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 ar-

chaeology, it is possible to independently reconstruct plutonium production histories

using isotope ratio measurements during inspections combined with detailed reac-

tor 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 soft-

ware that can be used to estimate plutonium production with nuclear archaeology.

ONIX is a nuclear burnup software that computes the changes in the isotopic compo-

sition 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 bur-

nup 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 pro-

vide 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 iso-

tope 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 de-

tailed 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.