Design and application of MTR fuel assemblies in new proposed inverted mode

MTR fuel is one of the most used fuel types in research reactors. Loading high positive excess reactivity enables a research reactor to be in service for an extended period of time without refueling. This in turn ends up to asymmetric burn up distribution in axial direction. Minor modifications in present standard fuel assembly design allow use of each fuel assembly in dual directions along vertical axis. Thus, a smoother axial burn up distribution can be achieved. Detailed calculations showed a real improvement in neutron economy and extra positive reactivity is gained.