Figure 24. PNAR-FC cross-sections. A horizontal slice is shown on the left and a vertical slice is shown on the right. The two diagrams are not on the same scale. 

PNAR uses reflected neutrons from a spent nuclear fuel assembly to induce fission and characterize total fissile content.  The PNAR system surrounds a fuel assembly on all sides with polyethylene to reflect emitted spent nuclear fuel neutrons.  Embedded in the polyethylene are four fission chambers (1 on each side).  PNAR evaluates the ratio of two neutron counts.  This first count is with the system as just described.  The second measurement is recorded with a cadmium layer inserted between the fuel assembly and the polyethylene.  Total fissile material content is characterized as a weighted average of the mass of U-235, Pu-239, and Pu-241.  Figure 24 illustrates horizontal and vertical cross-sectional views of the PNAR system.  PNAR has also been studied with He-3 neutron detectors instead of fission chambers.

Source: J.J. Gerhart, C. Freeman, J.L. Conlin, H.O. Menlove, and S.J. Tobin, "Passive Neutron Albedo Reactivity with Fission Chambers," Journal of Nuclear Materials Management   40:3 (2012).