Fissile materials dilution

Besides fission products, the various forms of known but newly formed elements in the spent nuclear fuel, there is a small but significant amount of fissionable, or fissile, material in the SNF. This is quite important. There is some unused, unfissioned U-235 that has become too dilute to use. Like natural uranium ores in which chain reactions do not... 

Contains about 25 wt% of U-238 to form the stable pyrochlore phase. The U-238 will also dilute U-235 (a fissile material which is a decay product from Pu-239) in the future to minimize the possibility of a criticality ... 

A tabulation of minimum critical values, including mass, volume, dimension, concentration, and enrichment, starts the second lecture, which continues with the Influence of density and dilution Of fissile material. The balance of toe session is devoted to interaction between subcrltical units, the criticality of arrays, array moderar tion and reflection effects, and the effective isolation of units by neutron-absorbing materials. 

An HTGR fuel element is a composite structure. Coated microspheres of enriched uranium are bonded together with fertile microspheres in a carbonaceous matrix to form rods. These rods are then inserted into hexagonal graphite blocks to form the finished clement. Both the rods and elements contain fissile material in dilute form and present no special criticality problems. Safety limits can be met by maintaining adequate spacing between units and the fabrication equipment requires no special design features to achieve the desired production rates. 

An overview of the uranium and plutonium certified reference materials that are used for safeguards and fissile material control for bulk and particle analysis was presented at a conference organized by the European Joint Research Centre (JRC) (Jakopic et al. 2012). There are three main types of CRMs single isotope standards for isotope dilution measur ents CRMs for isotope ratio calibration and mass discrimination correction and reference materials for age dating. As far as uranium is concerned, there are (almost pure) mono-isotopic standards and... 

---