New Design of the Differential Die-away Self-interrogation Instrument for Spent Fuel Assay

The Differential Die-away Self-Interrogation (DDSI) techniqueis being studied by the Next Generation Safeguard Initiative(NGSI) for the purposes of determining fissile mass, and eventuallyPu mass, in spent fuel assemblies. The technique utilizesambient neutrons primarily from the spontaneous fission of244Cm to interrogate the fissile materials (primarily 235U, 239Pu,and 241Pu) in the assembly. The time separation of neutron detectionevents from spontaneous fission (early gate) and inducedfission (late gate) enables the independent measurement of fertileand fissile masses in spent fuel. In-field testing of the DDSItechnique with prototype hardware and commercial spent fuelassemblies will occur in 2014. The original NGSI DDSI designwas annular and surrounded the fuel assembly on all four sidesand required the operator to insert the assembly from above. Anew side-entry design is being implemented because it is expectedto be more acceptable to facilities and regulators responsible forspent fuel management and it is not expected to deteriorate theperformance of the instrument significantly. The DDSI in thiscontext has three banks of 3He detectors surrounding the fuelassembly on only three sides. The efficiency and die-away timeof this updated design