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Let's say that you have a commercially/privately-operated jet aircraft. It's not something that's going to be used by a single pilot, or someone making mail runs, or a bush pilot; it's more of the type of thing that would be run by a commercial airline, or a UPS-style delivery company. You will see why.

Now, unlike most jet aircraft, this one runs on a nuclear jet engine; specifically, a fission model, and one of the indirect air cycle variety, in which the reactor is not exposed to the inside of the engine but instead heats it via a series of fluid loops (likely liquid metal or sodium).

Advantages to this include:

  • time aloft is no longer limited by fuel supply; instead, it is limited by crew endurance

  • an absence of greenhouse gas emissions

  • the reactor can be used to provide electricity for the rest of the aircraft

  • a vehicle with this type of propulsion can operate in zero-oxygen atmospheres, as it does not rely on a hydrocarbon-oxygen combustion reaction

However, this model of aircraft is commercially owned and operated. That means that, instead of it being some kind of experimental vehicle piloted only by test pilots, there are going to be thousands of the things, and they're not going to be exclusively flying over test ranges anymore.

Also, these things are pretty big - imagine, say, a 737, with the absolute bare-minimum size being something like a DC-3.

Given that this aircraft runs on a fission engine, which is radioactive if breached, as well as that it runs on fuel that is probably quite valuable to any hijacker, what safety features or operational standards would it require in order to become commercially-viable?

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    $\begingroup$ Fusion or blimp or not going to happen - the thing will crash, eventually, so it requires significantly better tech level than we have atm, something like specific nanotech/machines. So which technology level is available for the situation - it needs to be added to the q $\endgroup$
    – MolbOrg
    Commented Aug 17, 2021 at 8:34
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    $\begingroup$ Nuclear safety is an extremely complicated subject, that has multiple disciplines within it. There would be an enormous amount work going into making such a plane safe. For example, the UK Office of Nuclear Regulation publishes an overview document detailing the 36 conditions which must be met before they will licence an operator (onr.org.uk/documents/licence-condition-handbook.pdf), and there are further conditions which apply to nuclear vehicles. It may help if you could say if you are focusing on the engineered systems, the human systems or the organisational systems. $\endgroup$ Commented Aug 17, 2021 at 9:47
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    $\begingroup$ I don't see this happening without something that directly deals with the fundamental problem like fallout eliminating nanites or non-radioactive (or shorter-term radioactive) fission processes. $\endgroup$
    – DKNguyen
    Commented Aug 17, 2021 at 14:39
  • $\begingroup$ Can we call self-destruct button a safety feature? $\endgroup$
    – Alexander
    Commented Aug 17, 2021 at 18:39
  • $\begingroup$ Are we assuming a magical reactor? Nuclear powered aircraft were tried and were non-viable with a fission pile. $\endgroup$
    – jdunlop
    Commented Aug 19, 2021 at 21:46

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The nuclear engine would have to be in a black box.

Normally used to store flight recorders, the (not actually black) black box is very durable and extensively tested against hostile conditions.

Now to the testing, which is insanely intensive:

  1. The black box gets shot out of an air cannon at 3,400 times the force of gravity (or 3,400 Gs, if you’re cool/still like to quote Top Gun). It hits an aluminum target at about the force of a jumbo jet hitting the earth.
  1. For five brutal minutes, it’s crushed with 5,000psi (pounds per square inch) of pressure to ensure it can withstand a sustained impact.
  1. To test it against fire, the box sits inside a 2,000 degree fireball for an hour. Without sunscreen.
  1. Then, testers do a full-on Jacques Cousteau and drop it into a pressurized saltwater tank, simulating the water pressure at 20,000ft below the surface. For 24 hours. In a slightly less-pressurized environment, it must then survive 30 days completely submerged in saltwater.
  1. And if all that wasn't enough, a 500lb weight with a quarter-inch pin sticking out is dropped on the box from 10ft up to make sure it won’t puncture.

If all that goes well, then the unit is run through a series of diagnostic tests to see if it still works.

