Closed U-Pu and Th-U cycle in sixteen selected reactors: Comparison of major equilibrium features

Highlights: • In this study sixteen selected reactors are compared in closed U-Pu and Th-U cycles. • Equilibrium cycle is evaluated, as a Bateman burnup matrix eigenvalue. • The eigenvalue is obtained also in subcritical reactors. • Equilibrium excess reactivity, fuel composition, and neutron spectrum are discussed. • Novel method is applied for the excess reactivity break-down. - Abstract: Sustainability of a nuclear fuel cycle can be strongly increased by fuel recycling. Not all reactor types have sufficient neutron economy to enable this recycling. Nevertheless, even if the neutron economy is not sufficient, simulation of repetitive recycling, with constant imposed power and fuel cycle parameters, results in converged fuel composition. The final equilibrium state represents an eigenvalue of the respective Bateman equations and strongly differs between the reactor types. Equilibrium reactivity, as a product of the neutron spectrum and fuel composition, determines inherent neutron economy of the reactor and thus its potential for closed fuel cycle and legacy waste burning. In this study the performance of sixteen selected reactor types, eight thermal and eight fast, was evaluated in both U-Pu and Th-U equilibrium fuel cycles. The reactor types were selected so that all major designs and spectra are covered. Even though the equilibrium composition and spectrum mutually influence each other, the general spectrum shape is determined by the relative strength of coolant and structural materials scattering properties. EQL0D v2 MATLAB procedure coupled to the SERPENT 2 code was used for the simulation. Several simplifying assumptions have been applied to enumerate the eigenvector of Bateman matrix; the reprocessing losses were zero, the FPs were neglected and instantaneously replaced by either ²³⁸U or ²³²Th feed, the reactors were represented only by an infinite lattice, and the generated power was fixed at nominal value independently from the criticality or subcriticality level and fissile fuel share. In the obtained equilibrium closed cycle every core acts, per definition, as an iso-breeder. Burned and produced masses are the same for each isotope. As expected, some of the reactors are subcritical and thus cannot be operated with such a fuel cycle in reality. Still the obtained eigenvalues well characterize each reactor performance. To provide additional insight into the equilibrium behavior unique reactivity break-down method was applied. Furthermore, two options for equilibrium reactivity increase were discussed and two major safety related parameters evaluated. The general conclusion of this study is that the U-Pu cycle profits more from the spectrum hardening and has better neutron economy, where more neutrons are produced but also parasitically captured. On the other hand, the Th-U cycle is less sensitive to the spectrum hardening and has better neutron efficiency, where the lower neutron production is compensated by the lower parasitic captures.