I think it was the liquid core that mentioned spraying radioactive exhaust everywhere. [quote name=Nuclear thermal rocket From Wikipedia, the free encyclopedia]Robert Zubrin has proposed an alternative liquid-core design, the nuclear salt-water rocket, wherein water is the working fluid and neutron moderator. The nuclear fuel is not retained, drastically simplifying the design. However, by its very design, the rocket would discharge massive quantities of extremely radioactive waste and could only be safely operated well outside the Earth's atmosphere and perhaps even entirely outside earth's magnetosphere.