Pellet shattering process for the ITER disruption mitigation system - Part I: Development of a discrete element code for modeling the dynamic fragmentation of cryogenic materials

The ITER disruption mitigation system is based on the shattered pellet injection technology. The principle of operation is to form and accelerate cm-sized cryogenic pellets up to about 800 m/s towards a shattering unit, where the pellets disintegrate into fragments entering the plasma for the mitigation process. The effectiveness of this mechanism is strongly dependent on the fragmentation characteristics. Therefore, it is important to know the role of impact conditions, namely the pellet material, velocity, impact angle, etc. The ITER DMS task force has launched a program to investigate the pellet fragmentation experimentally and through numerical simulation. As part of this study, Fraunhofer EMI has developed numerical tools to simulate and analyze the complex fragmentation process. By using synthetic diagnostics, the experiments serve to calibrate and validate the developed models as well as to estimate the mechanical properties of these cryogenic materials, which are unknown and hard to characterize. The validated models will then be used to extend the experimental database and predict the resulting fragment plumes for various shattering scenarios. The goal is to optimize the design of the shatter unit and the impact conditions to maximize the DMS effectiveness. This paper gives an insight into the developed modelling approach, based on the discrete element method, which enables the consistent simulation of the dynamic fragmentation of brittle-like materials.