Why has Britain done a U-turn on plutonium?

Why does the UK government want to convert its 112 tonnes of plutonium — the largest civilian stockpile in the world — into mixed-oxide fuel (MOX), when just a few months ago it announced the closure of the only MOX production facility in the country because of a lack of demand? Energy minister Charles Hendry announced last week that converting the plutonium to MOX was still the plan, but a new £3-billion (US$4.7-billion) plant would only be built if it could be shown that it was both affordable and offered value for money.

Although at first glance this may seem fiscally prudent, when taken with other recent events it starts to look nonsensical. Just a few weeks ago, the Nuclear Decommissioning Authority (NDA), responding to the government’s own request to consider how to deal with the stockpile, advised that the cost of making MOX is greater than its value as a fuel. And therefore, the most cost-effective plan would be to continue storing the plutonium until a geological disposal facility becomes available for it to be buried underground. So what’s going on? To make sense of it all, Nature peers through the looking glass of the British nuclear industry.

Why would the UK government want to reprocess plutonium into fuel?

Partly, because the plutonium is a huge embarrassment. To continue storing it is not only a constant reminder of how ill-thought-out British nuclear policy has been over the past decades, but it also poses a risk to non-proliferation. So converting it into MOX would transform a huge pile of waste — which the NDA euphemistically calls a “zero value asset” — into something potentially useful, while making the plutonium far harder to weaponize were it to fall into the wrong hands.

So why is it closing the only plant that can do that?Ostensibly, because, in the wake of the Fukushima nuclear disaster, the Sellafield MOX Plant no longer has any customers. But in truth it never really worked. It was built to convert the UK plutonium stockpile into MOX, but, having been plagued by technical failures and delays, it has managed to produce only around 2.5% of its intended quota since it started operating ten years ago. With such a low throughput, it would have been unable to convert the UK stockpile fast enough to provide MOX for new reactors. And so in 2010 the NDA did a deal with ten Japanese utility companies to process their smaller amounts of plutonium and supply them with MOX, a deal that ultimately sank beneath a tsunami.

Why build a new one?

Because the problem hasn’t gone away. Britain still needs to get rid of the plutonium and, despite the failings of the Sellafield plant, MOX production is a proven technology. The one other plant in the world, the Melox plant in Gard, France, has produced around 1,700 tonnes of MOX since it was built in 1995. So, currently, the only other options are to do nothing — keeping the plutonium in storage until geological disposal becomes possible sometime after 2075 — or to process it so that it can supply a fast reactor, such as the kind that, with almost comic timing, GE Hitachi offered to build at Sellafield last week. The problem with this is that, despite 30 years in development, these reactors are still an unproven technology.

Is it worth it?

According to the NDA, not if you care about making a profit. With the low cost of uranium and the high cost of producing MOX, the reprocessed fuel would probably have to be given away. But profit isn’t everything. Indeed, as long-term plutonium managing solutions go, MOX production gets the NDA’s backing. And that is the message Hendry is putting out, that although the government is not quite ready to do anything with the plutonium yet, when it is ready it will be taking the MOX route. And if that means not making a profit, that’s a small price to pay for getting rid of such an albatross. It’s a brave decision, and one that gets the backing of both the Royal Society and the government’s former chief scientific adviser David King at the University of Oxford’s Smith School of Enterprise and the Environment. 

Will it ever actually happen?

Despite all the caveats, we could see a MOX plant approved sooner rather than later. Tied into the decision is a proposal for Britain’s next-generation reactors, which would use MOX as fuel, thereby requiring a MOX plant to be in production before they come online. What remains to be seen, however, is whether a new MOX plant will be able to show that the UK nuclear industry has learnt from the mistakes and failures of the Sellafield plant and to get rid of the plutonium once and for all.

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Walk-Through-Wall Effect Might Be Possible With Humanmade Object, Physicists Predict

Tunnel vision. A superthin sheet of carbon atoms suspended could quantum tunnel between two positions, one slightly bowed and one so bent it contacts the metal plate. An atomic force microscopy image of a graphene membrane (inset) 

If you've ever tried the experiment, you know you can't walk through a wall. But subatomic particles can pull off similar feats through a weird process called quantum tunneling. Now, a team of physicists says that it might just be possible to observe such tunneling with a larger, humanmade object, though others say the proposal faces major challenges.

If successful, the experiment would be a striking advance toward fashioning mechanical systems that behave quantum mechanically. In 2010, physicists took a key first step in that direction by coaxing a tiny object into states of motion that can be described only by quantum mechanics. Tunneling would be an even bigger achievement.

So how does quantum tunneling work? Imagine that an electron, for example, is a marble sitting in one of two depressions separated by a small hill, which represent the effects of a sculpted electric field. To cross the hill from one depression to the other, the marble needs to roll with enough energy. If it has too little energy, then classical physics predicts it can never reach the top of the hill and cross over it.

Tiny particles such as electrons, however, can still make it across even if they don't have enough energy to climb the hill. Quantum physics describes such particles as extended waves of probability—and it turns out that there is a probability that one of them will "tunnel" through the hill and suddenly materialize in the other depression, even though the electron can't occupy the high ground between the two low spots.

It sounds unlikely, but scientists and engineers have amply demonstrated quantum tunneling in semiconductors in which electrons tunnel through nonconducting layers of material. (In fact, some types of magnetic hard drives rely on tunneling for reading out data.) And the Nobel Prize-winning scanning tunneling microscope relies on electrons tunneling through a forbidden no man's land between a tiny fingerlike probe and a conducting surface. Still, no one has ever seen a macroscopic object tunnel through an obstacle.

