From the physics point of view, the main differences among reactor types arise from differences in their neutron energy spectra. The basic classification of nuclear reactors is based upon the average energy of the neutrons, which cause the bulk of the fissions in the reactor core. The neutron energy spectrum also influences fuel breeding. As was written in LWRs, the fuel temperature also influences the rate of nuclear breeding (the breeding ratio).
Instead of increasing fuel temperature, a reactor can be designed with so-called “spectral shift control”. The main idea of the spectral shift is based on the neutron spectrum shifting from the resonance energy region (with lowest p – resonance escape probability) at the beginning of the cycle to the thermal region (with the highest p – resonance escape probability) at the end of the cycle. In pressurized water reactors, chemical shim (boric acid), as well as burnable absorbers, are used to compensate for an excess of reactivity of reactor core along the fuel burnup (long-term reactivity control). From the neutronic utilization aspect, compensation by absorbing neutrons in poison is not ideal because these neutrons are lost. For better utilization of the neutrons, these neutrons can be absorbed by fertile isotopes to produce fissile nuclei (in radiative capture). These fissile nuclei would contribute to obtaining more energy from the fuel.
The spectral shift method can offset the initial excess of reactivity. There are many different ways of such regulation in the core. Spectral shift control can be performed by coolant density variation during the reactor cycle or by changing the moderator-to-fuel ratio with some mechanical equipment. Some of the current advanced reactor designs use for spectrum shift movable water displacers to change the moderator-to-fuel ratio. A decrease in reactivity caused by fuel burnup is simply compensated by the withdrawal of these movable water displacers while changing the moderator-to-fuel ratio. This makes it possible to exclude chemical shim from the operational modes completely. This method promises significant natural uranium savings (up to 50% of natural uranium).
See also: Teplov, P.; Chibiniaev, A.; Bobrov, E.; Alekseev, P. The main characteristics of the evolution project VVER-S with spectrum shift regulation. 2014.