Optimization of the Combined Delayed Neutron and Differential Die-Away Prompt Neutron Signal Detection for Characterization of Spent Nuclear Fuel Assemblies - 11419
2011
The Next Generation Safeguards Initiative (NGSI) of the U.S. Department of Energy (DOE) has funded multiple laboratories and universities to develop a means to accurately quantify the Plutonium (Pu) mass in spent nuclear fuel assemblies and ways to also detect potential diversion of fuel pins. Delayed Neutron (DN) counting provides a signature somewhat more sensitive to 235U than Pu while Differential Die‐Away (DDA) is complementary in that it has greater sensitivity to Pu. The two methods can, with care, be combined into a single instrument which also provides passive neutron information. Individually the techniques cannot robustly quantify the Pu content but coupled together the information content in the signatures enables Pu quantification separate to the total fissile content. The challenge of merging DN and DDA, prompt neutron (PN) signal, capabilities in the same design is the focus of this paper. Other possibilities also suggest themselves, such as a direct measurement of the reactivity (multiplication) by either the boost in signal obtained during the active interrogation itself or by the extension of the die‐away profile. In an early study, conceptual designs have been modeled using a neutron detector comprising fission chambers or 3 He proportional counters and a ~14 MeV neutron Deuterium‐Tritium (DT) generator as the interrogation source. Modeling was performed using the radiation transport code Monte Carlo N‐Particles eXtended (MCNPX). Building on this foundation, the present paper quantifies the capability of a new design using an array of 3 He detectors together with fission chambers to optimize both DN and PN detections and active characterization, respectively. This new design was created in order to minimize fission in 238 U (a nuisance DN emitter), to use a realistic neutron generator, to reduce the cost and to achieve near homogeneous spatial interrogation and detection of the DN and PN, important for detection of diversion, all within the constraints of a single practical instrument. Both DN and PN detections are active techniques using the signal from the most prominent fissile isotopes of spent nuclear fuel that respond the best to a slow neutron interrogation, 235 U, 239 Pu and 241 Pu. The performance is characterized against a library of 64 assemblies and 27 diversion scenarios at different burnup (BU), cooling‐time (CT) and initial enrichment (IE) in fresh water.
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