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Comment Re: Batteries just don't store enough energy... (Score 1) 219

Also, there's a lot of diversity in terms of aircraft electrification that one can take, it's not an all-or-nothing thing. There's lots of different proposals for varying degrees - for example, high bypass with electric turbofans, using onboard electricity to spin the compressor so that you don't have to have a turbine, and so forth.

Comment Re: Batteries just don't store enough energy... (Score 5, Insightful) 219

Yes. Also, you can't ignore comparative efficiencies of engines. Or engine mass to weight ratios. Or the length of time to market, and the expected level of battery change during that time period. Or side benefits (for example, the ability to have small, very light engines was made use of in one NASA experiment that placed numerous small engines along a wing, causing an effect that created drastically more lift at low speeds and allowing for a much shorter takeoff distance).

And beyond that, you can't ignore economics. Having reduced range but getting your fuel at a fraction of a cost may ultimately prove to be more desirable. It's a very complex issue that one can't just make all-encompassing statements based on a single figure like "energy density of batteries vs. energy density of fuel".

Anyway, this is hardly Elon's first time to mention it. Years ago he mentioned that he wants to be the first person to have an electric plane break the sound barrier. If there's anything one can say about Elon, it's that he sure doesn't set the bar low...

Comment Re:Icebergs float on glaciers (Score 5, Informative) 36

No, it does not form "one huge crystal". Nitrogen ices at these temperatures have little structural integrity. It was well known before we got to Pluto that if we saw any sort of relevant topography, we'd know immediately that it was from water ice, as nitrogen ices are so weak that they'd just flow slack over time.

Comment Flawed argument and Environment (Score 1) 257

This slowdown in economic activity causes a recession...until once again their price goes up

If you were talking about a normal 'fiat' currency then yes I would agree. However if you have an absolutely fixed supply of currency then deflation is not an abnormal condition but the steady state. In such a condition I'm not sure that your argument holds because there is no point in holding off for when prices start to rise because they will not, at least not be any significant amount.

Even if you are right and steady-state deflation encourages people to hold off on purchases until they really need something perhaps this is not a bad thing. Reducing consumption is a good thing to do given the limited resources of the planet. As for the stability of the economy look at the UK recessions. The US is a relatively new country which had a rapidly developing and changing economy over the period you give also the measure used changes with different periods in the article you linked. If you look at the UK list then, except for the great depression, there is no real difference in the depth of the recessions but there may be some indication that there were fewer, but longer, recessions before 1931 when the UK came off the gold standard. So I don't see the evidence to support your assertions.

Comment Re:Icebergs float on glaciers (Score 5, Informative) 36

Nitrogen ices at these temperatures, while crystalline, have rather low viscosity. If you put weight on them, they slowly diffuse around it until the object either sinks or is buoyantly balanced out. The latter happens in the case of water ice.

Also, it's worth noting that it's not pure nitrogen ices, it's a nitrogen-carbon monoxide-methane eutectic. Nitrogen is the most common component, however. Also, there are multiple crystal phases that can be taken, depending on the conditions. Nitrogen ices are most famous for having some rather "explosive" phase transitions between different states.

Comment Re:sunfire / in my stellerator / makes me... happy (Score 1) 98

Fast neutron cross scattering sections in the couple MeV range barely vary over more than the range of 1-10 barns

1-10 barns is, of course, by definition, an order of magnitude. There is a massive difference between 10 barns and 1 barn. Tenfold, to be precise. ;)

More to the point, you can't just combine all cross sections like that. The energy imparted from an elastic collision isn't the same as from an inelastic collsiion, which isn't the same as an (n, gamma), and so forth. Elastic collisions are particularly low energy, particularly the higher Z the target. Taking them out of the equation yields much greater differences between materials in the range of a couple MeV. The upper end of the neutron energies are "somewhat" similar (up to about one order of magnitude), but down below 6 or 7 MeV or so there's quite a few orders of magnitude difference.

Likewise, total cross sections have no bearing on the accumulation of impurities in the material. The particular cross sections are relevant not only in terms of reaction rate, but also what sort of impurities you tend to accumulate and what effect they have on the properties of the material. Which of course varies greatly depending on what exactly they are.

Integration of annealing cycles into blanket design is not brought up enough in some design studies, but is a consideration to help

It's not a side issue, it's a fundamental issue to the design of a material designed for high temperature operation under a high neutron flux.

Blanket design is extremely constrained by tritium breeder ratio to ensure more tritium is produced than used, which squeezes volume allowed to be used by coolant, ... but they have much lower neutron flux to worry about. Gen 4 reactor designs are in the 500-1000 C temperature range, exceeding in some cases what is thought reasonable for fusion blanket design. ... Blanket replacement is considerably more complex than fuel replacement in a fission reactor

Perhaps they've been heading in a different direction since I was last reading on the topic, but I was under the impression that a prime blanket material under consideration was FLiBe. Which operates in a temperature range of 459-1430C, and is its own coolant. That doesn't change what the first wall has to tolerate, but as for the blanket itself, you have no "structural properties" to maintain, and cooling is only limited by the speed that you can cycle it.

