Chemistry fans often refer to Oct. 23 as “Mole Day,” since the numbers 10 and 23 are part of a basic constant in chemistry, the mole. This unit describes how many atoms exist in a given sample of any substance, so scientists use moles to simplify lots of calculations. For example, when an average nuclear reactor first starts up its core has about 120,000 kilograms of uranium in its fuel. A mole of uranium weighs about 238 grams, so a brand-new core has about 504,000 moles of uranium. A plant scientist or NRC specialist would base some core calculations on a more exact definition of moles in the core.