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I'm writing about a nuclear-powered tank. Yes, I know about what the problems associated with it are. No, the people in the setting don't care about radioactive contamination of their environment, or the cost of these things, or the fact that the tank will likely be obsolete in a decade - none of those things.

Does present-day nuclear technology enable a nuclear reactor with the following characteristics to be built? If not, what technologies would be required to make such a reactor?

  • Weight: 3 metric tons/3,000 kg

  • Power output: 1500 KW/2011.5 HP

  • Power-to-weight ratio: 2 metric tons/MW

  • Volume: 2 cubic meters/2000 liters

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    $\begingroup$ 1500 W? Two horsepower? With a weight similar to two actual horses? Why not use the horses then? (For reference, the proposed characteristics are quite comparable to a late 1960s Soviet BES-5 "Buk" atomic thermoelectric generator; except that the Buk was much lighter and twice as powerful. If the Soviet engineers could make thirty of them in the 1960s and 1970s, I am sure we can make more today.) $\endgroup$
    – AlexP
    Commented Oct 15, 2021 at 23:59
  • $\begingroup$ @AlexP My apologizes, that was a typo. 1500 KW. $\endgroup$
    – KEY_ABRADE
    Commented Oct 16, 2021 at 0:01
  • $\begingroup$ The power-to-weight doesn't match the power and the weight... $\endgroup$
    – AlexP
    Commented Oct 16, 2021 at 0:04
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    $\begingroup$ According to the rules, "The question must refer to a specific, pracitcal issue with worldbuilding and be answerable." I'm not sure this is practical or answerable. Further, what you're really asking is, "Is this design believable." From a realistic standpoint, given the last 70 years of nuclear design, no - it's not believable. But Isaac Asimov's Foundation series had coin-sized nuclear reactors and people have happily believed that (suspension of disbelief) for decades. So, what's the problem here? $\endgroup$
    – JBH
    Commented Oct 16, 2021 at 2:12
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    $\begingroup$ I would hate to be in a world with a nuclear powered tank. Instead of looking at the power, look at the waste. The tank battle near Kursk saw nearly 500 tanks put out of action / destroyed. If they had been nuclear powered, the fallout would have been intense. As it was, the report is that in many places of the battle, the ground was so damaged by the fuel fires that nothing would grow there for years. $\endgroup$
    – David R
    Commented Oct 16, 2021 at 15:06

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The reactor itself, is doable...

The amount of highly enriched uranium required to sustain criticality is not massive. Although it can be done with a few dozen kilograms (less if you use plutonium), you would never actually be able to build a reactor this small for practical reasons:

  • More fuel is needed to extend the life of the core.
  • Reactivity decreases as the hot fuel expands, so you need more fuel to compensate. Maybe even neutron reflectors too.
  • If you want to start and stop the reactor, control rods are needed too.
  • Coolant channels make the core larger to cool the core and extract useful heat,
  • Optional instrumentation channels to measure neutron flux (can be done external to core).
  • Optional neutron source to make start-up go quicker and easier.

Still, even with these limitations, a reactor weighing less than half a tonne is possible. The Russians built reactors weighing less than 300Kg, with thermal outputs in the tens of Kw, so it's certainly possible. If you need to scale up power output, you would need to design the fuel geometry to have a high surface area, probably go for some cermet fuel, use a high thermal capacity coolant (probably molten metal or salt), and you would really need to pump a lot of coolant through it. All of this pushes you in the direction of using a fast reactor reactor than a thermal reactor, and the molten metal/salt can be used at low pressures which means you don't need massive thick pipes, keeping size and weight down. Of course the problem arises in that to run the core at at least 1.5Mw (probably closer to 5Mw when accounting for thermal inefficiencies), you are going to need to extract a lot of heat from that core, hence the real problem.

...but the power conversion isn't...

The reactor is only one part of the system. You also need to take the heat produced by the core, and produce useful electricity and shaft power to drive your tank. Step one of that is a main coolant pump, which for the kinds of flow you are talking about, is already going to be at least the size of a washing machine. Then you need to take that hot flowing molten metal/salt coolant, and do one of two things:

If you use a high temperature Brayton cycle, then you will use a heat exchanger to heat some gas, run it through a turbine, then allow it to expand, and run it through a compressor before repeating the cycle. unfortunately, most gasses do not have a high thermal capacity, so your heat exchanger is going to be massive. No way it could be fit within 2 cubic meters.

