Direct Determination of the Total Fissile Content in Irradiated Fuel Elements, Water Containers and Other Samples of the Nuclear Fuel Cycle

A non-destructive assay system for the direct determination of fissile material is presented which is not influenced by the presence of high level fission product radioactivity (_ 100 Ci). In order to explain the basic principles of this system, some neutron transport properties in hydrogeneous materials are discussed which lead to the specific attenuation of the source neutrons. The source neutron contribution is equivalent to 5 mg U-235 in the small sample system and 60 mg U-235 in the medium-size system. estimated to be 1 g U-235 in a conceptual NDA design for radioactive waste barrels. Some typical aspects from the measurement of medium-size waste boxes are reported. The system was originally developed for the assay of irradiated HTR pebble bed fuel materials. It was calibrated with U-235 and is well applicable to uranium from the thorium cycle too. In order to estimate the applicability to the U-Pu cycle, the spontaneous neutron emission of irradiated fuel-elements is compared with the neutron emission of fissile material in the source neutron field. According to the calculations, the system appears to be applicable to the U-Pu cycle and the medium-size asay system may be directly suited for the measurement of irradiated fuel rods. In general by the use of this type of NDA system, the solid radioactive material inputs and outputs of a reprocessing plant appear to be accessible to direct and fast NDA control. This could improve the overall accuracy of material balance and strengthen the confidence in an improved containment/ surveillance control system at such facilities. It is