Advantages of the Known-M Method Application for Measurements of Plutonium

There are plenty of nondestructive assay (NDA) techniques to measure plutonium-bearing materials. The Known-Multiplication (Known-M) method is an extension of conventional neutron coincidence counting which allows an appreciably decreased number of reference materials (RM) required for adequate calibration of a measurement instrument. One of the potential advantages of the method is a capability to measure impure plutonium items using conventional coincidence counters. The method exploits a predetermined dependence of neutron leakage multiplication on Pu-239 effective mass in order to solve point model equations. The key point of the method is to define this dependence - function: M = f(m239). The paper represents results of Known-M method testing on different Pu-bearing material measurements. Here the mentioned function has been determined using MCNP calculations for cylindrical models. The approach accounts for geometry, chemical form and isotopic composition of an item to be assayed. The Known-M measurements give accurate results comparable to those of multiplicity counting technique for remarkably less time especially for neutron counters with low detection efficiency. The paper considers also influence of moisture, and presence of neutron scattering and absorbing materials. The authors scrutinize different aspects of the Known-M method application for confirmatory measurements of various plutonium materials at Russian nuclear facilities.