Mechanical spectral shift reactor

This patent describes the method of operating pressurized-water nuclear reactor fueled by material having both fissile and fertile constituents in such manner that the neutron spectrum is hardened at the beginning of normal reactor operation and during prolonged reactor operation which results in the gradual depletion of reactor fuel. The neutron spectrum is gradually shifted toward a more moderated state to compensate for the depleted fuel. The reactor includes a pressure vessel enclosing a reactor core and having an inlet and an outlet for circulating a water coolant moderator in heat transfer relationship with the core. The core comprises fuel assemblies disposed therein for generating heat by nuclear fission. The fuel assemblies have a fuel enrichment which provides a measure of excess reactivity at the beginning of core life which is later drawn upon to lengthen core life. The reactor is provided with neutron-absorbing control rods which are vertically movable into and out of the core so that movement of the control rods into the core will substantially decrease reactivity and withdrawal of the control rods from the core will substantially increase reactivity, the control rods when inserted into the core displacing an equivalent volume of the water coolant moderator. The reactor is also provided with neutron-spectral-shift displacer rods which have a substantially lower absorptivity for neutrons than the control rods