system
Given the following post, respond as if you're commenting on Reddit:
human
Are nuclear propulsion systems used for spacecraft?  If not, why not?
gpt
No nuclear propulsion systems have yet to be used in space, though some systems have been tested on Earth. They haven't been used for a variety of reasons.

First, not enough research has gone into them. Nuclear systems are expensive to develop and nobody's done the work for it. Second, most nuclear propulsion systems have a fairly high mass, even if they have a high thrust to mass ratio. Meaning they require either on orbit assembly or heavy lift launch to be useful.

Third, launching nuclear materials is risky because launch failures do happen occasionally. Currently the only nuclear materials that are launched are radioisotope thermoelectric generators (RTGs) which keep their nuclear fuels in ceramic form within extremely strong containment vessels capable of surviving almost all conceivable launch vehicle failures. Potentially a two-step launch process could be used for nuclear propulsion systems whereby an unfueled system is launched and the fuel is launched inside a heavy, strong containment vessel and then transferred on orbit. However, such a system would be extremely costly and inefficient.

Fourth, there's no mission need that yet justifies the performance benefits of nuclear propulsion. Unmanned missions can generally afford to take their time to get to their destinations. For high delta V requirements other more efficient thrusters such as ion engines or hall thrusters make more sense since they are smaller, cheaper, and more proven.

Fifth, nuclear technology is heavily politically laden today. Merely researching it would require access to extremely sensitive materials, adding extreme regulatory and political burdens to the process. Building and testing a nuclear salt water rocket would require access to weapons grade Uranium and would involve operation of a very experimental reactor design and the creation of a lot of extremely hazardous radioactive exhaust. Currently we don't have the capacity to develop and test such a system entirely in orbit, and developing it on Earth would be far too expensive and dangerous. Building a nuclear pulse rocket would involve creating actual nuclear bombs. Building even a nuclear thermal rocket (which has seen a modicum of research) would involve sensitive materials and experimental reactors.

If we had more massive spacecraft and more pressing time constraints (such as asteroid/comet diversion interceptors or manned interplanetary spacecraft) nuclear propulsion systems might be more warranted and might see sufficient development to become reality.

TL;DR nuclear propulsion systems are too expensive, unproven, heavy, dangerous, and politically encumbered.