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Consider a steel mill or some setting with no human exposure. Can uranium be used as a heat source in place of electricity or gas or coal? Isn't it inefficient to use it in reactor?

Edit example

https://www.world-nuclear-news.org/Articles/Chinese-nuclear-heating-project-starts-up

  1. Get uranium

  2. Heat water

  3. Use the steam

Even though they are using a reactor, it would probably be cheaper to use the steam directly rather than the added electricity step in some cases. It would have to be used for preheating if the temp is low.

Also, uranium powder ignites, so that is additional energy.

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    $\begingroup$ @DKNguyen - the question is asking whether the heat from the nuclear pile could be used instead of arc furnaces, gas, or coal. Not just whether it could be used as a heat source (obviously it can) but whether it could be used in a smelter or forge. $\endgroup$ – jdunlop Jul 26 '20 at 23:32
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    $\begingroup$ @Cadence - I imagine it's like a lights-out shop. People truck in inputs and take away outputs, but no one is present inside the building. The OP, I think, is just removing "the radiation produced would be unsafe" from consideration. $\endgroup$ – jdunlop Jul 26 '20 at 23:34
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    $\begingroup$ It should be pointed out that given your edit, steam is 100C, and even less suited to blast furnace temperatures than using decay heat directly. $\endgroup$ – jdunlop Jul 27 '20 at 5:38
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    $\begingroup$ @jamesqf Unenriched uranium can be used in a nuclear reactor, just not in one that uses light water. You need a moderator and coolant that absorbs less neutrons, like graphite or heavy water. en.wikipedia.org/wiki/Natural_uranium $\endgroup$ – JanKanis Jul 27 '20 at 13:44
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    $\begingroup$ Re, "inefficient to use it in a reactor." FWIW "nuclear reactor" does not mean "electricity generating station." A nuclear reactor is a vessel in which nuclear reactions are allowed to happen. Some nuclear reactors are primarily intended to produce steam to drive the engines of war ships. Some nuclear reactors are intended only to produce exotic radionuclides (for medicine, for scientific research, for use in nuclear weapons, etc.) and the resulting heat is just wasted. $\endgroup$ – Solomon Slow Jul 28 '20 at 15:44
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Yes, as a gas

I see and appreciate the comments that "you can't get hot enough to melt steel", but proposed vapor-core reactor designs can go well above that temperature. They've been proposed for nuclear rocket propulsion (OUTSIDE Earth's atmosphere, thank you very much).

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    $\begingroup$ Using uranium tetrafluoride or uranium hexafluoride seems somewhat at odds with the idea of using unprocessed uranium, but this is fascinating. $\endgroup$ – jdunlop Jul 27 '20 at 20:51
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    $\begingroup$ Worth noting, though, that the metallic uranium option using microdroplets (as opposed to uranium vapour(!) at 5000K which would destroy practically any containment vessel) requires that the reactor be operating in microgravity - ie. not on the surface of a planet. $\endgroup$ – jdunlop Jul 27 '20 at 20:52
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    $\begingroup$ The issue here is that the vapor jet it makes will contain radioactive uranium which will be absorbed by the steel you are smelting... that means your steel will be radioactive which is probably not a good thing... $\endgroup$ – Nosajimiki Jul 27 '20 at 21:34
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    $\begingroup$ @MarkI A centrifuge doesn't produce microgravity. You need to be in freefall for the liquid metallic version of this proposed reactor type to work. The other reason why it's not used more generally is because you need a material capable of withstanding great heat and radioactive ablation while not having too large a neutron cross-section. $\endgroup$ – jdunlop Jul 27 '20 at 22:14
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    $\begingroup$ Also, @MarkI - source? I think you're confusing the sunk ships from nuclear tests (still on the seafloor and unsalvaged) and pre-atomic battleships sunk in WWII that were salvaged for low-background steel for a number of years. Steel with fission byproducts is going to be much more radioactive than finished battleship steel that was subjected to a nuclear blast. $\endgroup$ – jdunlop Jul 27 '20 at 22:16
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No

Uranium melts at 1132 degrees C.

Iron melts at 1538 degrees C.

