Radioactive decay of isotopes like uranium, thorium, and potassium is a major well-established source of Earth's internal thermal energy, making this a matter of common scientific knowledge that requires no citation.
The claim that radioactive decay contributes significantly to Earth's internal heat budget is a well-established, textbook scientific fact. While papers like [1], [6], and [8] discuss radiogenic heat and Earth's energy budget, basic consensus facts of this nature are classified as common knowledge.