Accelerator-driven reactors

Reactor heartAn ADS reactor (Accelerator Driven System), also known as a sub-critical system, bares little resemblance to current reactors. It is made up of two distinct parts: a particle accelerator and a reactor. The accelerator provides the reactor with neutrons, enabling the reaction to take place. This is why reactors are called sub-critical: the nuclear reaction cannot take place in a chain as in traditional reactors. If the supply of neutrons is stopped (i.e. when the accelerator is interrupted), the reactor also stops.

The accelerator (for example, a cyclotron) accelerates protons until a fairly high energy is reached. These protons are then fired onto a heavy metal target (generally molten lead or bismuth lead) situated in the reactor. The reaction called spallation then takes place inside the reactor, generating a significant flow of rapid neutrons. These neutrons will then interact with the fuel contained in the core of the reactor, provoking a small number of fission reactions. Such as system could, of course, produce energy (beyond that required to supply the accelerator), but its main advantage is that it allows fuels other than uranium 235 or plutonium 239 to be used. This could be, for example, plutonium-enriched thorium (more abundant than uranium), produced by current stations.

Although an installation like this will only be operational in Mol from 2020, a "reduced model" was inaugurated in March 2010. Called GUINEVERE, this model and consists of a small particle accelerator, built by French teams, coupled to the CEN research VENUS reactor. 

Other than plutonium and uranium which can be recycled as fuels, waste from a reactor such as those used in Belgium represents around 50kg of radioactive waste per tonne of fuel used. This waste is divided into two categories, the products of fission and actinides. The first are the direct result of the fission of heavy nuclei from the fuel (uranium and plutonium) by neutrons. Most of these (around 46kg per 50kg of waste) have a short life, i.e. have a radioactive period which is less than 30 years; this is the case for cesium 137 and stronium 90. Other fission products (around 3kg) have a long-life; this is the case for example of technetium 99, iodine 129 and cesium 135. However, capturing neutrons by the nuclei of reactor fuels is not always followed by fission, far from it. This simply produces nuclei which are heavier than those of uranium, called actinides, of which the best known is plutonium 239, which is fissile and also serves as a fuel. The others, called minor actinides (the last of our 50kg of waste) include neptunium, americium, and curium isotopes. They are unstable and often have very long lives, remaining toxic for thousands of years.

Transmutation is the realisation of the alchemists' age-old dream: changing one matter into another. More scientifically speaking, it involves transforming one nucleus into another through a nuclear reaction provoked by bombarding it with particles. The advantages of the process then become clear: transforming long-life radioactive isotopes into isotopes with a much shorter life, or even into stable elements which release no radioactivity. Is this possible? In theory, yes. In practice, huge difficulties remain to be overcome, not the least of which is managing to sort out the different types of waste on an industrial scale.

The particle which is best suited to conducting transmutation reactions is obviously the neutron, as it is not electrically charged and it is already available in the reactors where it permanently causes transmutations ... most of them are unsought after, as we have seen. The best method of recycling therefore consists of re-injecting waste into an installation of the same type as that which produces them.  This time, however, nature is harnessed, rather than being left to take its course. This is the case of ADS reactors, as it is possible to calibrate the flow of neutrons. A true work of art! Thus, for example, technetium 99, which has a half-life of 200,000 years could, by absorbing a neutron, be transformed into technetium 100, which has a half-life of a few seconds, and then transforms into stable ruthenium. A similar reaction would transform iodine 129 into stable xenon. In other cases, transmutations do not lead to stable elements but allow us to reduce the life-span, and hence storage time.

© 2007 ULiège