Pebble Bed Reactors for Once through Nuclear Transmutation

Pebble bed reactors fuel elements are specially suited for withstanding very high burn-up values. This is one of the main characteristics of a transmutation fuel. The higher the burn-up achieved in the fuel, the smaller the number of reprocessing steps needed for obtaining a high transmutation rate. In this paper, a new transmutation cycle option will be introduced, based in a single pass of the fuel through the reactor, with no reprocessing steps in the cycle. With a maximum burn-up of 700 MWth, the fuel transmuted is nearly 75% of the actinides originally charged in the reactor. For a minimisation of the radiotoxicity in the fuel element, the 25% of the mass of actinides remnant in the fuel after transmutation have to be low radiotoxic isotopes. For example, Pu, which has a long half life, and by α disintegration, it converts to U, beginning the Uranium cycle. In the paper, the optimum spectrum for minimising the final radiotoxicity is studied, and the possibility of initially drive the reactor as a critical reactor, with a last subcritical step, is analysed. In the calculations, it is shown that a good reduction in radiotoxicity of the fuel can be achieved by this strategy, with no reprocessing steps during the transmutation phase. The final disposal of the fuel elements, which have very good confinement characteristics, is also an additional factor for the use of Pebble Beds as Transmutation reactors.