Methods and systems for migrating fuel assemblies in a nuclear fission reactor

A method of controlling a nuclear fission traveling wave reactor, the method comprising:

• for a nuclear fission traveling wave burnfront propagating along first and second dimensions in a nuclear fission reactor core, determining a desired shape of the nuclear fission traveling wave burnfront along the second dimension within a plurality of nuclear fission fuel subassemblies according to a selected set of dimensional constraints;
• and
• determining a migration of selected ones of the plurality of nuclear fission fuel subassemblies along the first dimension from respective first locations in the nuclear fission reactor core toward respective second locations in the nuclear fission reactor core in a manner responsive to the desired shape.