Updated: 31 Mar 2011

In case you’re getting spooked by the “new” news of plutonium traces (updates by MIT NSE Nuclear & Info Hub here) found on five locations around the Fukushima nuclear plant, and there is bound to be more spills from the press on this ahead  … you might like to have a heads up by reading the following series of linked articles on what is plutonium, and the risks to human health and more:

From Plutonium and Health: “No humans have ever died from acute toxicity due to plutonium uptake. … Although dangerous, plutonium is not “the most toxic substance known to man.” On a weight-by-weight basis, plutonium is less toxic than the unforgiving bacterial toxins that cause botulism, tetanus, and anthrax. And yet, plutonium’s position is frighteningly high on the lethal ladder. A few millionths of a gram (or a few micrograms) distributed through the lungs, liver, or bones may increase the risk for developing cancer in those organs. Airborne, soluble chemical compounds of plutonium are considered so dangerous by the Department of Energy(DOE) that the maximum permissible occupational concentration in air is an infinitesimal 32 trillionths of a gramper cubic meter! By comparison, the national standard for air concentrations of inorganic lead is 50 millionths of a gram per cubic meter, which suggests that inorganic lead is a million times less dangerous by weight than plutonium.”

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Plutonium in the Environment (By the MIT NSE Nuclear Information Hub)

Much recent news and many questions directed at the blog have centered around the detection of plutonium in soil and water surrounding the Fukushima reactors. This post will outline our current knowledge of the situation, as well as potential impacts on the environment and on human health.

Measurements to date

On March 21 and 22, five soil samples around the site indicated the presence of isotopes of plutonium. Two of these samples contained the isotope Pu-238. Typically, a large ratio of Pu-238 to the other isotopes indicates that the material has been produced in a reactor. This is because the production of Pu-238 requires one of these processes to take place:

  • Successive neutron captures in U-235 produce U-237. U-237 decays to Np-237, with a half-life of 6.75 days. Np-238 captures another neutron and decays to Pu-238.
  • A fast moving neutron causes a Pu-239 nucleus to eject an additional neutron.

This second reaction can take place during the detonation of a nuclear weapon, but is rare. The first is next to impossible in a nuclear detonation, as the weapon blows itself apart before the necessary series of captures and decays can occur. This is why the detection of Pu-238 at two sites indicates that material at those sites came from a reactor. There is not sufficient information at this time to determine whether the material originated from the MOX-fueled unit 3, or from one of the other cores.

Because Pu-238 was not detected at the other three sites, it is thought that the plutonium at those sites is a remnant of past nuclear weapons tests. For reference, the natural rate of plutonium decay in Fukushima City is 0.61 Bq/kg, or 0.61 decays per second in each kilogram of soil. The quantities being measured on the reactor site are at roughly double this same level, according to TEPCO.

Transport Pathways

As stated, the pathway which was taken by the plutonium to the soil of the reactor site is not clear at the present. However, it is generally transported via one of two pathways:

  • Adhesion to particulate matter, like smoke.
  • Solution or suspension in water.

Whichever route led to the disposition of plutonium on the reactor site, it would be difficult for such plutonium to be transported over great distances. Its high mass means that it is not easily aerosolized, even by fire. Mention has been made of the fact that plutonium will, under the right conditions, burn. However, this burning occurs during plutonium metal’s conversion to plutonium oxide. As the plutonium within each reactor is already in an oxide form, it has no such tendency to burn. Finally, plutonium is not very water-soluble. Under optimal conditions, the solubility of plutonium metal in water is around 55 microgram/L. The solubility of plutonium oxide is even lower.

Could plutonium be transported away from the reactor site, under the current conditions? Potentially, yes. However, it would likely be in minute quantities that have no impact on human health.

