The neutron, which easily penetrates nuclei, is the perfect tool for transmutation, but it takes a lot of neutrons to transmute waste. Nuclei are transmuted one by one but even the tiniest sample of material contains an almost infinite number. Only reactors are able to supply a sufficiently intense flow of neutrons. But when neutron capture is not followed by fission, actinides are generated, replacing those destroyed.
Transmutaion efficiency is generally low. For example, in a powerful reactor generating 1 GW of electricity, the neutrons only transmute 4.5% of the nuclear fuel into plutonium and fission products after three to four years of irradiation. Intense neutron fluxes are needed to enhance transmutation efficiency, together with high probabilities (known as cross-sections) of neutron capture by the nuclei to be transmuted.
Light water reactors (LWRs) are not very good for transmuting actinides because more neutron captures take place than fissions. Transmutation in pressurized water reactors (PWRs) has not shown much promise either. Compliance with safety regulations means that high efficiency levels cannot be achieved, even with special fuel. The multirecycling of americium on its own would require modifications to the reactor and the nuclear fuel cycle. This type of recycling cannot be done with the current reactor fleet.
In a fast neutron reactor the fuel tolerates larger proportions of minor actinides than in a PWR: 2.5 to 5% of the mass of the fuel compared with 1%. The feasibility of minor actinide transmutation in these fast reactors has now been established.
After SUPERPHENIX was decommissioned in 1998, PHENIX was one of the few fast neutron reactors (FNRs) in operation and the only one available in France. It was built in 1974 to produce electricity, was renovated and restarted for the transmutation research programme, and was decommissioned in 2010. At present France has no transmutation facilities.
In what form should radioelements for transmutation be inserted into a reactor core? New types of fuel have been designed to optimise transmutation performance. Two methods have been evaluated for introducing minor actinides into reactor cores: a homogeneous method, where the elements to be transmuted are distributed throughout the fuel, and a heterogeneous method where these elements are concentrated in special assembles. Fuels based on americium or other long-lived elements have been successfully inserted into reactors. The test programme will last until 2008.
In principle ADS reactors controlled by an accelerator deliver better performance. Although FNRs are capable of burning their own actinides by recycling them, ADSs can burn a core consisting only of actinides.
There are many technological barriers to be lifted before these systems can see the light of day. European projects and research are currently in progress. The CNRS is actively involved in them, with the study of subcritical reactor cores (MUSE experiments), high-intensity proton injector (IPHI) projects, and targets for neutron production by spallation.
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