For over the last 13 years, I've been a huge supporter of the Gas Core Nuclear Rocket (GCNR) concept, ever since I saw the Discovery TV program "Rocketships," narrated by Kate Mulgrew.
On the show was a scientist named Dr. Steve Howe, I became an instant fan of his after listening to him on that show. He is one of the foremost experts on nuclear rocket propulsion, and has worked on the GCNR.
Here's more info on the GCNR from Wikipedia:
The final classification (of nuclear thermal rockets) is the gas-core engine. This is a modification to the liquid-core design which uses rapid circulation of the fluid to create a toroidal pocket of gaseous uranium fuel in the middle of the reactor, surrounded by hydrogen. In this case the fuel does not touch the reactor wall at all, so temperatures could reach several tens of thousands of degrees, which would allow specific impulses of 3000 to 5000 seconds (30 to 50 kN·s/kg). In this basic design, the "open cycle", the losses of nuclear fuel would be difficult to control, which has led to studies of the "closed cycle" or nuclear lightbulb engine, where the gaseous nuclear fuel is contained in a super-high-temperature quartz container, over which the hydrogen flows. The closed cycle engine actually has much more in common with the solid-core design, but this time is limited by the critical temperature of quartz instead of the fuel stack. Although less efficient than the open-cycle design, the closed-cycle design is expected to deliver a rather respectable specific impulse of about 1500–2000 seconds.
Nuclear Lightbulb concept:
A nuclear lightbulb is a hypothetical type of spacecraft engine using a Fission reactor to achieve Nuclear propulsion. Specifically it would be a type of Gas core reactor rocket that separates the nuclear fuel from the coolant/propellant with a quartz wall. It would be operated at such high temperature (approx. 25,000°C) that the vast majority of the electromagnetic emissions would be in the hard ultraviolet range. Fused silica is almost completely transparent to this light, so it would be used to contain the uranium hexafluoride and allow the light to heat reaction mass in a rocket or to generate electricity using a heat engine or photovoltaics.
This type of reactor shows great promise in both of these roles. As a rocket engine it, like all nuclear rocket designs, can greatly exceed the power density of a chemical rocket. However, it also does not involve the release of any radioactive material from the rocket, unlike other nuclear designs which would cause nuclear fallout if used in a planetary atmosphere (e.g. Project Orion). As a method to generate electricity, nuclear lightbulbs are extremely efficient because higher-temperature heat contains more Gibbs free energy than the low-temperature heat produced in current fossil-fuel plants and water-cooled nuclear reactors.
