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Let us assume that in the next 50 years one of the large nation-states of our world will build nuclear powered planes, for intercontinental flight. Supposedly, the nuclear planes would be bigger, faster, and need less oil in a world where good oil sources are dissapearing (so, assume that peak oil is true and that by 2070 the peak has come and gone). Also, nuclear planes would be a stepping stone to regular suborbital space planes, then to LEO space planes, that would be used to build nuclear pulse propulsion ship (AKA, orion bomb drive). The nation-states have deep pockets and are commited to develop the nuclear planes. We can, if necessary, assume that the nation-state in question can ignore public opinion for a long time, maybe it's a dictatorship like Soviet Union was and can ignore international outrage if something goes wrong.

Having said that, the government does not want a Chernobyl in the sky and would like for the nuclear plane to be as safe as possible. One of the main dangers of such a plane is the radiation emitted by the reactor: that will hurt the crew, irradiate the cargo and damage the control systems. How can the designers contain the radiation without using massive shielding like that used in well-built nuclear power plants? My first idea was to use a tungsten carbide sphere around the reactor to reflect the neutrons back to the fissile material and be able to achieve sustainable fission with less material while at the same time getting rid of the dangerous hot neutrons. I am aware that T.Carbide is heavy but is better then the meters of concrete and lead nuclear power plants use.

The nation-state is ignoring the other danger of the nuclear plane - it crashing and irradiating neighborhoods, there isn't much that can be done about that except placing the airports and the routes far from the cities and critical farmlands.

Would that work? If it won't, why, and what would work?

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    $\begingroup$ Have you researched what was planned and written about this back in the 1950s? American plans went far enough to actually fly a B-36 with a submarine reactor aboard (though as far I know the reactor wasn't operated in flight). $\endgroup$
    – Zeiss Ikon
    Commented Oct 7, 2019 at 17:07
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    $\begingroup$ There were also hot fire tests of a nuclear ramjet design, in Idaho, in the 1960s. $\endgroup$
    – Zeiss Ikon
    Commented Oct 7, 2019 at 17:08
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    $\begingroup$ "Also, nuclear planes would be a stepping stone to regular suborbital space planes" This isn't a valid assumption. The kind of propulsion systems you're talking about require an atmosphere to generate thrust. There's little to no commonality between nuclear powered aircraft and the kind of propulsion systems you need for either SSTO applications, and non whatsoever with the technologies required for nuclear powered spacecraft. $\endgroup$
    – Morris The Cat
    Commented Oct 7, 2019 at 17:54
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    $\begingroup$ Also, if a tungsten carbide sphere would allow a nuclear plant to achieve criticality with less shielding and less material, a nuclear plant would have been built with one. The people who design nuclear power plants aren't slow on the uptake; designs don't deliberately throw away advantageous ideas unless they don't work as intended. (Also, it's worth noting that tungsten is denser than lead, and they have similar radiation-shielding profiles per unit volume.) $\endgroup$
    – jdunlop
    Commented Oct 7, 2019 at 18:16
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    $\begingroup$ @Alexander Depending on the fuel type used, a crash would possibly not be significantly more dangerous than a traditional plane crash. If Thorium is used, the worst decay products are gone after a few weeks. $\endgroup$
    – Ryan_L
    Commented Oct 7, 2019 at 18:17

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Short answer:

If it would work, someone would have done it already

As was pointed out in the comments, this was explored in the 1950s and 60s, and an airborne nuclear reactor was built and run. The insurmountable problem (as you cover in the question) was that shielding a nuclear reactor requires heavy materials. This is a property of radiation, and is unavoidable.

The whole purpose of the NB-36H was to determine if sufficient shielding to protect the crew and cargo could be provided while still allowing the plane to fly under the power of the reactor. Not only was this not the case, but the heat exchanger used to heat the air for the jet engines (not used in the NB-36H, but in ground testing) mildly irradiated the jetwash, which would probably be unacceptable to flyover countries.

It's possible that metamaterials might be useful in constructing lightweight radiation shields:

Research from North Carolina State University shows that lightweight composite metal foams are effective at blocking X-rays, gamma rays and neutron radiation, and are capable of absorbing the energy of high impact collisions. The finding means the metal foams hold promise for use in nuclear safety, space exploration and medical technology applications.

