A first approach towards DEM analysis of plasticity in pebble beds

In the solid blanket concept, the tritium breeder and the neutron multiplier are both used in the form of pebble beds. Due to their discrete nature, the Discrete Element Analysis (DEM) allows determining the macroscopic behaviour of the bed as a result of the microscopic interactions among pebbles. At the Karlsruhe Institute of Technology, a Discrete Element Code (KIT DEM code) is constantly under development to support the R&D on the solid breeding blanket. As a further extension of the code, a first approach towards implementing plastic deformation inside pebble beds was investigated. By simulating the plastic deformation of each single pebble, the macroscopic plastic strain of the bed is defined jointly with the rearrangement of the particles in the assembly. Among the approaches reported in literature, the Thornton theory was selected and implemented. In order to validate the method, a comparison with experiments already reported in literature was carried out. Numerical results are in good agreement with the experiment in terms of maximum stress and residual strain. However, due to the approximation introduced by the theory, a perfect linear behaviour which differs from the experimental results is obtained during the plastic loading phase. In this sense, Finite Element analyses were carried out to adjust the correlation describing the plastic loading occurring during uniaxial compression.