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In this article on nasa.gov (https://www.nasa.gov/press-release/nasa-darpa-will-test-nuclear-engine-for-future-mars-missions) it mentions that NASA will test a nuclear rocket engine for future crewed Mars flights.

I thought that it was forbidden to test nuclear rockets because of the nuclear ban treaty (https://en.m.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treaty)

How does NASA have permission to test the nuclear rocket engine?

The Rocket fan
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The nuclear test ban treaty bans testing nuclear weapons. It does not ban nuclear reactors. A nuclear rocket engine of the type proposed by NASA would be a thermal nuclear rocket: https://en.wikipedia.org/wiki/Nuclear_thermal_rocket

This is effectively a nuclear reactor where the propellant acts as the coolant. The propellent (often liquid hydrogen) is heated to very high temperatures inside the reactor and is then allowed to escape this provides thrust as well as carrying away the reactor heat, whilst leaving the reactor core intact inside the rocket engine.

Atmospheric nuclear tests were banned in the US in 1963, but nuclear rocket engines were physically tested after that in the US under the NERVA program: https://en.wikipedia.org/wiki/NERVA

Multiple tests of the NERVA thermal nuclear rocket were carried out at the Nevada test site in the 60’s some at more than 1 GW power and producing a specific impulse of 811 seconds (compared to chemical rockets which are limited to around 450 seconds at best).

