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In Scifi there is often the issue that all the fancy technology like Mechas and Lasguns need a lot of energy. Chemical sources are limited by volumetric and regular power density. Batteries and capacitors even more so. Compact fusion, fission and antimatter devices produce problemeic amounts of waste heat, are insanely complex and dangerous. Using tanks with power plants which regularly recharge their robot swarms or just have constant power cords to them is interesting and feasible, but I'm interested in alternative solutions.

Alpha and Beta-Voltaic devices turn particle radiation directly into electricity. This should help a lot with the wast heat and issues of huge generators. This question from Physics SE deals with the question why we are ten using them currently outside of low power and long term operations. This list of issues is from an answer.

  1. Power produced is non-adjustable. The battery produces power at nearly constant rate (slowly decaying with time). It cannot be increased and if not consumed (or stored) the power is lost.
  2. Low power density. 63Ni for instance produces ~5 W/kg (and kg here is just mass of radioactive material, the actual battery would be at least order of magnitude heavier). There are, of course isotopes with power densities much higher but they encounter other problems.
  3. Semiconductor damage. If we try to increase power by using isotopes with higher decay energies we find that the high energy electrons damage semiconductors, reducing service life of batteries to times much shorter than isotope halflife. Alpha particles, especially, damage the p-n junctions, so even though (for instance) 238Pu produces 0.55 W/g of alpha radiation, it is mainly used in the thermoelectric schemes rather than in direct energy converters.
  4. Gamma radiation. Many isotopes has gamma emission as a secondary mode of decay. Since this type of radiation is difficult to shield, this means that the selection of isotopes usable for batteries is limited only to pure beta emitters.
  5. Bremsstrahlung. Electrons braking produces this type of radiation, that had to be shielded. Again, this limits our selection of isotopes to those with relatively low decay energies.
  6. Low volume of production / Economics. Many isotopes cost too much to be practical in wide array of applications. This is partly explained by low volume of production and partly by production process which will be costly at all volumes because it requires energy consuming isotope separation and special facilities for working with radioactive materials. For instance, tritium (one of the materials for betavoltaics) costs about $30 000 per gram and its world annual production is 400 g (from wikipedia).
  7. Safety / Regulations / Perception: 63 grams of 63Ni constitute more than 3500 curie of radioactivity, which would definitely require regulations for handling and probably would not be allowed inside a single unit for unrestricted civilian use. We know that when properly used betavoltaics are safe. But what about im-proper use / improper disposal / potential for abuse? At any rate, current perception of nuclear power by general public is not that good, so marketing nuclear batteries will present certain challenge.

The answer mentions that advancements in technology could help with these issues. So, these are the advancements concerning this technology in my setting.

  1. Pick the right isotope for the mission. Story wise this is an amazing source of tension. Furthermore carrying chargeable units to get energy boosts for certain applications later in the mission is a solution. In the beginning of the mission the device is far above is normal energy needs.

  2. "There are, of course isotopes with power densities much higher but they encounter other problems." This is great. What are these other problems? Can they be dealt with? Especially isotopes with half-life's of hours, days and weeks are interesting for many applications.

  3. As I'm more interested in high powered short lived applications, this isn't a big issue. In case it matters, maintenence and redundant systems are the answer.

  4. & 5. Advanced bio and nanotechnology made radiation damage much less relevant. Its an annoyance, nothing more.

  5. Fusion economy makes the energy cost of manufacturing appropriate isotopes much more acceptable.

  6. Critics of nuclear power may discuss their issues with my Gundam's main death ray.

So are there any other issues preventing Alpha and Beta-Voltaics from being used form everthing from weapon power celks to synthsects and Mechas?

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  • $\begingroup$ Typical semi truck is 500 hp or 373,000 watts. Your battery would require 74.6 tonnes of isotope, and 746 tonnes of battery to produce this. A diesel engine is under 2 tonnes. Maybe you just want a diesel in your mech? Also, you should probably check your budget. I'm guessing 74.6 tonnes of radio-isotope battery is going to be pretty seriously expensive. $\endgroup$ – puppetsock Jan 24 at 15:40
  • $\begingroup$ @puppetsock Which isotope are we talking about? Isotope can mean anything from Plutonium to Tritium and there are significant differences in energy density and output. $\endgroup$ – TheDyingOfLight Jan 24 at 16:05
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    $\begingroup$ I think you capped the main problem with Alpha and Beta-Voltaics. They maybe high on Specific Energy, but low on Power density, and there is no practical way around it. $\endgroup$ – Alexander Jan 24 at 17:41
  • $\begingroup$ Stephen's answer is great, but I wanted to address your handwaving of problem #3 - the higher-power your application, the faster semiconductors would get damaged, because you'd have much higher decay energies, and the additional mass/volume you'd require to add in redundancy sufficient to deal with it would reduce/remove the benefits. $\endgroup$ – jdunlop Jan 24 at 18:37
  • $\begingroup$ @jdunlop if the principle radiation source were "merely" alpha or beta particles, shielding isn't too hard to arrange. $\endgroup$ – Starfish Prime Jan 24 at 18:42
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TLDR: this is a powerful plausible power source, but it's probably not as revolutionary as it needs to be.

