How to make a nuclear reactor that can’t have a meltdown

via DVICE Atom Feed by Evan Ackerman on 3/16/11

How to make a nuclear reactor that can't have a meltdown

The word “meltdown” defines our worst fears about nuclear reactors, and with good reason: without complex and redundant cooling systems, reactors can run out of control, generating so much heat that they melt their own fuel, releasing massive amounts of radioactivity in the process. But a new generation of reactors promises to be much safer, even to the point where a meltdown is a physical impossibility.
Reactor Safety 
Generally, nuclear power plants rely on redundant safety systems, both active and passive, to prevent a meltdown in case of an accident like an earthquake or tsunami. Japan’s stricken Fukushima Daiichi nuclear plant is what’s called a light water reactor, or specifically a boiling water reactor, because the heat generated in the core of the reactor is used to boil water into steam, powering a turbine to generate electricity. 
Immediately after the earthquake, the reactor successfully shut down, meaning that control rods were inserted into the core to disrupt the nuclear reactions directly. However, there’s still a lot of heat contained in the core, which is still boiling water and making steam, which raises pressure in the reactor and makes things dangerous. To keep itself cool, the reactor depends on a continuous supply of water, and the problem is that the pumps to supply this water haven’t been functioning. This means that the reactor gets hotter, more water turns into steam, and the pressure inside increases (making it more difficult to pump water in), and eventually enough water gets turned into steam that the fuel rods themselves get exposed to air, which can cause them to melt. This may be what is currently happening at Fukushima Daiichi. 
The root of the problem at Fukushima Daiichi is that the reactor relies heavily on active safety systems, meaning that the safety systems don’t work well (or at all) without things like pumps and generators, which themselves rely on external power. More modern reactors (Fukushima Daiichi was built in 1970) try to incorporate passive safety systems. For example, some reactors suspend their control rods over the core on electromagnets with giant springs behind them, ensuring that the rods will shut the core down the instant power is lost. Other reactors have backup cooling systems that are just giant tanks of water on towers, and explosive valves can be used to pump water into the core using gravity. 
Even with passive safety systems, though, accidents can still cause reactors to overheat to the point of meltdown, especially in sustained disaster conditions like those in Japan. The next generation of nuclear reactors, called Gen IV reactors, promise to be significantly safer and more efficient while producing less hazardous waste than the current generation, and one design, called a pebble bed reactor, may even be incapable of having a meltdown at all.
The Pebble Bed

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A pebble bed reactor (or PBR) doesn’t use long rods of fuel pellets like most reactors. Instead, it uses a bunch of fuel “pebbles,” which come in varying sizes, from slightly smaller than tennis balls down to marbles. The pebbles are made primarily of graphite, and contain up to nine grams of uranium dispersed in sand-size grains throughout the pebble. To start a reaction, all you have to do is pile a bunch of pebbles together in a container until you get a critical mass of them, and they begin heating up.

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The core of a PBR contains about 380,000 pebbles, which cycle continuously in and out of the reactor. Every 30 seconds, a pebble drops out of the reactor and is inspected for damage and to make sure it’s still got enough fuel left inside. If so, it’s put back in the cycle, and if not, it’s pulled out and a fresh one is put in its place. On average, a single pebble will cycle through the reactor 10 or 15 times over a few years before being removed.

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While a PBR is operating, helium is pumped through the spaces between the pebbles to carry away heat. The helium then flows through a turbine, and that’s where the electricity comes from. So far, a PBR isn’t that different from a conventional nuclear reactor: you put fuel in, it heats up, and you use that heat to produce electricity. What makes a PBR potentially unique, though, is that because of its design, it’s capable of passive, inherent safety that makes a meltdown physically impossible.

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No Meltdowns 
Let’s just skip directly to the worst-case scenario, like in Japan, where failure of the coolant system caused the reactor to overheat uncontrollably. In terms of what would happen to a pebble bed reactor, this means that there’d no more helium coolant. So, okay, as you might expect, the reactor would start to get really, really hot. As nuclear fuel heats up, the uranium atoms start to move faster, making it harder for them to absorb extra neutrons and split, reducing the reactor’s power. This is what’s called negative feedback, and while it takes place in all reactors, the low fuel density of the pebbles magnifies it in a PBR. As the PBR continues to heat up, the negative feedback gets stronger and stronger until at about 1600 degrees Celsius, the core stabilizes at an “idle” temperature. This temperature is a solid 400 degrees short of what it would take to cause any damage to the fuel spheres or reactor vessel, which are made of a special kind of super strong graphite. 
The upshot of all this is that a pebble bed reactor can have the entirety of its supporting infrastructure power down, blow up, get flooded, get stolen, run out of gas, or otherwise fail, all while the entire staff is on vacation, and the only thing that happens is that the PBR will warm up to its idle temperature and… Stay warm. No meltdowns, no explosions, no radiation leaks. The reactor will just sit there and radiate the heat it produces until you cool it back down or take the fuel out. This scenario was tried once, in a prototype PBR in Germany: they shut off the coolant and removed the control rods and watched, and nothing bad happened. A later inspection of the reactor and fuel pebbles showed no damage. 
Of course, it’s important to understand that PBRs aren’t completely safe, and come with their own risks, including the potential for radioactive dust from pebbles rubbing against each other in the core and the difficulty of managing the circulation of the pebbles themselves. And PBRs still produce radiation, which is always dangerous, along with waste materials, although it’s worth mentioning that the waste is already contained inside the pebbles, rendering it much safer, and it’s so hard to get outof the pebbles that it’s useless as a weapon. But the point is that PBRs seem to be safe in a lot of ways that conventional nuclear reactors definitely aren’t.

