1.4. Foreseen Technical Challenges Associated with TRU Recycling

- to evaluate the neutronic feasibility of achieving the equilibrium with respect to TRU generation state by multiple recycling of TRU from the spent fuel - to evaluate the effect of the sustainable fuel cycle with zero net TRU generation on the repository - to identify and address the potential challenges associated with multi-recycling of TRU in LWRs

- to perform economic analysis of the sustainable LWR fuel cycle and compare it with conventional once through fuel cycle and alternative two-tier system of LWR and

Advanced fast spectrum Actinide Burner Reactor (ABR).

As discussed earlier, TRU transmutation in thermal spectrum reactors and in particular in LWRs imposes a number of potential design challenges.

Some of the MA have relatively small cross-sections for neutron absorption even in thermal spectrum in addition to relatively long half lives. As a result, these nuclides (typically Cm244, Cm246, Cm247, Cm248, and Cf252) require relatively long time and high concentrations to saturate in the thermal spectrum. Furthermore, changes in TRU isotopic vector with burnup and with the number of TRU recycles result in degradation of TRU transmutation efficiency, shorten the fuel cycle length, and lead to increased TRU loading requirements.

A harder neutron spectrum, as a result of increased TRU concentration in the core and changes in TRU isotopic composition, may lead to positive void reactivity coefficient. This would ultimately limit the maximum TRU concentration. Lower worth of the core reactivity control materials is also expected as a result of the neutron spectrum hardening.

The effective delayed neutron fraction (βeff) is expected to be smaller for TRU containing LWR core because practically all Pu and MA isotopes have lower βeff than U235 in conventional LWR core. The change in the fuel and moderator temperature reactivity feedbacks and prompt neutrons lifetime decrease are probable additional consequences of the harder neutron spectrum. As a result, some of the core safety criteria may be violated during the reactor response to rapid reactivity initiated transients.