On March 20, 1978, Fugen attained the first criticality loading 22 Mixed OXide (MOX) fuel assemblies. This was the first achievement of the prototype Advanced Thermal Reactor (ATR), which was constructed colligating Japan's technology under a national project to ensure the energy security in Japan. The good news was reported to INFCE, which was founded by the proposal of U.S. President James Carter (who took office in 1977) to support both the peaceful utilization and non-proliferation of nuclear power. By referring to the thesis on ATR project, experts in the U.S. favorably evaluated Fugen as "a mirror of Japan's nuclear policy," which was based on peaceful utilization of nuclear power and plutonium recycling by Japan's self-development. ATR, which was initially born in the concept of utilizing natural uranium, shifted its role in the plutonium utilization strategy according to the energy circumstances with ATR's excellent core flexibility. With regard to the time of Fugen's first criticality, ATR played a role as "plutonium burner."

Since the first criticality, Fugen has been operating safely with MOX fuel for more than 25 years, demonstrating the technical feasibility of ATR as the first Japanese-made nuclear power plant by completing the nuclear fuel cycle. At the same time, Fugen has been playing the leading role in the nuclear fuel cycle and established the basis of plutonium utilization both in technological and political sense. Fugen has diametrically embodied Japan's nuclear policy. The achievements of Fugen's plutonium utilization contributed to the deep understanding of the peaceful utilization of plutonium in Japan, both nationally and internationally.

While the construction of the ATR demonstration plant delayed, plutonium utilization programme of LWRs, one of the major option for the plutonium utilization, had advanced. The prospect that LWR could substitute for ATR in the utilization of plutonium led to the cancellation of the ATR project. Consequently, the operation of Fugen is scheduled to be terminated in the end of March 2003, and the ATR project is also to be terminated in the end of September 2003. However, the visible and invisible achievements of Fugen are expected to be put to full use in the future for plutonium utilization in LWR, full MOX LWR, as well as FBR, which will play central role in the nuclear fuel cycle in Japan.