Temporal Gamma-Ray Spectrometry to Quantify Relative Fissile Material Content

As the number of safeguarded nuclear sites increases across the world, and the amount of safeguarded nuclear material increases, the capability of nuclear materials control and accountably by domestic and international bodies must advance as well. This research introduces a new method to quantify relative fissile material content for future applications in international safeguard efforts, and then successfully demonstrates its effectiveness using empirical fissile material data. Previous non-destructive assay techniques combine empirical measurements with nuclear data to estimate the relative fissile material quantity, which inherently injects additional uncertainty into the measurement; the temporal gamma-ray spectrometry method presented in this research does not require any nuclear data for an estimate, instead relying solely upon empirically measured fission data. Previous methods also use a ratio of responses from different energy peaks to fission- normalize the measurements, which potentially adds systematic bias to the measurements due to changing detector efficiency at different energies. The temporal gamma-ray spectrometry method in this research fission-normalizes the measurements by using the response of the peak at a reference time, thereby eliminating a potential source of measurement bias. Current delayed gamma-ray spectrometry methods under development are generally accurate and precise to within 10% for spent nuclear fuel assembly models. Idealized modeling of delayed gamma-ray spectrometry by previous researchers resulted in estimates accurate and precise within 3%. The presented temporal gamma-ray spectrometry method has been shown in empirical studies on low-mass targets to be accurate and precise within 2% -- comparable performance in empirical studies using non-idealized irradiation and counting protocols. This approach has the potential to be expanded to provide an additional measurement technique in the non-destructive assay of spent nuclear fuel assemblies, potentially increasing the precision, accuracy, and confidence in the estimates from systems in development. The method does not require any additional hardware than the methods being considered by the NGSI effort, and uses the same data as other delayed gamma-ray spectrometry techniques.