I big ball of hot metal isn’t useful A nuclear reactor converts heat to kinetic energy (usually by heating water into steam to push a turbine). When the core melts down, it’s gotten so hot that that melts or damages all the machinery used to transfer heat, cool the reactor or contain the core.
Please read my post directly above yours. Why does the machinery need to contain the reaction instead of being external to it? On earth this may be a problem as gravity will pull it down and dilute it with other material, but in space it will just sit there and glow, like a mini star.
A big molten ball heat source might be useful in a fixed location - moon or Mars, or a space station perhaps. After all, how exactly do you tether a molten ball? Even if it’s taffy not liquid, what mechanism do you use to attach it? Then with Mars, how do you avoid contaminated dust blowing all over?
You’re still back to a controlled hot source (carbon rods, container, coolant/steam pipes, etc.) for a functional reactor. Even using it as a rocket heat source needs a means to feed and expel the propellant. The savings on the moon is that you don’t need a complex sheilding mechanism - just put it deep in a crater using the crater wall as shielding, away from the settlement. In space, you need some sort of harness to make random corrctions to position and attitude, not to mention a cable attaching it to the space station(?) using the output power. One concept might be using tidal pull to keep reactor and station separate, but I could see some level of oppostion to a giant molten radioactive ball in Earth orbit, so it’s probably more suitable for a remote outpost.
As others say, the problem in space is that closer to the sun, you have to fight solar power - perhaps put up a big reflective heat shield, one of those mylar blankets to block out the sun so the radiators for the reactor are always in shadow (as was done as a good kludge repair on the SkyLab). However, unless like SkyLab yo are in an orbit where there is a cycle of shadow for a while, eventually the blanket too will get to a temperature equilibrium and no longer be as good a shield. you are always fighting entropy.
Space station maneuvering thrusters to stay near it? Power cables to solar panels that gather energy from the radiation from the core?
Remove the rods, let it melt down. No container or coolant. Use the cube square law to get the size for the temperature you need of the core.
I guess that’s a concept - free floating fission star… But are there actually “Solar” panels that can capture that energy? I suppose you could position some devices so the “boiler” is a black panel facing the core, attached to a steam generator and a long pipe to some remote radiator/condensers. Since it’s not physically attached to the core it would need its own positional adjustment system.
I suppose it could be taken a step further, and every so often shoot another pellet of uranium at the core to keep it fed.
Another consideration is critical mass. Alleged critical mass for an atomic bomb was about 10kg - but that was refined to the more unstable isotope, and needed pressure to be held together long enough for the reaction to occur. The more stable the reactor isotope mix, the bigger the critical mass - but the goal is to avoid it getting so hot internally that it boils/flies apart - which is why earht-based reactors have rods and coolant. It would be an interesting physics thought experiment what is the eventual steady state - if any - to a free-floating fisioning blob with minimal ability to shed heat. You need to balance the energy lost from heat radiation (and nuclear particles) heat with the amount generated by fission… Would you end up with a small explosion, a “red giant” effect where the heat causes the center uraium to boil inside the molten bubble… would that expansion stop the core from flying apart? How viscous is molten uranium? Does it have surface tension?
it’s a whole different world when there is no convection to carry away heat, not necessarily intuitive. .
Yeah. Spending substantially all of human history at the bottom of an atmosphere and a gravity well really messes with our grasp of what’s “normal” in the universe. Adding a water ocean to the mix only makes it worse.
Sure, with present day science professionals can calculate past all that bias. But for us armchair experts, or at least this particular armchair expert, the bias runs deep.
This looks like the first step to see if it’s feasible. Can we create this? Either self sustaining or if needed with some maintenance such as adding pebbles of fuel periodically. What fissionable materials are the most likely candidates? We already know we can have a semi-long lived useful hot radioactive core for RTG’s. But if we had to scale up the power output beyond what can be contained.
I still don’t fathom how you think you can get useful power from a reactor core in meltdown. “Meltdown” means that the fuel and control elements, monitoring and cooling system, and containment structure literally melted, resulting in a pile of radioactive slag that is almost certainly in some kind of runaway fission condition. (There are reactor designs where the radioactive elements are actually carried within a molten salt that is essentially its own inner coolant loop but it is designed that way to limit reaction rates and heating so as to be controllable and not melt the heat exchanger to the outer ‘working’ loop.)
