Assessing a Comprehensive Material Control and Accounting Approach for
Safeguarding Pebble-fueled Reactors

Pebble-fueled reactors (PFRs) are a category of nuclear reactors that will require accurate nuclear material accounting and control (NMAC) measures to implement effective international safeguards. Current concepts for NMAC measures place heavy emphasis on measuring burnup of individual fuel pebbles to help discern pebble characteristics such as fissile material quantities. Well-characterized data from a burnup measurement system (BUMS) could help correlate to masses of fissile material via burnup analysis codes but this correlation may not be precise due to pebbles experiencing a wide range of neutron fluxes and irradiation times while traversing the reactor core via different paths. This variability in pebble trajectory and duration could lead to two pebbles exhibiting similar fuel burnup profiles yet having different fissile material content within. The work presented herein focuses on the justification and development of a complementary nuclear material control (NMC) approach that could function in concert with a burnup measurement system (BUMS) to effectively implement a combined NMAC approach for the eventual application of international safeguards. This study first analyzed variations in fuel pebble burnup profiles for two types of PFR designs to better understand the variability in fuel transmutation which is based on pebble trajectories through the reactor core. Second, this study is evaluating and experimentally verifying the applicability of candidate NMC technologies as a complementary measure to pebble burnup measurements. The NMC approach that is discussed herein accounts for the limitations of individual pebble identification and considers pebbles to be classified by types and accounted for in such a manner using extrinsic and/or intrinsic features to each pebble. An NMAC system is then discussed for eventual incorporation in future safeguarded PFRs but further work is needed for full private industry implementation.