lowering reactivity, and fuel economy is improved.

Abstract: A fuel assembly and a reactor core using same which are able to increase size of the fuel assembly with ensuring thermal margin and reactor shut down margin.A distance between centers of adjacent fuel assemblies is about 23 cm, which is enlarged about 1.5 times of conventional fuel assemblies. A thickness of water gap region is about 16 cm, which is relatively thinner than that of prior art. While, H/U ratio is about 5 as same as that of the prior art, and decreasing amount of non-boiling water in the water gap region is arranged in a channel box as water rods. Consequently, a ratio of transversal cross section area of the water rods to transversal cross section area of the fuel rods becomes about 0.6, and local power peaking factor can be decreased and thermal margin can be increased. Further, the transversal cross section area of the water rod is selected to be 15 cm.sup.2 so as to ensure the reactor shut down margin by reducing excess reactivity.

Abstract: A second fuel rod positioned at each corner of a channel box and second fuel rods adjacent to the former are formed to have a smaller outer diameter than that of ordinary first fuel rods, so that a pitch between the second fuel rods is narrower than a pitch between the first fuel rods. Making the outer diameter of the second fuel rods smaller than that of the first fuel rods reduces the power per unit length of the second fuel rods. The narrower pitch between the second fuel rods than the pitch between the first fuel rods provides two effects. First, a unit lattice cell becomes so small as to avoid an increase in the H/U ratio. Secondly, a new moderator region is formed between the second fuel rods and the first fuel rods adjacent thereto, the moderator region acting to intensify thermal neutron flux around those first fuel rods. These two effects enable a further reduction in the power per unit length of the second fuel rods.