Fugen was a prototype Japanese nuclear test reactor. Fugen was a domestic Japanese design for a demonstration Advanced Thermal Reactor. It was a heavy water moderated, boiling light water cooled reactor. The reactor was started in 1979 and shut down in 2003. As of 2018, it is undergoing decommissioning. It is located in Myà Âjin-chà Â, in the city of Tsuruga, Fukui. The name "Fugen" is derived from Fugen Bosatsu (Samantabhadra), a Buddhist deity.
The reactor was the first in the world to use a full MOX fuel core. It had 772 assemblies, the most in the world. It has received the title of a historic landmark from the American Nuclear Society.
The design boils ordinary water like a boiling water reactor (BWR) but uses heavy water as a moderator as in a CANDU reactor. The electrical output was 165 MW and the thermal output was 557 MW.
The plant is located on a site that covers 267,694 m<sup>2</sup> (66 acres); buildings occupy 7,762 m<sup>2</sup> (1.9 acres), and it has 46,488 m<sup>2</sup> of floor space. It employed 256 workers.
During dismantling operations it was found that walls with controls did not have the necessary strength at 25 of 34 points.