Radionuclides in fresh nuclear fuels

Uranium as the main starting material for nuclear energy production is a naturally radioactive element, composed of the three long-lived isotopes U, and In undisturbed natural deposits, these isotopes appear in a secular decay equilibrium with their daughter products of the 4n+2 and 4n+3 series, which are presented in a simplified version in Figs. 3.1. and 3.2. 

The isotope enrichment process does not only lead to an increase in the concentration in the fuel, but also in a more than proportional increase in the concentration, as can be seen from Table 3.1., where the mass concentrations of the actinide isotopes in different types of fresh nuclear fuel are compared. As a 

The isotopic composition of the plutonium fraction of mixed-oxide fuels strongly depends on the production history (previous fuel enrichment and irradiation) of the individual plutonium batch supplied from the reprocessing plant. For 

Up to now, reprocessed uranium has only been used for the fabrication of a limited number of test fuel assemblies. For this type of fuel, the uranium fraction from the spent fuel reprocessing process is again subjected to an isotope enrichment procedure to obtain a content which is sufficiently high for reactor operation (3.8% in the example shown in Tables 3.1 and 3.2.). Besides the naturally occurring isotopes and this material contains mainly gener- 

Likewise, the enhanced a activity of reprocessed uranium (greater by a factor of 4 to 10 than that of natural uranium) has to be considered in fuel manufacturing. The resulting complications can, in principle, be reduced by optimized fabrication logistics, so that the time elapse between isotope enrichment and insertion of the fuel into the reactor would amount to less than half a year. 

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