What materials would be significantly better than currently commonly used radioactive materials (Uranium/Plutonium) for energy generation?
I am looking for some material which could've been brought (so it doesn't need to occur naturally or commonly) to Earth long ago, kept hidden for thousands of years and now uncovered.
Molot has the right answer, but I'd just like to chime in with numbers because that is what I do.
Chemical reactions don't release nuclear binding energy; instead they only release the energy of molecular bonds, so they have limited bang for their buck. For example, burning octane (a component of gasoline) through the following chemical reaction
2C$_8$H$_{18}$ + 25O$_2$ $\rightarrow$ 16CO$_2$ + 18H$_2$O
yields 17.0 MJ/kg of reactants. This is about the recommended calorie intake for an active adult male, or the kinetic energy of a tank's main gun.
Fission reactions (Uranium and Plutonium) generate energy by splitting an atom into smaller parts. A portion of the binding energy from each atom that is split is released. Because mass and energy are equivalent, the binding energy of large atomic nuclei is manifested as a measurable mass. When such a large nucleus is split, the resulting fission products have a lower total mass than the original particle, by about 0.1%; mass was converted to energy.
Here are the numbers for U$^{235}$. Various other Uranium, Plutonium, and Thorium isotopes have similar energy profiles; you won't get a significant boost to energy density by switching out your fission fuel.
The average energy released by a single atom during fission is 202MeV, broken down as:
For 235 g of reactants, that means you get 1.74e13 J output, or 73.9 TJ/kg. This is about equal to the energy from Little Boy, or all the gasoline carried by an Airbus A380.
Fusion is sort of the converse of Fission; smaller particles merge to form larger particles; as they merge they lose mass. This works because of the binding energy curve. The binding energy is highest at around iron, with ~56 nuclides.
Thus, any reaction with daughter nuclei closer to Iron will produce more net energy. Fusion is more productive than fission because there is a (visible) large delta in binding energies between hydrogen isotopes and ultra-stable Helium-4; and also because the reactants are much lighter per atom.
The best fusion reactions from a specific energy standpoint involve fusing $^2_1$H with either $^3_1$H or $^3_2$He. The former of these two reactions releases most of its energy in a hard-to-capture neutron, but the latter develops 18.3 MeV per fusion as kinetic energy in an alpha particle and proton. Both can be captured and used electrostatically, giving a specific yield of 353 TJ/kg. This is about the energy released in a large thunderstorm, or a 20 meter meteor strike.
Whereas the previous reactions converted parts of their mass that were equivalent to nuclear binding energy to usable kinetic energy, matter-anti-matter reactions convert all of their mass to energy. This conversion is done simply by the famous equation $E=mc^2$. Thus, $c^2 = 8.99\times10^{16}$, so anti-matter yield is 89.9 PJ/kg, which is about the energy released in a magnitude 9+ earthquake, or 1 second of solar radiation on the Earth.
On the minus side, the anti-matter reactions are complex. Electron annihilation releases gamma rays, which are hard to capture and use. The reaction between protons is even more complex, as each proton is in turn composed of various quarks. Some of that energy will be carried away in unrecoverable neutrinos, and most of the rest in high energy gammas. I'm a bit down on anti-matter as an energy source for these reasons; there is a lot of research and engineering before we could recover the energy from this reaction.
Yes, antimatter can release about 3 orders of magnitude more energy per kilogram than nuclear fission, although I certainly hope the aliens left a tech manual explaining how to recover it. Also, don't be the explorer that slides the lid off the tomb filled with anti-matter...
Nothing beats energy density of matter-antimatter reaction, with famous $E=mc^2$ equation. Actually, if we will measure energy density the way we do for fuels we burn, it will be $E=2m_ac^2$ where $m_a$ is mass of antimatter - because for each unit of antimatter we will annihilate the same mass of matter - and matter is everywhere, essentially free.
For now, storing antimatter is only possible for short durations. CERN managed to store antihydrogen for 16 minutes and that's current world record, it seems. But there is a patent application for storing antimatter in an unpowered ways, trapped in fullerene molecule. This sounds feasible and, in theory, could be stored a long time.
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Well,
there is a correlation between the energy density a radioactive material has and its decay rate: The higher the decay rate, the higher the energy density.
To store store it for some thousand years, it needs a half life of the same order of magnitude or larger. Also, α radioactivity has typically more energy per decay than other types of radioactivity. So you need an α radioactive material with a half life of several thousand years. Besides Plutonium you can choose some other transuranium isotopes (of Americium, Curium, Berkelium or Californium) or you can use pure Radium. There are also interesting isotopes of Lead and Bismuth available.
An exotic alternative constitutes the isotope 250Cm that decays by spontaneous fission, emitting even more energy than a typical α emitter, and that has a half life in the right order of magnitude.
You can find isotope data on the German language wikipedia here: Liste der Isotope/7. Periode.
EDIT: Handle with care! Some of the suggested isotopes are able to sustain a nuclear chain reaction and careless handling may cause a criticality accident.
While I would agree with the existing answers that antimatter wins on a strict energy density ranking, I would like to propose an alternate material for consideration.
Super heavy elements inhabiting the island of stability.
In general as the atomic mass increases for elements (the nucleus becomes larger with more protons and neutrons) they become more unstable with shorter half lives (with a corresponding increase in radioactivity). However, it is theorized that at some large threshold the large number of protons and neutrons in the atoms nucleus becomes more stable making the elements last for longer than a few microseconds before it begins decaying.
The super heavy elements might have some interesting properties, the most useful of which may be a very small critical mass allowing for much smaller nuclear weapons.
