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Tuesday, 26 April 2016

On this day in science history: Chernobyl nuclear plant explosion

In 1986, in Pripet, Russia, one of the four reactors at the Chernobyl nuclear plant exploded in the world's worst civil nuclear catastrophe. It sent a cloud of radioactive dust over Europe. 

The disaster began during a systems test on 26 April 1986 at reactor number four of the Chernobyl plant.  There was a sudden and unexpected power surge, and when an emergency shutdown was attempted, an exponentially larger spike in power output occurred, which led to a reactor vessel rupture and a series of steam explosions. These events exposed the graphite moderator of the reactor to air, causing it to ignite. The resulting fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area, including Pripyat. The plume drifted over large parts of the western Soviet Union and Europe. From 1986 to 2000, 350,400 people were evacuated and resettled from the most severely contaminated areas of Belarus, Russia, and Ukraine. According to official post-Soviet data, about 60% of the fallout landed in Belarus.

The abandoned city of Pripyat with Chernobyl plant in the distance, by Jason Minshull [Public domain], via Wikimedia Commons
Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. A report by the International Atomic Energy Agency examines the environmental consequences of the accident. Another UN agency, UNSCEAR, has estimated a global collective dose of radiation exposure from the accident "equivalent on average to 21 additional days of world exposure to natural background radiation"; individual doses were far higher than the global mean among those most exposed, including 530,000 local recovery workers who averaged an effective dose equivalent to an extra 50 years of typical natural background radiation exposure each. Estimates of the number of deaths that will eventually result from the accident vary enormously; disparities reflect both the lack of solid scientific data and the different methodologies used to quantify mortality—whether the discussion is confined to specific geographical areas or extends worldwide, and whether the deaths are immediate, short term, or long term.

Thirty-one deaths are directly attributed to the accident, all among the reactor staff and emergency workers. An UNSCEAR report places the total confirmed deaths from radiation at 64 as of 2008. The Chernobyl Forum predicts the eventual death toll could reach 4,000 among those exposed to the highest levels of radiation (200,000 emergency workers, 116,000 evacuees and 270,000 residents of the most contaminated areas); this figure is a total causal death toll prediction, combining the deaths of approximately 50 emergency workers who died soon after the accident from acute radiation syndrome, nine children who have died of thyroid cancer and a future predicted total of 3940 deaths from radiation-induced cancer and leukemia.

In a peer-reviewed publication in the International Journal of Cancer in 2006, the authors (following a different conclusion methodology to the Chernobyl Forum study, which arrived at the total predicted death toll of 4,000 after cancer survival rates were factored in) stated, without entering into a discussion on deaths, that in terms of total excess cancers attributed to the accident:

The risk projections suggest that by now Chernobyl may have caused about 1000 cases of thyroid cancer and 4000 cases of other cancers in Europe, representing about 0.01% of all incident cancers since the accident. Models predict that by 2065 about 16,000 cases of thyroid cancer and 25,000 cases of other cancers may be expected due to radiation from the accident, whereas several hundred million cancer cases are expected from other causes.

Also based upon extrapolations from the linear no-threshold model of radiation induced damage, down to zero, the Union of Concerned Scientists estimates that, among the hundreds of millions of people living in broader geographical areas, there will be 50,000 excess cancer cases resulting in 25,000 excess cancer deaths.

For this broader group, the 2006 TORCH report, commissioned by the European Greens political party, predicts 30,000 to 60,000 excess cancer deaths. The environmental advocacy group Greenpeace reports the figure at 200,000 or more.

The Russian founder of that region's chapter of Greenpeace also authored a book titled Chernobyl: Consequences of the Catastrophe for People and the Environment, which concludes that among the billions of people worldwide who were exposed to radioactive contamination from the disaster, nearly a million premature cancer deaths occurred between 1986 and 2004. The book, however, has failed the peer review process. Of the five reviews published in the academic press, four considered the book severely flawed and contradictory, and one praised it while noting some shortcomings. The review by M. I. Balonov published by the New York Academy of Sciences concludes that the report is of negative value because it has very little scientific merit while being highly misleading to the lay reader. It characterized the estimate of nearly a million deaths as more in the realm of science fiction than science.

