Carlo Rubbia, a Nobel Prize nuclear physicist and former director of CERN published a proposal for a sub-critical reactor based on a proton cyclotron accelerator with a beam energy of 800 MeV to 1 GeV, and a target with thorium as fuel and lead as a coolant. The spent fuel is bombarded to force it to release more energy than is input, so the fuel is reduced to some new material. The problem is, that the 'window' where the energy is injected has to also retain the generated energy without blocking the input. Perhaps predictably, no-one has yet solved this tiny containment problem......