Figure 30. View of the SINRD unit internals used in experimental measurements at LANL

SINRD relies on solely on the neutron emissions of the spent fuel without any active interrogation. SINRD uses four fission chambers.  Three of the fission chambers are covered with different filtering materials that absorb neutrons in different energy ranges.  The neutron energy spectrum from the spent fuel was quantified by subtracting the count rate in different fission chambers.  A ratio of the difference between the count rates of two fission chambers to the count rate another of another fission chamber was used to minimize systematic uncertainties.  The filtering materials of the fission chambers were specifically selected to make the fission chambers sensitive to neutron energy ranges corresponding to resonances in the neutron absorption cross sections of U-235, Pu-239, and Pu-241.  Thus, depressions in the neutron energy ranges quantified by the SINRD detector correspond to the amount of U-235, Pu-239, and Pu-241 in the spent fuel.   This allows SINRD to quantify the amount of those fissile isotopes in the fuel.   

Source: J. Hu, H.R. Trellue, S.J. Tobin, T.J. Ulrich, A.M. LaFleur, C.R. Freeman, H.O. Menlove, and M.T. Swinhoe, "The Performance of Self-interrogation Neutron Resonance Densitometry in Measuring Spent Fuel," Journal of Nuclear Materials Management 40:3 (2012).