Metal actinide fuel is typically an alloy of zirconium, uranium, plutonium, and minor actinides. Calixarenes also form complexes with cadmium, lead, lanthanides and actinides. Examples are the lanthanides and actinides. There are two series of rare-earth metals, the lanthanides and the actinides. The same rule also applies to "f-f" transitions in centrosymmetric complexes of lanthanides and actinides. Uranium and thorium were the first actinides discovered. Second, plutonium must be separated from other actinides. A fast reactor is more efficient for using plutonium and higher actinides as fuel. These fast reactors are better suited for the transmutation of other actinides than thermal reactors. Fermium is produced by the bombardment of lighter actinides with neutrons in a nuclear reactor. This energy release profile holds true for thorium and the various minor actinides as well. Silicon-34 also occurs naturally as the result of radioactive decay of actinides. Thus actinides higher on the periodic table will more readily undergo hydrolysis. Like all actinides, einsteinium is rather reactive. Traces of the minor actinides are present in spent reactor fuel.