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The first PBR was built in Germany in the mid 60s. As an experimental reactor, it had some design issues, but even so, people working there only received about 1/5 as much radiation as they would if they were working at conventional plant. A follow-up was constructed, but it had some additional design issues and a few minor incidents (mostly related to human error) led to its closure in 1989.

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Nuclear Future 
It’s definitely true that pebble bed reactors are, at this stage of their development, less familiar to the power industry than more conventional designs. They’re also more expensive to construct while having only about 1/30 the power density of other reactors. But China, at least, is optimistic about their potential, and already has one test PBR and is planning on building thirty more in the next ten years, and possibly hundreds more by 2050. Part of the reason that China likes PBRs (besides their safety) is that their high operating temperature can be used to efficiently crack steam into hydrogen, which can be piped off and used as an alternative fuel. 
Really, the worst part of the disaster in Japan, as far as the industry goes, is that it’s going to make it that much harder to convince the public that nuclear power can be safe, clean, and efficient. To put it in perspective, in 2008 Next Big Future calculated how many people are killed per terawatt-hour of electricity generated. On average, there are 161 fatalities related to energy generation from coal for each one of those terawatt-hours, which comprise a quarter of the energy we use on Earth. 36 people die per TWh of oil energy, which is 40% of our energy use. Nuclear power has a deaths per TWh rate of only 0.04 while producing 6% of our energy, which makes it about ten times safer than solar power once you take into account how many people fall off roofs while installing it, and twice as safe as hydro power. 
It’s certainly true that nuclear power comes with its own host of issues, from reactor safety all the way down the line to spent fuel storage. It’s also true that nuclear accidents are terrible, frightening things. But the fact is that nuclear power is a viable, and even a necessary, alternative to fossil fuels, especially as we start thinking about exploring and colonizing other planets. When we go to Mars for the first time, we’re not going to be relying on solar power. We’re going to have compact, safe, and clean nuclear power along with us, because that’s what makes sense. And it’s not just the future: by embracing new technology, we can have the safe and clean nuclear power of tomorrow, today. 
There’s lots more info on pebble bed reactors from Wikipedia, and you can check your facts at MIT. There’s also a detailed discussion of modern PBR safety in a 2009 Nuclear Engineering International article, with links to some PBR criticism as well, and a story on the Chinese PBR from Wired.

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Cube made of 512 LEDs does 3D with calculus, not glasses

via Engadget by Tim Stevens on 3/21/11

Cube made of 512 LEDs does glasses-free 3D for real (video)

No goofy active shutter glasses, no headache-inducing parallax barrier screens, no optical trickery here. This is a pure 3D display — unfortunately done at a resolution of just 8 x 8 x 8. It’s a hand-built LED cube created by Nick Schulze, powered by Arduino, and driven largely by Matlab. Yes, Matlab, an application you probably deleted less than three minutes after signing off on your calculus final. We can’t help you find that installation disc again, but we can encourage you to enjoy the video of this 3D matrix of blinkenlights after the break, and you can get the full details on how to build your own at the other end of that source link. 

Scientists separate plasma from blood with working biochip

via Engadget by Sean Hollister on 3/21/11

Disposable biotech sensors won’t let you diagnose your own diseases quite yet, but we’ve taken the first step — a research team spanning three universities has successfully prototyped a lab-on-a-chip. Called the Self-powered Integrated Microfluidic Blood Analysis System (or SIMBAS for short, thankfully), the device takes a single drop of blood and separates the cells from the plasma. There’s no electricity, mechanics or chemical reactions needed here, just the work of gravity to pull the fluid through the tiny trenches and grooves, and it can take as little as ten minutes to produce a useful result. It’s just the first of a projected series of devices to make malady detection fast, affordable and portable. Diagram after the break!

