Checking of uranium enrichment in fuel pins

Before guaranteeing that fuel elements will behave satisfactorily one must check the homogeneity of 235U enrichment of the uranium in each pin. In order to perform such checks, the authors chose the method based on the thermal and epithermal neutron activation of 235U with delayed gamma counting. An automatic system was developed during the first half of 1975. The pin, moving at a constant speed, passes through an irradiation unit and then a measurement set-up. The former contains two sources, each with 350μg of 252Cf. The measurement set-up consists basically of an annular scintillator and three photomultipliers. The signals from the photomultipliers are treated by an electronic unit associated with a computer. Counting is carried out over regular time intervals which can be varied from 0.1s to 9.9s. The pin speed can be varied from 0.5m/min to 6m/min. The count rate is higher than 300000 counts/s. Under these conditions, the fissionable material content of a pin is known with a very high degree of precision and a pellet whose enrichment differs by more than 5% (relative) can be distinguished. This checking procedure is reliable and fast, and the first system is in operation at Saclay, where it is used for checking the fissionable material content and the enrichment of LWR fuel pins. (author)