The basis of most nuclear and thermal calculations at WTI is knowledge of the composition and condition of the radioactive waste. For spent fuel assemblies, the temporal development of the nuclide composition of the fuel is determined using burnup programs, such as TRITON from the SCALE program package. Based on this, the neutron and gamma source strengths can be determined for shielding evaluations, and the decay heat can be calculated for verifying safe heat dissipation. The burnup programs used at WTI are validated against radiochemically analyzed samples of spent fuel and the understanding of the methods and programs is continuously developed through participation in international benchmark experiments. Another focus of the department's work is the evaluation of the long-term behavior of fuel assemblies during extended interim storage and the assessment of potential impacts on shielding, criticality safety and activity retention. The third focus of the department's work is the planning and optimization of cask loadings. Based on the knowledge of neutron and gamma source strengths and the decay heat per fuel assemblies, the loading of available casks can be planned by comparing with the permissible values for a given fuel element inventory. WTI has planned and documented the loading of a large portion of the CASTOR® containers stored in central and on-site interim storage facilities. The methods used can also be applied to the loading of final disposal containers.