PWR-CANDU6 BİRLEŞİK YAKIT ÇEVRİMİ VE CANDU6’DA URANYUM + TORYUM KULLANIMI
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In this study, burnup analyses were made for PWR-CANDU6 combined fuel cycles in which PWR spent fuel is used as fuel in CANDU6. Besides, for the once-through fuel cycle, use of uranium + thorium (U+Th) in CANDU6 were looked into and compared to natural U or Slightly Enriched U (SEU) cases. The fuel cycles were compared in terms of Natural U Requirement (NUR) and Nuclear Resource Utilization (NRU). For the PWR-CANDU6 combined cycle, two primary recycling scenarios were focused on. The first scenario involves the Complete Coprocessing (CC), which is the easiest and most secure way to recover U and Pu content of spent fuel. In CC, all U and Pu in PWR spent fuel are recovered together (while in the standard reprocessing U and Pu are obtained as separate streams). Resultantly, the product of CC is a pure U+Pu mixture with a total fissile content of 1.4 to 1.5 wt%, and in order to reuse it in a PWR, it is necessary to blend it with a fissile makeup. However, a mixture of U+Pu with that fissile content can directly be used to fuel a CANDU. The other scenario is known as DUPIC (Direct Use of PWR spent fuel In CANDU). The DUPIC approach does not involve any element separation process; so, PWR spent fuel containing not only U and Pu but also almost all fission products and minor actinides is used as fuel in CANDU. In addition, use of Th-added fresh U fuel in CANDU6 on a once-through cycle was investigated. For this purpose, two fuel models were considered. One is “homogenous-bundle” containing a homogenous mixture of (U-Th)O2 formed by blending various slightly-enriched U fuels with Th (in Th mass ratios; 10%, 30% and 50%), used in all fuel elements of all bundles throughout the core. The other model is “mixed-bundle” which contains only ThO2 in 4 or 7 fuel elements in the center of a bundle, while the other elements comprise UO2 only. The results with U+Th fuels enable not only to observe the effect of Th use in CANDU6 but also to compare it to the results from the combined-cycle cases. For burnup computations, CANDU6 full-core geometry and the non-linear reactivity model with MCNP5 and MONTEBURNS codes were used. With burnup values at hand, NUR and NRU (and natural U saving) were calculated for different fuel compositions in each case. At the combined PWR-CANDU6 cycle; for PWR spent fuel with a discharge burnup of 33000, 40000 and 50000 MWd/tU; the additional burnups achieved in CANDU6 have been found to be 25981, 27021 and 27919 MWd/tHM, respectively for CC; and 16717, 16195 and 14926 MWd/tHM, respectively for DUPIC. Both the CC and DUPIC scenarios affect NUR and NRU positively. In general and with respect to NUR and NRU, the CC cycle is more advantageous than the DUPIC cycle since in that case CANDU fuel is free from fission products and minor actinides. As for U+Th fuel in CANDU6 on the once-through cycle, the higher the Th fraction in U+Th fuel, the higher the 235U fraction required to obtain the same discharge burnup. As a result, an increase on NUR and a decrease on NRU are observed. At very high discharge burnups, as Th fraction goes up, NUR begins to decrease, and NRU begins to increase.