The SSCR reactor is a pressurized water reactor that controlled with spectral shift control, by using variable concentrations of heavy water / light water to control reactivity. In the presence of heavy water in the reactor coolant, the neutron population moves to higher energy levels and remains longer at resonant energies, so that excess neutrons are mainly absorbed in the fertile material, rather than in the control poison. This means that during an SSCR operation, more fissile material is produced and burned on-site than in a conventional PWR.