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First let me say that I know almost nothing but the basics about nuclear fission. I know more about nano robotics but I've got no degree in the matter, just a rather in depth curiosity and a lot of hours logged in researching the matter. I am pretty much clueless but who knows, maybe I'll get lucky.

So the question, or rather the idea: I know one of the current running issues for nanobots/nanites/etc. is the fact that there is a lot needing to be crammed into such a small package including a power supply. This problem is exacerbated if the idea is that these nanobots are going to be performing complex tasks that we might not be able to do otherwise, at least not easily.

So I have been thinking: what if the nanobots were their own battery? Now I know that by the law of conservation of energy this is impossible but follow me here for a minute.

The process of nuclear fission the end result of the process is minuscule compared to that in which was used to make it and is also radioactive with a extensive half life. Now what I'm wonder is if this radiation is usable as an energy source, and if so would a nanobot created with such material could used that radiation as a source of energy thus in concept be self powering

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    $\begingroup$ Hi, and welcome to Worldbuilding! I tried to clarify your question in my edit, but the last paragraph eludes me. What do you mean that the end result of nuclear fission is minuscule compared to what was used to make it? Mass loss as a result of nuclear fission is only a fraction of a percent. $\endgroup$ – jdunlop Sep 21 '19 at 7:46
  • $\begingroup$ I always expect that nanobots will become more biological like bacteria and viruses rather than actual robots in the future. It makes sense for nanobot functions to use a potential self-replication process alongside the natural energy that already feeds current bacteria and virusses. It also hands you a good way to limit them by building genetic limits in how many times they can reproduce to prevent problems. $\endgroup$ – Demigan Sep 21 '19 at 17:04
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There are a couple of structural issues with this idea.

As LDutch pointed out, radioactive decay on the scale of something constructed of at most a few thousand atoms of material is not going to give you much power, but that's only the start of your issues.

Most radioactive material does not have great structural properties, and even if the material you choose is an artificial isotope of something more useful (like carbon, maybe), its decay products aren't going to have the same properties, so as the components of your radioactive nanobot decay, they're going to stop working.

There's also the fact that you can't usefully capture energy produced by the material your nanobots are made of without enclosing them in non-radioactive material, which would defeat the point.

And finally, if you have a tiny piece of radioactive material that happens to have robotic characteristics, what you have is a fuel flea. The electrostatic build up from the radiation created by its materials' decay would fling your nanobot around far too readily to allow it to do anything useful, even if it could capture the energy inherent in nuclear decay, which it cannot.

So, for many, many reasons, this idea cannot work in anything approaching real-life science.

TL;DR: a nanobot cannot practicably be made of radioactive material, and could not be self-powered using radioactive decay

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  • $\begingroup$ Fuel fleas. Whoever came up with that name was having a good time! That's hilarious :) $\endgroup$ – Megha Sep 29 '19 at 3:34
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Here's the funny thing, nanobots are so small and their energy requirement so minuscule, that it can actually harvest the energy from its surrounding more easily. Nanobots don't actually utilize energy in the conventional way making them more efficient. For eg: you want a boat to move across water with a payload, you would require motor, fuel and steering, apart from a separate control system that requires a battery. Now the same on a nanobot scale, won't require so many components. Heck, just the signal from the controller is powerful enough to drive the nanobot like an RFID. Instead of fan shaped propeller, it would have a flexible tail fin. No need for any fuel, or motor even, since the tail can be moved with either using heat to expand the two sides of the tail alternatively or use static charge or even em-coils.

The reason for this is that the forces we usually neglect in macro devices, become driving forces in a nanobot. A good example of this would be surface tension v/s gravity. Also the property of materials also changes significantly at that scale. As such fission, let alone being efficient, is simply unnecessary. Nanobots are not their own battery, rather they are their own energy harvesters.