But Mika Sillanpää and colleagues at Aalto University in Finland say it might be possible to do just that using a tiny widget that resembles a trampoline as they reported 8 November in Physical Review B. Researchers would fashion the micrometer-wide trampoline out of graphene, a superstrong, superflexible sheet of carbon only one atom thick. They would suspend the membrane—small but much larger than the atoms and molecules that are the usual domain of quantum physics—over a metal plate. When experimenters applied an electrical voltage, the membrane would have two stable positions: one in which it bows slightly in the middle and one in which it bends enough to contact the plate below. In the Finnish team's design, the electrical and mechanical forces on the membrane create an energy barrier between these two positions. If researchers could lower the membrane's energy by cooling it to a temperature of less than a thousandth of a degree above absolute zero, then the only way it could get between the two positions is quantum tunneling. The experimenters could then observe the membrane's change of configuration by looking for a change in the system's capacitance, a measure of how well it can store electrical charge. Sillanpää says achieving the low temperatures required may take several years, but the team is moving forward with an experiment.

Quantum tunneling in a mechanical system is "the kind of holy grail that people are looking for now," says physicist Walter Lawrence of Dartmouth College, but the experiment is likely to be difficult. Gil-Ho Lee, a physicist at Pohang University of Science and Technology in South Korea, says the proposed experiment would be an important first step toward demonstrating quantum tunneling. But he cautions that it might not be conclusive because the membrane might perform similar flip-flops when it absorbs a little extra energy in the form of heat. "A more sophisticated test must be done," Lee says. He says that searches for quantum tunneling in electrical systems known as Josephson junctions faced similar issues in the 1980s before experiments eventually confirmed tunneling.

So why can't you use quantum tunneling to walk through a wall? Quantum mechanical calculations show that for something as big as a person, the probability is so small that you could wait until the end of the universe and most likely still not find yourself on the other side.

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The Physics of Wine Swirling

Meet the new flavor of wine: fruity with a hint of fluid dynamics. Oenophiles have long gotten the best out of their reds by giving their glasses a swirl before sipping. A new study has revealed the physics behind that sloshing, showing that three factors may determine whether your merlot arcs smoothly or starts to splash.

Twirling a wineglass gently creates smooth arcs in the liquid that then circle, coating the sides of the glass. The gesture isn't just for appearances, says study co-author Martino Reclari, who studies fluid dynamics at the École Polytechnique Fédérale de Lausanne in Switzerland. Scientists and enthusiasts alike have long known that the swirling motion mixes oxygen into a red, enhancing its flavor.

One evening over their own bottle of wine, Reclari and colleagues decided to tackle the physics of this oenological routine. The team filled up small cylinders in a range of sizes with different volumes of a cheap merlot, then set them spinning. To keep things uniform, the researchers employed gyrating machines, commonly used to mix liquids precisely in biology or chemistry labs. This week, at the annual meeting of the American Physical Society's Division of Fluid Dynamics in Baltimore, Maryland, the group reported a mathematical formula explaining how wine sloshes.

Unlike the flavor of a perfectly aged pinot, Reclari says, the factors at play aren't overly complicated. Three factors seemed to determine whether the team spotted one big wave in the wine or several smaller ripples: the ratio of the level of wine poured in to the diameter of the glass; the ratio of the diameter of the glass to the width of the circular shaking; and the ratio of the forces acting on the wine. Those forces affecting the wine were the centrifugal force pushing the liquid to the outside of the glass and the gravitational force shoving the liquid back down.


By tweaking these factors a notch—for instance, by pouring a bit more wine into a glass or shaking that glass in tighter circles—Reclari and colleagues mastered the art of unusual wine waves. Their creations in the video above included the wine lover's standard, a single, smooth crest, all the way to four miniwaves that built in quick succession. Curiously, however, if the researchers kept all three ratios identical, they began to spot the same waves forming again and again, even in cylinders of very different sizes. "If you have a very small glass or a very big glass and you put in the same parameters, you will have exactly the same shape of the wave," Reclari says.

He and colleagues also landed on another important discovery: how overly enthusiastic wine swirlers manage to splash their drinks, possibly staining their sweaters. Just like an ocean crest, wine waves begin to break, turning frothy, if they're moving too quickly, he says. The breaking acceleration for a merlot is about 40% of the force of gravity, the team concluded, or nearly 4 meters per second. That acceleration, in turn, is dependent on the volume of wine in the glass, the force of shaking, and other factors.

The team's formula is useful for more than just helping a wine taster "impress his friends," Reclari says. When growing bacterial cultures, biologists often mix cells in with nutrients in one big jar, then swirl, much like an aficionado over the latest vintage. That rotation distributes the bacterial food throughout the slurry and also removes excess carbon dioxide. Knowing just how liquids slosh in such jars may help lab technicians optimize their growing methods, he adds.

The team's analysis is "simple" but does "make sense," says Vladimir Ajaev, an applied mathematician at Southern Methodist University in Dallas, Texas. And the study illustrates well how seemingly everyday physics, such as the swirling of a glass of wine, might help scientists and engineers develop better lab tools: "At first it might seem like a matter of curiosity," he says. "But then it turns out there are some specific applications."

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