Please create an account to participate in the Slashdot moderation system

 



Forgot your password?
typodupeerror

Comment Re:sunfire / in my stellerator / makes me... happy (Score 1) 94

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) 94

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) 94

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) 94

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 Re:Venus (Score 1) 304

Plant cultivation is far, far harder on Mars, for many reasons.

1) Natural light: the solar constant is 1/5th as much on Mars as on Venus, and you're guaranteed to have dust clinging to your greenhouse glazing. More on this later.

2) Electricity: Same for solar power. And fission power systems (as opposed to radiothermal, which is far too weak) are 1) a rather expensive line-item to your development costs, 2) heavy to transport, and 3) complex (complexity is not good when it comes to operation in space). Beyond this, most people vastly underestimate how much power it takes to grow plants under lights - you need 1-2 orders of magnitude more area of solar panels than the area of plants you can grow. And the size of the LED lighting systems you'd need is very significant in its own right. Plants consume way more light to grow than most people give them credit for. The real world isn't The Martian where one can grow potatoes on normal room lights ;)

3) Room: Abundant, practically unlimited space comes free with a Venus colony. Space is extremely expensive on a Mars colony - it's a pressure vessel. Another downside to limited space: plants don't like it. It leads to humidity and temperature instabilities and buildups of gases like ethylene that are far more poisonous to plants than carbon monoxide is to humans. These gases break down, particularly in sunlght, so in big areas they're not a huge problem - but in confined spaces, they can deform and kill your plants readily. Pests and diseases also thrive much more in confined spaces.

(My comments on plants come from experience: I grow a small "jungle" in an indoor environment, entirely on artificial light)

So, while it is of course possible to grow plants on Mars, it's far, far easier on Venus.

As for opressiveness, once a wall is opaque, you can't really perceive how thick it is.

Indeed, I wasn't talking about wall thickness :) Just the issue of being enclosed in small spaces. Most designs call for integrating as many windows as they can, but that's always going to be limited - windows are a lot heavier for a given amount of surface area and can't be shielded for radiation exposure.

And I'm not sure how attractive Venus would be in comparison

So, you don't get a landscape, that's true - the surface isn't visible there. But at the desirable altitudes, there is still a "view", the clouds are dynamic there. A few kilometers further up and it's just a continuous haze (which may lead to rainbow effects below, there are some papers debating this ;) ), but in the "earthlike" layers clouds will come and go. Like living among the clouds on Earth.

But no, you don't get a landscape outside. Your landscape is the Garden of Eden you make inside, surrounded by clouds. :)

There's also those ever-present lightning storms all around you - that's going to be noisy, and a serious maintenance issue

The current state of research isn't "ever-present lightning". Again, unfortunately our knowledge of Venus is so poor compared to Mars, so it's hard to make definitive statements. But lightning appears to be "about" as common on Venus as it is on Earth.

Another thing that we need to learn more about is atmosphere variation. We've seen what appears to be significant variations in sulfur levels on Venus over time - it seems that the sulfur may be the result of frequent or continuous volcanic activity. So how the atmosphere will vary over time is an important question to be able to answer before we can send humans.

And how do you plan to prevent lightning strikes through your habitat?

Again, we don't know the distribution of lightning between a) different altitude layers, b) different latitudes, and c) over time. We actually don't know at this point if it's ever a risk at all - and if it ever is, whether it's avoidable or not. If it's not avoidable, then yes, one would need lightning protection (I presume faraday cage-style rather than any sort of ion shield), which would add mass and require a more difficult testing regime. If it is avoidable, or is never a problem: then there's no issue.

Definitely need more data on this one before we can send humans! It's time to stop neglecting Venus.

but since you're in the middle of the cloud layer they won't actually be getting anywhere near as much sunlight as they would in orbit, maybe not even as much as they would on Earth or Mars

Actually no :) The light levels at acceptable flight altitudes (~51-55km) are comparable to Earth on a clear day (except that you also have almost as much light also reflecting up at you as coming down at you). Depending on the frequency, it blocks about half of the light from the sun - but twice as much light hits Venus. Mars, however, gets 40% as much light as on Earth - when the dust isn't blowing. Sometimes you get dust storms which can last for months, easily enough to kill plants from lack of sunlight.