The nuclear power engine would need similar testing, to ensure that the plane could fall out of the sky and crash and the nuclear material not leak.

It should also be made very hard for someone to steal the waste. A safecracker could do it in time, but not quickly.

The weight requirements would be significant, but the extra power should let you run more powerful engines, so it balances out.

The waste design and flight path would need to be such that it was less vulnerable to terrorism.

Some risk is understandable. Hospitals and industry people already use a lot of radioactive material and have crappy security, and we haven't had a dirty bomb made yet.

That said, the flight pattern and waste disposal should be done to minimize the risk. They shouldn't fly over countries that might abduct them, and shouldn't land in countries that might have people attack them. They should make sure the waste is stored in ceramics or vitrified glass form, making it hard to weaponize it into a dirty bomb.

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Special disaster-resistant fuel.

triso fuel

A fission reactor means you need fissile elements. It is possible to process these elements to minimize the chance of them escaping into air or water in case of a disaster, or even the chance of their participating in a runaway reaction. An example: triso fuel.

Tristructural-isotropic (TRISO) fuel is a type of micro fuel particle. It consists of a fuel kernel composed of UOX (sometimes UC or UCO) in the center, coated with four layers of three isotropic materials deposited through fluidized chemical vapor deposition (FCVD). The four layers are a porous buffer layer made of carbon that absorbs fission product recoils, followed by a dense inner layer of protective pyrolytic carbon (PyC), followed by a ceramic layer of SiC to retain fission products at elevated temperatures and to give the TRISO particle more structural integrity, followed by a dense outer layer of PyC. TRISO particles are then encapsulated into cylindrical or spherical graphite pellets. TRISO fuel particles are designed not to crack due to the stresses from processes (such as differential thermal expansion or fission gas pressure) at temperatures up to 1600 °C, and therefore can contain the fuel in the worst of accident scenarios in a properly designed reactor.

These are in little fuel jawbreakers. Don't eat them! Well, one will probably be ok. In any case, the coating keeps the fuel under control. There is much riffing on this - QUADRISO fuel etc.

A dirty bomb is still possible. They would need to crack and process the coated fuel to allow radioactive materials to disperse with the dirty bomb. That would take some doing.

It also occurs to me that using an unusual but legitimate fuel like thorium or unenriched uranium metal could limit the usefulness to terrorists who want to capture these materials to make a nuclear bomb. Bomb plans are probably pretty specific as regards starting materials.

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  • $\begingroup$ With that last idea (using thorium or uranium) keep in mind we're talking aircraft.. weight will be a consideration ! $\endgroup$
    – Goodies
    Commented Aug 17, 2021 at 18:35
  • $\begingroup$ @Goodies - if you can think of a lighter fission fuel please please post it! I $\endgroup$
    – Willk
    Commented Aug 17, 2021 at 19:03
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    $\begingroup$ TRISO reactors can run at much higher temperatures than more conventional nuclear reactors, because the SiC pellet shells melt at a way higher temperature than uranium metal, and will be able to keep the molten uranium contained. The hotter the heat source of a thermal turbojet, the more the gas passing through the engine can expand, and thus the more efficient the engine is- so a TRISO reactor might be a good choice for a nuclear jet engine, even ignoring the "what if the plane crashes" consideration. $\endgroup$ Commented Aug 17, 2021 at 20:45
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In addition to the other mentioned items

You are going to need a lot of shielding.

Fission reactions emit a lot of neutrons. Neutrons need a lot of shielding, but gamma radiation is an even worse problem. Figure on a 60 cm thick wall of lead to reduce gamma radiation by a factor of 1000. Given the intensity of radiation in the core, I would probably specify 100 cm of lead. This of course will be insanely heavy to fly around with.

Throw away all high-pressure designs.

The extra weight to have a flying containment vessel would be insane. Most of our commercial reactors are based on pressurized reactors, so you have to use a design for which there is limited experience. E.g., the molten-salt designs proposed for nuke-planes for the USAF. Note that limited design experiences means additional cost and delay in order to get a license to even fly a test plane, no matter if your genius engineers could get the design right the first time.

