Determination of the 235U content in uranium oxide particles by fission track analysis

One of the most important tasks of nuclear safeguards is to control the activities in nuclear facilities worldwide. These activities include the enrichment of 235U. To identify enrichments above the maximum authorised level in commercial enrichment plants (usually 5% 235U) swipe tests are performed and the 235U enrichment is determined in the sampled uranium oxide particles. A method is described that allows to measure the 235U content in small particles (around 1 μm in diameter) by the use of fission tracks. It is based on the fact that in uranium fuel, only 235U atoms are fissile with thermal neutrons and contribute to the amount of fission tracks counted for individual particles. Standard uranium oxide particles with a defined diameter and isotopic composition were prepared and used to calibrate the method. Results show that the detection efficiency has a non-linear dependence on the number of fission tracks per particle. The enrichment in 235U for single particles can be determined when good size measurements of the particles in a sample are available.