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The nuclear lightbulb is a closed-cycle gas-core nuclear propulsion system. It uses quartz to separate the nuclear fuel from the propellent, thus eliminating the radiation problems with open-core designs. Unfortunately, quartz melts at <2000 degrees Celsius, limiting the temperature of the engine.

If the quartz was replaced with graphene, or a graphene-quartz mixture, how high could the exhaust velocity be raised?

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How transparent is graphene or graphene-reinforced-quartz to UV? For a nuclear lightbulb to work the answer needs to be "Completely."

The basic idea for a "Nuclear Lightbulb" Nuclear Thermal Rocket calls for the reactor vessel to be transparent to UV radiation. It's a concept, not a thoroughly evaluated design, so everything but the basic physics is speculative. The idea is that you operate a fission reactor in a gas (really plasma) phase inside a transparent pressure vessel. The fissionables might be mixed with a fluorescing compound. If you run the reactor hot enough, radiation (which scales with the fourth power of temperature) becomes the dominant mode of energy transfer, primarily in the form of UV light. You pass your reaction mass - likely hydrogen doped with something to improve its UV absorption - over the outside of the reactor vessel. It's heated by the UV, conceptually to much higher temperatures than possible with solid core NTRs.

The concept hinges on the reactor vessel being so perfectly transparent to UV radiation that you can pass gigawatts of UV light through it without it absorbing them and therefore heating and melting. Additionally you need to run a plasma-phase fission reaction inside it (good luck with your neutron economy) and somehow protect it from that ferociously hot and corrosive material.

It's an intriguing concept and seductive in its simplicity and the amazing performance it promises, but the materials science to support it doesn't exist and likely never will. No material is 100.0000% transparent to UV, even before you etch the surface with a fissioning plasma.

There have been other concepts under the "Nuclear Lightbulb" umbrella with no pressure vessel. In those you trap your fissioning plasma with magnetic fields and run the fuel around it using clever swirl and flow regimes to prevent the two from mixing. Computational models (initial studies) have been run but they all show some escape of the plasma into the fuel; your exhaust would be highly radioactive.

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