If you use a lower temperature Rankine cycle, then you will use a heat exchanger to boil water then drive a steam turbine. Water has a high heat capacity, so the heat exchanger will be smaller. However in my experience, heat exchangers for a nuclear plant still tend to be at least twice as large as the reactor itself. Furthermore, steam turbines are not small. It's difficult to find weight specs, but this source gives a craneage weight for an 1.5Mw turbine as 3.4 tonnes. You may be able to shave a bit off that with fancy materials and clever engineering, but I doubt you could get it down low enough for the system to weigh less than 2 tonnes.

Both of these would then need to reject the waste heat from the system. This would require forced air convective coolers on your tank. Calculating the size of these is non trivial, so I'm not going to comment on how big they would need to be.

...and safety is impossible.

We often make fun of safety for ruining our fun, but there is no point building a tank if the crew is dead 10 minutes after they roll out of the garage. Shielding this reactor in such a confined area is going to be impossible, and a 5Mw reactor would produce a non-negligible amount of radiation. The shield itself would probably need to be at least half a meter thick (probably larger), and may require it's own active cooling. I just don't think it's doable.

Conclusion

If you are desperate to have a nuclear powered tank, I would suggest the following options:

  • The tank is an ultra-heavy monster tank. At 100s or 1000s of tonnes of tank, I would suggest that scaling laws make nuclear power more viable. Of course, this raises other problems.
  • The tank is a smaller, unmanned drone tank designed for long term deployment. Smaller 300Kg systems are viable, so you might be able to make a smaller unshielded drone. It's still not going to have a fantastic power to weight ratio, so the only advantage to going nuclear is if you want something that can operate without re-fuelling for long periods.
  • Artistic licence. Make the rest of your work engaging and thematic enough that nobody cares that a nuclear powered tank is unreasonable.
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  • $\begingroup$ I might compare with steam-powered cars (and the last few steam locomotives, to an extent). Aside from fuel source, they have many of the same problems. A nuclear reactor is just a really exciting steam source after all. $\endgroup$
    – TLW
    Commented Oct 22, 2021 at 2:43
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This ought to be doable, but the technology may not be published

Proposals for "miniaturized nuclear plants" generally involve megawatts of power produced by reactors that are too large for your purposes. (And they still sound like a very bad idea) The one I linked is perhaps typical in power, and is 9 feet in diameter and 65 feet long, though Wikipedia indexes some designs with lower output that are rather obscure.

The most obvious reason for a lower limit to size is that uranium has a critical mass, around which you need to put a wide range of measures to contain the reaction and extract the power. I think it's plausible to suppose that if you had something with a much lower critical mass, that you could scale down most of the other machinery accordingly. Note that fissile materials with low critical mass are what you might call "nuclear initiators" and not very well documented in the literature. There are some transuranic isotopes for which the heaviest isotopes that would be most stable are not even published. The discussions typically run in the direction of californium "nuclear hand grenades". Anyway, if this were true, maybe you could scale down the dimensions on that thing I linked above by, oh, a factor of 3, and the mass and power output by a factor of 27. At which point you can plausibly ride around in something about the size of "a big tank".

The weight of the reactor is an issue. If we scale down to pi (3/2 ft)^2 * 20 ft, something around 140 cubic feet x (62 pounds / ft^3) x D. = about 9000 pounds x D. Odds are a nuclear reactor won't float on water, so... you have to miniaturize a bit more. But that should lower the power from the hypothetically smaller critical mass of your isotope. Still, your isotope may (should) have more binding energy than uranium, so this can be done, to a point. Getting the rest of the way to viability here implies going closer to critical runaway reaction than a sensible nuclear reactor would, and hoping for the best.

Oh, you also need a good heat sink. 1500 KW is like 15,000 old time 100W light bulbs (remember how hot those were to the touch?), and the waste heat is going to be in that ballpark. Assuming good emissivity, the Stefan-Boltzmann law gives P = 6E-8 W/(m^2 K^4) A T^4 where A is area and T is temperature. So for a square meter of area you want 3E+13 W/K^4 * T^4 and T is a little over 2000 K. You can bump that up to ten square meters but T is still over 1000 K. You can insulate the cabin with Space Shuttle tile, but you might have to have plow the earth to get rid of the heat (or have autonomously mobile heat-tentacles that continually quench themselves in enemy soldiers and property) before your reactor melts.

