The temperature at the feed-water nozzle of the reactor pressure vessel continues to decrease (to 205 °C) at Unit 1, and at Unit 3 (42.8 °C), and it remains stable at Unit 2 (105 °C).  
 

In the case of human subjects, this involved injecting solutions containing (typically) five micrograms of plutonium into hospital patients thought to be either terminally ill, or to have a life expectancy of less than ten years either due to age or chronic disease condition.[68] This was reduced to one microgram in July 1945 after animal studies found that the way plutonium distributed itself in bones was more dangerous than radium.[69]
 
Eighteen human test subjects were injected with plutonium without informed consent. The tests were used to create diagnostic tools to determine the uptake of plutonium in the body in order to develop safety standards for working with plutonium.[68]
 
The episode is now considered to be a serious breach of medical ethics and of the Hippocratic Oath. More sympathetic commentators have noted that while it was definitely a breach in trust and ethics, "the effects of the plutonium injections were not as damaging to the subjects as the early news stories painted, nor were they so inconsequential as many scientists, then and now, believe."

Flammability
Metallic plutonium is a fire hazard, especially if the material is finely divided. In a moist environment, plutonium forms hydrides on its surface, which are pyrophoric and may ignite in air at room temperature. Plutonium expands up to 70% in volume as it oxidizes and thus may break its container.[99] The radioactivity of the burning material is an additional hazard. Magnesium oxide sand is probably the most effective material for extinguishing a plutonium fire. It cools the burning material, acting as a heat sink, and also blocks off oxygen. Special precautions are necessary to store or handle plutonium in any form; generally a dry inert gas atmosphere is required.

L'article rappelle que les sondes Voyager sont alimentées au Plutonium, qui fournit encore 300W de puissance 34 ans après leur lancement.

ar Chizy Nomiyama et Shinichi Saoshiro
 
TOKYO (Reuters) - Les techniciens japonais s'efforçaient samedi d'évacuer les flaques d'eau radioactive qui ont irradié trois des leurs et retardé les travaux en vue de refroidir les réacteurs de la centrale nucléaire accidentée de Fukushima-Daiichi.
De l'eau radioactive a été décelée dans les bâtiments de trois des six réacteurs de la centrale, à 240 km au nord de Tokyo. Jeudi, trois techniciens ont souffert de brûlures après avoir été exposés dans le réacteur n°3 à un niveau de radioactivité 10.000 fois supérieur à celui mesuré d'ordinaire dans un réacteur.
 
"Il devient très important d'évacuer l'eau accumulée dans les unités abritant les turbines, avant que la radioactivité ne remonte", a déclaré un haut responsable de l'Agence de sûreté nucléaire japonaise, Hidehiko Nishiyama.
 
Le séisme de magnitude 9 et les vagues géantes du tsunami ont fait plus de 10.000 morts et 17.000 disparus le 11 mars. Malgré un tel bilan et des destructions très étendues, une bonne part de l'attention se tourne, depuis la catastrophe, vers le risque d'une fonte du coeur de réacteurs à Fukushima, qui pourrait avoir des conséquences catastrophiques.
 
Nishiyama avait déclaré tout d'abord que la forte radioactivité mesurée au réacteur n°3 signifiait que la cuve du réacteur était peut-être endommagée, mais il a dit par la suite que cela pouvait provenir d'opérations de dégagement de vapeur menées pour faire retomber la pression, ou d'une fuite d'eau dans les tuyaux ou les valves.
 
A Vienne, l'AIEA (Agence internationale de l'énergie atomique) a déclaré vendredi que la situation n'avait guère évolué au cours des dernières 24 heures à Fukushima.
 
"Certaines évolutions positives se poursuivent mais il reste des incertitudes, fortement préoccupantes", a dit Graham Andrew, de l'AIEA, en ajoutant que la forte radioactivité constatée venait peut-être de la vapeur.
Plus de 700 techniciens se relaient pour stabiliser les réacteurs de Fukushima-Daiichi et les travaux avancent pour relancer les pompes à eau qui permettront de refroidir les barres de combustible.
 
FORTE RADIOACTIVITÉ A 30 KM DE LA CENTRALE
 
Deux des six réacteurs ne présentent a priori plus de danger désormais mais les quatre autres ne sont pas encore stabilisés et émettent de temps à autre de la vapeur et de la fumée. Malgré tout, l'Agence de sûreté nucléaire nippone a déclaré samedi que la température et la pression s'étaient stabilisées dans l'ensemble des réacteurs.
Lorsque l'exploitant de la centrale, Tepco, avait de nouveau raccordé les réacteurs à des câbles électriques, à la fin de la semaine dernière, certains avaient pensé que la crise serait bientôt terminée. Mais deux semaines après le séisme, la persistance d'une forte radioactivité aux abords des réacteurs endommagés continue de ralentir les opérations.
 
