Plutonium in the environment

Plutonium in the environment is an article which is part of the actinides in the environment series. Almost all plutonium present in the environment is due to human activity because the majoity of plutonium isotopes are shortlived on a geological timescale.^[1] It has been argued that some natural plutonium (the very long lived ²⁴⁴Pu isotope) can be found in nature.^[2]. This isotope has been found in lunar soil,^[3], meteorites,^[4] and in the Oklo natural reactor.^[5] But in general it is normally considered that the bulk of all plutonium is man made. According to one paper on marine sediments for plutonium in marine sediments bomb fall out is responsible for the majority of the ²³⁹Pu and ²⁴⁰Pu (66% and 59% respectively of that found in the english channel) while nuclear reprocessing is responsible for the majority of the ²³⁸Pu and ²⁴¹Pu present in the sea (bomb tests are only responsible for 6.5 and 16.5% of these isotopes respectively).^[6]

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Bomb detonations

About 3.5 tons of plutonium have been released into the environment by atomic bomb tests. While this might sound like a large amount it has only resulted in a very small dose to the majority of the humans on the earth. Overall the health effects of the fission products are far greater than the effects of the actinides released by a nuclear bomb detonation. The plutonium from the Pu fuel of the bomb is converted into a high fired oxide which is carried high into the air. It slowly falls to earth as global fallout and is not soluble, hence as a result it is difficult for this plutonium to be incorporated into an animal if taken by mouth. Much of this plutonium will become tightly absorbed onto sediments of lakes, rivers and oceans. However, about 66% of the plutonium from a bomb explosion is formed by the neutron capture of uranium-238; this plutonium is not converted by the bomb into a high fired oxide as it is formed more slowly. As a result this formed plutonium is more soluble and more able to cause harm when it falls to earth.^[7]

Some of the plutonium can be deposited close to the point of detonation. The glassy trinitite formed by the first atom bomb has been examined to determine what actinides and other radioisotopes it contained. A recent paper^[8] reports the levels of long lived radioisotopes in the trinitite. The trinitite was formed from feldspar and quartz which were melted by the heat. Two samples of trinitite were used, the first (left hand side bars) was taken from between 40 and 65 meters of ground zero while the other sample was taken from further away from the ground zero point.

The ¹⁵²Eu and ¹⁵⁴Eu was mainly formed by the neutron activation of the europium in the soil, it is clear that the level of radioactivity for these isotopes is highest where the neutron dose to the soil was larger. Some of the ⁶⁰Co is generated by activation of the cobalt in the soil, but some was also generated by the activation of the cobalt in the steel (100 foot) tower. This ⁶⁰Co from the tower would have been scattered over the site reducing the difference in the soil levels.

The ¹³³Ba and ²⁴¹Am are due to the neutron activation of barium and plutonium inside the bomb. The barium was present in the form of the nitrate in the chemical explosives used while the plutonium was the fissile fuel used.

It is interesting to note that the ¹³⁷Cs level is higher in the sample which was further away from the ground zero point. This is thought to be because the precursors to the ¹³⁷Cs (¹³⁷I and ¹³⁷Xe) and the cesium to a lesser degree are volatile. The natural radioisotopes in the glass are about the same in both locations.

In this paper a sample of the glass was digested and the plutonium extracted from it, and the mass ratio of the isotopes was calculated from the radiometric measurements. In light green the isotopic signature for the plutonium used for making the bomb is shown, and on the right in dark green the signature of the plutonium in the trinitite glass is shown. It is very clear that ²³⁸Pu and ²⁴¹Pu were generated during the detonation, so it is reasonable to conclude that some ²⁴⁰Pu was formed during the detonation.

As the ²³⁹Pu/²⁴⁰Pu ratio only changed slightly during the detonation, it has been commented^[9] that this isotope ratio for the majority of atom bombs (In Japan the ²³⁹Pu/²⁴⁰Pu ratio in soil is normally in the range 0.17 to 0.19)^[10] is very different than from the bomb dropped upon Nagasaki, so the forest soil^[11] and the lake sediment layer containing the local fallout from the second world war bomb is very different from the layers due to global fallout from bomb tests conducted later.^[12]

Bomb safety trials

One form of release of plutonium into the environment has been safety trials in these experiments nuclear bombs have been subjected to simulated accidents or have been detonated with an abnormal initiation of the chemical explosives. An abnormal implosion will result in a compression of the pit which is less uniform and smaller than the designed compression in the device. Such an abnormal implosion could result from an accident which triggers one or more of the detonators which trigger the chemical explosive charges.

As a result of these experiments (where no or very little nuclear fission occurs) plutonium metal has been scattered around near the site of the experiment. While some of these tests have been done inside holes in the ground, other such tests were conducted in open air. A paper on the radioisotopes left on an island by the French nuclear bombs tests of the 20th century has been printed by the International Atomic Energy Agency and a section of this report deals with plutonium contamination resulting from such tests.[3]

Other related trials were conducted at Maralinga, South Australia here both normal bomb detonations and "safety trials" have been conducted. While the activity from the fission products has decayed away almost totally (as of 2006) the plutonium remains active. A report (warning it is very big) can be read at [4] while a smaller report can be seen at [5].

Atomic batteries

Space

Another potential source of plutonium being introduced into the environment is the reentry of artificial satellites containing atomic batteries. There have been several such incidents, the most prominent being the Apollo 13 mission. The Apollo Lunar Surface Experiment Package carried on the Lunar Module re-entered the atmosphere over the South Pacific. Many atomic batteries have been of the Radioisotope thermoelectric generator (RTG) type.

Chain reactions do not occur inside RTGs, so such a nuclear meltdown is impossible. In fact, some RTGs are designed so that fission does not occur at all; rather, forms of radioactive decay which cannot trigger other radioactive decays are used instead. As a result, the fuel in an RTG is consumed much more slowly and much less power is produced.

