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Nondestructive Waste Assay Using Combined Thermal Epithermal Neutron Interrogation

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AR #1104

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Tech ID: 1568
Project Overview
 Improved transuranic (TRU) and low-level waste assay accuracy was needed to meet increasingly stringent shipping and disposal regulations. Conventional waste assay methods can give rise to very large errors, an order of magnitude or more, depending on the waste form and TRU isotope distribution. To improve assay accuracy, work under this activity focused on developing an instrument to assay problematic wastes in 55- and 83-gal drums. The combined thermal/epithermal neutron (CTEN) interrogation device incorporates several new developments that provide matrix density/distribution, fissile material composition/distribution information, high uncorrelated or singles neutrons corrections, and self-shielding corrections in many types of wastes. The CTEN should therefore be able to reduce the uncertainty in the measurements on waste drums containing lumps and heterogeneous wastes.
Technology Description
The widely used neutron system, the differential-dieaway technique (DDT) method, uses thermal neutrons to `interrogate' fissile isotopes in waste drums. The resulting (induced) fast fission neutrons are detected in cadmium-shielded 3He detectors that are insensitive to the interrogating thermal flux. The CTEN method is similar to the DDT method, but interrogates the sample with both thermal and epithermal neutrons. This is achieved partly by the addition of 4He detectors which have a faster response than 3He and can detect fast fission neutrons in the presence of the epithermal interrogating flux, and a by a re-design of the moderating cavity so that thermalization occurs more slowly. Because epithermal neutrons are more penetrating in fissile material than thermal neutrons, the differential response can be analyzed to detect the occurrence of self-shielding by fissile material and measure the size of the effect. Self-shielding occurs when discrete lumps of fissile material are present, and can result in assay errors of several hundred percent. A fully operational CTEN device will perform all the functions of existing DDT/PAN devices with the added capability of being able to identify and assay lumps of fissile material. The CTEN also includes features not available in the current generation of DDT devices: a neutron multiplicity measurement capability, increased active neutron detection efficiency, list-mode processing to address high single neutron count rates, and a capability to detect non-uniform matrices and SNM distributions.

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