A Three Mile Island en 1979 - le pire accident nucléaire survenu aux Etats-Unis - il n'avait fallu que quatre jours pour stabiliser le réacteur dont le coeur avait partiellement fondu. Nul n'avait été blessé et aucune radioactivité anormale n'avait été enregistrée en-dehors du réacteur touché.
A Tchernobyl en 1986, il avait fallu des semaines pour "stabiliser" ce qui restait du réacteur accidenté et des mois pour nettoyer le site des substances radioactives et recouvrir le réacteur détruit d'un sarcophage d'acier et de béton.
 
A Tokyo, samedi matin, une radioactivité de 0,22 milliSievert par heure était enregistrée, soit six fois la normale dans cette ville de 13 millions d'habitants. Un tel degré de radioactivité ne présente cependant rien d'inquiétant pour la santé des populations.
En revanche, un fonctionnaire du ministère des Sciences a confirmé que le niveau de radioactivité quotidienne dans une zone située à 30 km au nord-ouest de la centrale de Fukushima avait dépassé la dose d'une année de radiation naturelle.
Dans le nord du Japon, plus de 250.000 personnes vivent toujours dans des centres d'hébergement. Les secouristes, exténués, continuent de fouiller les décombres des villes et villages, retrouvant ici et là des corps. L'aide afflue aux sinistrés et le téléphone, l'électricité, les services bancaires ont recommencé à fonctionner, bien qu'irrégulièrement.
 
Avec Kazunori Takada, Kiyoshi Takenaka, Phil Smith à Tokyo, Jon Herskovitz à Kamaishi, Eric Faye pour le service français

To the Editor:

The detection of radioactive iodine 131 in Tokyo’s drinking water (“Anxiety Up as Tokyo Issues Warning on Its Tap Water,” front page, March 24), in amounts considered unhealthy for children, makes clear that potassium iodide must be administered if children are to be adequately protected against thyroid cancer caused by ingested and inhaled iodine 131. Interdiction of milk supplies, though important, is plainly insufficient.

Japan’s apparent preparedness with potassium iodide contrasts with the situation in the United States. In response to 9/11, Congress passed a law to create stockpiles of potassium iodide for populations within a 20-mile radius of nuclear reactors, rather than the 10-mile radius within which the Nuclear Regulatory Commission offers it to states that request it.

But the N.R.C., which had opposed the law, fought successfully to keep it from taking effect. In 2008 President George W. Bush’s science adviser, John H. Marburger III, declared that potassium iodide was not needed beyond the 10-mile radius, and that the law therefore would not be implemented.

The events in Japan demand that the Obama administration act quickly to reverse this unjustified rejection of a sensible law.

Peter Crane
Seattle, March 24, 2011

It Could Happen Here
By FRANK N. VON HIPPEL
Published: March 23, 2011

IT will be years before we know the full consequences of the disaster at the Fukushima Daiichi nuclear power plant in Japan. But the public attention raised by the problems there provides an opportunity to rethink nuclear-power policy in the United States and the rest of the world — and reduce the dangers of a similar disaster happening elsewhere.

From one perspective, nuclear power has been remarkably safe. The 1986 Chernobyl accident will ultimately kill about 10,000 people, mostly from cancer. Coal plants are much deadlier: the fine-particulate air pollution they produce kills about 10,000 people each year in the United States alone.

Of course, for most people this kind of accounting is beside the point. Their horror over even the possibility of a meltdown means that the nuclear-power industry needs constant and aggressive regulation for the public to allow it to stay in business.

Yet despite the 1979 accident at the Three Mile Island nuclear plant in Pennsylvania, the Nuclear Regulatory Commission has often been too timid in ensuring that America’s 104 commercial reactors are operated safely. Nuclear power is a textbook example of the problem of “regulatory capture” — in which an industry gains control of an agency meant to regulate it. Regulatory capture can be countered only by vigorous public scrutiny and Congressional oversight, but in the 32 years since Three Mile Island, interest in nuclear regulation has declined precipitously.

In 2002, after the commission retreated from demanding an early inspection of a reactor, Davis-Besse in Ohio, that it suspected was operating in a dangerous condition, its own inspector general concluded that it “appears to have informally established an unreasonably high burden of requiring absolute proof of a safety problem, versus lack of a reasonable assurance of maintaining public health and safety.”

Even before Three Mile Island, a group of nuclear engineers had proposed that filtered vents be attached to buildings around reactors, which are intended to contain the gases released from overheated fuel. If the pressure inside these containment buildings increased dangerously — as has happened repeatedly at Fukushima — the vents would release these gases after the filters greatly reduced their radioactivity.

France and Germany installed such filters in their plants, but the Nuclear Regulatory Commission declined to require them. Given the influence of America’s example, had the commission demanded the addition of filtered vents, they would likely have been required worldwide, including in Japan.

