Jōyō (常陽) is a test sodium-cooled fast reactor located in Ōarai, Ibaraki, Japan, operated by the Japan Atomic Energy Agency. The name comes from the previous country name of the area around Ibaraki. It was made with the purpose of doing tests on and advancing the development of that type of reactor, as an irradiation test facility for construction materials. It also does tests with the nuclear fuel as well as activation experiments. The reactor has gone through 3 different core changes. The current core provides the neutron flux of 4×1015 cm−2s−1 for E>0.1 MeV.