A SECOND LOOK AT NEUTRON RESONANCE TRANSMISSION ANALYSIS AS A SPENT FUEL NDA TECHNIQUE

摘要

Neutron Resonance Transmission Analysis (NRTA) is measurement technique capable of quantifying plutonium in spent fuel. Having first been explored in the mid-1970s for the analysis of individual spent-fuel pins, a second look is now underway to investigate the suitability of the NRTA technique for assaying complete spent nuclear fuel assemblies using advanced simulation and modeling methods. The technique is similar to neutron time-of-flight methods used for neutron cross-section measurements but operates over only the narrow 0.1-20 eV range where strong, distinguishable resonances exist for both the plutonium (~(239,240,241,242)Pu) and uranium (~(235,236,238)U) isotopes of interest in spent fuel. Additionally, in this energy range resonances exists for six important fission products (~(99)Tc, ~(103)Rh, ~(131)Xe, ~(133)Cs, ~(145)Nd, and ~(152)Sm) which provide additional information to support spent fuel plutonium assay determinations. Initial modeling shows excellent agreement with previously published experimental data for measurements of individual spent-fuel pins where plutonium assays were demonstrated to have a precision of 2-4%. Within the simulation and modeling analyses of this project scoping studies have explored fourteen different aspects of the technique including the neutron source, drift tube configurations, and gross neutron transmission as well as the impacts of fuel burn up, cooling time, and fission-product interferences. These results show that NRTA may be a very capable experimental technique for spent-fuel assay measurements. The results suggest sufficient transmission strength and signal differentiability is possible for assays through up to 8 pins. For an 8-pin assay (looking at an assembly diagonally), 64% of the pins in a typical 17×17 array of a pressurized water reactor fuel assembly can be part of a complete transmission assay measurement with high precision. Analysis of rows with up to 12 pins may also be feasible but with diminished precision. Preliminary analysis of an NRTA simulation has demonstrated the simplicity of the technique. This work is part of a larger effort sponsored by the Next Generation Safeguards Initiative to develop an integrated instrument, comprised of individual nondestructive assay techniques with complementary features, that is fully capable of determining Pu mass in spent fuel assemblies.