Rejuvenation of the CANDU Spent Fuel in (D-T)-Driven Hybrid Reactors

The possibility of Canada deuterium uranium reactor (CANDU) spent-fuel rejuvenation in deuterium-tritium (D-T)-driven hybrid reactors having 17.8 cm of fissile zone thickness is investigated for various plasma chamber dimensions (DR = 18.7, 118.7, 218.7, and 418.7 cm) with a linear fusion neutron source (plasma dimension is assumed as DR/2) under different first-wall loads (P_w = 2, 4, 6, 8, and 10 MW/m²). The behavior of the spent fuel is observed during 36 months for discrete time intervals of t = 15 days and by a plant factor of 75%. The fissile fuel zone is considered to be cooled with three different coolants: gas (He or CO₂), Flibe (Li₂BeF₄), and natural Li.

As a result of the calculation, in the case of the first-wall load and the plasma chamber dimensions being selected high, although the first-wall material had been damaged considerably by the high neutron flux (displacements per atom > 100 and He > 500 parts per million for P_w > 2 MW/m² over 3 yr of operation) and the maximum temperature in the centerline of the fuel rod (T_m) had reached the melting point (T_m > 2600°C for P_w > 6 MW/m² and DR > 1 m), it was observed that the neutronic performance of the hybrid reactor improved unnegligibly. For DR = 18.7 cm, at the beginning of rejuvenation, the tritium breeding ratio values were 1.18, 0.85, and 1.26 for gas, Flibe, and natural Li, respectively, and by the end of the rejuvenation had increased to 1.26, 0.93, and 1.32 for 2 MW/m² and to 1.60, 1.28, and 1.58 for 10 MW/m². In addition, the blanket energy multiplication M increased to 5.47, 4.92, and 5.02 for 2 MW/m² and to 8.89, 9.32, and 7.58 for 10 MW/m² from 4.64, 3.90, and 4.37, respectively. Only for Flibe, when the DR value is preferred at ~1 m, the M values increased to 5.82 and to 15.0 from 3.92 for 2 and 10 MW/m², respectively. Under the same conditions, the average cumulative fissile fuel enrichment (CFFE) values indicating the rejuvenation performance increased to 1.67, 2.13, and 1.57% for 2 MW/m² and to 5.78, 7.69, and 5.39% for 10 MW/m² from 0.418%, respectively. For Flibe coolant, while the same CFFE value is 11.2% at DR = ~1 m, it is 11.4% at DR = ~4 m. The contribution of a large plasma chamber (DR > 1 m) to neutronic performance can be neglected. The best rejuvenation performance and neutron economy have been shown by Flibe. However, Flibe has shown a bad capability until the rejuvenation time in terms of tritium breeding. For Flibe, to breed enough tritium, the values of the first-wall load and required rejuvenation time must be larger than 6 MW/m² and 2 yr, respectively.

For all cases, the denatured character of the initial fuel charge remains denatured for all investigated cases over the whole plant operation period in a hybrid reactor although the Pu quality increases continuously during the rejuvenation process. In addition, our calculations have proven that the effects of the important fission products (¹³⁵Xe, ¹⁴⁹Sm) and plasma densities up to 10²¹ (D + T)/cm³ can be neglected for the neutronic performance of the hybrid reactor, which rejuvenates CANDU spent fuel.