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An active neutron interrogation method has been developed for the measurement of 235u content in fresh fuel assemblies. The neutron Coincidence Collar uses neutron interrogation with an AmLi neutron source and coincidence counting the induced fission reaction neutrons from the 235u. This manual describes the system components, operation, and performance characteristics. Applications of the Coincidence Collar to PWR and BWR types of reactor fuel assemblies are described. I. INTRODUCTION 235 The neutron Coincidence Collar was developed to verify the U content in reactor fuel assemblies. This instrument uses an AmLi neutron source to interrogate the entire cross section of the fuel assembly. The AmLi neutrons are 238 low enough in energy so that the primary fission rate in the U is negligible. The U enrichment of the exterior fuel rods can be measured using passive gamma-ray techniques; however, because of absorption problems the interior rods cannot be measured with this approach. An autoradiograph, with x-ray film placed between the fuel pins, has been used by the International Atomic Energy Agency (IAEA) and Brumback at Argonne National Laboratory (ANL) to give semiquantitative verification of the enrichment of the interior fuel pins. However, this approach is both time consuming and intrusive. P Several years ago, we developed an active neutron technique (the neutron poc Col la*-) to verify " 3 U content by neutron interrogation and fast neutron 4 counting using He detectors. This approach has limited sensitivity in the interior regions of pressurized water reactor (PUR) fuel assemblies because of thermal-neutron penetrability problems and geometric considerations. Recent advances with the Active Well Coincidence Counter 3 have made it possible to apply this same technical approach to the verification of full light water reactor (LWR) fuel assemblies. The method involves neutron interrogation with an AmLi neutron source and coincidence counting the induced fission reaction 235 neutrons from the U. The coincidence counting separates the fission neutrons, which originate from U, from the random neutrons used in the interrogation. This "Coincidence Collar" approach has the following advantages over the previously developed neutron "Collar". 1. The AmLi neutron source strength requirement is a factor of 10 smaller, reducing transportation and handling problems. The prior Collar uses a 5 x 10 5 n/s source and the Coincidence Collar uses a 5 x 10 4 n/s source. 2. The sensitivity to the removal of interior fuel pins in an assembly is at least a factor of two better with the Coincidence Collar because of fast neutron multiplication and coincidence counting enhancement and the source detector geometry. 235 3. In addition to verifying the U in the active interrogation mode, ?38 the Coincidence Collar can verify the U in the passive mode and 9\A give a rough confirmation of U content. 4. The Coincidence Collar uses the same electronics as the high-level neutron coincidence counter (HLNCC), so the IAEA has an inventory of t...