In the US-APWR, large forged steel pieces are adopted to minimize the length of the welds requiring in-service inspections. Improved designs are applied for the head adaptors of the reactor vessel head to reduce residual stresses. In addition, design and material improvements in Mitsubishi’s reactor vessel heads have enhanced their corrosion-resistance, thereby improving their “preventive maintenance” function. The number of fuel assemblies in the reactor core has been reduced while output is increased. The 14’, open-lattice fuel assembly design contributes to the efficient use of neurons, resulting in excellent fuel economy and, therefore, the need for less fuel.