Fugen was a domestic Japanese design for a demonstration Advanced Thermal Reactor. With thorium, it is possible to breed using a thermal reactor. The organization specialized in special Breeder reactors and the Advanced Thermal Reactor. In thorium fuel cycle thorium-232 absorbs a neutron in either a fast or thermal reactor. Thus, even mass isotopes tend to accumulate, especially in a thermal reactor. A thermal reactor is a nuclear reactor that uses slow or thermal neutrons. This variant featured a cast-iron thermal reactor to reduce exhaust emissions and re-tuned exhaust ports.