Recycle transmutation of MA and LLFP using BWR for sustaining geologic disposal

Abstract

The actinides and fission products produced in nuclear fuels constitute an important part of the HLW. Therefore, methods for reducing the radiotoxicity of the MA and LLFP in HLW are presently under investigation. The purposes of this study are to evaluate the effectiveness of MA transmutation by taking advantage of neutron spectrum hardening due to void fraction along BWR axial direction; to understand the effectiveness of LLFP transmutation in BWR considering the large capture cross section of FP in thermal region; and to evaluate the macroscopic characteristics of longer residential period of LLFP target in the high burnup BWR core. Conceptual B/T BWR supposed in this study was reactor which the performance comparable to the current BWR. In MA transmutation case, the calculation was focused on varying the void fraction of 0 to 40% along the axial direction, which were directly associated to the lower and upper region of the BWR core. The performance of B/T BWR was evaluated in which four components of MA ( 237Np, 241Am, 243Am, and 244Cm) with fixed fraction were blended with UO 2 in B/T fuel. While, for LLFP transmutation, the B/T BWR was assumed to have two homogeneous regions: {1} the region for UO 2 driver fuel (99% of fuel weight), and {2} the region for LLFP ( 99Tc and 129I) target capsules (1% of fuel weight), in which metallic Tc rods and iodine in the form of CeI 3 was contained in cylindrical target capsules. The evaluation functions are {1} fission-to-transmutation ratio, [F/T ratio] MA, and {2} transmutation fraction, Tf LLFP. Results show that the hardening neutron spectrum due to increase of void fraction in B/T BWR would result a higher [F/T ratio] of MA transmutation performance. Np and Am would be effectively loaded in the upper region of the core, while Cm could be loaded in any region of the core. At the EOC of equal or more than 50 GWd/Mg(HM), technetium has a higher transmutation fraction compared to iodine. To obtain higher LLFP transmutation fraction, the residential time in the LLFP targets in the core, should be kept for long time, for instance about 10 to 30 years. For that purpose, it was proposed that the number of B/T BWR system for LLFP treatment corresponds to the residential time of the LLFP target, i.e. 10 to 30 units.