Reliable prediction of the characteristics of irradiated light water reactor fuels is needed for many aspects of the reactor operation and for the nuclear fuel cycle.

Modern fuel assemblies are both heterogeneous in the distribution of fuel rods with different enrichments, in the application of gadolinium rods and in the coolant density conditions for neutron moderation. Therefore, the 3D programme system KENOREST was developed for reactivity and full inventory calculations of LWR fuel, both with square and hexagonal assembly grids. In KENOREST, the 3D Monte Carlo code KENO and the 1D GRS burn-up programme system OREST comprising the 1D spectrum code HAMMER and 0D full inventory depletion code ORIGEN are directly coupled to the 3D reactivity and inventory calculation system.

The objective is to achieve a better modelling of plutonium and actinide build-up or burnout for advanced heterogeneous fuel assembly designs. Further objectives are directed to reliable calculations of the reactivity behavior during burn-up, the pin power distributions and the reactor safety parameters for such fuel assemblies.
In the most recent version of KENOREST, a multiregion model of the pellet was developed for a physically more detailed description of the radial burnout in the fuel rod. An automated convergence control for multiplication factors and pin power distributions is implemented. Updated neutronic ORIGEN libraries based on point data evaluations and additional neutronic reactions solve the Tritium, the Carbon-14 and other problems in the LWR inventory calculations.