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    $\begingroup$ "No need for any fuel, or motor even, since the tail can be moved with either using heat to expand the two sides of the tail alternatively or use static charge or even em-coils": This is what a motor is. $\endgroup$ – AlexP Sep 21 '19 at 15:31
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    $\begingroup$ There's a nice set of animations of molecular motors which use chemistry in this short vid, it always amazes me what nature has already devised. $\endgroup$ – Tantalus' touch. Sep 21 '19 at 16:38
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    $\begingroup$ @AlexP Sorry for the way i worded it there, I was referring to a traditional generator-motor assembly in reference to the boat example, I should have worded it preperly. $\endgroup$ – Deepon GhoseRoy Sep 22 '19 at 11:13
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There exist in and around the chernobyl nuclear power-plant radiothropic fungus capable of generating chemical energy from radiation, using the pigment melanin. Theoretically nanites would be able to use a similar chemical to perform radiothropism and generate energy. That being said, there are caviates for this, fistly in order to recapture radiation from the reaction you would need a high concentration of nanites (even alpha particles are likely to simply pass though a single nanomachine with ease) , secondly flox is right here in that radiological power-sources are probably a better bet than nuclear ones since complex control mechanisms would be needed to stop a melt-down or biosphere contamination with a fission reaction and while possible with nanomachines there's also a much higher chance of failure than with conventional systems, next compared to a conventional thermal generator your efficiency is likely to be abysmal, finally radiation will likely do damage to a lot of potential nano-machine systems.

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As has been already pointed out, fission is not a good idea, since the mass of the system supplying it would be way bigger than the mass of the nanobot.

An RTG doesn't seem suitable either, though.

In the past, small "plutonium cells" (very small 238Pu-powered RTGs) were used in implanted heart pacemakers to ensure a very long "battery life".

However, a true nanobot is made of between a few thousand atoms, and a few million. This means that, if the RTG is of comparable size, the RTG cannot work well, because statistics work against you: you will have too sparse decays to continuously supply heat to the RTG.

Consider plutionium: with an half life of 87 years, it means that if you start with 1000 atoms, 500 will be decayed in 87 years, meaning about 6 atoms per year! If you instead use polonium, with an half life of about 4 months, you will have about 125 decays per month, or 4 per day. If you use 1 million atoms, you are still out of luck with plutonium, while polonium will ensure you a very short life.

Long story short: radioactivity won't work for nanobot.

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The problem with fission is that it delivers too much energy to a finely-tuned collection of atoms. It would be a bit like feeding a high explosive into a car engine, and would be more likely to shatter the whole engine than to do useful work. Fission byproducts are not completely predictable, so there could be a few different highly energetic smaller atoms that the nanite would have to deal with, not to mention the chance of neutron activation of its atoms.

However, there is another nuclear option other than fission or fusion. Certain atoms can absorb gamma radiation, jolting their nucleus into a higher energy configuration, and unlike most other atoms, they can retain this extra energy for a long time, not nanoseconds, but much, much longer. Metastable Tantalum-180 has never been observed to spontaneously decay, giving it a half-life of 10^15 years or more, while others may have half-lives from hours to years.

Metastable-2 Hafnium-178 can hold 1.33GJ of energy per gram, has a half- life of 31 years, and there are disputed claims that it can be stimulated to release its energy on demand. With other elements having lower energy capacities and decay modes that include excitation of their electrons, nuclear batteries have been hypothesized, with energy densities a million times higher than any chemical energy storage system known, and the potential exists for them to be switchable.

See https://en.wikipedia.org/wiki/Nuclear_isomer.

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Nuclear energy is possible, but use decay instead of fission.

Fission is a nuclear process, and you can use it to generate heat for power, however it would be fairly complex to use this to power your nanobot, which may increase it's size significantly such that it becomes bigger than a nanobot.

Fission requires an unstable element, such as Uranium, that will split into two smaller atoms, releasing energy. However, your nanobot would need to retain fuel, capture the energy, remain shielded, control the reaction properly, keep it moderated (hot enough) to sustain the reaction, keep it from melting etc.

By the time you create systems to deal with these issues your nanobot may be the same size as a modern nuclear reactor.

However you may not require an actual fission reaction to supply you with power: Radioactive decay may be a better candidate. This is distinguished from fission in that the atom remains the same, but as it is unstable it emits alpha, beta or gamma radiation instead.

In fact radioactive decay generators, using thermo-couples, are already in use in probes and satellites as the mechanisms to get power from decay heat is a lot simpler, has less weight and much smaller and longer lasting (requiring also less 'fuel').

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  • $\begingroup$ Radioactive decay doesn't leave atoms the same.. It's not induced fission, but alpha particles are two protons and two neutrons. The neutrons don't change the chemical properties of the atom, but the protons sure do. That's why radioactive decay gives unstable isotopes a decay chain. $\endgroup$ – jdunlop Sep 21 '19 at 7:41

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