Impact of Plutonium on Human Health

As a radiation hazard, plutonium is a danger when ingested or inhaled. This is because it’s an alpha emitter. Alpha particles, while they can be stopped by the skin or a sheet of paper, can severely injure very delicate structures of the body, such as the alveoli in the lungs, or the lining of the gastrointestinal tract. Plutonium is a bone-seeker, but is not efficiently absorbed by the body because of its low solubility in the body’s fluids. The vast majority of ingested plutonium (greater than 99%) is excreted within a week of ingestion. Between 5% and 60% (estimates by different agencies vary) of inhaled plutonium stays within the body, with the rest being exhaled immediately.

In addition, plutonium is chemically toxic like other heavy metals. A number of estimates have been circulated regarding how much plutonium is fatal to humans, many of which have no evidence to support them. Experiments using lab rats have indicated that 50% of those rats die within a month after injection of 700-1000 micrograms of plutonium per kilogram of body weight. This would translate to 47.7-68.2 mg of plutonium injected into a 150-pound person. Since the efficiency of plutonium uptake by inhalation or ingestion is low, the dose needed to actually cause illness or death would be correspondingly higher.

It’s unknown whether the results of experiments on rats translate directly to human exposures. No human has ever died from acute uptake of plutonium. Our information on the health effects of plutonium on humans is derived from the case studies of plutonium workers, who sustain very low doses over a period of decades; a series of studies on chronically ill patients; and the histories of atomic bomb survivors, whose doses are confounded by exposures to a whole host of other radioactive isotopes.

Sources: Journal of Radiological Protection, Los Alamos National Laboratory, TEPCO,  American Chemical Society.

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Wednesday, March 30, 2011 Japan Times
ANALYSIS
By JUN HONGO
Staff writer
Revelations that low amounts of plutonium, a component of nuclear bombs, were detected in soil near the Fukushima No. 1 plant sent shock waves across the nation Tuesday.
But experts say despite plutonium’s dangers and the mounting fears, there is little risk of the deadly radioactive particles spreading to a wider area.
The plutonium leak “will have no impact on the surrounding residential areas,” Hironobu Unesaki, a professor at the Kyoto University Research Reactor Institute, told The Japan Times. The nuclear engineering expert added the continued dispersal of radioactive iodine and cesium is much more worrisome than the plutonium leaks.
Tokyo Electric Power Co. announced the plutonium find late Monday during a hastily arranged news conference. Officials said they found plutonium-238, -239 and -240 during a study conducted a week ago in and around the power plant.
A few hundred grams of soil were taken from five locations between 500 meters and 1 km from reactors No. 1 and No. 2, they said. Although traces of plutonium were found, Tepco stressed the contamination levels pose no health hazard.
Detected so far are levels of radioactive decay ranging between 0.18 and 0.54 becquerel per kilogram of soil — about the same amount observed in Japan after the nuclear tests carried out in the Pacific in the 1950s and 1960s.
“This does not pose any (threat) to human health,” an official of the Nuclear Industrial Safety Agency in Tokyo said Monday. But he also acknowledged that because plutonium is produced within the central parts of reactors, the five levels of the containment mechanism — designed to be airtight — have been breached since the tsunami knocked out the power plant’s electricity supply.
Tepco officials claim the plutonium leak is small, but people were still alarmed.
This is because plutonium-239 has a half-life of 24,000 years, unlike the eight days for iodine-131 and 30 years for cesium-137, the two major radioactive substances that have contaminated vegetables and tap water as far away as Tokyo.
In addition to the long-lasting risks plutonium poses, the radioactivity it emits is in alpha particles, compared with the beta particles from iodine-131. Alpha particles are known to pose a greater risk to health.
For example, in data compiled by the Japan Atomic Energy Agency, 21 mg of potassium cyanide is considered a lethal dose to an average male weighing 70 kg. When it comes to plutonium, 13 mg is enough to kill.
The U.S. Nuclear Regulatory Commission explains on its website that if one drinks or eats plutonium oxide, most of it will pass through without the body absorbing it. But if inhaled, usually between 20 and 60 percent is retained in the lungs. Plutonium entering from an open wound may also move directly into body parts and organs, the commission said.
Even Tepco is aware of the dangers and talked of the hazards related to plutonium in its 2010 nuclear power pamphlet, noting the substance “can cause cancer once retained in liver and bones.”
But one crucial characteristic limiting the spread of plutonium is that it weighs 20 times more than water and 2.5 times more than iron.
Based on such data, Chief Cabinet Secretary Yukio Edano assured reporters at a news conference Tuesday that plutonium will not become airborne and therefore spread to a wider region.
While Kyoto University’s Unesaki said the area found to have traces of plutonium will require appropriate management, he agreed that the heavyweight particles will not spread over a wide area.
“Even in the case of Chernobyl, in which traces of iodine and cesium were found thousands of kilometers away and all over Europe, plutonium was found only within a radius of 30 km from the nuclear power plant,” Unesaki explained.
The spread of plutonium at Chernobyl was due in part to the reactors’ use of solid graphite as neutron moderators, which started a ferocious fire and a strong updraft. The reactors in Fukushima use light-water as moderators.
For such reasons, even if the reactors in Fukushima were to experience a hydrogen explosion that completely obliterates all safety measures — which is impossible from an engineering point of view, according to Unesaki — the spread of iodine and cesium will do much more damage than plutonium.
So far all parties, including Tepco, the government and the nuclear safety agency, have been unable to find the plutonium leak. Possibilities include reactors No. 1, 2 and 3, which were in operation when the earthquake hit, and any of the fuel rod pools adjacent to all six reactors.
The leak may also have come from reactor No. 3, which uses MOX fuel, which is known to contain weapons-grade plutonium. Thirty-two of the 548 fuel elements in reactor No. 3 use the mixed plutonium and uranium oxide fuel, according to Tepco.
Some people expressed strong concern over using such highly toxic fuel at the aging Fukushima plant. But Tepco opted to go forward, concluding it was a more efficient way of using limited resources. Tepco began producing energy from MOX fuel last October.
But Kyoto University’s Unesaki advised that instead of fearing an unlikely catastrophe, Tepco and others should keep their focus on the task at hand.
“It’s hard to believe that conditions of the nuclear reactors will abruptly deteriorate at this point. Instead of fearing an explosive outbreak of radioactive particles, there should be more focus on not allowing the ongoing leaks to continue for an extended period of time,” he said

