Radium Rocket Engine

For traveling longer ranges in space chemical reaction based propulsion is not feasible. Here I propose a nuclear propulsion system using Ra₂₂₄ (Radium 224 isotope). The idea is to fission Radium to emit alpha radiation and Radon 220 which is a noble gas. The basic mechanism of all rocket engine is to emit high gas from a nozzle. In this rocket design the gas is Radon 220 and heat source is the fission reaction of Radium 224. Radon is a heavy atom which would increase the efficiency of the rocket engine. The fuel is solid therefore the volumetric efficiency of the fuel is also high. 

Thorium 232 is quiet abundant on Earth and is not radioactive like Uranium.   Turning Thorium 232 into Thorium 228 is not easy but can be achieved. Later Thorium 228 can be turned into Radium 224 which is much easier. Here is the Thorium decay cycle.

This rocket engine can even be used on earth. Rocket emission Radon 220 decays to stable Pb 208 (Lead) with a half life of last than 12 hours. Lead is not environmentally good but dispersion on a large area would be less problematic compared to other Nuclear rocket engine emissions that have much longer half lives and pollute the land for centuries.