Algae holds promise for nuclear clean-up

Organism's ability to distinguish strontium from calcium could help in dealing with nuclear waste.

Richard A. Lovett

Strontium-eating bacteria could help clean up after nuclear accidents.CLAUDE NURIDSANY & MARIE PERENNOU/SCIENCE PHOTO LIBRARY

Common freshwater algae might hold a key to cleaning up after disasters such as Japan's Fukushima nuclear accident, scientists said yesterday at a meeting of the American Chemical Society in Anaheim, California.

The algae, called Closterium moniliferum, are members of the desmid order, known to microbiologists for their distinctive shapes, said Minna Krejci, a materials scientist at Northwestern University in Evanston, Illinois. But the crescent-shaped C. moniliferum caught Krejci's eye because of its unusual ability to remove strontium from water, depositing it in crystals that form in subcellular structures known as vacuoles — a knack [un truc] that could include the radioactive isotope strontium-90.

Strontium is very similar in properties and atomic size to calcium, so biological processes can't easily separate the two elements. That makes strontium-90 a particularly dangerous isotope: it can infiltrate milk, bones, bone marrow, blood and other tissues, where the radiation that it emits can eventually cause cancer.

"That's what makes strontium-90 one of the dominant health risks of spent fuel for the first 100 years or so after it leaves the reactor," says Krejci. The radioisotope has a half-life of about 30 years.

Unfortunately, reactor waste and accidental spills can contain up to ten billion times more calcium than strontium, making it very difficult to clean up the strontium without also having to dispose of a mountain of harmless calcium. "We need a highly efficient and selective method of separating it," says Krejci.

Enter C. moniliferum. The organism has no particular interest in strontium: it mostly collects barium. But strontium is midway between calcium and barium in size and properties, so any of it that happens to be around gets crystallized as well. Meanwhile, even though calcium is far more abundant than either of the other two elements, it is different enough to barium that it gets left behind.

The result is a crystal that is mainly composed of barium, but is massively enriched in strontium.

How do they do that?

Much of Krejci's research so far¹ has focused on trying to work out how the algae generate the crystals, with an eye to making the process even more strontium-selective. For the moment, she knows that the organism isn't purposefully bringing excess barium and strontium through its cell walls. Rather, she says, the crystals appear to form because the vacuoles in which they collect are rich in sulphate. Barium and strontium have relatively low solubility in sulphate solutions, so any barium and strontium that make their way into these vacuoles easily precipitate out to form crystals.

Microbiologists don't know whether the crystals have any function for the organism. Perhaps they are simply waste, forming by accident in vacuoles that serve as storage depots for sulphate, said Krejci.

Whatever purpose the crystals serve, Krejci's research has found that it is possible to enhance the uptake of strontium by tailoring the amount of barium in the algae's environment. This, she says, means that it might prove possible to seed nuclear waste, or a spill of radioactive material, with barium to encourage the algae to grab the strontium — easy to do, she says, because "it would only be a small amount" of barium.

It might also be possible to improve the process by tinkering with sulphate levels in the environment, thereby changing the amount of sulphate in the vacuoles. "Once we learn about how the cells respond to conditions, we can think of more elegant ways to manipulate them," says Krejci.

Once isolated by the bacteria, the strontium could be sequestered in high-level nuclear waste repositories, while the rest of the waste could go to a less expensive lower-level repository, saving space and money. Currently, Krejci says, there are hundreds of millions of litres of stored nuclear waste in the United States alone, much of which contains strontium. "So we know it's a big problem," she says.

Radiation exposure

Krejci and her colleagues have not yet tested how well the algae survive in the presence of radioactivity. But even if the organisms respond poorly, she says, they would probably live long enough to start removing strontium, because the process begins quickly. "The cells precipitate crystals within 30 minutes to an hour," she says. And if more are needed, "they are easy to culture".

