Principles of material accountancy are presented for a typical tritium fuel cycle under special consideration of the in-vessel inventory which for the time being can only be estimated roughly. For this purpose we pursue the selection and testing of suitable algorithms: For the process simulation we apply the stochastic version of the Karlsruhe Tritium Model. It is demonstrated that with the help of statistical methods which have been developed three years ago for the purpose of nuclear material safeguards, the measured data can be analyzed such that questions put forward by the operator as well as the licensing authority can be answered.