Comparing DGSMC, FIER, and FISPACT Simulations to Experimental Delayed Gamma-Ray Spectra for Nuclear Safeguards Development

Present safeguards practices of irradiated nuclear fuel in reprocessing plants utilize destructive analysis methods to measure the composition from small samples. As a faster alternative, the Japan Atomic Energy Agency/Integrated Support Center for Nuclear Nonproliferation and Nuclear Security (JAEA/ISCN) is developing the delayed gamma-ray spectroscopy (DGS) nondestructive assay technique. DGS is especially promising for evaluating the fissile nuclide composition using the relative contributions to the gamma-ray spectrum above ~2700 keV from short-lived fission products. The JAEA/ISCN is developing an inverse Monte Carlo (IMC) verification method that analyzes differences between the measured spectrum and expected spectrum produced from Monte Carlo simulations of the instrument and process. This work evaluates the viability of the JAEA/ISCN DGS Monte Carlo (DGSMC) as for the IMC method by comparing simulated spectra to experimental spectra from multiple interrogation timing patterns and fissile nuclides, as well as FIER and FISPACT calculations under the same conditions. All codes show < 10% average spectral biases compared to experimental short-irradiation timing pattern spectra from 235U, increasing to an inconsistent spread of 8%–53% for long-irradiation timing patterns. The 239Pu comparisons are limited by the experimental spectra, but also show an inconsistent range of differences of 3%–74%, with DGSMC matching best on average. Preliminary, first-order approximations of 235U: 239Pu composition evaluation can reach ~15% using all matching gamma rays for any code. This can be reduced to ~5% by using those simulated peaks within 25% of the experimental spectra, but this greatly limits the number of peaks that can be used. From this, we conclude that DGSMC is a viable simulation code though improvements to the nuclear data fission yields must be performed to better match the experimental spectra and DGSMC can be improved for efficiency.