Gija Geme, a chemist at the University of Central Missouri in Warrensburg, organized the symposium at which Krejci presented her work. Geme, who grew up in an area of Russia not far from Chernobyl and so has a personal interest in nuclear clean-up, was one of the few people at the meeting who knew the significance of Krejci's presentation in advance: the talk's title, focusing on biomineralization, did not mention Japan, radioactivity or nuclear accidents.

"It's a hot topic right now," says Geme. "But when I put this symposium together, there was no tragedy [in Japan]. I was looking for any studies about sequestration of metals that would be of significance to society."

Geme urges Krejci's team not to spend too much time trying to discover precisely why the algae does what it does before they start testing the process with nuclear wastes.

"Sometimes, just getting it out is very, very important," she says. "I would like to see field studies using actual waste as soon as possible."

http://www.nature.com/news/2011/110330/full/news.2011.195.html

Japan faces up to failure of its earthquake preparations

Systems for forecasting, early warning and tsunami protection all fell short on 11 March.

David Cyranoski

TOKYO

Japan has the world's densest seismometer network, the biggest tsunami barriers and the most extensive earthquake early-warning system. Its population is drilled more rigorously than any other on what to do in case of earthquakes and tsunamis.

Yet this month's magnitude-9 earthquake surprised the country's forecasters. The grossly underestimated tsunami destroyed the world's deepest tsunami barrier and caught people by surprise. And the early-warning system for earthquakes largely failed. What went wrong?

The first problem was the earthquake forecast. Japan's seismic hazard map, the latest version of which was released in March 2009, breaks the offshore area of northeastern Japan into five seismic zones and envisages seven different earthquake scenarios. Each is assigned a probability based on the historical record of earthquakes. The southern Sanriku offshore region, which included the origin of this month's earthquake, was given a 30–40% chance of rupturing in the next 10 years and a 60–70% chance in the next 20 years.

Click for larger image

As earthquake forecasting goes, these are very high numbers. "That basically means it could happen any day," says Yoshinori Suzuki of the Earthquake Disaster Reduction Research Division within the science ministry, which coordinates the map-making. But the fault was expected to unleash an earthquake of around magnitude 7.7 — about as large as any in the historical record for the area (see Nature 471, 274; 2011).

For a separate fault segment offshore from the Fukushima Daiichi nuclear plant, the same forecasting approach postulated only a magnitude-7.4 earthquake, with a less than 2% chance of occurring over the next 10 years and less than 10% over the next 50 years. The government of Fukushima prefecture even refers to the seismic hazard map to boast on its website: "With firm geological foundations and major earthquakes rare, Fukushima is a safe and secure place to do business." What the risk maps didn't allow for, however, was the coupling of segments that allowed the rupture to propagate for some 500 kilometres, unleashing an earthquake of magnitude 9.0 (see 'False comfort').

Japan's earthquake forecasting has had its successes. In 2003, the magnitude-8.3 Tokachi-oki earthquake occurred right in the middle of a forecasted hotspot. But for the most part, earthquake forecasting, which really took off in Japan in the 1980s and 1990s, has had mixed results, with many devastating quakes hitting outside the expected zones. "We would like to see more hit the marks," says Kyoto University's James Mori.

Despite the surprisingly powerful earthquake, Japan's earthquake-resistant buildings seemed to hold up well. "There was shaking damage but not much considering how strong the earthquake was," says Mori. It was the tsunami that did most of the damage, overwhelming barriers and years of preparation.

The world's deepest tsunami barrier, a 2-kilo- metre-long edifice at the mouth of Kamaishi Bay on the northeast coast, was completed in 2008 after 30 years, at a cost of more than ¥120 billion (US$ 1.4 billion). Anchored to the sea floor 63 metres down and rising 8 metres above the water, the 20-metre-thick break- water was designed to withstand the impact of a tsunami like the one from the 1896 Sanriku earthquake, which produced waves rising to nearly 40 metres in some areas.

