Isotope of which element is used in atomic reactor?

Isotope of which element is used for the treatment of goitre ?

The atomic mass of an element is the weighted average of the atomic masses of different isotopes of that element .This explains,why atomic masses of many elements show large departures from integer values.However ,even if we consider msases of individual isotpes,they are not strictly integer multiples of the mass of a hydroen atom.How do yo account for this fact?

(true/false) Isotope of iodine is used for making tincture iodine, which is used as a medicine

Which isotope of carbon is used as a standard reference to express atomic masses ?

Which of the following is used as moderator in nuclear reactor?