Even then, the issue of channeling the heat to the jet engines potentially circumvents this protection, because any heat exchanger is necessarily going to be exposed to the reactor core, and its medium will become irradiated.

Your best near-future hope may be to wait for cheap, practical fusion, and build a truly colossal aircraft. The neutron flux per unit power from a fusion reactor is much lower than a fission reactor, so your energy density is higher per unit mass. But fission is almost certainly a non-starter. The nuclear-crazy 50s and 60s gave us all the information we need to conclude that.

Edit: Another comment mentioned the nuclear ramjet engine. Project Pluto definitely would work, with the right materials, but the quantity of radiation and fallout expelled from the engine would make it totally unacceptable for any use but the one for which it was envisioned - a loitering cruise missile meant as a nuclear deterrent.

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    $\begingroup$ As a further addendum, a 747's engines output at takeoff is ~90MW. The airborne experimental reactor output 1MW, while the requirements for its jet engines summed to 11MW. So even if the shielding had been sufficient, a larger reactor would have been necessary. $\endgroup$
    – jdunlop
    Commented Oct 7, 2019 at 17:53
  • $\begingroup$ Couldn't you use drones instead of manned aircraft? Electronics might need less shielding than humans. $\endgroup$
    – Richard Smith
    Commented Oct 8, 2019 at 16:24
  • $\begingroup$ Re "If it would work...", not necessarily. Why go to the expense of building nuclear powered airplanes when jet fuel is cheap & plentiful? Conversely, nuclear power works perfectly well for ships (e.g. NS Savannah: en.wikipedia.org/wiki/NS_Savannah ) there are few if any nuclear powered cargo ships in operation. $\endgroup$
    – jamesqf
    Commented Oct 8, 2019 at 18:03
  • $\begingroup$ @RichardSmith - the OP made it clear he wanted crews and radiation-sensitive non-crew cargo. $\endgroup$
    – jdunlop
    Commented Oct 8, 2019 at 18:17
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    $\begingroup$ @jamesqf - there are lots of nuclear-powered ships in operation... in the Navy. The NS_Savannah was a proof of concept. And that's my point. Regardless of economics, either the USSR or the US military would have built a nuclear powered plane to demonstrate the viability of the idea, before mothballing it because it's much more economic to use traditional fuels. $\endgroup$
    – jdunlop
    Commented Oct 8, 2019 at 18:19
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The US and the USSR both extensively researched nuclear powered aircraft in the past.

As traditionally designed, these aircraft wouldn't be a bridge to space planes, since they still need air to function. Radically different technology would be required to get to space. Nuclear rocket technology could get to space, and has also been tested. In general, such technology wouldn't be reasonable to use for flights inside the atmosphere. Air breathing engines are simply far more efficient. However, it would be possible to build an engine that could convert from using a nuclear reactor to heat an onboard propellant, rather than air, and bridge from atmospheric to orbital flight.

The primary downsides of traditional designs for space planes have been the issues of crew shielding and crashes, neither of which were ever resolved, and neither of which have any real means of resolution based on modern technology. Shielding is heavy, and there's no way to stop a reactor hitting the Earth at supersonic speeds from being a mini-Chernobyl. Nuclear rockets, furthermore, tend to irradiate their exhaust. The only ways around these issues are to not have people on board, and to not care about nuclear pollution. Those are certainly possible, but mean that fission-powered nuclear aircraft won't ever really be a viable replacement for all the things we currently use fuel-burning aircraft for.

As such, any reasonable nuclear powered flight will have to rely on fusion, rather than fission. Electric engines would be reasonable for atmospheric aircraft, and it's likely that it would be possible to build a thermal engine, as well. Current direct-thrust research leans towards high specific impulse low thrust engines, which likely makes it unsuitable for any atmospheric flight. Fusion, compared to fission, creates relatively little radiation, and no mini-Chernobyls when aircraft inevitably crash. However, we have yet to realize any net energy production through fusion power. In order to produce a reasonable aircraft in 50 years, we'd need some major breakthrough to happen in the near future, as substantial refinement and miniaturization of a fusion power source would need to happen before it was reasonable to put one on an aircraft.