The issue here is the distinction (and trade-off) between power density and energy density. To use the example of $\mathrm{^{63}Ni}$, 63g of source material contains one mole of nuclei, each of which will eventually decay to release (on average) 17keV of energy. That's a total energy density of

$$ \mathrm{ 6 \times 10^{23} \ \times \ 17 \ \times \ 1.6 \times 10^{-16} \approx 1.7 \times 10^9 \ J}$$

1.7 gigajoules is good going for something about the same volume as a C-size battery. A standard alkaline C battery might hold 8Ah of charge at 1.5V, for a total energy content of 43kJ, so the nuclear battery holds $\mathrm{10^5}$ times as much energy as a standard battery. An equivalent volume of petrol weighs about 5g and contains 232kJ of chemical energy; an equivalent mass of petrol contains 2.7MJ, so 620 times less than the nuclear battery. The fact that the number 620 is not huge is worrying for this as a revolutionary new power source, because it comes with many engineering challenges, as described.

This particular battery will deliver that power continuously, with an exponential decay, such that you extract 90% of it over three hundred and fifty years or about 11 giga-seconds, which brings the power density back down to the levels described (~300mW for the 63 gram C-size battery). That's not much use for a mecha or a death ray: we need that power on a timeframe of hours or days, as you say. Different isotopes have wildly different half lives, but the beta decay energy is generally the same order of magnitude; as is (in order-of-magnitude terms) the density of the material. So the total energy content of the battery is broadly similar across different power ratings.

Let's charitably assume that we can pick an isotope with a 100 kilosecond half life (ie a little over a day), so $\mathrm{10^5}$ times less than $\mathrm{^{63}Ni}$, and that we can solve all the engineering challenges associated with dealing with the hundred-thousand-fold increase in power and still capture all the energy (now 30kW) safely and effectively. The resulting battery delivers about 40 horsepower for a day, which is enough to power the Mercedes Simplex (production dates 1902-1909), and eighteen of them would power a Tesla model S. This is awesome by any objective standard... but is it really revolutionary?

The classic 'big mechanical thing' that generally gets pulled out as the closest thing we have to a mecha is Bagger 293, the 14.2 kiloton coal excavator that's powered externally with a 16.3 megawatt supply. To power this relatively simple mecha with our nuclear batteries, we need a bank of just over 500 of them, weighing 34kg. Realistically these should be considered as fuel pellets which are consumed and extracted to be re-purified every day. 33kg of horrifically radioactive fuel per day is not an unreasonable ask for a mecha - it's a sphere about 20cm in diameter - although we've handwaved away potentially several orders of magnitude of efficiency in looking at theoretical maxima. But mainly we're overlooking the fact that we can already get that much energy from 32 tons of diesel, which is also not a huge ask for a machine that size, and is probably a lot easier to transport, handle and engineer for.

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  • $\begingroup$ You can't increase the energy output of these batteries by 10^5 regardless of what isotope you use. They will melt long before you get that much energy out of them. And the electrical generation methods will saturate long before that. $\endgroup$ – puppetsock Jan 24 at 18:53
  • $\begingroup$ You couldn't increase it to this level and keep it in the shape of a C-size battery, no. But by breaking it up into smaller pellets and embedding it in a heat-sink you could control the heat (and probably even capture it for additional energy extraction). No one is suggesting that this power source is easy to work with :-p $\endgroup$ – Stephen Jan 28 at 10:39
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I'm more interested in high powered short lived applications

Your "batteries" start discharging as soon as they're assembled, and so have no shelf life to speak of and are unrechargeable. To manufacture them, you need something like a particle accelerator, or a specialist nuclear reactor. Their discharge curve is an exponential decay function, so the highest power bit is also the shortest lived bit and is available right at the point at which the desired isotope is decanted from whatever mechanism produced it in the first place. It also means that you need some other storage mechanism to smooth it out for most purposes.

Compact fusion, fission and antimatter devices produce problemeic amounts of waste heat, are insanely complex and dangerous.

Radioactive waste wrapped in power extraction mechanisms is still radioactive waste. High power radiation sources are also going to be hot. They may not be complex, but they are the product of a necessarily short manufacture and logistics chain that terminates in some large, expensive, inconvenient, insanely complex and dangerous equipment.

You've also made every battery its own little "dirty bomb" just waiting for a suitable amount of force to spread it around, unless you've packaged it very well which will make it bulkier and heavier and more expensive, none of which are really desirable.

As an aside, safely shipping radioactive waste around is problematic in itself, but it also makes it more difficult to detect suspicious radiation sources in transit. Generally, any radiation sources strong enough to be interesting are also going raise eyebrows, at the very least. If you've got loads of the things floating around, it becomes a bit easier to smuggle hazardous materials (eg. fissiles) without people noticing.

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