Koji Fujima, a specialist in tsunami wave propagation at the National Defence Academy in Yokosuka, says that this and other structures along the coast gave people a false sense of security. "The region probably gets 2- or 3-metre tsunamis more than once a decade, and people know that the breakwaters will protect them from those," says Fujima. With the hazard map forecasting earthquakes in the magnitude-7.5 range, people would have anticipated a maximum tsunami of 4–5 metres.

Tsunami risk underestimated

Faith in the barriers seems to have undermined Japan's legendary tsunami-preparedness drills. In northeastern Japan as elsewhere, university professors, research institutes, non-governmental organizations and local civic groups carried out several drills each year to train people in how and where to evacuate. "We were working as hard as we could to educate people," says Fujima.

Yet people apparently became relaxed about tsunami risks, says Yoshiaki Kawata, a disaster-management expert at Kansai University. A tsunami originating in Chile last year triggered an evacuation warning to 1.68 million people in northeastern Japan. Only 62,000 sought shelter, says Kawata.

"People thought the breakwater was enough," says Fujima. But he adds that "there was no way it could protect them" against the tsunami on 11 March, although it did diminish the wave. Rising an estimated 15–20 metres at sea and 50 metres at some points after hitting the shore, even higher than the 1896 wave, it destroyed the tsunami barriers at Kamaishi and elsewhere and has killed an estimated 20,000 people who had failed to find safe, higher ground. It also swamped emergency generators at the Fukushima Daiichi plant, disabling the cooling system (see page 555). Built in the 1960s, the plant was designed to withstand a tsunami of no more than 5.7 metres.

The early-warning system operated by the Japan Meteorological Agency, designed to alert people when an earthquake will create shaking at or above level 5 on Japan's energy intensity scale (severe enough to crack walls), fell short as well. Based on a seismic reading taken a few seconds after an earthquake hits, the system provides up to tens of seconds of warning before the major shaking begins. On 11 March it delivered accurate warnings to areas near the epicentre. But the greater Tokyo region, where many areas experienced level-6 shaking, received no warning. Bullet trains and nuclear reactors, which have their own warning systems, shut down promptly, as designed.

The problem, according to Kyoto University's Masumi Yamada, was that the system assumes a 'point source' for an earthquake. In this case, the point source led to an estimate of a magnitude-7.2 quake. But as the Sanriku rupture ripped hundreds of kilometres of fault line parallel to the coast, unleashing ever more energy and causing slips of 20 metres or more near the Tokyo region, the system didn't correct itself. The frequent aftershocks also confounded the system, which generated several false alarms and missed large aftershocks.

"The system seems to break down around a magnitude-8 quake," says Yamada. In April, she will start a three-year collaborative project with the Japan Meteorological Agency to convert the point-source warning system to a dynamic one that works in two dimensions.

Japan's disaster defences can certainly be improved, says Fujima, but he thinks that people should recognize that there are limits to what can be done against a "once in a thousand or two thousand years earthquake". Sturdier breakwaters could be built in areas where the tsunamis hit hardest, but they are expensive and could never fully protect against the biggest waves. "People probably should just stop building in the areas where large tsunamis will come," he says.

Kawata, however, puts his faith in better engineering. He agrees that the most effective way to avoid damage is to have people live out of reach of tsunamis. But he envisages houses (and nuclear plants) built on an artificial coastline supported by 10-metre-high concrete pillars. "There are a lot of things we have to do urgently. If we have a vision and we pool our energies, we can do it."

http://www.nature.com/news/2011/110329/full/471556a.html

TP dissection souris mâles et femelles : faire un dessin d'observation légendé

dia dissection : http://www5.ac-lille.fr/~svt/telechargements/diss_app_uro/diss_app_uro_gen_souris.pps

FT dissection : liste de fiches techniques : http://pedagogie.ac-toulouse.fr/svt/serveur/bankact/index.php?

photos légendées : http://espace-svt.ac-rennes.fr/applic/dissect/souris/souris17.htm

sujets bac : http://eduscol.education.fr/cid47782/evaluation-des-capacites-experimentales.html

