Assessment of self-interrogation safeguards Signatures for pebble bed reactor fuel

• Investigates neutron self-interrogation as a non-destructive analysis technique for safeguards of spent fuel pebbles. • Develops a numerical method equating the delayed gamma emissions from induced fissions to the key fissile actinides mass • Demonstrates feasibility of a 4π passive counting of end-of-life pebbles for materials accountability. • Achieves mass prediction precisions of < 13 % for 235 U, 238 U, 239 Pu, 241 Pu fissile isotopes. Many next generation reactors propose the use of nontraditional nuclear fuel in the form of pebbles filled with thousands of fuel particles. While these next generation pebble bed reactors have been in development for decades, recent support for nuclear energy has bolstered the process, with many reactor designs proposed for deployment in the coming decade. An issue facing pebble bed reactors is safeguarding the fuel itself. Research on the burnup measurement systems is still evolving for developing a nondestructive assay method to quantify the amount of fissile material present in a used fuel pebble, creating a challenge for international safeguards design. The study presented here investigates the potential of neutron self-interrogation of spent fuel pebbles as an innovative method to implement materials accountability in these advanced reactors. Through reactor physics and fuel burnup simulations, spent fuel pebble material compositions are found and a method is developed to equate the delayed gamma-ray emissions resulting from the fissions induced by the pebble neutrons to the mass of key fissile actinides. The feasibility of this self-interrogation method is assessed, leading to the conclusion that the method is suitable for use in a passive counting mode employing a 4 π detection geometry. As an example, the mass of 235 U, 238 U, 239 Pu and 241 Pu can be predicted at a precision of 4.1%, 0.86%, 13% and 13%, respectively, when measuring 100 end-of-life spent fuel pebbles over approximately 12 days.