We have developed an instrument to measure neutron multiplication by statistical analysis of the timing of neutrons emitted from fissionable material. This instrument is capable of repeated analysis of the same recorded data with selected algorithms, graphical displays showing statistical properties of the data, and preservation of raw data on disk for future comparisons. In our measurements we have made a comparison of the covariance to mean and Feynman variance to mean analysis algorithms to show that the covariance avoids a bias term and measures directly the effect due to the presence of neutron chains. A spherical assembly of enriched uranium shells and acrylic resin reflector/moderator components used for the measurements is described. Preliminary experimental results of the Feynman variance to mean measurements show the expected correlation with assembly multiplication.

Total Measurement Uncertainty (TMU) in neutron assays of transuranic waste (TRU) are comprised of several components including counting statistics, matrix and source distribution, calibration inaccuracy, background effects, and neutron multiplication error. While a minor component for low plutonium masses, neutron multiplication error is often the major contributor to the TMU for items containing more than 140 g of weapons grade plutonium. Neutron multiplication arises when neutrons from spontaneous fission and other nuclear events induce fissions in other fissile isotopes in the waste, thereby multiplying the overall coincidence neutron response in passive neutron measurements. Since passive neutron counters cannot differentiate between spontaneous and induced fission neutrons, multiplication can lead to positive bias in the measurements. Although neutron multiplication can only result in a positive bias, it has, for the purpose of mathematical simplicity, generally been treated as an error that can lead to either a positive or negative result in the TMU. While the factors that contribute to neutron multiplication include the total mass of fissile nuclides, the presence of moderating material in the matrix, the concentration and geometry of the fissile sources, and other factors; measurement uncertainty is generally determined as a function of the fissile mass in most TMU software calculations because this is the only quantity determined by the passive neutron measurement. Neutron multiplication error has a particularly pernicious consequence for TRU waste analysis because the measured Fissile Gram Equivalent (FGE) plus twice the TMU error must be less than 200 for TRU waste packaged in 55-gal drums and less than 325 for boxed waste. For this reason, large errors due to neutron multiplication can lead to increased rejections of TRU waste containers. This report will attempt to better define the error term due to neutron multiplication and arrive at values that are more realistic and accurate. To do so, measurements of standards and waste drums were performed with High Efficiency Neutron Counters (HENC) located at Los Alamos National Laboratory (LANL). The data were analyzed for multiplication effects and new estimates of the multiplication error were computed. A concluding section will present alternatives for reducing the number of rejections of TRU waste containers due to neutron multiplication error.