Hot answers tagged

63

The answer is in wikipedia The photograph on the right shows two unusual phenomena: bright spikes projecting from the bottom of the fireball, and the peculiar mottling of the expanding fireball surface. The surface of the fireball, with a temperature over 20,000 kelvin, emits huge amounts of visible light radiation (more than 100 times the intensity ...


47

The efficiency of a thermoelectric generator is around 5 - 8%. The efficiency of a large steam turbine power plant aproaches 40%. In fact the thermodynamic efficiency of a large steam turbine power plant is over 90%, so it's about as efficient as anything could be. The maximum possible efficiency of a steam driven engine is given by the idealised model ...


44

Everything is made of tiny things called atoms. All atoms have a tiny center part called the nucleus. Some atoms have an unusual type of nucleus that, every once in a long while, randomly explodes, sending tiny pieces in all directions. Normally those tiny pieces just bounce around until they join another atom. However, if you have a bunch of the right kind ...


27

I'm not sure what all you've read on them, but I'll try to clarify at least a few things. I would certainly disagree with several of your assertions. For starters, you say "...they don't produce anything you could feasibly use as a source of material for nuclear weapons." Thorium reactors use Thorium as a fertile fuel that transmutes into fissile U233. ...


23

I was going to comment on other people's answers, but this was going to become too long. Almost everyone fails to separate Thorium (which is a fuel type) and reactor type. Safety is a function of the reactor type, and molten salt in particular for this question. Does the fuel choice impact ultimate reactor safety? Yes, but to a limited extent. So how ...


23

It's all a question of if they need it. Most that are staying within a couple AU of the sun can get sufficient power from solar panels. It's when they start getting further away that they use an RTG. For example, New Horizons, which launched in 2006 (which is considered to be 'modern' when you only launch a few probes per year) is going to Pluto, so it ...


23

There are many ways to detect a nuclear explosion, and there are people working to detect it. The Comprehensive Nuclear-Test-Ban Treaty Organization or CTBTO is one such organization. They are using a global network called the International Monitoring System. This is capable of detecting any nuclear detonations anywhere on the Earth (underwater, or in the ...


20

which experiment gave scientists the reason to believe nuclear fission/fussion existed Fusion was first. Francis William Aston built a mass spectrometer in 1919 and measured the masses of various isotopes, realizing that the mass of helium-4 was less than 4 times that of hydrogen-1. From this information, Arthur Eddington proposed hydrogen fusion as ...


20

This seems wrong, or the uranium mine would explode whenever there is a rainfall. A natural nuclear "reactor" probably existed at Oklo, Gabon The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a nuclear chain reaction took place. The heat generated from the ...


18

Per this article on the subject: http://theenergycollective.com/nathantemple/53384/how-shutdown-and-core-cooling-japanese-reactors-likely-functions Even with rods inserted, the reactor continues to produce heat equivalent to about 3% of its full power level. This is not the same as taking a pot off the stove and letting it cool. There are still ...


18

If you throw a bunch a uranium ore in one blob, nothing happens. If you chemically purify the ore so that the only element present is uranium, still nothing happens. The runaway chain reaction needed for a uranium-powered bomb involves U-235, an isotope having three fewer neutrons than the most common natural isotope U-238. According to Wikipedia, The ...


18

my question is about whether it's possible in principle The answer is yes. and whether anyone tried it. The answer is by all chances, no. So, how come? The effect The thermoelectric effect for electricity generation (called the Seebeck effect) is the phenomenon that a voltage is generated at a temperature different across the ends of a conductor: ...


15

edit: I originally had some points about the inefficiency of RTGs, but after some more research prompted by @Jeremy I found that it's not really a valid point when they're used appropriately for the spacecraft's mission. The RTGs used by Galileo at Jupiter generated 300W of power, whereas the solar panels that will be used by Juno at Jupiter will generate ...


15

There are many reasons for this situation. 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. (Mentioned by DumpsterDoofus) low power density. ${}^{63}\text{Ni}$ for instance produces ~5 W/kg (and kg here is just mass of ...


15

Short summary In fact, in typical reactor, neutron needs to travel quite a lot before it initiates next fission, if during these travels it encounters control rod it is "lost" and chain reaction slows down. Neutron needs to travel because it needs to lose energy (or in other words slow down), this is because modern reactors are designed in such way that ...


14

If you read Wikipedia page about corium, they say that critical mass can be achieved locally. But if you are concerned about a critical mass allowing a nuclear explosion, the difficulty in nuclear weapon design, as told here, is to achieve the criticality fast enough. If you do not achieve criticality fast enough, your material heats and its interaction ...


12

I don't know about extracting mass-energy from inside the black hole; it seems pretty inefficient to me. Hawking radiation isn't that powerful (as well as still being highly hypothetical), but building a Dyson sphere around a star is better. Remember, black holes, are, well, black. If there was enough Hawking radiation, they would cease to be black and would ...


12

The real problem with RTGs is that the US stopped making Pu238 in the 80s and has been very slow to start up production again, purchasing all our spacecraft Pu238 from the Russians (who have now also run out). I don't know about the byproducts from the breeder reactors, but Pu238 itself is actually not that dangerous to handle, and only toxic if ingested.


12

Why could they not have a 30 meter pit below a reactor filled with water with a trap door holding the reactor up? If you read about the Molten-Salt Reactor, you will find that it has something rather similar to this. In the image below, item number 13 is a freeze valve. If the reactor overheats, the plug melts and the molten core flows down to the tanks ...


12

Assume that you have a fission of an atom of $U^{235}$, and that we look at one of the neutrons produced. Although the neutron itself is sub-atomic, the "size" of the space needed for the fission neutron to slow down through collisions with the moderator atoms, avoid capture by control rods or reactor structure, find another atom of $U^{235}$, collide ...


11

In principle, the drop in the Gibbs energy when the uranium gets converted to the fission products is available for doing useful work. While a steam engine will not come close to the maximum possible efficiency attainable (which is very close to 100%), a thermoelectric device will have much worse performance, as pointed out in detail in the other answers. ...


10

Starting in the fifties, there was a lot of work (see RDD-8, V.C.1.g) trying to build a pure fusion weapon for mainly two reasons: they promised to be cleaner than conventional thermonuclear devices (important for peaceful uses and some of the not-so-peaceful ones) and they wouldn't need relatively scarce fissionable materials. As you can use staging to ...


10

This is a cost to benefit question and can only be answered by a guess in a physics board. There is a new generation of small compact reactors that could be used for powering apartment buildings The new reactor, which is only 20 feet by 6 feet, Seems compact enough, so it is not size but weight that is important, since this weight has to be lifted. ...


9

In addition to the above it might be noticed that coal fired power stations are often inland despite their similar requirement for lots of water. The difference being in the weight of fuel required to run the power station. A significant desirable fact of nuclear power is the very high energy density of the fuel compared to coal. So we have more ...


9

This would guarantee a meltdown. They're trying to get heat out of the core because---thought the fission chain reaction has been suppressed---various unstable fission daughters continue to decay. Adding hot lead would add heat to the system and not stop this behavior. Total disaster. -- If the core does slag out, it will probably end in a hot (...


9

As far as a catastrophic plasma disruption is concerned, it might be a problem for the first wall, and hence the ability to use the reactor. But despite the high energy per nucleon, the desity of the plasma is extremely low, the total energy is dominated by the energy in the magnetic fields, and thats not tremendous. Of course you do have the radioactivity ...


9

One aspect is the concern for if the spacecraft were to fail to launch correctly and ended up crashing back to earth. In such cases, the nuclear radiation pollution could be severe if it ended up crashing in inhabited areas.


8

This was (one of the few) facts reported clearly and correct in German news. The Zirconium metal of the fuel tubes reacts with water vapor at elevated temperatures (above 1200 °C if I remember correctly) The reaction is simply: Zr + 2 H2O => ZrO2 + 2 H2 This is proof, that parts (at least) of the fuel rods were not immersed in water, but it does not ...


8

Hmm, lets see. The melting point of lead is fairly low, 327.46 °C and it is a good absorber of radioactivity. I think the problem with the reactors is not the heat per se, but the exposure of the fuel rods to the air without cooling because of escaping steam not being replenished by cooling water. The remaining steam etc may blow out the thick container ...



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