In order to get power from a reactor, you have to keep it just on the edge of criticality (where it is producing just enough neutrons to sustain spontaneous fission) and and then run a coolant (water, helium, molten salt or lead) through and around the fission core to extract the thermal energy produced by absorbing neutrons, then deliver that thermal energy to a turbine or some other means of doing mechanical work, typically through heat exchange to a outer ‘working’ (non-radioactive) loop. A meltdown core is out of control and useless for any purpose.
First of all, stars are powered by nuclear fusion; this is an entirely different process that works on completely different principles than fission, and has actually really low power mass density, which is why it is so difficult to reproduce on Earth in a compact plasma with net power production. (Per unit mass, a human produces more power than the luminosity of the Sun.) The Sun and other stars release their energy as an approximate blackbody spectrum (actually higher in the UV and X-ray bands than an equivalent blackbody) and what we get on the surface of the Earth is mostly infrared and visible light.
A fission reactor releases energy mostly in the form of kinetic neutrons, which are then absorbed by other fuel elements, coolant, moderators, et cetera where the momentum is converted into thermal (random) molecular motion. To get a controllable amount of energy from a fission reaction and not consume the fuel elements rapidly, it has to be run at the edge of criticality; otherwise, it produces wildly fluctuating rates of reactions, consuming the fissile material at exponential rates and producing radioisotopes which will absorb and ‘poison’ the fission chain, eventually causing it to be unstable. A nuclear fission bomb is actually an extreme version of this where the ‘pit’ is designed to undergo a highly supercritical reaction, delivering a massive amount of energy in a very short period of time; a few tens of nanoseconds or “‘shakes’ of a lamb’s tail” as one whimsical nuclear scientist on the Manhattan Project described it.
(Note that a fission reactor cannot explode in the manner of a nuclear bomb; it takes a very intentional design to create the conditions for that kind of supercritical state, and explosions like the Chernobyl #4 reactor were due to superheated steam dissociating into hydrogen and oxygen, which while violent was not a nuclear detonation.)
There is no such thing as a “fission star”, it would not be safe or useful to have an unstable fission core in meltdown towed behind your hypothetical spaceship, and RTGs are undergoing spontaneous decay but not sustained nuclear fission chain reactions.
Stranger
One way of getting rid of heat by radiation only is to use a working fluid like potassium vapour, which condenses at 759 degC.
Potassium vapour turbines have been built:
Whether a melted down core in space (“fission star”) would be stable is an interesting question. That’s what I was wondering, whether it could reach a steady state or if the internal heat not escaping would cause the center to vaporize? It’s the same principle as spontaneous combustion in a damp haystack or collection of oily rags - a steady reaction produces heat that cannot escape the center fast enough.
A molten blob in space will not have the convection to circulate the hotter parts to the surface, unless it has noticeable gravity - I hope we’re not planning on building something that big. Even a rocky iron core surrounded by fissible material - the core will just get hotter and hotter until it vaporizes. Perhaps we get a big bubble of molten uranium around a gaseous core until a wall section fails and the gas escapes in one direction sending it off the other way; then it collapses to repeat the process, so we have a randomly wandering star.
There was a steady-state natural reactor that ran for a while in a uranium deposit in Africa millions of years ago, however it apparently never melted down and suggestions are it was not particularly hot, it just emitted enough radioactive particles (neutrons?) to leave unusual radioactive byproducts whose half-life and presence indicated that they were recent occurence, not part of earth’s formation.
I assume you do not want your nuclear reactor to self-destruct, which is what it sounds like a melted-down core is, pretty useless. Whence designs like the gaseous-core nuclear rocket and gas-fuelled reactors in general. The problem there is not to keep the core from melting down (the core is already vaporized), it’s to keep it from destroying the entire reactor, so that you can continue to extract useful power.
First of all, stop writing “fission star”; it’s like saying “gumbo cake”. It’s just not a thing. Second, if you had fuel elements that are just melted together (not diluted in a molten salt) then they are going to undergo uncontrolled spontaneous fission at varying rates as the puddle heats and expands, and then cools and contracts, but producing prompt neutrons at unpredictably varying rates. It would not be useful for power production, safe to tow along, and there would be no way to reliably confine it. It’s a completely unworkable idea.
