There are three naturally occurring radioactive series - the thorium series, the actinium series, and the uranium series.

有三个天然存在的放射系列，即钍系、锕系和铀系。

youdao

But it occurs to me that one of the effects of the adoption of thorium technology would be to make a series of other metals, like tantalum and the rare earths, rather cheaper.

FORBES: An Odd Thought: Thorium Reactors Would Make Tantalum And Rare Earths Cheaper