The accident raised concerns about nuclear power worldwide and slowed or reversed the expansion of nuclear power stations. The accident also raised concerns about the safety of the Soviet nuclear power industry, slowing its expansion for a number of years and forcing the Soviet government to become less secretive about its procedures. The government coverup of the Chernobyl disaster was a "catalyst" for glasnost, which "paved the way for reforms leading to the Soviet collapse".

The final shutdown of the undamaged last reactor on the site took place ceremoniously on 15 Dec 2000.

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Tuesday, 19 April 2016

Unexpected discovery leads to a better battery

An unexpected discovery has led to a rechargeable battery that's as inexpensive as conventional car batteries, but has a much higher energy density. The new battery could become a cost-effective, environmentally friendly alternative for storing renewable energy and supporting the power grid.

A team based at the Department of Energy's Pacific Northwest National Laboratory identified this energy storage gem after realizing the new battery works in a different way than they had assumed. 

The journal Nature Energy published a paper today that describes the battery.

"The idea of a rechargeable zinc-manganese battery isn't new; researchers have been studying them as an inexpensive, safe alternative to lithium-ion batteries since the late 1990s," said PNNL Laboratory Fellow Jun Liu, the paper's corresponding author. "But these batteries usually stop working after just a few charges. Our research suggests these failures could have occurred because we failed to control chemical equilibrium in rechargeable zinc-manganese energy storage systems."

A range of different batteries
After years of focusing on rechargeable lithium-ion batteries, researchers are used to thinking about the back-and-forth shuttle of lithium ions. Lithium-ion batteries store and release energy through a process called intercalation, which involves lithium ions entering and exiting microscopic spaces in between the atoms of a battery's two electrodes.

This concept is so engrained in energy storage research that when PNNL scientists, collaborating with the University of Washington, started considering a low-cost, safe alternative to lithium-ion batteries - a rechargeable zinc-manganese oxide battery - they assumed zinc would similarly move in and out of that battery's electrodes.

After a battery of tests, the team was surprised to realize their device was undergoing an entirely different process. Instead of simply moving the zinc ions around, their zinc-manganese oxide battery was undergoing a reversible chemical reaction that converted its active materials into entirely new ones.

Liu and his colleagues started investigating rechargeable zinc-manganese batteries because they are attractive on paper. They can be as inexpensive as the lead-acid batteries because they use abundant, inexpensive materials (zinc and manganese). And the battery's energy density can exceed lead-acid batteries. The PNNL scientists hoped they could produce a better-performing battery by digging deeper into the inner workings of the zinc-manganese oxide battery.

So they built their own battery with a negative zinc electrode, a positive manganese dioxide electrode and a water-based electrolyte in between the two. They put small, button-sized test batteries through the wringer, repeatedly charging and discharging them. As others had found before them, their test battery quickly lost its ability to store energy after just a few charging cycles. But why?

To find out, they first performed a detailed chemical and structural analysis of the electrolyte and electrode materials. They were surprised to not find evidence of zinc interacting with manganese oxide during the battery's charge and discharge processes, as they had initially expected would happen. The unexpected finding led them to wonder if the battery didn't undergo a simple intercalation process as they had previously thought. Perhaps the zinc-manganese battery is less like a lithium-ion battery and more like the traditional lead-acid battery, which also relies on chemical conversion reactions.

To dig deeper, they examined the electrodes with several advanced instruments with a variety of scientific techniques, including Transmission Electron Microscopy, Nuclear Magnetic Resonance and X-Ray Diffraction. The instruments used were located at both PNNL and the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science user facility located at PNNL. 

