DEM is used to study the interparticle force distri­bution and translate the microscopic information into macroscopic information such as stress-strain re­sponse. This method is important for estimating the overall properties of pebble beds, such as yield strength and crush probability. The use of DEM for
fusion reactor blanket analyses was started at the Uni­versity of California, Los Angeles (UCLA)124,131,205 and has been continued by KIT.91,119,132,133

A standard experiment, used by both groups, to validate DEM is UCTs; see corresponding section above. Figure 31 from An et al.205 shows three cycles of a pebble bed with an initial packing factor of 60.3%. With increasing cycle number, the pebble bed becomes stiffer. The strong dependence of the stress-strain curves on the packing factor was also stated by Gan and Kamlah.119

DEM analyses were also compared with the SCA — TOLA experiments.124 Some features were well described (see Figure 32), but thermal creep was not satisfactorily predicted.

With increasing load on the pebble bed, the number of contacts between pebbles, Nc, increases, as shown in Figure 33.91 An important quantity to assess the frac­tion of crushed pebbles during blanket operation is the

Although the present DEM codes have proven to be very helpful for the understanding of the interaction between pebbles, there is still consider­able development work required until quantitative results for small thermally loaded pebble-bed geo­metries can be expected. The method will certainly not be applicable for large components in the near future because of computational costs, but the improved understanding of the micromechanism will be beneficial for the improvement of the con­tinuum codes.