Image of the Day: storms rock the sun

Image of the Day: storms rock the sunvia DVICE Atom Feed by Megan Wollerton on 12/22/10

The entire sun experienced a series of violent explosions back in August. And thanks to a new sun-observing satellite, NASA captured the whole thing. Not only are the images amazing, but they also helped researchers determine that solar activity occurring simultaneously may not always be a coincidence.

Photo: stunning 648 megapixel image of the Milky Way

via DVICE Atom Feed by Kevin Hall on 11/2/09

Photo: stunning 648 megapixel image of the Milky Way

Physicist Axel Mellinger spent nearly two years traveling 26,000 miles across South Africa, Texas and Michigan. What does he have to show for it? Well, he’s cobbled together a stunning 648 megapixel panorama of the Milky Way as seen from Earth, using 3,000 individual photographs. The Central Michigan University professor wants to make the image available for planetariums, as it’s large enough to serve educational purposes. It even shows stars that are 1,000 times too faint to be seen by the human eye, so this is a Milky Way like you’ve never seen.

Here’s a larger view, the largest Mellinger offers for personal use. (Before you blame him, the prof spent countless man hours putting this thing together. It only makes sense that there’s a cost to admission, and I bet that fee is well worth the price. There’s an interactive version you can play with here.)

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From the University of Chicago:

Piecing together 3000 individual photographs, a physicist has made a new high-resolution panoramic image of the full night sky, with the Milky Way galaxy as its centerpiece. Axel Mellinger, a professor at Central Michigan University, describes the process of making the panorama in the forthcoming issue of Publications of the Astronomical Society of the Pacific. An interactive version of the picture can viewed on Mellinger’s website.

“This panorama image shows stars 1000 times fainter than the human eye can see, as well as hundreds of galaxies, star clusters and nebulae,” Mellinger said. Its high resolution makes the panorama useful for both educational and scientific purposes, he says.

Mellinger spent 22 months and traveled over 26,000 miles to take digital photographs at dark sky locations in South Africa, Texas and Michigan. After the photographs were taken, “the real work started,” Mellinger said.

Simply cutting and pasting the images together into one big picture would not work. Each photograph is a two-dimensional projection of the celestial sphere. As such, each one contains distortions, in much the same way that flat maps of the round Earth are distorted. In order for the images to fit together seamlessly, those distortions had to be accounted for. To do that, Mellinger used a mathematical model-and hundreds of hours in front of a computer.

Another problem Mellinger had to deal with was the differing background light in each photograph.

“Due to artificial light pollution, natural air glow, as well as sunlight scattered by dust in our solar system, it is virtually impossible to take a wide-field astronomical photograph that has a perfectly uniform background,” Mellinger said.

To fix this, Mellinger used data from the Pioneer 10 and 11 space probes. The data allowed him to distinguish star light from unwanted background light. He could then edit out the varying background light in each photograph. That way they would fit together without looking patchy.

The result is an image of our home galaxy that no star-gazer could ever see from a single spot on earth. Mellinger plans to make the giant 648 megapixel image available to planetariums around the world.

Axel Mellinger, via University of Chicago, via Examiner, via io9

MIT dreams of fully autonomous greenhouse, will definitely make it happen

via Engadget by Darren Murph on 3/22/09

You know what’s hard to find these days? Consistency and reliability — in anything, really. But we’ve learned that when MIT touches something, it not only gets done, but it gets done right. Thus, we’re absolutely elated to hear that a few of its students have dreamed up a fully autonomous greenhouse, utilizing real plants, sensors and gardening robots to ensure the greenest, most healthy crop possible. In fairness, we’ve already seen oodles of robotic plant tending apparatuses, but this is just something special. Thus far, gurus have used “re-imagined versions of iRobot‘s Roomba” in order to tell what a plant needs and then respond accordingly, and apparently, things have been going quite well early on. Check out a demonstration vid just past the break.

[Via MAKE]

Continue reading MIT dreams of fully autonomous greenhouse, will definitely make it happen

Deep Flight submarine is not for the claustrophobic

via DVICE by Michael Trei on 3/21/09

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Most of what lies under the world’s water remains a mystery, so it’s not surprising that mega wealthy individuals with an adventurous spirit have moved on from building crazy multi-million dollar yachts, to submarines.

This mini sub was created for venture capitalist Tom Perkins by Hawkes Ocean Technologies, presumably to go along with his 290 foot sailing yacht The Maltese Falcon. Built to withstand ocean depths of up to 400 feet, the carbon fiber electric submersible can seat two, and stay submerged for up to four hours.

Any thought of going deep into the ocean like this makes me want to stay firmly planted of dry land, but if it’s any comfort, Hawkes Ocean Technologies was busy building a sub for Steve Fossett to explore the 35,000 foot deep Mariana Trench before he died in a plane crash.

Hawkes can build a Deep Flight sub for your yacht too, starting at around $1.3 million.

CNN, via Like Cool