Note that solar panels don't have to be outside the envelope, if the envelope is transparent (which I've been assuming thusfar). They can even be built into structural elements (for example, solar roofs on shelters or walkways). It'd cost under 10% of the power, and in turn they'd be shielded from winds, lightning (if a risk), icing (if a risk), corrosion, etc, and your wiring needs would be greatly reduced. I really don't see a point to having anything outside the envelope except for the return rocket (even that's not 100% necessary, but probably a lot easier than a rocket-sized drop-bay ;).

If the ambient pressure is ~1atm, then you have roughly as much air above you as you would on Earth, but without a magnetosphere you're going to be counting on that air to block a lot more radiation.

I read a paper about this before but can't be bothered to dig it up again ;) Okay, okay, just a second.... hmm, this may have been the one. They simulated the Carrington Event and one previous one that was even stronger, and found that even they wouldn't be problematic at 62km (let alone a more realistic 53-54km, which has orders of magnitude more atmosphere over it). That is to say, they calculate 0,09Gy. Radiation therapy in humans is 45-80Gy. A CT scan is 0,008 Gy. So it's like getting a dozen CT scans, but nothing like undergoing radiation therapy. And that's at a much higher altitude than people would actually live at. Long-term GCR at actual colony height, according to their graphs, would be about 1e-8Gy/20h, or 4,4e-6Gy/year - not at all "dangerous". Levels are indeed higher than on Earth, but they're not problematic like they are on Mars. You don't need added shielding, you're sitting under a mass of shielding equivalent to a ~5 meter tall column of water. And the atmosphere above you creates a small induced magnetic field to boot.

Comment Re:Better transistors? (Score 2) 275

And if they're having a significant reduction in power consumption, then adding more cores gets all the easier.

Its always seemed to me that the best approach to processing is to offer a variety of cores and let the scheduler handle what to put where. You can have one or two extremely fast cores, half a dozen moderate speed cores, and dozens or more low speed cores - why insist that all cores be the same in "general purpose" computing?

Comment Re:Caller ID Blocker (Score 2) 225

Back when I lived in the states (I've never gotten a single telemarking call here in Iceland) I've often been tempted to respond with, "Why should I buy your product when I'm going to kill myself as soon as I get off the phone?" Suddenly making their job waaaay more stressful than they expected when they picked up the phone.

Never did it, but... ;) Honestly, I just couldn't get myself to be that mean to them, they're just normal people on the other end working menial, low paying jobs.

Comment Re:Fusion energy is impractical (Score 4, Interesting) 94

Fast neutrons can impact any isotope and destroy it in that regard, but that says nothing about the long-term structural stability of the bulk material. Different materials have different annealing properties. More to the point, slow neutrons can do the same thing, just in a different manner (that is, (n, gamma), instead of (n, random-ions-and-neutrons)). Fast neutrons are overall more damaging (and of course more penetrating... although we're not talking about spallation neutrons here with energies up into the GeVs, we're only talking 14,1 MeV) - but they're not some sort of whole different ball game. I am, of course, assuming you're talking about structural issues. If you're talking about from the perspective of how radioactive it will become, tell me, how hot does beryllium get under heavy bombardment? Boron carbide? Graphite? I could keep going. In fact, I did, further up the thread.

There are many reasons to complain about various designs, but your over-generalized statement is anything but some kind of universal rule. And really, the sort of flexibility of materials that fusion allows versus fission more than compensates for having to deal with higher neutron energies.

Comment Re:This is completely awesome (Score 5, Informative) 94

Interestingly enough, for d-t fusion, the neutrons are not an unwanted waste product, but actually essential. Tritium doesn't grow on trees, you have to make it. And more importantly, d-t fusion only gives off one neutron, and it takes one neutron captured by 6Li to breed 1 tritium (you can also make tritium from 7Li bombardment and not consume the neutron, but due to the cross sections and energies involved its usually not as interesting). So if you use one neutron to make the fuel that produces one neutron, and you can't capture 100% of the neutrons, you're in trouble! You get around this by using a lithium-beryllium blanket, as beryllium is a good neutron "multiplier" (capturing one high energy neutron and yielding two lower energy neutrons). It's also rare, expensive as heck and its dusts are highly toxic, but it's consumed at a tiny rate, so it's mainly just an initial cost (heavy elements like lead can also be used as multipliers but they're not very effective in this context, their cross sections don't extend down as far as beryllium and their (n, Xn) reactions where X>2 don't make up for it). So basically, while you lose some neutrons to unwanted reactions, you overall end up producing enough to produce enough tritium for your reactor to consume. The key point is, you want the neutrons to be hitting your reactor, they're doing you a service ;)