Combining these techniques revealed manganese oxide was reversibly reacting with protons from the water-based electrolyte, which created a new material, zinc hydroxyl sulfate.

Typically, zinc-manganese oxide batteries significantly lose storage capacity after just a few cycles. This happens because manganese from the battery's positive electrode begins to sluff off, making the battery's active material inaccessible for energy storage. But after some manganese dissolves into the electrolyte, the battery gradually stabilizes and the storage capacity levels out, though at a much lower level.

The team used the new knowledge to prevent this manganese sluff-off. Knowing the battery underwent chemical conversions, they determined the rate of manganese dissolution could be slowed down by increasing the electrolyte's initial manganese concentration.

So they added manganese ions to the electrolyte in a new test battery and put the revised battery through another round of tests. This time around, the test battery was able to reach a storage capacity of285 milliAmpere-hours per gram of manganese oxide over 5,000 cycles, while retaining 92 percent of its initial storage capacity.

"This research shows equilibrium needs to be controlled during a chemical conversion reaction to improve zinc-manganese oxide battery performance," Liu said. "As a result, zinc-manganese oxide batteries could be a more viable solution for large-scale energy storage than the lithium-ion and lead-acid batteries used to support the grid today."

The team will continue their studies of the zinc-manganese oxide battery's fundamental operations. Now that they've learned the products of the battery's chemical conversion reactions, they will move on to identify the various in-between steps to create those products. They will also tinker with the battery's electrolyte to see how additional changes affect its operation.

This research was supported by DOE's Office of Science and used resources at the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science user facility located at PNNL.

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Tuesday, 12 April 2016

Supernovae showered Earth with radioactive debris

An international team of scientists has found evidence of a series of massive supernova explosions near our solar system, which showered Earth with radioactive debris.

The scientists found radioactive iron-60 in sediment and crust samples taken from the Pacific, Atlantic and Indian Oceans.

The iron-60 was concentrated in a period between 3.2 and 1.7 million years ago, which is relatively recent in astronomical terms, said research leader Dr Anton Wallner from The Australian National University (ANU).

"We were very surprised that there was debris clearly spread across 1.5 million years," said Dr Wallner, a nuclear physicist in the ANU Research School of Physics and Engineering. "It suggests there were a series of supernovae, one after another.

"It's an interesting coincidence that they correspond with when the Earth cooled and moved from the Pliocene into the Pleistocene period."

A supernova. NASA/ESA [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
The team from Australia, the University of Vienna in Austria, Hebrew University in Israel, Shimizu Corporation and University of Tokyo, Nihon University and University of Tsukuba in Japan, Senckenberg Collections of Natural History Dresden and Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany, also found evidence of iron-60 from an older supernova around eight million years ago, coinciding with global faunal changes in the late Miocene.

Some theories suggest cosmic rays from the supernovae could have increased cloud cover.

A supernova is a massive explosion of a star as it runs out of fuel and collapses.

The scientists believe the supernovae in this case were less than 300 light years away, close enough to be visible during the day and comparable to the brightness of the Moon.

Although Earth would have been exposed to an increased cosmic ray bombardment, the radiation would have been too weak to cause direct biological damage or trigger mass extinctions.

The supernova explosions create many heavy elements and radioactive isotopes which are strewn into the cosmic neighbourhood.

One of these isotopes is iron-60 which decays with a half-life of 2.6 million years, unlike its stable cousin iron-56. Any iron-60 dating from Earth's formation more than four billion years ago has long since disappeared.

The iron-60 atoms reached Earth in minuscule quantities and so the team needed extremely sensitive techniques to identify the interstellar iron atoms.

"Iron-60 from space is a million-billion times less abundant than the iron that exists naturally on Earth," said Dr Wallner.

Dr Wallner was intrigued by first hints of iron-60 in samples from the Pacific Ocean floor, found a decade ago by a group at TU Munich.

He assembled an international team to search for interstellar dust from 120 ocean-floor samples spanning the past 11 million years.

