Criticality safety analyses are performed at WTI to ensure that the nuclear chain reaction, which is desired in a nuclear reactor for energy production under controlled conditions, can be safely excluded for fissile material outside a nuclear reactor. These analyses are performed using 3D Monte Carlo programs, such as SCALE/KENO and MCNP®, primarily for the storage and transport of spent fuel assemblies in CASTOR® casks. They also include the assessment of potential changes to casks and fuel assemblies under incident or accident conditions. Additionally, analyses are performed for the final disposal of fuel assemblies in specially developed disposal containers. Unlike the safe heat dissipation or shielding of ionizing radiation, the effectiveness of the technical and structural measures intended to ensure subcriticality cannot be verified by measurements. Therefore, the analyses and the calculation methods and programs used are subject to particularly high requirements. WTI participates in the advancement of the state of science and technology in this area by contributing to national and international expert groups, attending conferences and participating in benchmark projects.