To produce spherical fuel or absorber elements for high temperature reactors a mixture of coated nuclear fuel or absorber particles and graphite molding composition is molded into spheres, carbonized in a furnace having gas flushing and calcined in a vacuum. There are attained high throughputs without addition of transportation aides by employing as resin binders a thermosetting synthetic resin, hardening the resin at 110.degree. to 170.degree. C. and subsequently allowing the spheres to roll for 1 to 10 hours through an oven which is inclined around 2.degree. to 12.degree. to the horizontal. Thereby the oven must exhibit an increasing and decreasing temperature profile, the flushing gas introduced from both sides and be removed in a temperature zone of 400.degree. to 500.degree. C.