There will of course be unwanted neutron captures, but when you engineer it you're choosing specifically what materials are going to be bombarded, so you can pick materials with low neutron capture cross sections and which capture to isotopes that are either stable or have short half lives. Concrete is great for how cheap it is (light elements in general are, and concrete is mostly made of light stuff). As far as metals go, aluminum is great where heat loads or mechanical stresses aren't excessive. Beryllium is even better, as well as stronger and lighter... but see the aforementioned issues with it. Steel is "okay", usually fine if you're careful about what you alloy it with. You generally want to avoid titanium. Graphite is superb if you run it hot enough (otherwise you risk Wigner energy problems). Composites likewise, although they're more temperature limited. Most common ceramics are made of light elements, which makes them very good to use, although those with heavy elements (like tungsten carbide) should be avoided. Tungsten in general should be avoided unless necessary. Some ceramics like boron carbide/nitride are highly heat and corrosion tolerant, high compressive strength, huge neutron absorbers and don't yield dangerous byproducts, which lets them fit multiple roles at once - so long as there's little tensile or shear stresses. In some cases you may want more of a neutron "window", wherein things like zirconium or lead would be good - particularly specific isotopes of them if you're willing to pay for enrichment. It all depends on the operating environment and geometry.

Comment Re:sunfire / in my stellerator / makes me... happy (Score 5, Interesting) 94

The "50 years away" stuff is a really unfair criticism. The amount of progress that's occurred in the past several decades is many orders of magnitude - JT-60 has even gotten to Q=1.25, which means they were getting 25% more power out than they were putting in to maintain the reactor in steady-state operation.

Part of the reason that this concept got started was because of a big mistake early on with the ZETA program. Unbeknownst to them, A) heavy electron bombardment of their detectors was leading to false spectral shift readings, making them think that the temperature was much hotter than it was, and B) there was a possible method to create neutrons that they were unaware could be significant - heavy localized acceleration of ions causing spallation impacts. The unfortunate part was, by coincidence, (B) happened to produce roughly the amount of neutrons that would be expected by (A). So they thought that they were just a short step away from a viable fusion reactor, when in reality they weren't even close. Due to the more primitive technology at the time, not only did they not have detailed computer models that could have warned them to expect the neutrons, but they also didn't have a convenient way to measure neutron energies (it was this that later proved their early conclusions wrong). Their lack of computer models also meant that they were unaware of how much of a problem drift would be.

It's a very different situation today. There's really no question that we can viably produce fusion power today. The real question hanging over our heads is, what is it going to cost? How can we engineer a system to produce power affordably? And that's the real question that's going to take a lot of work to figure out. One thing is for sure, though: the higher the magnetic fields you can get for a given cost, the vastly easier it becomes. And these new high temperature superconductor tapes could push us leaps and bounds even beyond ITER, whether you go with a stellerator, a more traditional tokamak, or really anything else that employs magnetic fields. It's very encouraging for the field to see a route that already looked to be on a positive path get such a "bonus".

Comment Re:This is completely awesome (Score 3, Insightful) 94

There will never be an "energy independent world". But what one can accomplish is of course highly dependent on how much energy can be provided for a given amount of money (where the concept of "money" is basically an IOU for human labour... all costs, eventually, trace back to human labour)

Of course, cheap energy costs can have disadvantages... it all depends on how we choose to use it. For example, with our greater ability to "make things", it would be quite possible that mining would dramatically increase. On the other hand, we could take a more modest quality of living improvement and dedicate more resources toward recycling and living with lower environmental footprints - even using the energy to drastically reduce our footprint (such as intensive light-driven grow ops, freeing up farmland). It all depends on the choices we make as a society.

All of that said... this is way premature. We don't even know that this sort of technology will - anytime in the remotely near future - prove to even beat current sources of electricity on price, let alone dramatically outcompete them. One can hope, however.