Before you got your nuclear forge hot enough to forge steel, you would literally have a meltdown.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica♦ Jul 27 '20 at 16:25
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    $\begingroup$ If the nuclear forge was designed to handle molten uranium, would this necessarily be an issue? What about molten uranium would preclude it from still being used as a fuel? $\endgroup$ – Colonel Thirty Two Jul 27 '20 at 18:08
  • $\begingroup$ @ColonelThirtyTwo Nothing, assuming that first bit. Designing the reactor to handle molten uranium is the hard bit. Filtering neutron poisons out of a molten salt reactor is much easier than the metallurgy required to do so with liquid uranium, which is why the molten fuel reactor research kind of dead-ended in the 1960s. $\endgroup$ – jdunlop Jul 27 '20 at 20:21
  • $\begingroup$ Does liquid uranium not undergo fission? The term "meltdown" refers to the containment melting down. While the proposition that liquid uranium would be problematic sounds reasonable to me, I think this answer should more directly establish that claim. $\endgroup$ – Acccumulation Jul 28 '20 at 0:57
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    $\begingroup$ Technically, it has nothing to do with containment, but rather the fuel melting. So, by definition, the uranium transitioning from solid to liquid would be a meltdown. I'm not certain how you'd engineer a reactor to handle the transition. Now, as I said to @ColonelThirtyTwo, they did some engineering thought experiments regarding starting with liquid metal fuel, but the metallurgical challenges were too great and other designs worked better. $\endgroup$ – jdunlop Jul 28 '20 at 1:06
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Maybe?

(But it had better be unmanned, because it ain't going to be stable)

As stated in @jdunlop 's answer, steel melts at around 1400-1550°C, while uranium melts at 1132°C. However, most reactors don't use pure uranium metal, they use uranium dioxide. This is a ceramic, rather than a metal, and melts at an astounding 2865°C instead of 1132. This is promising, although taking the fuel itself up to those temperatures is difficult. There are a couple main concerns:

1) Moving / using the heat

Sure, you could potentially get this fuel up to steel-melting temperatures, but how are you going to use that heat? Power reactors generally use pressurized water, which doesn't like to be above 315°C. No good. You could try using a molten salt to transfer the energy, but those reactors usually operate at around 600-800°C. One commonly considered salt, FLiNaK, could potentially work, although not that well, since it boils at 1570°C. You might be able to find another salt that wouldn't boil until a higher temperature, but beware of anything with chlorides, and actually good luck finding anything that will hold a superheated molten corrosive substance at that temperature. Some liquid metals could work (sodium has been used, although it wouldn't work here), but be aware that as it passes through the reactor, the neutron bombardment can transmute your metal: Copper, for instance, could become Nickel in a matter of days, severely changing your metal properties).

2) Keeping everything stable

One problem with running nuclear reactors is that the reactivity (sort of the balancing point) depends on the temperature of the fuel. Letting the reactivity tip even slightly towards positive or negative can result in a power surge that could melt your reactor. As your fuel heats up to operating temperature, you have to carefully position your control method to keep it from running away. If you're using a ceramic, you also have to be careful that the temperature change from room-temperature to operating temperature doesn't fracture your fuel, altering the shape and messing with your reactivity.

Your best bet is something like a TRISO fuel, which is like little pellets encased in a durable ceramic. You could run your heating fluid through these, and then use the fluid to heat your crucible. Or, if you're willing to throw some nuclear physics out the window, you could put these beads directly into the crucible, and use it as the reactor. The problem with this is that as the pellets move around, that will mess wildly with your reactivity. If you want a realistic scenario, though, your best bet is probably to use nuclear power to generate electricity somehow, and then use arc furnaces to melt your metal. Electricity is much more easily stored and controlled than raw nuclear energy.

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  • $\begingroup$ I definitely agree with this answer, +1. Avoiding pure uranium is a great way around the heating issue. Do you know if there are other good ceramic choices, or is uranium dioxide really the only one? $\endgroup$ – HDE 226868♦ Jul 27 '20 at 2:48
  • $\begingroup$ You can use water as long as theres a lot of it $\endgroup$ – user77172 Jul 27 '20 at 3:38
  • $\begingroup$ Also, it would not have to be the sole heating source- other things can assist $\endgroup$ – user77172 Jul 27 '20 at 3:46
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    $\begingroup$ @MarkI "You can use water as long as there's a lot of it" - The issue is that using water to transfer your heat gets you not much beyond 300°C, when your end goal is 1500°C, which isn't really that much of a benefit. That's 1/5 of the energy needed or less, and anyone sane would likely rather a 25% increase in coal usage over dealing with a nuclear reactor. The key is that you can't heat your steel hotter than the coolant for the uranium without some other method of heat transfer. No volume of water can increase that; it's solely dictated by the difference in temperature. $\endgroup$ – fyrepenguin Jul 27 '20 at 20:32
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    $\begingroup$ That's why the easiest it to do uranium -> water -> electricity -> heat. The water is an easier reactor design than trying to directly use the heat from the fission to raise the temperature of the iron, as a lot of answers here mention. $\endgroup$ – fyrepenguin Jul 27 '20 at 20:35
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Sure, if you are willing to design a new reactor. But it won't be cheaper.

