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I came across this Wikipedia entry about Project Pluto; a nuclear-powered ramjet that the U.S. was developing back in the day.

https://en.wikipedia.org/wiki/Project_Pluto

This missile would have been able to fly for weeks and worked on a similar principle to a turbine; superheat the air that comes in, the air expands and this generates thrust.

One thing mentioned in the article is that the radioactive material used as fuel has to achieve criticality.

[question 1] Could someone explain how this type of critical state does not result in a huge nuclear explosion?

[question 2] The article talks about using ceramics to handle the extreme temperatures inside the reactor. How hot can it get when in the pseudo-critical state? The amount of thrust produced by this contraption would have been enormous; just from heating air... so the temperatures must have been spectacular.

[question 3] Was there really no way to keep the craft from irradiating everything on its path? from the article "It was proposed that after delivering all its warheads, the missile could then spend weeks flying over populated areas at low altitudes, causing secondary damage from radiation" thats messed up...

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    $\begingroup$ There's nothing new about maintaining criticality without undergoing an explosion. How do you think nuclear power reactors work? $\endgroup$ – Emilio Pisanty Aug 26 at 7:24
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    $\begingroup$ As for your third question, that's part engineering, not physics, and part rant. Neither is on topic here. $\endgroup$ – Emilio Pisanty Aug 26 at 7:25
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Could someone explain how this type of critical state does not result in a huge nuclear explosion?

In general, you have three possible states for a lump of fissile material:

  • Subcritical, in which there is no chain reaction (i.e. the material decays essentially as if it were just a bunch of separate individual atoms);
  • Critical, in which there is a sustained chain reaction (i.e. the material is concentrated enough that decays from one atom can induce decays in other atoms at a constant rate); and
  • Supercritical, in which there is a runaway chain reaction (i.e. the material is concentrated enough that decays from one atom can induce decays in other atoms at an increasing rate).

Meltdowns and nuclear explosions happen when the matter enters the supercritical state. Quite a bit of nuclear engineering is devoted to keeping a lump of fissile material in the narrow range of conditions that keeps it critical, but not supercritical, by controlling the rate at which decays are allowed to induce other decays in the material. One common way to do this is by inserting other non-fissile matter into the lump that absorbs some of the decay products, slowing down the reaction.

The article talks about using ceramics to handle the extreme temperatures inside the reactor. How hot can it get when in the pseudo-critical state? The amount of thrust produced by this contraption would have been enormous; just from heating air... so the temperatures must have been spectacular.

I imagine the exact temperatures are probably classified; that said, the melting point of the turbine blades used in normal jet engines is around 1400 degrees Celsius (source: https://www.thenakedscientists.com/articles/interviews/how-do-you-stop-jet-engine-melting), so probably hotter than that.

But this doesn't really have anything to do with the fact that this missile happened to be powered by a nuclear reactor. Reading the article, the missile has to be capable of low-altitude supersonic flight. This makes the requirements quite different from those of a normal jet engine, which mostly operates under subsonic, high-altitude conditions, and also quite different from rocket engines, which do not usually enter the supersonic regime until high in the atmosphere (and also have a completely different propulsion mechanism, of course).

The reason a nuclear reactor was used here was not because that was the only way to reach high enough temperatures. There are plenty of other, cheaper and less dangerous ways to make things hot. Rather, the nuclear reactor's main benefit is the incredible energy density of its fuel. A small amount of fuel can power a nuclear reactor for an extremely long time, which would enable the missile to stay in low-altitude supersonic flight for weeks.

Was there really no way to keep the craft from irradiating everything on its path? from the article "It was proposed that after delivering all its warheads, the missile could then spend weeks flying over populated areas at low altitudes, causing secondary damage from radiation" thats messed up...

The key word here is could. The missile could be designed with enough shielding that this wouldn't happen, but 1) shielding is heavy, decreasing the longevity of the missile, and 2) it wouldn't be as effective as a weapon. After all, this is a weapon, a device intended to harm people. It would not be surprising if this was a conscious choice that was made to make it better at harming people.

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    $\begingroup$ Actually, you do want to be able to take the fissile material to a super-critical state for brief periods so that you can obtain the desired power level. You want to guard against a prompt super-critical state, i.e., the power-doubling time needs to be longer than a second. Reactors have super-critical power-doubling times on the order of several minutes thanks to delayed neutron emission of some of the fission products, with mechanisms/processes to become critical after reaching desired power levels. Bombs on the order of milliseconds, maybe less. $\endgroup$ – Bill N Aug 26 at 19:48
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[question 3] Was there really no way to keep the craft from irradiating everything on its path? from the article "It was proposed that after delivering all its warheads, the missile could then spend weeks flying over populated areas at low altitudes, causing secondary damage from radiation" thats messed up...

I have seen this claim in many places, but it fails the smell test.

I have looked around but could not find a good number on how much nuclear waste would be needed to cause a deadly dose when spread out over a given area of ground. Yet I suspect its somewhere in the grams per acre.

An acre is a square about 63 m on a side. This thing is supposed to be flying around at Mach 3. Mach 3 at sea level is almost 3700 km/h, or around 1000 m/s. So it's going to cross that 63 m in 0.063 seconds.

If we go with a gram per acre, which seems rather low, that means it has to give off 1 gram every 0.063 seconds, or about 16 grams per second. If this thing flies around for even one week, that would be 9,676,800 grams, well over 9000 kg of waste.

I mean that right there is obviously wrong right off the bat. If you consider that it has to maintain criticality during this entire time, in spite of spewing its own fuel out of itself the whole while, the entire concept looks a lot like rubbish.

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  • $\begingroup$ I don' think the damage from flying around was from waste, but from radiation damage from the unshielded reactor. Think of it like a radiation lamp being moved over the enemy's terrain. $\endgroup$ – zeta-band Aug 26 at 15:58
  • $\begingroup$ In that case, it's even worse. The neutron flux is going to be high, but it's going overhead so quickly the dose you would get is infantismal. There's simply not enough time to get any sort of load. $\endgroup$ – Maury Markowitz Aug 26 at 17:32

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