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Here is a Summary on Plutonium in Nuclear Fuel by the MIT Nuclear Information Hub

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From A Technical Paper Rocky Flats Public Exposure Studies on Plutonium (excerpts):

What are the problems with handling plutonium?

Plutonium metal is difficult to handle and store safely, because it is radioactive and “pyrophoric” meaning it oxidizes and can become very hot when exposed to air. It can ignite nearby flammable materials, causing fires that can result in plutonium exposure of workers and the public. In addition, workers must avoid storing more than a few pounds in close proximity to prevent runaway fission, an uncontrolled nuclear chain reaction. Such a reaction’s burst of energy, known as a “criticality event,” would not become a nuclear explosion, but could release radiation very dangerous to nearby workers. Such an event can also result in uncontrolled releases of both plutonium and fission products to the environment.

How are people exposed to plutonium?

Members of the public living or working near nuclear weapons production plants can be exposed when equipment fails, accidents happen or mistakes are made, causing releases that move off the plant site through the air or in water. Plutonium particles in the air can deposit on the soil, where adults or children may work or play, or on water, which may be a source for drinking, irrigation of crops or recreation. Particles can also deposit on vegetables or on grass eaten by cows that may later provide milk and meat for human consumption.

Why is plutonium a human concern?

Plutonium emits alpha radiation and low-energy x-rays, which are easily absorbed by tissue. The alpha radiation travels only about a quarter of an inch in air and cannot penetrate the skin. Therefore, if plutonium remains outside the body, it is generally not harmful. Plutonium is very toxic if it enters into the body because the alpha radiation can damage living tissue.

The larger the “dose” in the body, the greater the toxicity.

