2.1. Thorium Based Fuels If introduced into the LWR fuel cycle, thorium as a primary fertile material in the core has a good potential for reducing the TRU generation simply due to its lower than uranium position in the periodic table. Longer neutron capture and decay chains have to take place in thorium to result in Pu or MA isotopes. However, significant quantities of U233 and long lived radioactive Pa231 generated in a thorium fuel mitigate this advantage. U233 is a valuable fissile isotope that, if recycled, can improve fuel utilization and due to its continuous generation from the thorium can increase fuel burnup and reduce core reactivity swing during a fuel cycle. At the same time, U233 is a weapons usable material with relatively low spontaneous neutron source. Thus, its accumulation reduces attractiveness of the thorium based fuel cycle from the proliferation resistance standpoint. Addition of natural or depleted uranium to the fuel for the sake of limiting U233 concentration in uranium below the weapons usability limit will result in increased TRU generation. Although Th232 has a lower resonance integral than U238 its response to the fuel temperature increase (Doppler broadening) is greater, which results in a more negative fuel temperature reactivity coefficient than that of the uranium fuel – a desirable feature in case of transients with rapid reactivity increase. At the same time, the smaller delayed neutron fraction of U233 than of U235 has a negative effect on reactivity initiated accidents. This is particularly important for the Th-TRU mixtures where βeff is considerably smaller than that of the uranium fuel all through the in-core residence time. In the fuel performance area, thoria is found slightly more advantageous than UO2. ThO2 thermal conductivity is slightly higher than that of UO2 at comparable temperatures in addition to about 500 °C higher melting point [Oggianu S.M. et al.,] (Table 2.1.I). In ThO2 compound, thorium has its highest possible and the only oxidation state +4, which makes it very stable and almost chemically inert [Belle J. et al., 1984]. Therefore, thoria is expected to be a very durable waste form for the spent fuel. If the fuel recycling strategies are considered, however, reprocessing of the thoria fuel is somewhat complicated for the same reason and therefore it is expected to be more expensive than reprocessing of UO2 fuel. The presence of U232 isotope inevitably generated in Th fuel primarily as a result of (n,2n) reaction in U233 may further complicate reprocessing and add to its cost. U232 decay chain products emit highly
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