Optimization of the LWR Nuclear Fuel Cycle for Minimum Waste Production

Abstract

The once through nuclear fuel cycle adopted by the majority of countries with operating commercial power reactors imposes a number of concerns. The radioactive waste created in the once through nuclear fuel cycle has to be isolated from the environment for thousands of years. In addition, plutonium and other actinides, after the decay of fission products, could become targets for weapon proliferators. Furthermore, only a small fraction of the energy potential in the fuel is being used. All these concerns can be addressed if a closed fuel cycle strategy is considered offering the possibility for partitioning and transmutation of long lived radioactive waste, enhanced proliferation resistance, and improved utilization of natural resources. It is generally believed that dedicated advanced reactor systems have to be designed in order to perform the task of nuclear waste transmutation effectively. The development and deployment of such innovative systems is technically and economically challenging. In this work, a possibility of constraining the generation of long lived radioactive waste through multi-recycling of Trans-uranic actinides (TRU) in existing Light Water Reactors (LWR has been studied. Thorium based and fertile free fuels (FFF) were analyzed as the most attractive candidates for TRU burning in LWRs. Although both fuel types can destroy TRU at comparable rates (about 1150 kg/GWe-Year in FFF and up to 900 kg/GWe-Year in Th) and achieve comparable fractional TRU burnup (close to 50a/o), the Th fuel requires significantly higher neutron moderation than practically feasible in a typical LWR lattice to achieve such performance. On the other hand, the FFF exhibits nearly optimal TRU destruction performance in a typical LWR fuel lattice geometry. Increased TRU presence in LWR core leads to neutron spectrum hardening, which results in reduced control materials reactivity worth. The magnitude of this reduction is directly related to the amount of TRU in the core. A potential for positive void reactivity feedback limits the maximum TRU loading. Th and conventional mixed oxide (MOX) fuels require higher than FFF TRU loading to sustain a standard 18 fuel cycle length due to neutron captures in Th232 and U238 respectively. Therefore, TRU containing Th and U cores have lower control materials worth and greater potential for a positive void coefficient than FFF core. However, the significantly reduced fuel Doppler coefficient of the fully FFF loaded core and the lower delayed neutron fraction lead to questions about the FFF performance in reactivity initiated accidents. The Combined Non-Fertile and UO[subscript 2] (CONFU) assembly concept is proposed for multirecycling of TRU in existing PWRs. The assembly assumes a heterogeneous structure where about 20% of the UO[subscript 2] fuel pins on the assembly periphery are replaced with FFF pins hosting TRU generated in the previous cycle. The possibility of achieving zero TRU net is demonstrated. The concept takes advantage of superior TRU destruction performance in FFF allowing minimization of TRU inventory. At the same time, the core physics is still dominated by UO[subscript 2] fuel allowing maintenance of core safety and control characteristics comparable to all-UO[subscript 2]. A comprehensive neutronic and thermal hydraulic analysis as well as numerical simulation of reactivity initiated accidents demonstrated the feasibility of TRU containing LWR core designs of various heterogeneous geometries. The power peaking and reactivity coefficients for the TRU containing heterogeneous cores are comparable to those of conventional UO[subscript 2] cores. Three to five TRU recycles are required to achieve an equilibrium fuel cycle length and TRU generation and destruction balance. A majority of TRU nuclides reach their equilibrium concentration levels in less than 20 recycles. The exceptions are Cm246, Cm248, and Cf252. Accumulation of these isotopes is highly undesirable with regards to TRU fuel fabrication and handling because they are very strong sources of spontaneous fission (SF) neutrons. Allowing longer cooling times of the spent fuel before reprocessing can drastically reduce the SF neutron radiation problem due to decay of Cm244 and Cf252 isotopes with particularly high SF source. Up to 10 TRU recycles are likely to be feasible if 20 years cooling time between recycles is adopted. Multi-recycling of TRU in the CONFU assembly reduces the relative fraction of fissile isotopes in the TRU vector from about 60% in the initial spent UO[subscript 2] to about 25% at equilibrium. As a result, the fuel cycle length is reduced by about 30%. An increase in the enrichment of UO[subscript 2] pins from 4.2 to at least 5% is required to compensate for the TRU isotopics degradation. The environmental impact of the sustainable CONFU assembly based fuel cycle is limited by the efficiency of TRU recovery in spent fuel reprocessing. TRU losses of 0.1% from the CONFU fuel reprocessing ensure the CONFU fuel cycle radiotoxicity reduction to the level of corresponding amount of original natural uranium ore within 1000 years. The cost of the sustainable CONFU based fuel cycle is about 60% higher than that of the once through UO[subscript 2] fuel cycle, whereas the difference in total cost of electricity between the two cycles is only 8%. The higher fuel cycle cost is a result of higher uranium enrichment in a CONFU assembly required to compensate for the degradation of TRU isotopics and cost of reprocessing. The major expense in the sustainable CONFU fuel cycle is associated with the reprocessing of UO[subscript 2] fuel. Although reprocessing and fabrication of FFF pins have relatively high unit costs, their contribution to the fuel cycle cost is marginal as a result of the small TRU throughput. Reductions in the unit costs of UO[subscript 2] reprocessing and FFF fabrication by a factor of two would result in comparable fuel cycle costs for the CONFU and conventional once through cycle. An increase in natural uranium prices and waste disposal fees will also make the closed fuel cycle more economically attractive. Although, the cost of the CONFU sustainable fuel cycle is comparable to that of a closed cycle using a critical fast actinide burning reactor (ABR), the main advantage of the CONFU is the possibility of fast deployment, since it does not require as extensive development and demonstration as needed for fast reactors. The cost of the CONFU fuel cycle is projected to be considerably lower than that of a cycle with an accelerator driven fast burner system.