Human exposure occurs mainly by breathing contaminated air or ingesting contaminated food or drink. Breathing is generally the route of most concern. When plutonium particles are inhaled and lodge in lung tissue, they continue to give off radiation internally. They can remain in the lungs or enter the gastrointestinal tract and the bloodstream. About 80 percent of the plutonium that enters the bloodstream goes either to the liver, bone or bone marrow, where it is retained for years, damaging tissue nearby. That damage may later develop into cancer. Common forms of plutonium do not dissolve significantly in water or body fluids, so little ingested material is actually absorbed into the blood from the gastrointestinal tract. (Airborne) Plutonium particles can deposit on plants, but are not readily absorbed by the roots into the plants. Plutonium is usually insoluble in water, so plutonium particles that land on lakes and streams usually settle to the bottom in the sediment.

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Plutonium health risks may not be as bad as iodine | Japan reactor may be leaking plutonium, but it doesn’t travel very far, experts say

By Linda Carroll (msnbc.com 3/28/2011)

Each day it seems there’s more scary news from the stricken Japanese reactor. Monday brought the dreaded “p”-word: measurements from the reactor site turned up signs of plutonium leakage.

But experts say that plutonium isn’t the most dangerous of the isotopes seeping out of the reactor. Over the short term, radioactive iodine may be the most worrisome, they say.Whenever there’s a discussion about the dangers of nuclear power, people always bring up plutonium because it sticks around for a long time — hundreds of years, in fact. That sounds a lot scarier than the 30-year half life of cesium and the 8-day half life of radioactive iodine.“In reactor accidents iodine-131 comprises the vast majority of radioactive material,” said Yuri Nikiforov, a professor of pathology at the University of Pittsburgh Medical Center. “It’s easy to inhale and it can get into the soil and it can contaminate vegetables and milk.”

Radioactive iodine can blow the farthest

Because it’s the lightest of the radioactive elements spewing out of a damaged reactor, it can blow the farthest and therefore affect the most people, Nikiforov adds. It tends to accumulate in the thyroid, which is why radioactive iodine exposure can increase the risk of thyroid cancer.

Still, Nikiforov says, radioactive iodine is the shortest lived of the radioisotopes seeping from a reactor, so if you can avoid eating contaminated food for a few weeks, you’ll be fine.

Plutonium sticks around for a lot longer, but it’s very heavy so it’s not going to blow far from the reactor site. You could possibly develop lung cancer from inhaling large quantities of plutonium, says Andrew Maidment, an associate professor of radiology and chief of physics and radiology at the University of Pennsylvania.

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To find out more about plutonium, how it gets into the environment and the body and the health risks of exposure to plutonium, read the US EPA’s primer on Plutonium and Health Effects of Plutonium

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See also from Associated Content Japan Detects Plutonium Outside Fukushima; Asessing the Risks by Carol Bengle Gilbert (28 Mar, 2011)

Kyodo news agency is reporting breaking news that plutonium was detected in soil samples outside Japan’s Fukushima Daiichi plant. Tokyo Electric Power Company said March 28 that its own tests did not show plutonium in the soil but sent off samples for independent testing. It’s that independent testing by Japan’s Atomic Energy Agency and the Japan Chemical Analysis Center that now shows plutonium in soil samples collected from five different locations March 21 and 22.

Banners displaying breaking news updates on Kyodo’s website indicate that officials don’t know which reactor is the source of the radiation leak. The leaking of plutonium indicates presumptive fuel rod damage. Reactor 3 is the only reactor using a plutonium fuel mix called MOX; however, uranium fission creates plutonium as a byproduct, so the other reactors could also be the culprits.

What are the signs to look for in assessing the seriousness of Japan’s plutonium contamination as this breaking news develops?

Information shared on University of California at Berkeley‘s Nuclear Engineering Department public forum included Russian guidelines for plutonium exposure. These guidelines set out health risks from different exposures.

