Reactor meltdown

[THX1138]

If one were to place a nuclear reactor in orbit around the earth / On even placed it upon our moon, and run it with no cooling whatsoever so as to purposely have it go in to an uncontrolled meltdown what would it look like ?
Would it glow red ? Would it glow bright white like the spots on Ceres or would there be a thermonuclear explosion

[geckzilla]

That's a really odd question. We know Chernobyl's fuel melted straight down through concrete, steel, or whatever else was in its way. If it is enough to reach critcial mass then we get a firework. I kind of imagine it melting into a ball and then the heaviest matter making its way to the center, the fissile material being the heaviest, so an explosion is a good possibility. Prior to reaching criticality, yeah, it would glow. It would never be like Ceres's white spots, though, unless you think, say, snow and glowing hot metal blackbody spectrum are the same.

[Chris Peterson]

That's a really odd question. We know Chernobyl's fuel melted straight down through concrete, steel, or whatever else was in its way. If it is enough to reach critcial mass then we get a firework. I kind of imagine it melting into a ball and then the heaviest matter making its way to the center, the fissile material being the heaviest, so an explosion is a good possibility. Prior to reaching criticality, yeah, it would glow. It would never be like Ceres's white spots, though, unless you think, say, snow and glowing hot metal blackbody spectrum are the same.

I doubt it would become critical. Most of the mass of the reactor is non-fissile, and there are materials which are neutron absorbers that would get mixed in. Also, while self gravity would tend to pull the densest stuff towards the center, the force of gravity for such a low total mass would be very small. I think that convection inside the molten, or partially molten zone would dominate and keep things stirred up.

It would glow in the IR, for sure. I don't think that the entire structure would be hot enough to glow visibly, though. In fact, I think most of the structure would remain unmelted, so we'd have surviving structure hiding the actual melted core elements.

[geckzilla]

How would we even get the reactor up there in the first place? It would have to be built piece by piece and cooling methods wouldn't work in microgravity would they? The water would not want to stay contained. It might be harder to deal with the water than the fissile material...

[Chris Peterson]

How would we even get the reactor up there in the first place? It would have to be built piece by piece and cooling methods wouldn't work in microgravity would they? The water would not want to stay contained. It might be harder to deal with the water than the fissile material...

The water is pumped through the core. That's going to work exactly the same whether you're in microgravity or not. On Earth, the heated water is typically cooled by evaporation. That's not going to work well in space. So you'd need to pump the water through huge radiative fins.

Getting rid of heat in space is always a problem.