A Jan. 26 agreement to cooperate on fast nuclear reactor technology between the Japanese Atomic Energy Agency and Mitsubishi Heavy Industries Ltd. with American TerraPower LLC raises worrisome international security issues. The agreement’s immediate goal is building a 350-megawatt nuclear plant, called Natrium, in the U.S. state of Wyoming, to demonstrate TerraPower’s technology. Unlike fast reactor projects in the past, which were fueled with plutonium, a nuclear explosive, and that used “reprocessing,” or chemical separation to extract plutonium from spent or irradiated power reactor fuel, N…