Comment Re:Venus (Score 2) 304

For cargo delivery I'd expect an Earth gravity assist - no need to rush things, it's fine if it takes a few years.

As for humans, overall, between the reduced transfer times and the natural shielding while at the planet, you do make the radiation loads on people a lot lower. You're absolutely right that it's not an issue you can just ignore - but any benefit is still a benefit, and cutting your transit times by a third, that's a real benefit.

For some hard numbers I just crunched: for simplicity I assumed a spherical habitat. At 250 meters diameter floating at 53,5km (24C outside temperature) I calculate a maximum of ~800 tonnes lift using normal Earth air as a lifting gas (if enriched in oxygen there would be more lift; also lift increases for a given diameter with decreasing altitude). I estimate a wet mass of the ascent stage to get a full crew back to orbital rendezvous at about 300-400 tonnes (a bit lighter than a Falcon 9, due to the somewhat reduced gravity and pressure vs. Earth) - all depending on the propellant combination and rocket details. However, at the very least the LOX would be made locally, and probably the fuel (H2 or methane, presumably), so what you actually have to bring to Mars (assuming local fuel) would be around 30 tonnes (again, a bit less than Falcon 9 + Dragon). I estimate the skin of the habitat (at 0,25kg/m^2) at 15 tonnes. Double that to 30 tonnes for after you add in propulsion, ballonets, stringers, etc. Add one tonne of solar panels (any more would be overkill with such a high insolation), a few tonnes of walkways/ladders (internal, external) and an airlock, a tonne or so of tankage to hold water (for local needs and surface probe cooling), a couple tonnes of air processing hardware (CO2 scrubbing, O2 generation, etc), a couple tonnes of aeroponic plant support/growth hardware if you want lots of local greenery, half a dozen tonnes of housing space and furnishings for sleep, lab environment, half a tonne of lab and communications gear, a tonne of batteries and wiring.. you get the drift, all of your normal colony stuff. You end up probably in the ballpark of 80-100 tonnes delivered (not counting surface probes, which could be launched as their own missions on much smaller craft). The rest of the lofted mass is made up of what you produce locally from the atmosphere - breathing air, return rocket propellant, water, plant mass, etc (you'd still have the potential to make use of plenty of surplus lift to hold future expansions - more people, new mineral processing equipment, etc).

The airship is basically is its own entry system, if you bring a couple tonnes of hydrogen or helium for the initial in-space inflation; inflatables are being tested for reentry here on Earth, and the test systems have far lower cross section than this (high cross section is good when it comes to reentry - the bigger the cross section, the higher up you can begin your deceleration, the greater your surface area to radiate heat from, and the further away from your craft the primary shocks are; with something this big, reentry should impart only trivial heat loads - although detailed modeling and testing would be a must!). It would settle out at very high altitude (having little mass onboard) and start to sink as it produces water, propellant and swaps out its initial atmosphere for a locally produced one. Like with (most) Mars mission proposals, humans to a Venerian habitat would be delivered on a second mission after the habitat is fully established and ready for habitation (unlike Mars, on Venus they could parachute, glide, and/or propeller to their destination).

To put these numbers in perspective, Musk's Mars Colonial Transporter concept (from what we know of it so far) is designed to deliver 100 tonnes to the surface of Mars (aka, including landing).

As a plant nut and person who loves open spaces, a Venus colony environment appeals to me more than a Mars colony environment. As much as I'd love to be able to "go on exploration walks" outside like Mars allows (albeit in inconvenient bulky suits), I'd go stir crazy spending most of my time in a cramped "pressure vessel" habitat. Just picturing all of the greenery that one could have on a Venus colony... vanilla orchids in bloom, passionfruit dangling off of walkways, ripe mangos right outside one's bedroom... that would be a true paradise to me. And while Mars colonies basically scale up linearly, Venus colonies get easier and easier with scale. The mass of your habitat increases with the square of its radius but its lift increases with the cube of the radius. So once you get to the point that you can start producing plastics or other structural materials locally (the atmosphere contains everything you need to start a diverse plastics industry - even composites like carbon fiber), you have the potential to really build some massive cities.

Slashdot Top Deals

An engineer is someone who does list processing in FORTRAN.

Working...