Radiological Analyzation Of Spallation Target Production For An Accelerator-driven System

Spallation target is the most important part of accelerator-driven subcritical system(ADSS), playing a role of coupling neutrons created through spallation process to reactor. Under an irradiation of MW level, the traditional monolith target can’t bear the high intensity bombardment due to uniformly energy distribution and the difficulty to diffuse heat. To break out this bottleneck, a stationary granular target was proposed by P. Sievers of CERN. In this paper, we compared monolith target and granular target from four aspects of neutronics, energy deposition, radioactivity and decay time.This paper will first analyze those two targets from dual aspects of neutronics, neutron yield and spatial distribution and lateral neutron energy spectrum. Viewed from the neutron yield and spatial distribution prospects, the granular target produces more leaked neutrons than monolith target, especially in the lateral direction. Meanwhile, the granular target has minor neutrons leaked from the backward direction, which helps to minimize the radiation damage to the structures of beam tunnel. Viewed from the lateral neutron energy spectrum, the granular target has a harder neutron energy spectrum than monolith target, also with the higher mean energy of neutron that is precisely needed to transmute MAs.Viewed from the energy deposition prospect, the maximum of energy density in granular target is lower than the monolith target. Meanwhile the distribution of energy density is more homogenous in granular target along the axial direction. In addition, the higher ratio of surface to volume makes the granular target that is filled with helium easy to diffuse heat.Viewed form the induced activity prospect, the total induced activity in granular target is lower than monolith target, and this situation continuous after ceasing the operation. What’s more, it also has advantages on activity and damage, since the number of non-elastic reaction in granular target is lower than monolith target.Viewed from the decay time prospect, the surface gamma dose rate in granular target is higher than monolith target. It need more time to decay to recycling limit correspondingly. In general, they both need almost 50 years to decay to this limit and have minor difference.In summary, from the standpoints of neutronics, energy deposition, reduced activity and decay time, the granular target is more suitable to be as spallation target in comparison with monolith target.