Accelerator driven transmuters use a buffer region to protect the structural and the cladding materials of the transmuter from the radiation damage caused by the high-energy spallation neutrons, to accommodate the coolant channels of the self cooled targets, and to have an insignificant effect on the neutron utilization for the transmutation process. These functions are contradicting with respect to the buffer thickness. An extension of the target region in the axial direction (the proton beam direction) is also required to act as a neutron multiplier for the forward component of the high-energy spallation neutrons and a reflector to minimize the neutron leakage. The buffer assessment was performed as a function of its thickness with different proton energies for a self-cooled Lead-Bismuth Eutectic and a sodium-cooled tungsten targets. The analyses show that the number of generated neutrons per proton has a low sensitivity to the buffer thickness. However, the number of neutrons reaching the transmuter is significantly reduced as the buffer thickness is increased. The transmuter neutrons dominate the nuclear responses in the structural material outside the target buffer. The length of the axial target extension is determined as a function of the proton beam energy.
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