The Physics of Spallation Processes

2.5. THE “ENERGY-AMPLIFIER” 27spoil particular neutron energies that massively increase the capture probability: the socalledresonances. If the energy of the neutrons emitted in nuclear reactions is decreasedvery slowly, it is sure that sooner or later the neutron will be captured by the resonanceof the radioactive element that has to be destroyed. To do so, neutrons interact witha heavy element so that they are able to transfer only a small fraction of their energy:they will then lose energy in small steps. The element which is used is lead, which hasthe added advantage of having a small tendency to capture neutrons itself. This element,which is already ideal for transuranics transmutation, seems to be the key element forthe elimination of any dangerous waste. The principle has again been demonstrated atCERN, with the TARC experiment (Transmutation by Adiabatic Resonance Crossing)[Aba01, Aba02, Rev99]. The process is so efficient that it can be applied not only to fissionfragments transmutation, but also to activate stable elements and produce radioactiveisotopes to be used in medicine as tracers in radiological analysis or in cancer therapy.Computer simulation of the interaction of a single proton into a 334 ton lead block usedin the TARC experiment at CERN show that on the average 147 neutrons are producedthat, before being absorbed, will interact with the Pb atoms about 55,000 times. In sucha cloud it is very easy for a radioactive element to capture a neutron and become stable.The spallation reaction is not the only mechanism involved in the destruction of thetransuranics elements. In fact, the probability of breaking up a heavy nucleus like plutonium,instead of being captured by it, increases with the energy of the striking neutron.Therefore, it is necessary to maintain the neutrons produced in the cascade to a very highenergy. This is particularly favorable if liquid Pb is used as coolant, a solution that hasalready been adopted in the alfa class Russian submarines in the fifties, but that has tobe reproduced today using current standards observed by the industry in the west. Theuse of particle accelerators therefore seems to be the perfect solution to the problem ofradioactive waste, but like all technologies connected with the nuclei of elements, it hasto be studied in great depth and tested before being used on a large scale.The results of the research on the transmutation of radioactive waste allow us to foreseea different strategy of energy production and nuclear waste disposal for the future,which is much more suitable for future generations, because it allows the elimination ofcontamination risks associated with a possible failure of geological repositories. Transmutationtechniques can be applied to both military and civil waste: military plutoniumis in fact an excellent fuel for ADS. The demonstration of the transmutation techniquecould bring about the elimination of all plutonium in the next 50 years!2.5 The “Energy-Amplifier”On the basis of former ideas formulated starting from the fifties, in 1993 Carlo Rubbiatook up once again the challenge to operate particle accelerators not only for the transmutationconcept as discussed in the previous section, but also for energy production[Car93, And95]. The principle of the spallation is similar to that of fission: acceleratedprotons are sent to a target made of a heavy element like Pb, Hg or W whose nuclei canbe broken up or charged with energy. The interaction always results in the release of alarge quantity of neutrons (up to a few tens for each proton sent) that can then be used to