623 - Echelle moléculaire : Régulation physiologique de l’axe gonadotrope

[testo] et fonction reprod : p268-269

contrôle hormonal de l'activité testiculaire : p270-271

rappel 1S sur struct cerveau : localisation hypothalamus, hypophyse, adéno=anté // post=neuro :

http://asso.orpha.net/AFDI/upload/pict/Tigepituitaire.jpg

http://www.didiersvt.com/ts/media/jpg/atlas_end01.jpg

http://www.ulb.ac.be/sciences/biolhc/chap02/chap08/atlas_end01.html#

http://fr.academic.ru/pictures/frwiki/72/Hypothalamus.gif

http://fr.academic.ru/dic.nsf/frwiki/800032

http://www.sante.univ-nantes.fr/med/ticem/umvf/item57/site/html/2.html

doc p 271 : hormones GnRH = gonadolibérine, LH = gonadostimuline

dosages hormones : http://svt.ac-dijon.fr/schemassvt/article.php3?id_article=560

schématisation cybernétique : rappels de 1°S sur glycémie

L’originalité de ce système par rapport à la régulation de la glycémie réside dans le mécanisme de neurosécrétion par l’hypothalamus de l’hormone gonadolibérine ou lulibérine (GnRH).

transformer schéma 1S/ glycémie en tableau

Le panache radioactif qui s’échappe de la centrale nucléaire de Fukushima Daiichi depuis la série d’accidents du 13 mars 2011 devrait atteindre la France dès le 23 ou 24 mars. Les autorités écartent tout risque sanitaire.

Le panache radioactif qui s’échappe de la centrale nucléaire de Fukushima Daiichi depuis l’accident du 13 mars 2011 se répand peu à peu dans l’atmosphère. Initialement chassé par les vents d’ouest vers le Pacifique, il devrait rejoindre l’Europe le 23 ou 24 mars, selon les prévisions de l’Institut de radioprotection et de sûreté nucléaire (ISRN) et de Météo France.

A partir de l’estimation des rejets de substances radioactives par l’ISRN, Météo France a produit une simulation de la dispersion de ces éléments dans l’atmosphère jusqu’au 26 mars (voir ci-dessous).



Le panache est composé de gaz rares comme le xénon 133 et le krypton, et de particules radioactives en suspension dans l’air, telles que l’iode et le césium. Très peu réactifs, les gaz rares restent dans l’atmosphère sans se déposer au sol et leur concentration diminue au cours du temps.

En revanche les particules radioactives sous forme d’aérosols comme l’iode 131 (dont la période radioactive est de huit jours), ou le césium 137 (30 ans) se déposent progressivement au sol, plus rapidement en cas de pluie. C’est ce qu’on appelle les retombées radioactives, qui contaminent l’environnement.

Quels risques pour la France ?

Aucun, selon l’IRSN. Les concentrations attendues en substances radioactives devraient être de 1000 à 10 000 fois inférieures à celles mesurées dans l’est de la France après l’accident de Tchernobyl en 1986.

Les produits frais français sont eux aussi à l’abri. Toutefois, les produits alimentaires importés du Japon sont soumis à un contrôle systématique. Les autorités écartent donc tout risque sanitaire.

Après un séjour d’une semaine dans l’atmosphère, les particules radioactives du panache sont grandement diluées. A tel point que l’institut prévoit que ses balises réparties sur le territoire français ne pourront pas détecter leurs traces. Les mesures de la radioactivité sont toutefois accessibles sur internet via une carte interactive (cliquez sur la carte ci-dessous).

Et après ?

Il faut garder en tête que la simulation est issue de modèles numériques. Plusieurs facteurs peuvent modifier la donne, comme par exemple la venue de pluies ou, plus vraisemblable, l’incapacité à stopper rapidement les fuites radioactives à Fukushima. Ce 23 mars, les réacteurs n°2 et 3 fuient toujours.

Fabien Goubet

http://www.larecherche.fr/content/actualite-technologie/article?id=29492

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