There are technical difficulties to overcome before ADS can become economical and eventually be integrated into future nuclear waste management. The accelerator must provide a high intensity and also be highly reliable - each outage of the accelerator in addition to causing a scram will put the system under immense thermal stress. There are concerns about the window separating the protons from the spallation target, which is expected to be exposed to stress under extreme conditions. However, recent experience with the MEGAPIE liquid metal neutron spallation source tested at the Paul Scherrer Institute has demonstrated a working beam window under a 0.78 MW intense proton beam. The chemical separation of the transuranic elements and the fuel manufacturing, as well as the structure materials, are important issues. Finally, the lack of nuclear data at high neutron energies limits the efficiency of the design. This latter issue can be overcome by introducing a neutron moderator between the neutron source and the fuel, but this can lead to increased leakage as the moderator will also scatter neutrons away from the fuel. Changing the geometry of the reactor can reduce but never eliminate leakage. Leaking neutrons are also of concern due to the activation products they produce and due to the physical damage to materials neutron irradiation can cause. Furthermore, there are certain advantages to the fast neutron spectrum which cannot be achieved with thermal neutrons as are the result of a moderator. On the other hand, thermal neutron reactors are the most common and well understood type of nuclear reactor and thermal neutrons also have advantages over fast neutrons.