During the spallation process, the collision between the energetic particle and the target nucleus leads to direct reactions referred to as intra-nuclear cascade. In this cascade, small groups or individual nucleons (protons and neutrons) are expelled from the nucleus. At energies above a few GeV per nucleon, the nucleus can fragment. After the intra-nuclear cascade, the nucleus is in an excited state and subsequently releases ‘evaporates’ nucleons, mainly neutrons.

The spallation process is complicated and depends on the target thickness and the target materials. For thick targets high energy (> 20 MeV) secondary particles may take part in further spallation reactions. For some target materials, low energy (< 20 MeV) neutrons produced from cascade evaporation, can enhance neutron production. For heavier nuclei, high-energy fission may compete with evaporation. Examples of materials that undergo spallation/high-energy fission include lead, tantalum and tungsten. Some spallation target materials, e. g. thorium and depleted uranium may be fissioned by both high — and low- energy neutrons. Regarding target particles, deuterium and tritium produce more neutrons than protons in the below 1-2 GeV energy range but the low-energy part of the accelerator tends to get contaminated, resulting in higher maintenance costs.

The requirement for an ADS target is to convert a high-energy particle beam to neutrons at low energy. It is desirable for it to be of compact size, to couple to a surrounding blanket, operate in the 10-100 MW power range, and have high neutron production efficiency. Other requirements, in common with other nuclear devices, are that it should be reliable and of low cost, be safe and generate only a small amount of waste. Molten lead is a good choice for meeting these requirements. Lead-bismuth eutectic has also been considered because of its lower melting point than lead, but this eutectic produces polonium, the release of which may be a problem at high temperature.

The blanket (sub-critical assembly) surrounding the target multiplies the spallation neutrons for the transmutation of the minor actinides (MA) and LLFP. Taking account of many aspects, safety, operations, material cost and incinerator costs, keff values for the target in the range 0.9-0.98 are typically considered.

The different neutron spectrum modes have different advantages and disadvantages. The thermal cross-section for transmuting MA and fission products is larger than the fast neutron cross-section enabling core inventories to be reduced substantially, but the thermal neutron cross-section of the transmuted products is also large; so neutron capture is a problem. From the point of view of neutron economy, the fast reactor is better than the thermal reactor.

The Th-U fuel cycle is an attractive option for future ADS because it produces a relatively small amount of higher actinides compared with the U-Pu cycle. The Th-U cycle is safer from a weapons proliferation standpoint because of the existence of the hard gamma emitter in the U decay chain and because U can be diluted by depleted or

natural uranium in the start-up or feed fuel. Against these advantages, the Th-U fuel cycle has a less favourable neutron balance.