Examining 239Pu and 240Pu Nuclear Resonance Fluorescence Measurements on Spent Fuel for Nuclear Safeguards

In nuclear resonance ?uorescence (NRF) measurements, resonances are excited by an external photon beam leading to the emission of gamma rays with speci?c energies that are characteristic of the emitting isotope. NRF promises the unique capability of directly quantifying the concentration of a speci?c isotope in spent fuel without the need for unfolding the combined responses of several ?ssile isotopes as is required many in other measurement techniques. Despite this promise, the implementation of a practical measurement system is quite di?cult because of limitations in photon source and gamma-ray detector technologies. These limitations, combined with the low concentration in 239 Pu in spent fuel and its relatively weak NRF resonances would make a direct measurement of 239 Pu content using available technology very time-consuming. The e?ects of improved photon source technology as well as alternative detector types will be discussed in terms of the gain in statistical precision that can be achieved for a given measurement duration. Because of the di?culty in measuring 239 Pu via NRF, the strengths and energies of 240 Pu resonance states have recently been experimentally measured. As expected, signi?cantly stronger resonances were observed at excitation energies similar to those of 239 Pu. The impact of these stronger resonances on nuclear safeguards measurements will be discussed using a combination of MCNPX simulations that have been experimentally checked, and analytical model-based predictions. The modeling suggests that 240 Pu will generally be less di?cult to quantify via NRF than 239 Pu because of these larger cross sections, despite generally lower concentration in spent fuel. Therefore the time-scales, photon source and detector systems requirements for high-accuracy NRF measurements of 240 Pu in spent fuel will also be examined.