Year
2007
Abstract
A new passive neutron assay system has been designed for in-situ characterization of buried transuranic (TRU) waste. The system uses an array of high efficiency 3He neutron detectors embedded in a polyethylene slab. In its standard mode of operation, the system will be deployed at a dig face prior to excavation as a means of controlling Material At Risk (MAR). Total neutron counting is used to quantify the plutonium content of the excavation zone. This technique offers considerable advantages over the traditional environmental survey methods that are based on detection of low energy gamma-rays and x-rays. Such techniques work well for characterization of surface-embedded particulates such as fall-out contamination. However, the short penetration depth of the photon radiation limits the investigation to the upper few centimeters of soil. Gamma based surveys can therefore only provide 2-dimensional information on the TRU contamination. Such an approach is often undesirable for historical waste burial sites where the distribution of spatial isotopes is highly heterogeneous. By detecting the far more penetrating neutrons emitted from (alpha,n) interactions and spontaneous fission, the total neutron counting method can provide an appropriate solution to this 3-dimensional problem. Monte Carlo N-Particle (MCNP) modeling has been performed to determine the optimum geometrical arrangement of polyethylene shielding / moderating materials and assess the performance limits of the neutron assay system. Various types of soils were modeled with a wide range of density and moisture content. This work has demonstrated that the counter is capable of accurately quantifying plutonium over an excavation zone of 3 m x 3 m and up to 1 meter below the surface. The typical lower limit of detection is less than 1 gram of Weapons Grade Pu in a 15 minute count.