Neutrons in a highly diffusive medium : a new propulsion tool for deep space exploration?

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The recently completed TARC Experiment at the CERN-PS has shown how it is possible to confine neutrons by diffusion in a limited volume of a highly transparent medium for very long times (tens of milliseconds), with correspondingly very long diffusive paths (> 60 m neutron path ÒwoundÓ within a ~ 60 cm effective radius). Assume an empty cavity is introduced inside the previous volume of diffusing medium. The inner walls of the cavity are covered with a thin layer of highly fissionable material, which acts as a neutron multiplying source. This configuration, called Òn-HohlraumÓ, is reminiscent of a classic black-body radiator, with the exception that now neutrons rather than photons are propagated. The flux can be sufficiently enhanced as to permit to reach criticality with a ~ 1 mm thick Americium deposit, corresponding to a mere 1100 atomic layers. Such a layer is so thin that the Fission Fragments (FF) exit freely into the cavity. The energy carried by FF can be recovered directly, thus making use of the enormous enthalpy of the initial nuclear process. Such a direct utilisation of the FF energy has interesting applications in space, since for instance it permits to attain very high temperatures of the exhaust propellant gas (up to 0.5 million °K), impossible by chemical fuels. This opens the way to very fast, deep in space, manned, round-trip travel to distant planets and their satellites using low thrust, continuous propulsion of nuclear origin. The (so far conceptual) engine has potentials of low weight, high specific power, no moving parts and a simple geometrical structure. It overlaps in performance to the expectations of fusion engines, but in a more realistic way since it requires no major fundamental breakthroughs. If nuclear energy on Earth is competing with many other alternatives and is not without problems, in deep-space travel it offers unique possibilities, since for instance the energy supplied by 1 gram of Americium is about the same as that of…