Ultritium 342

From Memory Alpha, the free Star Trek reference

Ultritium 342 is a form of ultritium, one of many explosive materials available to the crew of the USS Enterprise-D in 2367 when they were trapped in a Tyken's Rift.

According to the database, this material is a "high-yield version of standard industrial ultritium, used in terraforming applications. Activated with microwave pulse ignition devices. Intended for use when nuclear explosives are not desirable." (TNG: "Night Terrors")

Atoms of this material appeared in an okudagram display of explosives listed in the Enterprise-D library computer.