Highly enriched uranium can continue fission even in regular water.
Work Step by Step
For a chain reaction to occur, the emitted neutrons from fission events must be slowed down, allowing them to be captured by other uranium nuclei. Ordinary water does not moderate neutrons as well as heavy water does. However, when the uranium is highly enriched, even ordinary water will do fine because there are many more fissionable nuclei that are available to prolong the chain reaction.