ONIX: An Open-source Burnup Code for Nuclear Archaeology

Abstract

All nuclear-armed states use plutonium in their weapon designs and have acquired stockpiles of military plutonium. Verifying existing stockpiles and their elimination is a key challenge for nuclear arms control and disarmament efforts. With nuclear archaeology, it is possible to independently reconstruct plutonium production histories using isotope ratio measurements during inspections combined with detailed reactor simulations. However, most of these simulation tools are proprietary or export-controlled. As a consequence, they lack transparency and not all parties engaged in processes involving plutonium production estimates might gain access to the same tools. Furthermore, nuclear archaeology is unable to provide complete estimates of plutonium production in reactors that also produced tritium, another isotope used in nuclear weapons and produced by nuclear-armed states.

This thesis introduces ONIX, the first open-source nuclear reactor simulation software that can be used to estimate plutonium production with nuclear archaeology. ONIX is a nuclear burnup software that computes the changes in the isotopic composition of different materials in a nuclear reactor. ONIX provides a coupling interface for the open-source neutron transport code, OpenMC and a module to solve the burnup equation. The software is also equipped with a module that automates methods of nuclear archaeology. The code is validated using results from two different bench-marks, and its results agree with other code packages within expected error ranges. In the context of nuclear arms control and disarmament processes, ONIX can provide plutonium production estimates in a fully transparent way that enhances mutual trust among parties.

To estimate plutonium production in reactors that have also produced tritium, a new method called production mode verification is introduced. It relies on the fact that the production of plutonium leads to a neutron spectrum in the reactor that is different from the spectrum that characterizes tritium production. Specific isotope ratios can be measured to detect such differences in neutron spectra. This work demonstrates the general feasibility of the method for heavy-water reactors using detailed reactor simulations. This method could be indispensable to estimate plutonium production in nuclear-armed states since they all produced tritium in nuclear reactors for weapons development.