Characterization Measurements of the Differential Die-Away Self-Interrogation

The Next Generation Safeguards Initiative (NGSI) Spent Fuel Project is developing several nondestructive assay instruments, such as Differential Die-Away Self-Interrogation (DDSI), to analyze spent nuclear fuel assemblies. The technical goals are to 1) detect replaced or missing pins; 2) determine plutonium and/or other fissile mass parameters; 3) verify initial enrichment (IE), burnup (BU), and cooling time (CT) of declarations; 4) determine heat; and 5) estimate reactivity. DDSI has been identified as a potentially viable option for fuel measurement and characterization in interim storage, a reprocessing facility, and/or long-term storage scenarios. DDSI utilizes the neutrons emitted from spontaneous fission within the measured sample to provide information about the fissile material in the sample. The difference between neutron arrival times in the detector is characteristic of the measured sample and provides information about the sample multiplication. Experiments were conducted at Los Alamos National Laboratory (LANL) for characterization of the instrument and verification and validation of MCNP simulations. A series of 252Cf sources was placed individually and together in the center of the instrument surrounded by air. The primary measurement goals were to characterize the instrument’s efficiency, dead time, and die-away time. Data were collected in list-mode and converted to Rossi-alpha distributions for further analysis. The signal was also split and sent to a shift register for comparison. Results were compared to simulated data to validate the simulations. Fresh fuel measurements with DDSI are scheduled to take place at LANL in 2015, and the measurements discussed here will support that work