Passive and active neutron signatures of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msup><mml:mi/><mml:mn>233</mml:mn></mml:msup><mml:mrow><mml:mrow><mml:mi mathvariant="normal">U</mml:mi></mml:mrow></mml:mrow></mml:math> for nondestructive assay

Abstract

The thorium fuel cycle is emerging as an attractive alternative to conventional nuclear fuel cycles, as it does not require the enrichment of uranium for long-term sustainability. The operating principle of this fuel cycle is the irradiation of ${}^{232}\mathrm{Th}$ to produce ${}^{233}\mathrm{U}$, which is fissile and sustains the fission chain reaction. ${}^{233}\mathrm{U}$ poses unique challenges for nuclear safeguards, as it is associated with a uniquely extreme $\ensuremath{\gamma}$-ray environment from ${}^{232}\mathrm{U}$ contamination, which limits the feasibility of the $\ensuremath{\gamma}$-ray-based assay, as well as more conservative accountability requirements than for ${}^{235}\mathrm{U}$ set by the International Atomic Energy Agency. Consequently, instrumentation used for safeguarding ${}^{235}\mathrm{U}$ in traditional fuel cycles may be inapplicable. It is essential that the nondestructive signatures of ${}^{233}\mathrm{U}$ be characterized so that nuclear safeguards can be applied to thorium fuel-cycle facilities as they come online. In this work, a set of ${}^{233}{\mathrm{U}}_{3}{\mathrm{O}}_{8}$ plates, containing $984\phantom{\rule{0.2em}{0ex}}{\mathrm{g}}^{233}\mathrm{U}$, was measured at the National Criticality Experiments Research Center. A high-pressure ${}^{4}\mathrm{He}$ gaseous scintillation detector, which is insensitive to $\ensuremath{\gamma}$ rays, was used to perform a passive fast neutron spectral signature measurement of ${}^{233}{\mathrm{U}}_{3}{\mathrm{O}}_{8}$, and was used in conjunction with a pulsed deuterium-tritium neutron generator to demonstrate the differential die-away signature of this material. Furthermore, an array of ${}^{3}\mathrm{He}$ detectors was used in conjunction with the same neutron generator to measure the delayed neutron time profile of ${}^{233}\mathrm{U}$, which is unique to this nuclide. These measurements provide a benchmark for future nondestructive assay instrumentation development, and demonstrate a set of key neutron signatures to be leveraged for nuclear safeguards in the thorium fuel cycle.

Topics & Concepts