Basically everything on the periodic table except extremely radioactive. Stereotypically known as the scary stuff that Fukushima and Chernobyl released from their damaged nuclear reactors poisoning the surrounding countryside forever.
The Fission Product releases from the spent fuel pool fire expelled many times the radioactive materials that one reactor could have released.
The fuel elements were ejected from the core during subsequent explosions and the fission products will be a hazard for many centuries to come.
The Fuel lava consists of various fission Products.