Non-destructive analysis of nuclear fuel by means of thermal and cold neutrons

The article describes the experimental procedures, detection methods and resulting information when thermal or cold neutrons are used for non-destructive inspections and quantitative analysis of fuel elements from nuclear power plants. Due to the high penetration capability of slow neutrons through natural uranium and different attenuation properties for the individual uranium isotopes, a high sensitivity in respect to the enrichment in U-235 exists. Both the structural integrity and the material distribution inside the pellets can be determined by neutron imaging methods. This holds for fresh fuel before insertion into the reactor core but also for spent fuel after reloading and storage. In the latter case, more demanding effort with respect to shielding is needed and adapted detection methods have to be applied because of the high radiation background from the fuel itself. The facility NEUTRA at the spallation source SINQ is able to manage the high quality neutron imaging procedures with fuel, the special set-up NEURAP allows it to do such measurements with pellets of high burn-up too