Simply by the fact of their extreme densities, atomic masses >300, compared to U-235 or Pu-239, would give them slightly higher energy densities compared to existing fission based fuels.
Using radioactive materials for passive heat production (without the need for a reactor) is the simplest method, but power is inversely proportional to half-life, so after thousands of years, there won't be much useful material left. With a half-life of 432 years, Americium-241 is probably the best choice here. It is currently being considered for space probes due to the shortage of Plutonium-238. Even after thousands of years, the remainder should be seperable without too much problems, as the decay product is Neptunium-237, which has a half-life of 2 million years (and is useful in its own right).
Current fission processes all start with uranium. Natural uranium consists of 99.3% U-238 and 0.7% U-235. Uranium-235 is fissile, but Uranium-238 is not. It will eat neutrons though, especially at higher energies, which means you can't get a chain reaction in natural uranium without a very clever reactor encasing it. For most purposes, this means the uranium will have to be enriched, increasing the percentage of U-235.
Except for bombs, practical enrichments still contain mostly U-238, which means you will produce Plutonium-239 as it absorbs neutrons. Pu-239 is itself a good fissile material and has a lower critical mass than U-235, though if it stays in a reactor for too long, it will grow fractions of Pu-240, Pu-241 and Pu-242, which will make it unsuitable for gun-type bombs.
All fissile isotopes, upon neutron-induced fission, will release roughly 0.1% of their mass as energy, so big gains can't really be made here. Fusion will give you about 0.5%, but there are other answers that describe that.
An alternative fuel cycle starts with Thorium-232, the only primordial isotope of thorium. Like U-238, it's not fissile, but when it eats a neutron, it will turn into Uranium-233, which is fissile.
This fuel cycle is considered (by some) to be better because it doesn't grow transuranium elements (such as plutonium) by design, and as fissile isotopes (U-233 in this case) that fail to fission (and instead eat the neutron) will have extra chances to do so (U-235, Np-237 (fast neutrons only), Pu-239), very few transuranics will be produced in practice.
Up to this point, we've discussed things that are available on earth right now, in sufficient quantities. Some transuranics have half-lives sufficient to stash them away for a long time, while still having useful properties.
Neptunium-237, mentioned earlier, has a half-life of 2 megayears, can be used in bombs and fast reactors, and can be used to breed Plutonium-238, which is, for most purposes, the best RTG material.
Curium-247 is an isotope of Curium that has a half-life of 15 megayears. It is fissile and has a critical mass much smaller than the currently used fissiles. This enables the creation of small bombs (suitcase nukes) and small reactors. If you leave the stuff lying around long enough, it will grow fractions of U-235 (half-life 703 megayears), Pu-239 (24 kiloyears) and Am-243 (7 kiloyears), with U-235 being the largest fraction by far. Amusingly, this produces chemically separable, pure U-235. This can also be done simply by storing Pu-239 for long enough.
Other curium isotopes may yield even better results, but only Curium-245, with a half-life of 8 kiloyears, can make even a claim at being storeable.
As far as where a hidden energy source could be kept hidden for thousands of years and now uncovered, I have a perfect answer and that is right where we found all the other radioactive elements. In pitchblende.
Pitchblende (Uraninite) is a radioactive, uranium-rich mineral and ore. It has a chemical composition that is largely uranium, but also contains oxides of lead, thorium, plutonium, polonium and rare earth elements. Marie Curie processed tons of pitchblende to come up with her discoveries of radium and polonium. If you were to discover a radioactive substance beyond group 7 elements, traces of it would be found here.
In the past when miners would find pitchblende ore, it was usually a disappointment. It meant the silver vein they were following had run out and now it was just lead and pitchblende, which had little commercial value, all the way up to the point it became worth a great deal to anyone interested in radioactive elements.
What better material to discover has riches planted from the past than something everyone thought through the centuries was utterly useless?
https://en.wikipedia.org/wiki/Uraninite
The need for a huge energy kick brings us to nuclear reactions, but fusion fuel is not very likely to fit the bill; we have fusion fuel and the problem lies in getting it to fuse to begin with. So we would need to retrieve the technology rather than the material.
Fission could do, but even then, the brighter an element burns, the shorter it does so. A radioactive substance that's still radioactive enough after a thousand years would not be all that radioactive.
I submit a third hypothesis which is still tolerably plausible: an anti-meta-material.
It is possible to create a structure (Dehmelt-Paul lattice) that is capable of trapping antimatter ions in a charged ionic lattice. In reality, with the materials and technology we have, it can't exist because building it would require extreme tolerances, and the anti-ions' thermal energy must be lower enough than the lattice constraints that the lattice itself doesn't implode.
But we could imagine a metamaterial where a positively charged lattice held free antiprotons in crystal-like "cells" at room temperature for an indefinite time. Then, when heated e.g. by a laser, the lattice would start collapsing and freeing antiprotons towards the laser itself; these could be electrically accelerated and used to trigger a controlled series of annihilations. You would get lots of radiation and a staggering amount of thermal energy. Then it's just a matter of using that heat to power a turbine, not unlike what's done in (fission) nuclear reactors.
Once the nature of the magic crystals is discovered (that they are about 5% antimatter in mass), to exploit them you only need a tuned laser and to be able to control an electric field. And you get an energy source that's hundreds of time more powerful than nuclear fission.
The crystals also double as fearsome weapons - crush a crystal with a powerful enough explosive, and enough of it will fracture and start an antimatter explosion, vaporizing the remainder of the crystal and instantly triggering an even larger reaction.
