Final answer:
Krypton is not used in airships because it is rare and expensive, not because it is inert or radioactive. The flammability of hydrogen and the scarcity and cost of helium and krypton have influenced their use in airships.
Step-by-step explanation:
Krypton is not used in airships primarily because it is rare and too expensive, not because it is radioactive or inert. Although krypton is a noble gas and does not react with other elements, this characteristic is not the main reason for its exclusion from airship use. In the past, airships like the Hindenburg used hydrogen, which, despite being very light and providing excellent lifting power, had the severe disadvantage of being highly flammable, as evidenced by the well-known Hindenburg disaster. Today, airships use helium, which provides substantial lifting power and is non-flammable, but even helium was once considered too costly and was not used in the Hindenburg due to export bans and price. Krypton, being even rarer than helium, would thus be impractical due to its cost and scarcity compared to other available options.