Your reactor design has to be suitable for zero or negative gravity.

Even though you don't plan on flying combat maneuvers or such-like, strong air-currents can result in negative gravity for a plane. It would be very nice if your plant is designed to cope with this condition instead of going super-critical during such an event. I know that some reactor designs would fail because they rely on gravity (e.g. coolant pools.)

Materials exposed to radiation are damaged by it.

Bombarding engineering materials with hard radiation causes both chemical and nuclear changes. All material in such areas will have to be designed to withstand the expected radiation exposure with tolerable degradation, and this almost means certainly means accelerated inspection and replacement schedules should be expected.

Even more redundant safety features

People like their redundant safety measures when nukes are located on the ground behind hardened bunkers. Before they let you fly, expect that the safety features required will be redoubled.

Don't forget your whole-plane parachute

People are going to be nervous about the nuke-planes. Hard to imagine that this won't be a licensing requirement once somebody decides it is a needed safety features. I am pretty sure I would lobby for it (after shorting your stock).

Some serious liability bond is in order

OK, say you are running your nuke-plane fleet and some terrorist takes one down with a missile. When you lose containment in the middle of the big city, you are going to face a huge class-action suit. Without such a bond, you are not going have investors or be licensed to operate. Not the safety equipment you were thinking of perhaps, but nonetheless essential.

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  • $\begingroup$ "Given the intensity of radiation in the core, I would probably specify 100 cm of lead" Put the engines on the end of the wings, so that distance is doing some of that attenuation for you. ;) $\endgroup$
    – nick012000
    Commented Aug 21, 2021 at 3:26
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Emergency stop

A nuclear reactor in an aircraft should have the most important safety feature any nuclear power source has: emergency stop. In this case, that stop could occur at 40.000 Ft, in an aircraft with passengers, and little gliding capability because of its weight.

At least three very large and strong parachutes would be needed, plus near water, to get the aircraft safely down.

Without parachutes, to be able to land safely at any time, consider propulsion that can run on both nuclear energy and fuel (Cerosine), a hybrid engine construct was introduced by the Russians when they developed Tupolev Tu-119, which succeeded their early nuclear prototype Tu-95.

Emergency Cooling

A nuclear power source needs emergency cooling and a relevant amount of cooling fluid at hand. In a submarine, cool water is abundant, it just needs a safe inlet. But in an airplane, the cooling installation and its coolant would add considerable weight to the aircraft.

Shielding

Weight issue too. See the other answers. Military prototypes failed on it, ref The Atlantic Jan'2019

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  • $\begingroup$ "But in an airplane, the cooling installation and its coolant would add considerable weight to the aircraft." Well, jet aircraft do have access to a functionally unlimited amount of freezing cold air once they're above 7000 to 8000 feet. $\endgroup$
    – nick012000
    Commented Aug 21, 2021 at 3:24
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The big problem with powering an aircraft using a fission reactor is radiation.

The substances used to stop the radiation from escaping: lead, water, or concrete, are all very heavy.

Consider the case of the NB36H, a US B-36 bomber modified to carry a small fission reactor. The reactor didn't actually power the aircraft, it was just a test to find out what happens when you put a reactor in an aircraft.

GE did some early work on a jet engine that used the heat from the reactor to power the engine, instead of burning fuel. Problem there was it tended to irradiate particles in the air, leaving a radioactive trail. Bases that operated such an aircraft would have had a real problem with residual radiation.

With the NB36H, the crew compartment was lined with 11 tons of lead, making it very heavy. And, while the NB36H did fly a number of times, and the shielding was found to be effective, the disaster that would result from a crash, including a runaway meltdown of the reactor core, put an end to this idea.

The Soviets had an almost identical program, the TU95LAL, which was abandoned for the same reason: the radioactive trail it left in the air, and the dire consequences of a crash.

Presumably a nuclear powered aircraft would use a similar shielding arrangement, but it would also face the dual problems of leaving a radioactive trail in the air, and the results of a crash.

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