Non-destructive assay techniques for irradiated fissile material in extended configurations

The paper presents a non-destructive assay (NDA) system for the direct determination of irradiated fissile material in extended waste boxes and fuel elements. It is based on active neutron interrogation with an Sb-Be neutron source and specific attenuation of the source neutrons relative to the fission neutrons before registration. Based on previous experience with a small sample system, construction and performance of this assay system are reported. The system operates in a hot cell in the presence of some 100 Ci of fission products. The count-rate, obtained from source neutrons, was finally equivalent to 60 mg 235U. This value indicates the lower detection limit of the system. One part of the system (I) is intended for small samples mainly for calibrating purposes. The samples stand there fixed during the measurement. In the other part of the system (II) the samples continuously move during the measuring turn. For waste boxes of 16.5 cm diameter and 25 cm high, the relative counting efficiency in system II is 50% compared with system I. Additional parameters were determined, which influence the measuring accuracy of the waste box. Different packing positions change the result by 5%, the addition of 500 g metal wool by 2% and the measurement of eight sub-samples as a whole by 4%. Performance is demonstrated with irradiated fuel elements of the AVR reactor at burnup values between 0 and 170,000 MW.d/t. Many types of fuel element from water reactors will fit within the dimensions of this measuring system II. (author)