Year
2013
Abstract
An integrated nondestructive assay (NDA) system combining active (neutron-generator) and passive neutron detection and passive gamma detection is being analyzed for estimating the amount of plutonium in a spent fuel assembly as part of the Next Generation Safeguards Initiative Spent Fuel Project. For the active part of the system, a 14 MeV DT generator emits neutrons on one side of the assembly. These neutrons travel through spectrum-tailoring material en route to the assembly. Eight independent neutron detectors are located on the three sides of the assembly where the generator is not located. Active signals are measured using the Differential Die Away (DDA), Delayed Neutron (DN) and delayed gamma (DG) techniques. Passive signals are measured using total neutron (TN) counts and both gross and spectral-resolved gamma counts (PG). To quantify how a system of several NDA techniques is expected to perform, the performance of all the relevant NDA techniques listed above were simulated as a function of various reactor conditions such as initial enrichment, burnup, cooling time, assembly shuffling pattern, reactor operating conditions (such as temperature, pressure, and the presence of burnable poisons) by simulating the NDA response for five sets of Light Water Reactor (LWR) assemblies produced as part of the NGSI Spent Fuel Project. This paper compares the performance of several exploratory model fitting options (including neural networks, adaptive regression with splines, iterative bias reduction smoothing, projection pursuit regression, and regression with quadratic terms and interaction terms) to relate data simulated with measurement and model error effects from various subsets of the NDA techniques to the total Pu mass. Isotope masses for spent fuel assemblies and expected detector responses (DRs) for several NDA techniques are simulated using MCNP, and the DRs become inputs to the fitting process. Such responses include eight signals from DDA, one from DN, one from TN, and up to seven from PG; the DG signal will be examined separately. Results are summarized using the root mean squared estimation error for plutonium mass in held-out subsets of the data for a range of model and measurement error variances. Different simulation assumptions leads to what we call different spent fuel libraries relating DRs to Pu mass. Preliminary results for training with one library and testing with another library are given.