[en] This paper discusses the development of the reactor designs for the Advanced Boiling Water Reactors (ABWR) and Simplified Boiling Water Reactors (SBWR). While there are marked differences in the design philosophy of the two products, an attempt was made to use the same technology and the same manufacturing practices wherever possible. This paper discusses the design of the RPV and internals of both designs and some of the key issues and decisions behind the designs. Also discussed are the Reactor Internal Pumps used in the ABWR and the Fine Motion Control Rod Drive used in both products. The RIP testing and development is summarized as are some improvements made over previous designs. The FMCRD testing, including an in-plant test is also discussed. The modifications made as the result of the in-plant test experience are also discussed. (author)