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  • $\begingroup$ The reactor needs to be below 3 metric tons, not the whole tank :) $\endgroup$
    – Trioxidane
    Commented Oct 16, 2021 at 12:14
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With the current parameters it's impossible

Nuclear reactors are big. Miniaturisation of such technology isn't easy, making a maximum of 2 liter volume reactor impossible. You need more for cooling alone.

The closest we can do are radioisotope thermoelectric generators. They are sometimes referred to as nuclear batteries and are commonly found on sattelites and space probes. This is because the fuel slowly decays and produces the warmth to keep it from freezing as well as a lot of electricity. It requires no moving parts and can go on for a thousand years, depending on the craftsmanship and fuel. Regardless of their name, they produce heat from nuclear decay which is transferred into electricity.

These are relatively small, but still not enough. They produce only hundreds of watts, so you need a lot of them. Watt per kilogram makes plutonium the best, at about 5,3 watt per kg. That means you need (1500000/5,3=) 283019 kg. We can assume some improvements of watt per kg with size, but that would still mean a normal reactor would be more feasible.

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  • $\begingroup$ Wikipedia says that plutonium is 0.57 watts/kg. That means that 1500000 watts can be provided with 2,631.578 kilos of plutonium - not 283,019. This is, of course, still a RIDICULOUS amount of plutonium. I like the idea though - maybe we could switch out Polonium-210, which, if pure, is 140 watt/kg (meaning only 10.714 kg of this incredibly toxic material), but has a 138-day half-life, or Strontium-90, which is worse than plutonium in terms of energy density but is much more easily produced and a lot more common - it's literally nuclear waste. +1 for you. $\endgroup$
    – KEY_ABRADE
    Commented Oct 16, 2021 at 10:35
  • $\begingroup$ @KEY_ABRADE Wikipedia says 0,57 watts per gram :). That means 570 watts of power per kg. This is the heat energy. With about 10% efficiency of these reactors at maximum, it's about 57 watt per kg. But this doesn't take into account the watts per whole reactor. In the wiki of radioisotope thermoelectric generators under the models you see the most efficient is 5,2 - 5,4 watt per kg, so it's reduced to about 1% of the original watt in heat. If we can do the same for the Polonium it'll be much better, so you're right about that. 1400 watt per kg at 1%. Just a 1071 required. $\endgroup$
    – Trioxidane
    Commented Oct 16, 2021 at 12:11
  • $\begingroup$ Strontium-90 isn't a pure-alpha emitter, so it's going to need more shielding, which adds weight. I'm also very concerned about the mass of the radiator you're going to need for this monstrosity. $\endgroup$
    – papidave
    Commented Oct 16, 2021 at 13:00
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Fusion is nuclear power too.

https://www.lockheedmartin.com/en-us/products/compact-fusion.html

The MIT ARC reactor is a torus 3 meters across and will in theory produce several hundred megawatts. So several orders of magnitude more power than you request but you will find something to do with it. Specifics on the Lockheed reactor are not as easy to find but they claim 100 MW and “fit in the back of a truck”. Tank should be ok.

I propose you cool your reactor with the same fans you use to produce lift: this is a hovercraft tank.. Of course rail guns will be the primary weapon as you have electricity to spare.

Electricity generation will use air as the working fluid: Some heated and compressed air will be released into a generator but much will be used directly to produce motive force via pneumatically driven lift fans - less lossy. An electrical motor will be available for taxiing with the fans off.

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In addition to the other replies: to get useful energy out of the reactor requires converting thermal energy to electrical or mechanical energy. Direct conversion to electrical energy (thermo-electric effect etc.) is orders of magnitude too ineficient for the power to weight and power to volume ratios you are looking at. So you would need something like a gas turbine - i.e. heat -> heat exchanger -> (liqiid->gas) -> turbine -> generator.

Assuming a very efficient heat transfer system, at ~1.5 MW you will be converting approximately 1 litre of water to steam every second. The dimensions required for an air-cooled heat exchanger are impractical so the water will be 'lost' after it has passed through the turbine (similar to the way that it is lost in an old-style steam engine). Thus, you would need to carry 3.5 tonnes of water for every hour of operation. So even ignoring the considerable difficulties of building a reactor of the dimensions and weight you have suggested, the 'working fluid' requirements appear to be impractical.

But if you could solve all the other problems, then maybe a nuclear-powered boat might be borderline feasible, as it wouldn't have to carry the working fluid around.

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