JP2873230B

PROBLEM TO BE SOLVED: To reduce the void coefficient of a fuel assembly aiming at high burn up, without decreasing reactivity, to flatten local output peaking, and to optimize the cross-sectional area of the moderator of a neutron moderator rod. SOLUTION: Normal fuel rods 2 and partly long fuel rods 3 are arranged in a 10×10 square grid pattern and three large water rods 4 are arranged diagonally in the center region where ten fuel rods can be placed. Two of the partly long fuel rods 3 are placed within the range of the projected areas of the water rods 4 at the outermost layer of the square grid pattern of fuel rods, and only the normal fuel rods 2 are placed on two layers inside the outermost layer. This constitution reduces the void coefficient and enhances the control rod value. Also, improvements are made in both reactivity losses and local output peaking, resulting in enhanced fuel economy and thermal tolerance. The cross-sectional area within the water rod is 9 cm , which also enhances fuel economy, channel stability, and core stability.