The first step was to extract all the iron from the ocean cores. This time-consuming task was performed by two groups, at HZDR and the University of Tokyo.

The team then separated the tiny traces of interstellar iron-60 from the other terrestrial isotopes using the Heavy-Ion Accelerator at ANU and found it occurred all over the globe.

The age of the cores was determined from the decay of other radioactive isotopes, beryllium-10 and aluminium-26, using accelerator mass spectrometry (AMS) facilities at DREsden AMS (DREAMS) of HZDR, Micro Analysis Laboratory (MALT) at the University of Tokyo and the Vienna Environmental Research Accelerator (VERA) at the University of Vienna.

The dating showed the fallout had only occurred in two time periods, 3.2 to 1.7 million years ago and eight million years ago. Current results from TU Munich are in line with these findings.

A possible source of the supernovae is an aging star cluster, which has since moved away from Earth, independent work led by TU Berlin has proposed in a parallel publication. The cluster has no large stars left, suggesting they have already exploded as supernovae, throwing out waves of debris.

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Wednesday, 6 April 2016

Star Wars-style lava planet discovered close to Earth

First they thought it was a water world, a planet larger than Earth covered in nothing but ocean. Then they thought it might be a diamond world, covered in mountains of graphite and diamond. Now, researchers think that near-by 55 Cancri e has an entire hemisphere engulfed in lava.

The planet orbits a sun-like star located just 40 light years away. It orbits its parent star about 100 times closer than Earth to the sun, completing a circuit in just 17.68 hours.

So close to its parent star, the planet is locked by gravity to show only one face to the star rather like the moon shows only one face to Earth. This means that one hemisphere of the planet is permanently sunlit, while the other is in perpetual darkness.

The planet has attracted a lot of interest since 2011, when it was discovered to cross the face of its star and block out some of its light. This allowed the planet’s atmosphere to be analysed. No water vapour was found, putting paid to the idea of it being a water world.

An analysis of the parent star, however, showed a higher than usual concentration of carbon-bearing elements. This led researchers to suggest next that 55 Cancri e could be a diamond planet with a landscape composed of graphite and diamond mountains.

The latest work involves observations of the planet with the Spitzer space telescope, Nasa’s orbiting infrared observatory. It shows that the temperature of the sunward facing hemisphere soars to 2500°C, while the permanently dark hemisphere reaches around 1100°C.

At these temperatures the hot side must be completely molten. At the terminator, the name for the boundary between the light and dark side (sorry, another Star Wars reference), their must be some form of lava shoreline as the molten rock solidify into landforms. In the twilight of the terminator region, the lava will be glowing red hot casting a hellish appearance across the alien landscape.

Lava flow. By Brocken Inaglory (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
Dr Brice-Olivier Demory of the University of Cambridge’s Cavendish Laboratory is the lead author of the paper announcing the new results. Although the work answers some questions about the nature of the planet, it raises others.

For example, despite the proximity of 55 Cancri e to its star and the tremendous amount of blinding sunlight it receives as a result, the temperature calculated from the infrared observations is higher than expected. So there must be another source of heat in the planet.

At eight times the mass of the Earth, it seems certain that the planet will contain a lot more radioactive elements than our world. As these decay, they would heat the interior, perhaps providing the extra heating.

One thing is certain, 55 Cancri e must now be a top target for the James Webb Space Telescope. This Nasa-built spacecraft is the successor to the Hubble Space Telescope and will be launched in 2018 by the European Space Agency. Its mirror will be more than seven times larger than the Spitzer’s. Although it works at somewhat different infrared wavelengths it will be able to study nearby planets such as 55 Cancri e in unprecedented detail.

But perhaps the best thing about the announcement of this discovery is that none of the astronomers felt duty bound to reference Mustafar, the lava planet on which Obi-Wan Kenobi and Anakin Skywalker fought their climatic light sabre battle in Star Wars: Revenge of the Sith.

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