concentration of fissile isotopes is not achieved due to impurities exceeding acceptable levels, all fissile and fertile isotopes of Actinoids can be "burned" in a subcritical reactor, thus closing the nuclear fuel cycle without the need for fast breeder reactors Disadvantages Each reactor needs its own facility (particle accelerator) to generate the high energy proton beam, which is very costly. Apart from linear particle accelerators, which are very expensive, no proton accelerator of sufficient power and energy at has ever been built. Currently, the Spallation Neutron Source utilizes a proton beam to produce its neutrons, with upgrades envisioned to Its cost included research equipment not needed for a commercial reactor. Economies of scale might come into play if particle accelerators (which are currently only rarely built to the above mentioned strengths and then only for research purposes) become a more "mundane" technology. A similar effect can be observed when comparing the cost of the Manhattan Project up to the construction of Chicago Pile-1 to the costs of subsequent research or power reactors. The fuel material needs to be chosen carefully to avoid unwanted nuclear reactions. This implies a full-scale nuclear reprocessing plant associated with the energy amplifier. If, for whatever reason, neutron flux exceeds design specifications enough for the assembly to reach criticality, a criticality accident or power excursion can occur. Unlike a "normal" reactor, the scram mechanism only calls for the "switching off" of the neutron source, which wouldn't help if more neutrons are constantly produced than consumed (i.e. Criticality), as there is no provision to rapidly increase neutron consumption e.g. via the introduction of a neutron poison. Using lead as a coolant has similar disadvantages to those described in the article on lead cooled fast reactors Many of the current spallation-based neutron sources used for research are "pulsed" i.e. they deliver very high neutron fluxes for very short durations of time. For a power reactor a smaller but more constant neutron flux is desired. The European Spallation Source will be the strongest neutron source in the world (measured by peak neutron flux) but will only be capable of very short (on the order of milliseconds) pulses. See also Accelerator-driven sub-critical reactor Alternative energy Thorium fuel cycle Breeder reactor, another type of nuclear reactor that aims for an energy profit by creating more fissile material than it consumes. Thorium-based nuclear power Muon capture Nuclear transmutation References A PRELIMINARY ESTIMATE