More recently, independent analysts have argued, based on risk analyses done for the commission, it is dangerous for the United States to pack five times more spent fuel into reactor cooling pools than they were designed to hold, and that 80 percent of that spent fuel is cool enough to be stored safely elsewhere. It would also be more expensive, however, and the Nuclear Regulatory Commission followed the nuclear utilities’ lead and rejected the proposal.The commission has even fought relentlessly for decades against proposals — and more recently a Congressional requirement — to distribute potassium iodide pills beyond the 10-mile emergency zones around American reactors, arguing that the probability of a large release of radioactivity was too low to justify the expense. And yet the American Embassy in Tokyo is handing out potassium iodide pills to Americans 140 miles from the Fukushima plant.

The commission’s defenders often argue that it must be cautious because increased costs from safety requirements could kill the nuclear power industry. But the cost of generating electricity from existing plants is actually low: the construction expenses have been paid off and running them is relatively cheap. Requiring the operators of plants to install new safety systems would not result in them being shut down.

Therefore, perhaps the most important thing to do in light of the Fukushima disaster is to change the industry-regulator relationship. It has become customary for administrations not to nominate, and the Senate not to confirm, commissioners whom the industry regards as “anti-nuclear” — which includes anyone who has expressed any criticism whatsoever of industry practices. The commission has an excellent staff; what it needs is more aggressive political leadership.

Fukushima also shows why we need to develop reactors that are more inherently safe. Almost all the world’s power reactors, including those at Fukushima, are descended from the much smaller reactors developed in the 1950s by the United States for submarines. As we saw in the Fukushima accident, they depend on pumps to keep them from catastrophic failure, a major weak point. New designs less dependent on pumps have been developed, but there has not yet been enough research to make certain that they would work effectively.

One promising design is the high-temperature gas-cooled graphite reactor; its fuel is in the form of small particles surrounded by layers of material that could contain their radioactivity if a cooling system fails. The United States built two such prototypes in the 1960s, and Germany built one in the 1980s. With the virtual end of new reactor orders in the United States and Western Europe, as well as their small generating capacity compared to current water-cooled reactors, they were not pursued further.

China, however, which accounted for over 60 percent of the world’s nuclear power plant construction during the past five years, is now planning two prototypes and, if these work, 36 more. Such a demonstration could help determine the commercial viability of gas-cooled graphite reactors worldwide, and the Department of Energy should offer the expertise of its national laboratories to help China make this effort a success.

ANOTHER area that requires review is unrelated to the Fukushima accident, but would benefit from some of the attention generated by the crisis — namely, the need to strengthen the barriers to misuse of nuclear-energy technology to develop nuclear weapons.

The unintended effect of much of governmental research and development has been to make nuclear proliferation easier. Most notably, over the past 50 years the developed world has spent some $100 billion in a failed effort to commercialize plutonium breeder reactors. Such reactors would use uranium more efficiently, but would also require the separation of plutonium, a key component in nuclear weapons.

Even though plutonium breeder reactors have yet to make it past the research and development phase into commercial production, enough plutonium has been separated from spent power-reactor fuel to make tens of thousands of nuclear weapons, creating an enormous security risk. The technology’s spread raises the possibility that it could be diverted to military purposes. In fact, this has already happened: in 1974 India tested a nuclear weapon design using plutonium that had been separated out for its breeder reactor program.

Meanwhile, General Electric has applied for a license to build a plant that would use lasers to enrich uranium for commercial use, which could provide yet another way to produce weapons-grade material. A coalition led by the American Physical Society, a professional organization of physicists, has petitioned the Nuclear Regulatory Commission to assess the risk that this technology poses to non-proliferation efforts before it issues a license. The commission, predictably, has been reluctant to do so.

It is critical to find more effective ways to control such dangerous nuclear technologies. In 1946, the United States proposed that uranium enrichment and plutonium be put under international control, a proposal that failed because of the onset of the cold war.

More recently Mohamed ElBaradei, the former director general of the International Atomic Energy Agency, made the more modest proposal to place such dangerous activities under merely multinational control, which would make it more difficult for any one country to divert the material to military ends. In fact, Urenco, the West’s most successful uranium enrichment enterprise, is already under the joint ownership of Germany, the Netherlands and Britain.

The United States should help shape this industrial model into an international one, in which all enrichment plants are under multinational control. Doing so would make it more difficult for countries like Iran to justify building national enrichment plants that could be used to produce nuclear weapons materials.

While new plants are unlikely to be built in the United States over the next 25 years, nuclear power provides 20 percent of our electrical power and is climate friendly. We therefore must make existing reactors safer, develop a new generation of safer designs and prevent nuclear power from facilitating nuclear proliferation. As tragic as the Fukushima disaster has been, it has provided a rare opportunity to advance those goals.

Frank N. von Hippel, a nuclear physicist, is a professor of public and international affairs at Princeton and co-chairman of the International Panel on Fissile Materials. From 1993 to 1994 he was responsible for national security issues in the White House Office of Science and Technology Policy.

According to the government's Nuclear and Industrial Safety Agency, radioactive iodine-131 at a concentration 1,250.8 times the legal limit was detected Friday morning in a seawater sample taken around 330 meters south of the plant, near the drain outlets of its troubled four reactors.