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.