Slarty
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  • Are thermal nuclear rockets dangerous? Like if something went wrong would it explode like Chernobyl again? – The Rocket fan Jan 25 '23 at 07:40
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    Thermal nuclear rockets should be very safe to fly. They will not be used to lift a rocket off the launch pad, they would only be activated in Earth orbit and beyond. During launch the reactor would have control rods held in place to prevent any significant nuclear activation. Testing is another matter, but it should be possible to design a test rig with multiple safety features (I'm not a nuclear expert). But there are still design challenges and regulatory challenges to be overcome. I suspect the advent of the SpaceX Starship may dent interest in the nuclear thermal rocket (we shall see) – Slarty Jan 25 '23 at 08:19
  • But do they still produce nuclear waste? – The Rocket fan Jan 25 '23 at 08:40
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    Any test rocket would remain on Earth and would have to be dealt with on Earth. Nuclear reactors run for many years and slowly become more radioactive as decay products build up in the fuel. But the operational lifetime (actually firing) of a nuclear rocket engine can probably be measured in minutes so the amount of dangerous decay products would be minimal. Any operational nuclear rocket would probably end up on the Moon or Mars so would be much less of a problem even than that. – Slarty Jan 25 '23 at 09:14
  • In short yes, but only the test engines used on Earth and even they would produce very little waste. – Slarty Jan 25 '23 at 09:16
  • It is good to know that the risk isn’t so high, but I can’t help thinking about Space: 1999 – The Rocket fan Jan 25 '23 at 10:13
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    @TheRocketfan Chernobyl didn't explode. Nor did Three Mile Island or Fukushima. They suffered a meltdown, something very different from a nuclear explosion. – David Hammen Jan 25 '23 at 14:12
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    @DavidHammen - Chernobyl did explode; mainly a steam explosion but it was the result of a runaway nuclear chain-reaction. – antlersoft Jan 25 '23 at 15:33
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    The issue isn't how safe they are to fly. It's how safe they are to crash. This isn't a bomb or a meltdown. Think nuclear sub dropped from space. – candied_orange Jan 25 '23 at 16:29
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    @TheRocketfan For the testing at least, I would be much more worried about an industrial accident like Kyshtym or Tokaimura than a serious meltdown like Chernobyl or Fukushima. We’ve actually gotten pretty good at avoiding meltdowns. Ignoring Fukushima, there haven’t really been any major meltdown incidents since the late 80’s. – Austin Hemmelgarn Jan 25 '23 at 18:07
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    @canddied_orange its not quite the same as a nuclear sub dropping from space. At least not a nuclear sub that has been running for many months or years. Such a reactor would have accumulated a range of highly radioactive fission products. A thermal nuclear rocket engine would have been run for a minute or two at the most before the flight so would just consist of uranium which is not very radioactive. – Slarty Jan 25 '23 at 18:36
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    Might be worth mentioning that the Earth's seas are currently awash in ships with nuclear power plants, including all US carriers, most of its subs, and at one point many of its cruisers. So sticking a nuclear generator in a craft to provide power for propulsion isn't exactly a strange new idea. – T.E.D. Jan 25 '23 at 19:40
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    @T.E.D. to be fair, none of those have a ground speed measured in 10's of thousands of kph. If a supercarrier runs into the ground, you have billions of dollars of damage, but little to no risk of a nuclear accident. A nuclear rocket running into the ground is a big ol' dirty bomb. These are a fantastic idea that is probably best left for interplanetary freight: launch an unfueled nuclear engine to space, fuel it in orbit, and use it to transition back and forth to the moon/Mars/etc. Use chemical rockets for launch and landing, leaving the nuclear freighter in orbit. – asgallant Jan 25 '23 at 21:54
  • @asgallant - An unmanned nuclear reactor settling the to bottom of the ocean (possibly in international waters nearby a major coastal city) seems like it could in fact pose a real problem as well. – T.E.D. Jan 25 '23 at 21:57
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    I know you said something like this already, but just to put some numbers on the spent fuel issue, You're talking about a test run that produces around one GW for a few minutes. A base-load, nuclear power station, of which there are several hundred operating in the world right now, typically produces multiple GW, 24 hours per day, 365 days per year. – Solomon Slow Jan 26 '23 at 02:57
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    @asgallent nuclear rockets will not be used to launch from the surface of the Earth. Although they are very efficient they don't produce enough thrust. A nuclear rocket running into the ground is a big hunk of purified uranium which is not pleasant but not that radioactive either, the half lives are 700 and 4400 million years for the isotopes present. – Slarty Jan 26 '23 at 08:46
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    I think Starship will rather increase interest in nuclear rockets, far from denting it. Nuclear rockets aren't going to be used for launching heavy equipment, and you need a lot of that before it becomes interesting for more than some small crews of humans to travel between Earth and Mars. But once Starship has established the infrastructure, it will be very attractive to shorten crew travel time between the planets, and the number of people affected will make it a far more worthwhile investment. – leftaroundabout Jan 26 '23 at 12:45
  • I wonder if any of those who worked on NERVA will live to see the groundwork they laid finally put to use. – Ian Kemp Jan 26 '23 at 17:40
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    @asgallant fueling a nuclear reactor in space is all well and good, but wouldn't we still have to launch the fuel on a rocket of some sort, taking the risk that something goes wrong during launch and there's a big boom? I'm not the paranoid "the sky is falling" type, just asking if there's any actual advantage to doing the final assembly in space vs on Earth. Refueling in space (every 30ish years, like a nuclear carrier) certainly makes sense. – FreeMan Jan 26 '23 at 18:15
  • Any nuclear engine would be launched with the fuel already onboard and with control rods locke in position. Re Starship it will completely change the calculus of space flight with 100 tonnes to play with the options for shielding become more attractive. It will be a different ball game to what we currently see with crews having to build some wendy house like structure from supplies to help shield themselves during a solar storm. – Slarty Jan 26 '23 at 18:28
  • @Slarty I believe that the idea of using a nuclear-powered rocket was ruled out at the very beginning of the Apollo project, was it the 1963 ban you mentioned? – Mat Jan 26 '23 at 20:00
  • @Mat Nuclear rockets were tested from 1964 and throughout the 1960's. They were originally considered to be a possible upper stage for Saturn V and had considerable support, but were cancelled by Nixon in 1973 along with further Apollo Moon shots. – Slarty Jan 26 '23 at 20:07
  • https://en.wikipedia.org/wiki/NERVA – Slarty Jan 26 '23 at 20:07
  • @FreeMan I'm talking about fueling it in space, not assembling the reactor there (though there could be advantages to that, but I do not know what they would be). – asgallant Jan 27 '23 at 16:29
  • @Slarty yes, I know. I'm just saying launching a fueled reactor seems like a Bad Idea. – asgallant Jan 27 '23 at 16:30
  • @T.E.D. sure, but the scale of the problem is completely different than disintegrating a fueled reactor in the atmosphere. – asgallant Jan 27 '23 at 16:31
  • Are you talking, @asgallant, about collecting radioactive material in space and creating the fuel rods there (including whatever refinement is necessary)? If not, then I think we're on the same page: Launch the reactor, launch the refined & built fuel rods, insert fuel rods into reactor once everything's in space. It's the launch of the refined fuel rods itself that causes the risk, right? Not whether they're installed in the reactor at the time of the launch... Again, I'm no Chicken Little, I'm just curious what the advantages would be to having them in separate launches. – FreeMan Jan 27 '23 at 16:37
  • @asgallant - Perhaps, but I'm guessing not in the way you think. The volume of water in the earth's oceans is about 1.37 billion cubic kilometers. The volume of the earth's atmosphere is about 51 trillion trillion cubic meters. – T.E.D. Jan 27 '23 at 18:55
  • @FreeMan you could mine and refine fuel in space, but that assumes mining operations in space. For launching the fuel from Earth, we could design containers for it that are more resistant to a Rapid Unscheduled Disassembly than a reactor core could possibly be. Anyway, this has gone way beyond the bounds of comments and should probably get moved to chat if anyone wants to continue it. – asgallant Jan 27 '23 at 19:09