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[Nadaka]

True of course, though I thought I saw a heat capacity value of 3.6 or 3.8 kj/kg*K somewhere. Of course the temperature also matters, but then I'm wondering why they're using the mix you refer to instead of, say, molten ceramics which have a significantly higher melting point.

Because molten ceramics are pretty close to being magma, and we all know how often magma works against you.

[PTTG??]

Let's say the weather's bad for just too long and they can't keep the temperature high enough. Would you rather the pipes are full of salt, or solidified obsidianlike ceramics?

[Zrk2]

...Never?

Anyway, about the thought that private ships with nuclear reactors could siezed by terrorists or whatnot, well CANDU reactors use non-weapons grade uranium, meaning their uranium can't make bombs.

[olemars]

No nuclear power plants use weapons grade uranium, most use fuel enriched to 4-5% U235, many even less (weapons grade is 90%+). More interesting when comparing reactor types is what kind of waste they produce. That's where Thorium reactors shine, especially the subcritical energy amplifier concept.

[Mattasmack]

True of course, though I thought I saw a heat capacity value of 3.6 or 3.8 kj/kg*K somewhere. Of course the temperature also matters, but then I'm wondering why they're using the mix you refer to instead of, say, molten ceramics which have a significantly higher melting point.

Hmm, if you can find that reference I'd be interested in seeing it.  Water's pretty hard to match in terms of specific heat, at least for other liquids at anywhere near the same temperatures.

Despite all the interesting and sometimes-crazy technologies being studied at universities and research labs, once things start to get built on an industrial scale manufacturers tend to be quite conservative.  Molten salt has been used as thermal storage for years, and was used as heat transfer fluid in a large-scale research system (Solar Two) 15 years ago.  But it is only now that they're willing to try using it as heat transfer fluid in a commercial plant (Archimede in Italy), despite the fact that it makes some aspects of the plant simpler.  Any other medium will have to be studied for a long time before it gets accepted on a large scale.

In current systems, the salt always remains in the liquid phase.  The high melting point of the salt is actually a problem.  The plant designers have to consider every operating (and failure) mode of the plant and make sure there's no way the salt can freeze.  If the salt freezes solid, in the piping or in the storage tank, you're pretty much stuck.  I'm not familiar with molten ceramics, but a higher melting point is not an advantage, unless they're being used specifically as phase change thermal storage.  And then the melting point has to be just right to match the rest of the plant systems.  -- ah, PTTG kind of beat me to it.  I'm not familiar with molten ceramics, what sort of melting point are we talking here?

[Virex]

Depending on the ceramic, anywhere from near 1000 K to over 3000 K. But you're right, I was looking at it from the point of energy efficiency, not from the point of what's practical. Even if it's possible, you're looking at prohibitively high costs for insulation and material costs for heat exchangers. Not to mention the cost of keeping the stuff hot during a maintenance shutdown (might be less of a problem if used for long term storage, but if your system is rated for a down time of 5 days and the maintenance takes 2 weeks you got a problem.)


Also the reason that it takes so long for a technology to diffuse into the industry probably has to do with the fact that most plants are designed to run for several decades or more. Engineers aren't too keen on using technologies that haven't proven themselves to be fully reliable. Especially in the case of liquid salt, I could imagine it's quite difficult to do unscheduled maintenance, because the salt has to be kept hot and probably also has to be kept flowing to prevent solidification. I also don't know the viscosity of it, but if it's high, heat exchangers and pipes also have to be designed to avoid hot spots. And even if the technology has proven itself, most plants are partial rebuilds of older plants or additions to existing sites, where the installation of a new technology like liquid salt heat transport would be very expensive compared to using the already existing steam networks.


Also it seems I got my numbers mixed up. Some salts have a heat capacity of 38 cal/mole*K, which I mistook for 3.8 j/g*K. That'll teach me to rapidly browse a document.

[PTTG??]

Just to clarify something, I wasn't talking about nuclear bombs, but rather dirty bombs.

[Mattasmack]

Ah, yeah, that's a bit beyond current technologies.  Most currently-operating plants use a heavy oil as heat transfer fluid that's limited to 390 C.  That one plant running with molten salt heat transfer fluid gets up to 550 C.  Parabolic trough plants probably won't go to much higher temperature, as the thermal radiation heat loss gets too bad.  To get to higher temperatures efficiently you need a higher concentration ratio of the sunlight, like you get with a central tower system.  Of course, then you're getting past the maximum temperature for steam cycles, so would need to revamp the power block and everything else too.  It may happen, but it would be years away.

Oh I agree, there's good reason for manufacturers to be conservative when it comes to building the plants.  And yeah, freezing of the molten salt in the pipelines has been a worry for years.  There are lots of articles in the literature talking about it and possible ways to thaw a frozen collector loop.  Freezing in the thermal storage tanks isn't such a risk; it would take a months for a large one to cool down enough to start freezing.

[ChairmanPoo]

Weren't there some kind of "small scale" nuclear batteries?
...gojira...

[Nadaka]

Weren't there some kind of "small scale" nuclear batteries?
...gojira...

Thermopiles, yes.

Toshiba was also planning on mass producing a self contained mini reactor the size of a semi trailer.

[Virex]

Beta batteries would also count.

[Tellemurius]

As i noted earlier yes.

[olemars]

And yeah, freezing of the molten salt in the pipelines has been a worry for years.  There are lots of articles in the literature talking about it and possible ways to thaw a frozen collector loop.  Freezing in the thermal storage tanks isn't such a risk; it would take a months for a large one to cool down enough to start freezing.

Reminds me of the insane Alfa submarines built by Russia/Soviet. They had nuclear reactors that used molten lead as a coolant. That gave them a lot of power for the size and a top speed over 40 knots submerged. Only problem was that if the coolant ever dropped below 125 degrees celsius, the reactor had to be replaced. Half the subs built of the class were scrapped due to unintended reactor shutdowns.

[Nadaka]

Everything I hear about the scenario makes it sound worse. I've heard experts go from stating "There's no conceivable risk" to "Well damn..." in under a week. I'm really hoping they can avoid a disaster, and barring that get everyone out of Fukushima Prefecture before things get too ugly.

Either way, I'm hoping that this provides a welcome incentive to research power sources other than nuclear energy. Solar Plants don't leak deadly matter and energy when they have problems, and we've explored the technology only to a fraction of the degree we have Nuclear Plants. I can't help but think that the power industry's endless iterations of using hot things to make steam to turn turbines is horribly inefficient and generally unsafe, when we should be skipping the middleman and creating electricity directly from chemical reactions utilizing a source of energy that is free and nigh-endless in supply.

I did a hell of a lot of chemistry while I was in school.

I am afraid that getting electricity out of a pure chemical reaction isn't practical at all. And in the cases where you can, the process of getting the chemicals into the right state uses more energy than you get out of it. Advanced photovoltaics can be more efficient that a heat engine, but they require exotic materials and nanoscale structures to do so.

Heat engines are about as good as you are going to get for most large scale practical applications. A sterling engine could be more efficient at high temperature differentials than a turbine, but they are more complicated to build.

[Zrk2]

No nuclear power plants use weapons grade uranium, most use fuel enriched to 4-5% U235, many even less (weapons grade is 90%+). More interesting when comparing reactor types is what kind of waste they produce. That's where Thorium reactors shine, especially the subcritical energy amplifier concept.

Ah, my wording was off. They use uranium that cannot be enriched to weapons grade.