Abstract: To provide a reactor core for light water cooled reactor, a fuel assembly and a control rod intended for Pu multi-recycling at a breeding ratio of about 1.0, or 1.0 or more while keeping the economical or safety performance to the same level as in BWR now under operation, that is, while minimizing the change for the incore structures and maintaining the void coefficient negative. A reactor core for a light water cooled reactor having an effective fuel-to-water volume ratio of 0.1 to 0.6 by the combination of a dense lattice fuel assembly constituted of fuel rods formed by adding Pu to degraded uranium, natural uranium, depleted uranium or low concentrated uranium, coolants at high void fraction of 45% to 70% and a cluster-type, Y-type or cruciform control rod.

Abstract: A reactor core for a boiling water reactor, a fuel assembly and a control rod intended for Pu multi-recycling at a breeding ratio of about 1.0, or 1.0 or more while keeping the economical or safety performance to the same level as in a boiling water reactor now under operation. The reactor has an effective water-to-fuel volume ratio of 0.1 to 0.6 by the combination of a dense lattice fuel assembly constituted of fuel rods formed by adding Pu to degraded uranium, natural uranium, depleted uranium or low concentrated uranium, and having coolants at a high void fraction of 45% to 70% and a cluster-type, Y-type or cruciform control rod.