What is the emergency propulsion on nuclear submarines?

Depends on what you mean by emergency propulsion. The Emergency Propulsion Motor (the EPM) on the USS Ethan Allen (SSBN-608) was a LARGE DC motor wthat was fixed to the propeller shaft. read more

A nuclear submarine has two independent fuels, nuclear and diesel fuel. The emergency battery is not a fuel, it’s an energy storage method, but for the purpose of this discussion, I will consider it to be a fuel. If the nuclear reactor goes off-line (scrams), there is no steam for propulsion and electrical generation. read more

The performance advantages of nuclear submarines over "conventional" (typically diesel-electric) submarines are considerable. Nuclear propulsion, being completely independent of air, frees the submarine from the need to surface frequently, as is necessary for conventional submarines. read more

Strategic submarines actually predated the nuclear-propulsion era, in that during the 1950s both the U.S. and Soviet navies developed missile-carrying diesel-electric submarines. The U.S. submarines were armed with Regulus cruise missiles, and the Soviet ships carried SS-N-3 Shaddock cruise missiles and SS-N-4 Sark short-range SLBMs. read more