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Comment Re:6.8 Billion (Score 1) 271

No, a RTG is distinctly different from a nuclear reactor in almost every single way. They do not involve chain reactions. They do not involve neutrons to any significant degree. Moderation, cross section calculations, etc don't even come into play. It's just a ball of material that stays hot due to capturing its own alphas. RTGs are not considered nuclear reactors. There is no wiggle room on this; they're an entirely different class of spacecraft power systems.

RTGs scale down quite well. They're also, however, about as far on the opposite side of the affordability spectrum as you could possibly get.

There have been actual nuclear reactors used on spacecraft in the past, as I wrote, primarily by the Soviets. But they're anything what you'd consider a cost effective design for civilian power generation.

Comment Re:6.8 Billion (Score 1) 271

Higher fissile number density = higher enrichment = nonstarter. Fine for submarines, not for civilian power. Re, reflector, you still have to deal with free path issues when determining overall reactor size. The more you're spending on inert mass relative to how much power you're getting, the worse your economics. Plus your reflector is contributing to (n, gamma) and other neutron consuming reactions (although it's possible to use a moderator that you need anyway (say graphite) as a reflector... although there are issues with that as well to deal with)

You'll note that I mentioned and agreed with the mass production argument - if fission power is going to have an actually sustainable renaissance, I would expect modular reactors to be the means. But I nonetheless questioned whether that could be enough to overcome the basic issues on top of the additional challenges that a small modular reactor imposes.

Comment Re:6.8 Billion (Score 1) 271

Energy density (with respect to time) - J/m^2-s or equivalent.

As for "Who cares?" The GP for one. Me for two. Most people on Earth as well. The more land that is used up, the less you have for other purposes, be that for humans (agriculture, forestry, mining, grazing, etc) or natural habitat. It hits doubly that reservoirs target land defined most notably by the following characteristics:

1) Large river
2) Deep ravine/basin
3) Significant altitude change

In short, they often tend to be the areas most important to wildlife, often locally-unique habitats, as well as the most scenic areas within a given location - areas responsible as well for significant mobilization of sediment and oxygenation of water.

Solar, by contrast benefits most from environments full of endless identical flat wastelands. The more mundane and barren, the better.

Comment Re:6.8 Billion (Score 1) 271

US or Russian naval officers would disagree with you.

See what I wrote above. You can make a reactor of any size. But you lose efficiency - both neutron efficiency and cost efficiency - the more you scale down. Nuclear sub reactors' scaledowns are aided by the use of highly enriched uranium as fuel, something you don't want to do with civilian nuclear plants. And note that even nuclear subs' reactors aren't "small". A Los Angeles class, for example, uses a 165MW reactor. And nuclear power plants, unlike subs, generally need to have multiple reactors so that they can be taken down for maintenance / fueling.

Comment Re:6.8 Billion (Score 3, Interesting) 271

The GP is correct. Solar farms are a pretty dense energy source - comparable (when the reservoir is included) to all but the highest head dams, and an order of magnitude or two more than a typical dam. And some designs can get even more dense, such as linear fresnel reflectors (which cover a higher percentage of the ground because of less issues with self-shading as the sun moves). Plus, solar can be paired with wind. Wind is a low energy density source with respect to total acreage, but very high with respect to actual surface area required on the ground.

Beyond this, a few notes. Much solar doesn't have to take up any new land at all, as one notes from rooftop solar (ideally industrual/commercial), parking shelters/covered walkways, etc. And places where solar plants are made are most typically desert areas. And there's a curious reversal in the desert when it comes to life: while shading terrain hinders life in moist areas, it encourages life in desert areas. In the desert, places that provide shade (ironwood trees, saguaro cacti, large rocks, etc) tend to turn into oases of life - not simply by providing relief from the blazing sun, but slowing down the rate of water loss from the soil. Now, this doesn't usually happen with solar plants because at this stage, most are kept cleared. But that does not have to be the case.

Comment Re:Assange running out of time (Score 1) 215

So you're perfectly happy to make up whatever context suits you. Why am I not surprised? Here, how does this sound for you?

Dear Mike,

As you know, I hate white people. Could you please make me some lists of candidates that don't involve any of them racist crackers? Thank you.

Allahu akhbar,

Comment Re:6.8 Billion (Score 4, Interesting) 271

From a physics standpoint, this is not true. Larger reactors help you have higher total neutron cross sections, both for elastic scattering / moderation and fission. A "small" nuclear reactor is defined by the IAEA as one that's less than 300MWe, although even reactors as big as 500MWe are sometimes referred to as "small". Per-reactor, not per-plant. Don't get me wrong, you can make reactors at any size - some companies are looking at modules as small as 25MW (per reactor). But it makes your already problematic economics even worse.

That said, I still do have more hope for small reactors than large ones, just simply from the standpoint of getting some degree of mass production and refinement through use. Still, the "nothing may go wrong" situation one faces with nuclear reactors and the "need to start from scratch if some flaw is developed in the basic design that prevents you from 'nothing may go wrong'" still bites.

Comment Re:6.8 Billion (Score 4, Insightful) 271

Nuclear power has always been a lot more popular on K Street than on Wall Street. At least these sort of overruns pale in comparison to some of the ones in Europe - one in the UK has now become the second most expensive thing ever made by man (after the International Space Station). Lots of nuclear plants on that list, too. One in Finland is now a decade overdue and commercial operation still isn't expected until 2018 - assuming there's not even more delays.

One of nuclear's biggest problems is, it doesn't work very well small. There are some "smallish" modular reactor designs, but as a general rule, nuclear plants are very large structures. Which means, you're not making a lot of them. Which means you don't retire the risk (both financial and safety) very quickly. Nuclear inherently contains a lot of both of those. It can take decades to learn what problems are. And when we redesign systems to start over with a new "generation" of nuclear power plants, that "ironing out the financial and safety kinks" process starts over.

It's unfortunate, but the very nature of fission means going through every element on the periodic table except the extremely short-lived/superheavy ones. Which automatically means facing very significant corrosion and containment challenges. The very nature of a high neutron flux means degradation on its own. The very nature of having exceedingly toxic materials means that you can't allow even tiny amounts to escape, and have to go to extreme levels to prevent serious problems like fires - and not only is your fuel source challenging from a chemical and materials standpoint, but it also can't be shut down quickly. Criticality can be, but the daughter product decays keep the core hot for a considerable length of time.

Nuclear is eminently doable from a technological standpoint. But like rocketry, a lot of things conspire to make it very difficult to do affordably and safely.

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