High-resolution X-ray diffraction imaging of 200 mm silicon wafers following rapid thermal annealing at a temperature of 1270 K has revealed the presence of many early stage sources of thermal slip associated with the wafer edge. Dislocation sources are primarily at the wafer extremity, though many are generated by damage at the edge of the bevel incline on the wafer surface. A smaller fraction of sources is associated with other regions of localized damage, probably relating to protrusions on the wafer support. The geometry of the latter is similar to that of dislocation sources generated by controlled indentation on the wafer surface. It is concluded that rapid spike annealing at high temperature does not suppress the nucleation of slip, but rather the rapidity of the process prevents the propagation of the dislocations in the slip band into the wafer.