Health Consequences of Plutonium Exposure

The Russians have published this information on the health consequences of varying levels of plutonium exposure:

* 370 kBq – fatalities during the first year after accident result from acute interstitial pneumonitis;

* 37 kBq…370 kBq – serious deterministic effects including disability caused by pneumosclerosis and high risk of lung cancer;

* 3.7 kBq….37 kBq – long-term lung pathologies;

* < 3.7 kBq – does not lead to reliable defined consequences.

Response to Plutonium Soil Contamination: Russia and USA

This is what the Russian decision-making criteria say about soil contamination with plutonium 239:

If soil contamination with plutonium reaches a density of .2 to .9 MBq m-2, permanent or temporary relocation is required. The document says that comparable U.S. guidelines call for permanent or temporary relocation at .22 MBq m-2.

Sheltering is called for if concentrations reach .008 to .09, whereas U.S. sheltering is triggered by a level of 2.2.

Russians take urgent protective actions when levels are between 10 and 70, whereas the United States mandates such actions when the concentration hits 22 MBq m-2.

As the situation develops, Japan will need to decide whether precautions are necessary to protect residents from plutonium exposure.

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Excerpt from “Plutonium and Health: How Great is the Risk?” by George L. Voelz

“Robert Stone, the head of the Plutonium Project Health Division at the Metallurgical Laboratory (Met Lab)in Chicago, made the earliest estimate of a permissible plutonium body burden—the total amount of plutonium that can be present in the body over a lifetime without causing ill effects—by scaling the radium standard on the basis of the radiological differences betweenradium and plutonium. Those included differences in their radioactivities and those of their daughter nuclei and the difference in the average energy of their alpha particles. Results indicated that, gram for gram, plutonium was less toxic than radium by a factor of 50, and the permissible body burden was therefore set to 5 micrograms, or 0.3 microcurie.9In 1977, however, the International Commissionon Radiation Protection (ICRP) described a new radiation-protection concept (ICRP 26, 1977) based on plutonium dose rather than plutonium deposition. The guideline for a maximum occupational dose is based on acalculated effective whole-body doseequivalent, and because it uses weighting factors, it does take into account organ doses. The overall guideline is that the maximum occupational plutonium dose is not to exceed an effective whole-body dose equivalent of 0.05 sievert, or 5 rem, annually from all types of occupationalradiation exposure—internal and external (see the box “Units of Radiation Dose”). Published between 1979 and 1988, a series of reports known collectively as ICRP 30 contains the derived annual limits of radionuclide intake for the protection of workers.

The ICRP protection concept requires calculation of organ doses. For plutonium, these doses are uncertain because the internal distribution of plutoniumvaries greatly from one case to the next and the microdistribution of dose within organs is poorly understood. Therefore, the effective dose equivalent for plutonium is calculated with standard models, as recommended byICRP 30 (see the box “Models Predicting Risk of Carcinogenesis”). This revised worker-protection guidance wasplaced into effect for DOE facilities at the beginning of 1989. In the spring of 1991, the ICRP published new recommendations(ICRP 60) according to which the occupational exposure limit will be reduced to 0.02 sievert, or 2 rem, per year, which includes external and internal radiation doses. So far, the United States has not adopted this latest recommendation.”

“No humans have ever died from acute toxicity due to plutonium uptake. …

As a precaution in setting radiation standards, the International Commission on Radiation Protection (ICRP) assumes that some risk may be involved in any exposure. Although dangerous, plutonium is not “the most toxic substance known to man.” On a weight-by-weight basis, plutonium is less toxic than the unforgiving bacterial toxins that cause botulism, tetanus, and anthrax. And yet, plutonium’s position is frighteningly high on the lethal ladder. A few millionths of a gram (or a few micrograms) distributed through the lungs, liver, or bones may increase the risk for developing cancer in those organs. Airborne, soluble chemical compounds of plutonium are considered so dangerous by the Department of Energy(DOE) that the maximum permissible occupational concentration in air is an infinitesimal 32 trillionths of a gramper cubic meter! By comparison, the national standard for air concentrations of inorganic lead is 50 millionths of a gram per cubic meter, which suggests that inorganic lead is a million times less dangerous by weight than plutonium.”