Which isotope is a fission product gas?

• A. Kr-88
• B. I-131
• C. Cs-137
• D. Xe-133

Answer: (A)

Fission product gases come from the fragments of a fission event that are volatile and remain as gases at room temperature. Noble gases like krypton and xenon are classic examples because they don’t react chemically and readily escape from the fuel as gas-phase fission products. Krypton-88 fits this pattern: it is an isotope of krypton, a noble gas, and would exist as a gas if produced in fission, making it a representative fission product gas. In contrast, iodine-131 is a volatile element but not a noble gas and tends to form different chemical forms; cesium-137 is a metal and not a gas; xenon-133 is also a noble gas fission product, but krypton-88 is the example highlighted here.