Plutonium

Plutonium is an artificially occuring element, number 94 on the Periodic Table, which can be used in the manufacture of atomic bombs. After realizing that his Japanese captors were familiar with the periodic table, Teerts once agaig tried to confuse them by referring to plutonium, which does not occur naturally. To his horror, they were familiar with it as well, and he realized he had inadvertantly betrayed one of the Race's atomic secrets.

When enrichment of uranium proved difficult, humans began to use plutonium in the construction of their atomic bombs.