Existing reactors: not hot enough

If all you want to do is use the thermal output of a reactor to pre-heat the steel or ore, that is possible with any reactor. They nearly all produce steam with the goal of producing electricity through a turbine, but that steam can be used just as well for other purposes. But as other answers mention, you won't get steam hot enough to melt steel from any existing reactor design.

In your own answer you mention the use of a heat pump to increase the temperature of the steam. But that won't work economically. The efficiency of a heat pump becomes lower the higher a temperature difference it has to bridge, and getting to a temperature to melt steel is so high that there probably won't be more than a few percent gain in efficiency compared to generating electricity and using that. And generating electricity has lower capital cost and is much more flexible.

Very high temperature reactor: getting close

But there are options other than steam. You might be interested in the very high temperature reactor design. That uses (in some of the variants) helium gas as coolant, and can have output temperatures of up to 1000°C, so that is getting closer to steel melting temperatures. Without molten salts you also avoid most of the corrosion issues. You could even use a heat pump with such a reactor as getting from 1000°C to 1600°C is much more feasible than getting there from around 300°C. Some jet engines operate at turbine inlet temperatures of 1600°C, so building compressors (which are just turbines working in reverse) that work at these temperatures is possible.

Given that uranium ceramics can be used as fuel, graphite as moderator, and helium as coolant, all of which don't mind temperatures that melt steel, I don't see any immediate physical reasons why a reactor couldn't be designed that operated at, say, 1600°C. It probably just takes a whole lot of engineering.

But building a new nuclear reactor is very expensive, creating a new design for a nuclear reactor and having it approved by the relevant authorities is very much more expensive, and designing your core to run at 1600°C only adds to the ginormous expenses. So unless such a reactor already exists in your world, there's no way this would be cheaper than using an electric cycle.

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  • $\begingroup$ It's also hard to imagine that you wouldn't have bigger efficiency losses from running a jet turbine compressor than just using an arc furnace. $\endgroup$ – jdunlop Jul 27 '20 at 17:36
  • $\begingroup$ Since something like 70% of nucelar energy is currently wasted as heat I think there could be applications (ignoring regulatory costs) where it would work $\endgroup$ – user77172 Jul 27 '20 at 19:16
  • $\begingroup$ @jdunlop I don't think so. Generating electricity is pretty inefficient (40% for coal, 60% for combined cycle gas, depending on the usable high temperature). Turbines and compressors themselves are pretty efficient. But using electricity to drive a heat pump is more efficient than using electricity as a heat source directly, unless the heat pump needs to bridge too large a temperature gap and its losses become too great. And in this case the turbine can drive the compressor directly, skipping the electricity. My guess is that a heat pump will be slightly more efficient than an electric cycle. $\endgroup$ – JanKanis Jul 27 '20 at 19:40
  • $\begingroup$ @MarkI The problem is that for nuclear, the fuel is only a small part of the total lifetime cost of a reactor. The largest part is the construction of the power plant itself. So it doesn't make economic sense to make the reactor much more expensive in return for a bit higher fuel economy. $\endgroup$ – JanKanis Jul 27 '20 at 19:46
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Radiation can be used to generate electricity through radioactive decay, but Uranium wouldn't be a suitable fuel choice https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator

Conventional power stations (coal, gas etc.) turn chemical energy into electricity by combusting fuel, heating water, and using the steam to turn a generator. That is an entirely different process to a nuclear power station, which generates energy from mass changes to do splitting/combining unstable nuclei. Uranium has very limited chemical energy, so it won't "burn", trying to put it into a conventional coal power station won't do anything at all and nuclear power is not what you're after in the question.

It is possible and practical to generate electricity from natural radioactive decay however. When radioactive materials decay naturally, they produce heat which can be turned into electricity. (In RTG style fuel cells this is usually done by thermocouples rather than steam turbines, but the principle is essentially the same: heat -> electricity). These not only work, but actually exist and are used in contexts such as space exploration. Uranium would be a bad choice of fuel for these as it's natural decay rate is very slow (millions of years), but an alternative radioactive fuel such as Plutonium can be used.

