Advanced Computational Models for Accelerator-Driven Systems

In the nuclear engineering scientific community, Accelerator Driven Systems (ADSs) have been proposed and investigated for the transmutation of nuclear waste, especially plutonium and minor actinides. These fuels have a quite low effective delayed neutron fraction relative to uranium fuel, therefore the subcriticality of the core offers a unique safety feature with respect to critical reactors. The intrinsic safety of ADS allows the elimination of the operational control rods, hence the reactivity excess during burnup can be managed by the intensity of the proton beam, fuel shuffling, and eventually by burnable poisons. However, the intrinsic safety of a subcritical system does not guarantee that ADSs are immune from severe accidents (core melting), since the decay heat of an ADS is very similar to the one of a critical system. Normally, ADSs operate with an effective multiplication factor between 0.98 and 0.92, whichmeans that the spallation neutron source contributes little to the neutron population. In addition, for 1GeV incident protons and lead-bismuth target, about 50% of the spallation neutrons has energy below 1MeV and only 15% of spallation neutrons has energies above 3MeV. In the light of these remarks, the transmutation performances of ADS are very close to those of critical reactors. This contributes to different research topics, including the following: