# Tag Info

22

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. ...

21

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 ...

19

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 ...

17

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 ...

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 ...

14

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 ...

13

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 ...

11

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.

11

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 ...

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

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 ...

9

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. ...

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

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 ...

8

Is that correct that there are no nuclear warheads in service made of U-235, as plutonium ones are much smaller & much more efficient Maybe. Who knows precisely what people use. But publicly available information from US warheads is that they moved away from Uranium to Plutonium fission devices soon after world war II. Is that correct, that ...

8

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 ...

7

The German THTR-300 Thorium High-Temperature Reactor operated for about 16,000 hours and the IAEA produced a report on its shutdown. So there are no physics barriers to thorium reactors: there is an existence proof for thorium reactors. That ends the relevant answer for this site. There are economic, engineering, social, political, technical, and ...

6

The probable answer is that to make a bomb one needs a very special design and very pure U235, in a sphere. So even if a critical mass forms in meltdown, a reactor does not have the geometry and purity for a nuclear bomb. No mushrooms. But it may continue to heat, acting as a reactor, and the problem is in conveying the heat away without building up steam ...

6

The funny thing is that in the case described in the paper, pretty safe isotope (I-129) was converted into deadly dangerous I-128, I would prefer to leave it as is :-) Transmutating random mix of isotopes would give you more random mix of isotopes (garbage in->garbage out), and it's a big question if this would lead to lower integral activity. So to ...

6

I don't know about extracting mass-energy from inside the black hole, it seeps pretty inefficient to me. Hawking radiation isn't that powerful (as well as still being highly hypothetical), 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 ...

6

A lot can be, and has been, written on the subject, but I'll give you the short and sweet version. Does nuclear chain reaction start in fuel pellets even before they being installed in reactor? -- No There are several reasons why this is so. The number of spontaneous fissions of $^{235}$U is minimal. The branching ratio for that mode of decay is \$7 ...

6

Nuclear power is the only way to make submarines work underneath the water for long distances without coming up because of the oxygen that all other types require. And batteries for electric engines are too heavy. Historically a submarine had to go up to just below sea level to get air through a snorcel for the engines that charge the batteries. Then it ...

6

We already store waste by combining it with molten glass. It doesn't take very much energy - obviously no more than making glass! The main reason for not doing it is political, it cost a lot to make that plutonium and it might come in handy to build bombs one day - so we don't want to waste it by reprocessing it. (We also have a small problem of persuading ...

6

Well, if you search the internet it seems there are kids out there that make the claim of having built a fusion reactor . I watched this link. Note that in .56 minute he gives a small description, and does not claim breaking even, but that he demonstrated fusion. It is a plasma that he obviously creates and manages to fusion some deuterium that is not ...

5

A heck of a lot of energy in one place always represents a local danger. The question of a more wide spread danger depends on a lot of details. Does the core reaction produce neutrons? How bad is the activation rate in the plant? Would a disaster event spread activated material over a wide area? In the absence of data I would guess "less dangerous than ...

5

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 ...

5

This is a graph of Binding Energy per nucleon for the various nuclei. The fission of heavier nuclei would lead to daughters with higher binding energy than the parent (graph slopes downward towards the high mass end). Daughters will be stable, hence energetics favors fission for heavy nuclei. In Response to comment: This answer address fissionable ...

5

When 235U captures a neutron, it forms the compound state 236U*, the excitation energy is given by E_ex = [m(236U*)-m(236U)]c^2 To find the energy of the compound state, the mass energies of 235U and n can be used - assuming that the neutron will be thermalised for this reaction to occur and thus it's kinetic energy is negligible. m(236U*) = ...

5

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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