In order to guarantee a sustainable supply of future energy demand without compromising the environment, some actions for a substantial reduction of emissions are nowadays deeply analysed. One of them is the improvement of the nuclear energy use. In this framework, innovative gas-cooled reactors (both thermal and fast) seem to be very attractive from the electricity production point of view and for the potential industrial use along the high temperature processes (e.g., production by steam reforming or I-S process). This work focuses on a preliminary (and conservative) evaluation of possible advantages that a symbiotic cycle (EPR-PBMR-GCFR) could entail, with special regard to the reduction of the HLW inventory and the optimization of the exploitation of the fuel resources. The comparison between the symbiotic cycle chosen and the reference one (once-through scenario, i.e., EPR-SNF directly disposed) shows a reduction of the time needed to reach a fixed reference level from 170000 years to 1550 years (comparable with typical human times and for this reason more acceptable by the public opinion). In addition, this cycle enables to have a more efficient use of resources involved: the total electric energy produced becomes equal to 630 TWh/year (instead of only 530 TWh/year using only EPR) without consuming additional raw materials.
At present, the 125 GWe of nuclear power in the European Union produce about 3000 tons of spent fuel annually, containing about 25 tons of plutonium, 2.5 tons of minor actinides (MA) and about 100 tons of fission products, of which 3.1 tons are long-lived fission products. Actual reprocessing of LWR fuel and a first recycling as mixed plutonium and depleted uranium oxide fuel (MOX) in LWR already contribute to a significant reduction of waste volumes and radiotoxicity. However HTRs have some characteristics which make them particularly attractive: intrinsic safety, cost-effectiveness, reduced thermal pollution, capability of increasing energy availability (with the use of Pu-Th cycle) and of minimizing actinides radiotoxicity and volume of actinides. In this paper particularly the last item is investigated. Symbiotic fuel cycles of LWR and HTR can reach much better waste minimization performances. It happens because of the specific features of HTRs cores that leads to an ultra-high burnup and, last but not least, the ability to accommodate a wide variety of mixtures of fissile and fertile materials without any significant modification of the core design. This property is due to a decoupling between the parameters of cooling geometry and of neutronic optimization. In our calculations we considered a pebble-bed HTR using a Pu-based fuel (deriving from reprocessing of classical LWR fuel and/or weapons grade plutonium) at the maximum technological discharge burnup. As results, we find, at EOL (End Of Life), a relatively small amounts of residual Pu and MA produced in terms of quantities and of radiotoxicities. Furthermore we used in our calculations a different type of fuel based on a mixture of Pu and Th to try to optimize the previous results and to increase energy availability. Calculations have been done using MCNP-based burnup codes, capable of treating 3-D complex geometry and ultra-high burnup
The management of radioactive waste is a key issue for the present and future use of nuclear energy. In this frame, high temperature reactors (HTRs) have, among others, the capability to burn actinides. After a short introduction on HTRs, the performances of two MC-based burnup codes (Monte Carlo continuous energy burnup and MONTEBURNS) in assessing the ability of these reactors to burn actinides are compared. These codes are necessary for performing ultra-high burnup calculations on HTRs. The best one, in this specific case, results to be MONTEBURNS. It was analysed using HTRs loaded with the following: (1) 1st generation Pu, 600 equivalent full power days; (2) 2nd generation Pu, 645 equivalent full power days; and (iii) 33% 1st generation Pu and 67% Th, 705 equivalent full power days. Finally, it is possible to conclude that HTRs can reduce time when the waste is considered dangerous. Even if the amount of reduction does not solve the whole problem, it represents an important step in the management of radioactive waste.
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