Light water reactor fuel assembly and core

In an MOX fuel assembly, water rods are arranged in a rotationally symmetry at each of corners of a fuel rod arrangement, and fuel rods are arranged at positions of second row from the outermost circumference and in adjacent with the water rod, to enhance the improving effect of a void reactivity coefficient per one water rod. In particular, each one water rod is disposed to each of the corners or at a lattice position in adjacent with the corner, to minimize the number of additional water rods required for improvement of the void reactivity coefficient. In order to substantialized the improving effect, it is necessary that the water rod has length identical with the region of the fuel rod filled with nuclear fuel materials. A void reactivity coefficient and a moderator temperature coefficient can be made identical with those of a uranium fuel assembly without greatly reducing a plutonium loading amount and without greatly increasing a linear power density. Further, in a BWR type reactor, the kind of enrichment degree of MOX fuel rods can be reduced. (N.H.)