Reconstructing nuclear histories with archives – hands-on lessons learned

Reconstructing how much fissile material was produced in nuclear facilities - nuclear archaeology - could become a key element in the verification of future arms control or disarmament agreements. The past production of plutonium can be modelled with reactor simulations, using information on both reactor design and operating history. That information would typically be provided by the inspected state and must be independently verified. In a first step, the available documentation of the reactor program can be thoroughly examined, for instance by studying its self-consistency. In a second step, forensic measurements, e.g., of samples from inside the reactor core, can be compared to data simulated according to the documentation. This paper addresses the first step, in particular how different types of information from the documentation can be assessed in an integrated manner to build confidence in a declared production history.  We explore a real-world scenario with the former nuclear research program from Karlsruhe, Germany, for which we gained access to the archives. The nuclear research program included a pilot reprocessing plant and two heavy water reactors, the FR-II and the MZFR. While the program was used for civilian purposes only, the fact that the reactors were moderated by heavy water makes them ideal candidates for this study, as this type of reactor is elsewhere used to produce plutonium. In our work, we examine the potential of consistency-checks between time-series data, describing the operational history, and documented activation measurement data taken during the decommissioning process. This paper updates our previous contribution to INMM from 2023 and sheds light on opportunities and obstacles of document-based nuclear archaeology.