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  • $\begingroup$ Re, "...generates energy from mass changes..." The same is true of a coal-burning plant or any other type of energy transfer. $\endgroup$ – Solomon Slow Jul 28 '20 at 15:20
  • $\begingroup$ @SolomonSlow - that is not true. Burning Methane CH4+ 2O2 -> CO2 + 2H2O has the same total mass before and after. The energy output has come from the difference in chemical potential between C-H and O=O bonds before compared to C=O and O-H bonds after. In nuclear power the energy is from the mass change related to E=mc^2, the total mass of atoms is fractionally different before/after the fission/fusion reaction. $\endgroup$ – David258 Jul 29 '20 at 9:32
  • $\begingroup$ en.wikipedia.org/wiki/… The total mass of a CO2 molecule and two H2O moleucles is less than the total mass of one CH4 molecule and two O2 molecules. The difference is expressed by the famous equation, $e=mc^2$, where $e$ is the amount of energy released by the reaction, $m$ is the so-called mass defect (i.e., the difference between the mass of the reactants, and the mass of the products,) and $c$, of course, is the speed of light. $\endgroup$ – Solomon Slow Jul 29 '20 at 11:03
  • $\begingroup$ @SolomonSlow from that source "Exothermic chemical reactions in closed systems do not change mass, but do become less massive once the heat of reaction is removed, though this mass change is too small to measure with standard equipment." The energy output in combustion is from the change in chemical potential, that energy (like all energy) has mass, but it is negligible. In nuclear reactions, the energy generation is caused by the mass change, not as a trivial side effect. $\endgroup$ – David258 Jul 29 '20 at 11:12
  • $\begingroup$ Re, "...closed system..." Yes that is correct. The mass of a closed system can not change. If energy can leave, then the system is not closed. Re, "caused by mass change" vs. "trivial side effect." Call it what you like. Energy has mass. A release of energy is a loss of mass. The two ideas can not be separated, so it really doesn't matter which one you choose to call "cause" and which one you choose to call "effect." $\endgroup$ – Solomon Slow Jul 29 '20 at 11:16
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It's surely possible, but inefficient. Like with conventional power plants you should do both, a process called combined heat and power (CHP).You create electricity and use the resulting hot steam for process or residential heat. This typically raises the energy efficiency into the 70% - 80% range instead of the 35%-61% typical for electricity production only. If you really cannot use electricity (but you can generate heat with it as well!) or need something very simple then you can forfeit the essentially free possibility to generate electricity, sure.

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Get uranium, heat water, use steam

Generally, that's how we make electricity today (at gross level of simplification). Steam is used to rotate turbines, they rotate generators.

OK, we can use steam for some other purpouse. Even in the real world, some reactors are used not (only or mainly) for electricity, but also for desalination of water, district heating and so on.

BUT:

"No electricity" requires some other means of controlling the reactor. A nuclear reactor lives on a very tiny edge between the chain reaction stopping completely and a medicore atomic bomb (the control systems strongly favor stopping the reaction if something is not right, see Chernobyl for what happens when control fails the wrong way). Most nuclear reactors have to be controlled on a seconds timescale (or better yet, less than a second). You can probably arrange some steam-valves-pistons controlling mechanism AND use a reactor design that has "negative power coefficient" (i.e. self regulates, to an extent), but I doubt one can obtain all the needed knowledge without using electricity.

"Steel mill" needs a sophisticated reactor design working above the iron melting point. And you will get a radioactive steel (unless you invent even more sophisticated design where you transfer heat and not radioactivity - at a temperature that severely limits your materials choice).

Continuity: a stopped reactor has to be cooled for months in order to keep it from melting because of the "decay heat". Sure, it can be done in purely mechanical way, but we don't have an established practice.

p.s. when talking about the nuclear power, the energy from uranium burning is completely negligible.

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    $\begingroup$ "A nuclear reactor lives on a very tiny edge between the chain reaction stopping completely and a medicore atomic bomb (see Chernobyl for what happens when control fails the wrong way)." There is a world of difference between a nuclear reactor melting down and a nuclear bomb exploding. $\endgroup$ – nick012000 Jul 27 '20 at 13:30
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    $\begingroup$ as @nick012000 says, reactor meltdowns are not remotely comparable to atomic weapons, they are completely different things. $\endgroup$ – eps Jul 27 '20 at 18:12
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    $\begingroup$ @fraxinus Designing a nuclear bomb to not blast itself apart before enough nuclear energy has been released is actually quite difficult. In a nuclear reactor that is not designed to be a bomb you won't get a nuclear bomb type explosion even if it goes prompt critical. At "best" the explosion will destroy the reactor and maybe the building and leak lots of radioactive material. That in itself is also very bad of course. $\endgroup$ – JanKanis Jul 27 '20 at 20:33
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    $\begingroup$ @JanKanis that's why I said "medicore" bomb. $\endgroup$ – fraxinus Jul 28 '20 at 6:11
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    $\begingroup$ @nick012000, The Chernobyl reactor didn't just quietly melt down: It exploded. It produced a multi-gigawatt pulse that super-heated the cooling water, and the resulting blast ruptured the reactor vessel and significantly damaged the building. Not different in principle from a nuclear weapon producing a pulse of energy that creates a blast by super-heating the surrounding air. The main difference is, the Chernobyl explosion was orders of magnitude less powerful because, as the engineers say, the reactor core "spontaneously disassembled" before a significant amount of the fuel was "burned." $\endgroup$ – Solomon Slow Jul 28 '20 at 15:31

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