Summary of the changes made. They include control rod changes so that inserting the rods does not produce a reverse effect, fixed neutron absorbers to inhibit unstable low-power operation, and extra protection systems. The void coefficient is reduced, although it is still positive, which is an undesirable characteristic. A side-effect of the changes is the need for higher enrichment of the fuel.

https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/appendices/rbmk-reactors.aspx