The last paper I read on the subject also suggested that for breeding purposes one needs not only beryllium (they were reporting really poor results with high-Z multipliers), but the optimum ratio (to my surprise) worked out to be significantly more beryllium than lithium. So building structural elements out of beryllium serves double purpose, you don't have the excuse of "I need to use steel because it's cheaper" - you need the beryllium either way. It's strong, low density, similar melting point to steel, but retains strength better with heat, and highly thermally conductive. Beryllium swelling from helium accumulation stops at 750C+ as helium release occurs. So pairing a beryllium first wall with a FLiBe-based blanket seems like a very appropriate option.

Please don't get me wrong, I'm not at all disputing the great amount of engineering work left to do. I'm just more optimistic that appropriate solutions will be found. Perhaps I'm just naive in that regard ;)

Comment Re:sunfire / in my stellerator / makes me... happy (Score 1) 98

So on average the fission reactor material only has about 10% of its atoms displaced over the lifetime, while the fusion reactor would have, on average, every atom displaced hundreds of times over the lifetime.

How can you make generalized statements like that? Cross sections vary by many orders of magnitude Fission reactors are generally made of steel, which is hardly setting any records in terms of low cross sections. The smaller the reactor, the less material you have to replace, and the more expensive the material you can use. And being "displaced" is not a fundamental universal material property effect, it depends on how the material responds to radiation damage, which varies greatly. Generally materials respond better at high temperatures (annealing), and fusion reactors operate of course at far higher temperatures than fission reactors.

I have trouble seeing how one would consider neutrons per square meter to matter more than neutrons per MeV. Because neutrons determine what you're going to have to replace, and energy determines how much money you get from selling the power to pay for said maintenance. You can spread it over a broad area and do infrequent replacements, or have it confined to a tight area and do frequent replacements, the same amount of material is effected. Some degree of downtime for maintenance is normal in power plants - even "high availablility" fission plans still only get ~85% uptime.

Comment Re:sunfire / in my stellerator / makes me... happy (Score 1) 98

Hmm, thought... and honestly, I haven't kept up on fusion designs as much as I should have... but has there been any look into ionic liquids as a liquid diverter concept? In particular I'm thinking lithium or beryllium salts. They're vacuum-compatible, they should resist sputtering, they're basically part of your breeding blanket that you need already... just large amounts, flowing, and exposed. Do you know if there's been any work on this?

Comment Re:sunfire / in my stellerator / makes me... happy (Score 2) 98

The plasma facing material faces a flux of 1 neutron per 17,6Mev. By contrast, nuclear fuel cladding faces a flux of ~2,5 neutrons per 202,5 Mev, or 1 per 81 MeV. It's certainly higher, but it's not a whole different ballpark. And yes, you're dealing with higher energy neutrons but in a way that can help you - you've often got lower cross sections (for example), and in most cases you want the first wall to just let neutrons past.

There's a number of materials with acceptable properties. Graphite is fine (no wigner energy problems at those temperatures). Beryllium is great, and you need it anyway. In areas where the blanket isn't, boron carbide is great. Etc. These materials aren't perfect, but they're not things that get rapidly "converted into dust" by neutrons. Really, it's not the first wall in general anyway that I'd have concerns about, it's the divertor. The issue isn't so much that it takes a high neutron and alpha flux and "erodes" fast - that doesn't change the reactor's overall neutrons per unit power output ratio, and if you have a singular component that needs regular replacement, said replacement can be optimized. The issue is that you have to bear such an incredible thermal flux on one component. Generally you want to spread out thermal loads, it makes things a lot easier.

Comment Re:Fusion energy is impractical (Score 1) 98

When a fast neutron hits an atom it knocks it out of its position and frequently changes it to a different element/isotope.

The same applies to slow neutrons, so....? Your average 14,1 MeV neutron is most likely to inelastic scatter down to the point where more exotic reactions than (n, gamma) are basically impossible (excepting a few specific cases, like 6Li(n,t)4He - again, not dangerous). Only a small percentage of your 14,1MeV neutrons (depending on the material they're passing through) have a chance of undergoing anything more than a standard (n, gamma) transmutation. Unless the system is specifically designed to cause that (for example, a beryllium multiplication in the lithium blanket). The standard case is inelastic scatter once or twice -> elastic scatter a bunch -> become partially or completely thermalized -> capture.

This turns a solid structural material into a radioactive powder

What happens depends entirely on what's being bombarded. Many materials are perfectly fine after long periods of exposure - slow or fast neutrons. Light ions in particular are usually either A) relatively unaffected (sometimes requiring sufficient heat for proper annealing, sometimes not), or B) incredibly good absorbers, leaving nothing dangerous behind. See a more detailed breakdown above.

Comment Block all adverts... (Score 5, Insightful) 39

Use a good browser plugin or some good backend rules, but block every single advert out there. That stops the "OHHH YOU GOTTA INSTALL THIS" vector that fools clueless visitors into downloading and running the trojan.

Good people install adblocking on every single computer they touch. Bad people allow ad's from websites.

Dear web admins.... WAHH. If you cant vet and host your ads yourself to make sure they are safe, you dont DESERVE your ad's to make it through.

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