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Accelerator Driven Systems (ADS) are an old technological idea: Relativistic proton accelerators deliver their beams onto massive heavy element targets, thus producing abundant neutron fluences. Placing this target into sub-critical nuclear fission assemblies is yielding substantial fission reactions, thus additional fission energy (Rubbia called such a system “Energy Amplifier”). This technology has recently attracted considerable attention due to advances in the construction of powerful accelerators. It allows the safe and cheap production of nuclear energy simultaneously with the destruction (Transmutation) of long lived radioactive waste, in particular plutonium and other minor actinides (neptunium and americium). The principles and the present-state-of-the-art are described, including first experiments to transmute plutonium this way. This technology needs, however, many more years of further “research and development” before large scale ADS's can be constructed. It may be even necessary to investigate the question, if all basic physics phenomena of this technology are already sufficiently well understood.

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Although the idea of subcritical electronuclear energy boosters was advanced many years ago (see, specifically, the review [1]), for several decades electronuclear systems had been viewed only as reactors for the production of plutonium or 233U in natural-uranium or uranium-depleted targets or thorium targets with multiplication factor/(eft << 1. It was assumed that each such reactor will be supplied with fuel from several nuclear power plants. Theoretical simulation and experiments suggested that the optimal energy of the bombarding protons is E = 1 GeV, since at other energies the energy value of a generated neutron increases: at high energy because of an increase in the cost of the accelerator and at low energy because of a rapid increase in ionization losses. For example, if at E = 1 GeV on the average about 10 MeV is expended to generate one neutron in an infinite natural-uranium block, then approximately 18 MeV are expended at E = 0.4 GeV and 45 MeV at E = 0.2 GeV.Physical and economic estimates showed that electronuclear breeding is a good method for utilizing weakly fissioning 238U and 232Th. This conclusion was a result of work performed at a seminar of specialists from several Russian and Ukrainian institutes, which operated for several years at the beginning of the 1970s in Obninsk. The seminar prepared detailed information that initiated planned investigations in this direction in our country. However, the advancement of the investigations was held back by the need to use accelerators with intense beams ~100 mA, the production and operation of which involved both purely technical problems and serious difficulties due to the requirement of radiation safety.The idea of using subcritical systems was rehabilitated by C. Rubbia, who proposed that the systems be limited to a closed "one accelerator -one reactor" scheme, where without any breeding the electronuclear setup produces energy and burns long-lived radionuclides. This approach makes it possible to decrease the intensity of the beam of accelerated particles to a level close to the level that has already been mastered. Once again an energy of 1 GeV is considered to be optimal. Experiments with a reactor in a proton beam [2, 3] have confirmed that the relative (per unit energy consumed) output of heat and the number of neutrons are indeed maximum near 1 GeV. However, in subcritical systems low-energy fission makes the main contribution to heat release and the neutron yield, and ionization losses are not as great as in the case of low Keff. For example, for 100 MeV bombarding neutrons, for which about 95% of the energy is expended on ionizing the medium, the relative energy gain isIt is nonetheless several times greater than 1. The switch from E = 1 GeV to E = 0.5 GeV decreases G by only approximately 5%. A rapid decrease of heat release occurs only at energies less than 0.2-0.25 GeV.Of course, the real relative energy gain G r = [, where the coefficient k a takes account of the energy losses in converting grid electricity into proton-beam energy and ...

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