RTGs are still a potential source of radioactive contamination: if the container holding the fuel leaks, the radioactive material will contaminate the environment. The main concern is that if an accident were to occur during launch or a subsequent passage of a spacecraft close to Earth, harmful material could be released into the atmosphere. However, this event is extremely unlikely with current RTG cask designs.

In order to minimise the risk of the radioactive material being released, the fuel is typically stored in individual modular units with their own heat shielding. They are surrounded by a layer of iridium metal and encased in high-strength graphite blocks. These two materials are corrosion and heat-resistant. Surrounding the graphite blocks is an aeroshell, designed to protect the entire assembly against the heat of reentering the earth's atmosphere. The plutonium fuel is also stored in a ceramic form that is heat-resistant, minimising the risk of vaporization and aerosolization. The ceramic is also highly insoluble.

The US Department of Energy has conducted seawater tests and determined that the graphite casing, which was designed to withstand reentry, is stable and no release of plutonium should occur. Subsequent investigations have found no increase in the natural background radiation in the area. The Apollo 13 accident represents an extreme scenario due to the high re-entry velocities of the craft returning from cislunar space. This accident has served to validate the design of later-generation RTGs as highly safe.

The Plutonium-238 used in RTGs has a half-life of 88 years, as opposed to the plutonium-239 used in nuclear weapons and reactors, which has a half-life of 24,100 years.

Pacemakers

Some heart pacemakers which are powered by RTGs using ²³⁸Pu have been made.

Nuclear fuel cycle

Plutonium has been released into the environment in aqueous solution from nuclear reprocessing and uranium enrichment plants. The chemistry of this plutonium is different to that of the metal oxides formed from nuclear bomb detonations.

One example of a site (military not civil) where plutonium entered the soil is Rocky Flats where in the recent past XANES (a X-ray spectrscopy) has been used to determine the chemical nature of the plutonium in the soil.[6]. The XANES was used to determine the oxidation state of the plutonium, while EXAFS was used to investigate the structure of the plutonium compound present in the soil and concrete.[7]

Chernobyl

Because plutonium oxide is very involatile, most of the plutonium in the reactor was not released during the fire. However that which was released can be measured. V.I. Yoschenko et al. reported that grass and forest fires can make the cesium, strontium and plutonium become mobile in the air again. (Journal of Environmental Radioactivity, 2006, 86, 143-163.) As an experiment fires were set and the levels of the radioactivity in the air downwind of these fires was measured.

Nuclear crime

One case exists of a German man who attempted to poison his ex-wife with plutonium stolen from WAK (Wiederaufbereitungsanlage Karlsruhe). WAK was a small scale reprocessing plant where he worked. He did not steal a large amount of plutonium, just some rags used for wiping surfaces and a small amount of liquid waste. This man was sent to prison for his crime. [8] [9] At least two people (besides the criminal) were contaminated by the plutonium. [10]. Two flats in Rhineland-Palatinate were contaminated. These were later cleaned at a cost of two million euro.

For photographs of the case and details of other nuclear crimes see [11] which was presented by a worker at the ITU. A general over view of the forensic matters associated with plutonium exists.^[13]

The details of how the two flats in Landau were cleaned has been recorded [12]. In addition it has been claimed that a house in Reading, Berkshire) has been contaminated with plutonium.[13][14][15]

Environmental chemistry

Overview

Plutonium like other actinides readily forms a dioxide plutonyl core (PuO₂). In the environment, this plutonyl core readily complexes with carbonate as well as other oxygen moieties (OH^-, NO₂^-, NO₃^-, and SO₄^-2) to form charged complexes which can be readily mobile with low affinities to soil.

• PuO₂(CO₃)₁^-2
• PuO₂(CO₃)₂^-4
• PuO₂(CO₃)₃^-6

PuO₂ formed from neutralizing highly acidic nitric acid solutions tends to form polymeric PuO₂ which is resistant to complexation. Plutonium also readily shifts valences between the +3, +4, +5 and +6 states. It is common for some fraction of plutonium in solution to exist in all of these states in equilibrium.

Binding to soil

Plutonium is known to bind to soil particles very strongly, see above for a X-ray spectrscopic study of plutonium in soil and concrete. While cesium has very different chemistry to the actinides, it is well known that both cesium and many of the actinides bind strongly to the minerals in soil. Hence it has been possible to use ¹³⁴Cs labeled soil to study the migration of Pu and Cs is soils. It has been shown that colloidal transport processes control the migration of Cs (and will control the migration of Pu) in the soil at the Waste Isolation Pilot Plant according to R.D. Whicker and S.A. Ibrahim, Journal of Environmental Radioactivity, 2006, 88, 171-188.

Microbiological chemistry

Mary Neu (at Los Alamos in the USA) has done some work which suggests that bacteria can accumilate plutonium because the iron transport systems used by the bacteria also function as plutonium transport systems.[16][17][18]

Biology

Plutonium ingested by or injected into humans is transported in the transferrin based iron(III) transport system and then is stored in the liver in the iron store (ferritin), after an exposure to plutonium it is important to rapidly inject the subject with a chelating agent such as calcium complex[19] of DTPA[20] [21]. This antidote is useful for a single one off exposure such as that which would occur if a glove box worker was to cut their hand with a Pu contaminated object. The calcium complex has faster metal binding kinetics than the zinc complex but if the calcium complex is used for a long time it tends to remove important minerals from the person. The zinc complex is less able to cause these effects.

Plutonium that is inhaled by humans lodges in the lungs and is slowly translocated to the lymph nodes. Inhaled plutonium has been shown to lead to lung cancer in experimental animals.

References