氦是钍和铀元素数千年放射性衰变的副产品。
Helium is the by product of millennia of radioactive decay from the elements thorium and uranium.
稀土矿山常常也含有放射性元素，比如铀和钍。
Rare earth mines often also contain radioactive elements, such as uranium and thorium.
理由是煤燃烧过后省些的煤灰含有放射性元素，尤其是铀和钍。
The reason is that the ash left over when coal is burned contains radioactive elements, notably uranium and thorium.
应用推荐