Reconstructing Nuclear Fuel Cycle Operations
with Nuclear Archaeology

A key challenge for future nuclear disarmament treaties lies in verifying the completeness of fissile material baseline declarations. One approach for this is nuclear archaeology, which aims at reconstructing the past fissile material production of a country. It is a set of methods to infer operational production histories of nuclear facilities, typically combining forensic measurement data with simulation models of the examined facilities. Hence, nuclear archaeology methods usually apply to the facility level so far. They do not take into account fuel cycle-level information that may also be contained in declarations, such as material flows between facilities. To provide a platform for such fuel cycle analyses, we develop Bicyclus, an open-source Python3 module that couples nuclear fuel cycle simulations with an inference framework. The user models a fuel cycle in Cyclus, an open-source simulator, and inputs measurement data and rough estimates of the operational parameters to be reconstructed. Examples of parameters are the capacity factor of a reactor or the efficiency of a reprocessing facility. Following this, the software reconstructs those parameters using Bayesian inference and Markov Chain Monte Carlo algorithms, and yields estimates of the produced fissile material. Furthermore, Bicyclus can be used in a measurement-independent mode. Here, the user specifies ranges of values for uncertain operational parameters. Then, a Quasi-Monte Carlo method is used to efficiently sample this parameter space and to obtain aggregated fissile material estimates and uncertainties. We showcase our approach with a hypothetical nuclear fuel cycle for military purposes. First, we modelled the fuel cycle in Cyclus and generated synthetic measurements of the high-level reprocessing waste and the depleted uranium. Using these measurements in Bicyclus, we reconstructed both operational parameters key to the plutonium and HEU production as well as the overall fissile material production. Last, we performed a measurement-independent estimate of the produced fissile material.