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  • $\begingroup$ Won't fusion generate hot neutrons too? $\endgroup$
    – Geronimo
    Commented Oct 7, 2019 at 20:55
  • $\begingroup$ @Geronimo Depends what you're fusing. There are so-call aneutronic fusion reactions that have been proposed for this and that -- but let's wait until we can realize a net energy gain from the easy deuterium-tritium reaction before we start attempting lithium-boron... $\endgroup$
    – Zeiss Ikon
    Commented Oct 8, 2019 at 16:23
  • $\begingroup$ A nuclear plane could be a bridge to spaceplanes by allowing you to build a big, fast, plane, capable of going very high, very fast, and then activating the space propulsion that will push something else. $\endgroup$
    – Geronimo
    Commented Oct 8, 2019 at 20:09
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As multiple people have stated, the americans (and probably russians) did experiment with atomic airplanes in 50-60ies. Though to elaborate a bit on this, the primary reason nothing came of it, was not due because it couldn't be done. Because of weight limitations in airplanes they were working on whats called a MSR (Molten Salt Reactor) that has a number of advantages over a regular reactor as we know it. It's safer, it's inherently stable and can't run out of control and it's potentially much lighter/smaller. More data here: https://en.wikipedia.org/wiki/Molten_salt_reactor

The last hurdle that was not solved before funding was cut (for other reasons), was removing the nuclear waste from the salt solution. Due to lack of funding and the unpopularity of nuclear energy further research was sadly not done. (some work has been done from private sector, though mostly theoretical as far as i know)

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Fusion plane.

All of those fission-reaction powered planes were covered in other answers. That is totally doable and not really science fiction. Probably a fission rocket is what the Russians were bragging about recently. Fission power uses the heat generation by the breakdown of heavy elements and in the process liberates additional radioactive materials - fission products. Fission is what we use now for nuclear power, and all the nuclear weapons have some fission component.

But you are in the future. In the future we will have figured out fusion power. Fusion power does not generate radioactive fission products or gamma rays. It just generates loads of heat and that is what you want. Your fusion engine heats the air and throws it out the back, just like a ramjet or one of these fission rockets.

The reason fusion power is not being used now is that it is hard to compress the starting materials enough to get it started. Hydrogen bombs use a fission explosion to achieve that compression. Without that you need giant lasers or other things not amenable to mounting on a plane. But maybe muons or some other tech will facilitate fusion in the future such that you don't need a Tokumak.

Fusion power is near future science fiction and using this form of energy to power a plane is totally plausible science fiction.

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  • $\begingroup$ Not really relevant to your answer but if the Russians had a FISSION rocket it wouldn't have thrown all that Strontium and Barium around when it blew up. $\endgroup$
    – Morris The Cat
    Commented Oct 8, 2019 at 16:19
  • $\begingroup$ @MorrisTheCat - A FISSION rocket is what I suspect they had which is why it did throw that stuff around. A fusion rocket would have thrown around some cool helium isotopes and a gentle hydrogen wind. $\endgroup$
    – Willk
    Commented Oct 8, 2019 at 17:17
  • $\begingroup$ Well there I go getting my Fissions and Fusions confused. That's embarrassing. $\endgroup$
    – Morris The Cat
    Commented Oct 8, 2019 at 17:20
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    $\begingroup$ @MorrisTheCat - Happens all the time in my lab. Makes for lots of employee turnover. $\endgroup$
    – Willk
    Commented Oct 8, 2019 at 17:45
  • $\begingroup$ Oh god, that sounds like an episode of Rick and Morty or something now. $\endgroup$
    – Morris The Cat
    Commented Oct 8, 2019 at 17:50
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You can easily stop alpha radiation, beta isn't that difficult, while gamma is nearly impossible to stop. So, you require something that emits (mostly) alpha radiation.

Your probably best candidate would be Po 210 (https://en.wikipedia.org/wiki/Polonium-210). It takes about 4 months to decay, while product is stable lead. Getting enough of material is a pain you have to solve yourself, likely just by scaling up existing operation - Russia exports several grams per month to US.

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