Yes, there were several ‘natural fission reactors’ active around 1.7 Bya near what is now Oklo, Gabon where a unique configuration of high grade uranium deposits with a greater portion of [SUP]235[/SUP] in naturally occurring uranium deposits than found today were in contact with groundwater which acted as a moderator with a negative void coefficient, the reaction rate became high enough for sustained criticality. When the reaction rate increased, the water would boil away, slowing or stopping these reactors, such that they were incidentally self-moderating. This does not happen with the meltdown of a reactor core with enriched uranium; it just goes way past criticality and the fission rate goes up exponentially until the pile melts into some puddle where the uranium is spread out and can’t sustain fission.
Stranger
OK, so let’s go with “fission blob”. Just as descriptive.
Thanks. The bit I read about the natural reactors did not include the details about water moderation, although it’s unclear how water moderation would work if the first step was to boil away all the water.
The blob, being in zero G will not spread out into a flat puddle (obviously) so the only thing that might make it “self-moderate” would be expansion.
My question is whether it would actually result in a gaseous bubble due to heat buildup. I guess the question there is the criticality issue. Too concentrated and it would undergo “runaway” too fast and simply explode, even if not at a rate comensurate with a for-real deliberate bomb; or whether this would be a slower process to the point where we simulate “flat puddle” with a thin liquid skin of fissible material around a gaseous bubble. The other question is whether it would work its way toward some steady state or whether it would be pulsing erratically, with or without the internal gas breaching the currounding bubble.
(Note we’re sort of sitting on one… the suggestion is that much of the earth’s core remains molten due to the very slow leak by conduction of heat of decay of radioactive materials in the core, combined with their very dilute nature.)
But ultimately the problem with any molten blob is that it is unlikely there is a useful mechanism to tether it, so it goes where it wants and all you can do is follow. An orbit may be confining is the larger sense, but if your goal is to be close enough to make use of it without being so close as to collide or become irradiated, a lot of highly reliable guidance on the ship/station following is needed.
So like a RTG in some ways. And could this fission blob could be made of non-chain reactive materials but depend on natural decay (like a RTG), but with enough mass to glow white hot? And would that be more stable than the chain reaction concept?
There is no way to answer this nonsensical question.
Stranger
We can at least say, as was discussed in another thread, that masses of alpha-emitters like plutonium-238 and polonium-210 do get glowing hot and have been used in spacecraft; what thermoelectric/thermodynamic/thermionic technology to use and how hot you can push it is another question. Maybe that is what you are thinking of.
Assuming this whole “fission blob” thing isn’t complete nonsense for just a minute. How do you deal with even the slightest of course corrections without it going its own way?
I think one problem with talking about a molten blob in space is that just the idea of a blob contains too much gravity well dweller thinking. There is very little to keep the blob together. Only the slightest disturbance will break it apart. Moreover, it is sitting in a vacuum. So will tend to evaporate since the surface pressure is zero. It is doubtful that a melted reactor would live a blob for more than a few seconds before permanently breaking apart.
Perhaps the question about building a suitable reactor might be cast in terms of building a reactor from highly refractory materials - with high enough melting temperature that the reactor can run at, or close to, white hot and yet remain tethered and safely operated.
But the entire idea is probably not answering the right question. What any spacecraft wants is energy in a useful form. That pretty much means electrical energy. Unless we are talking propulsion at energies larger than can be met by electrical means.
A big hot nuclear reactor for electricity is essentially assuming a heat engine in the chain. No matter what, you have to eventually reject the energy. A reactor at white heat might gain us greater efficiency in our heat engine - if we can work out how to harness it, and most importantly, deep the cold end cold. So there will be lots of radiators involved. So it is probably a marginal or non-existent gain. Especially when everything else gets hotter and harder to manage. Then again, optimising stuff in space does seem to throw up unexpected solutions.
Something as baroque as a white hot reactor and a huge array of solar panels might work as an alternative to a heat engine. Whether the array of solar has bought us anything compared to an array of radiators is not clear. Simplicity might be the largest gain.
Space station/ship thrusters. Follow it.