Researcher Offers a Technical Perspective on Plutonium in the Environment

Introduction

Plutonium is often inaccurately identified in the media as the "most toxic substance known to man." Indeed, the element and its compounds are hazardous, and measures must be employed to protect workers, the public, and the environment. Minimizing the potential for release of plutonium is a primary concern at TA-55. Plutonium already in the environment, however, receives comparatively little attention.

An accurate assessment of the hazard posed by environmental plutonium is difficult, but substantial advances in this assessment are reported. Areas of investigation include determining the amount of environmental plutonium, describing its distribution and migration, and evaluating its biological and health consequences. These topics are of increasing interest as a result of the expanding use of plutonium in mixed oxide fuels for power generation in other countries. This article attempts to summarize important aspects of the subject for readers of the Actinide Research Quarterly.

Quantity, Sources, and Distribution of Environmental Plutonium

Plutonium occurs naturally as a result of neutron capture and fission of uranium in pitch-blende ores. The resulting plutonium concentration, about 5 x10^-12 of Pu per gram of uranium, constitutes a negligible source of environmental plutonium.

Estimates place the total amount of man-made plutonium in the environment at 4.3 metric tons. This quantity results primarily from atmospheric testing of nuclear weapons and to lesser extents from reprocessing of nuclear fuel and destruction of thermoelectric generators from satellites reentering the atmosphere.

Figure 1: Plutonium in the environment takes a number of forms and can be distributed in a number of ways, depending on the particle size and the dispersal mechanism.

The total alpha activity of ²³⁹Pu from aboveground nuclear testing is estimated at 7400 tera (10¹²) Becquerels (TBq-see box) and accounts for about 3.3 metric tons of plutonium. The total alpha activity resulting from ²³⁸Pu in the environment is about 1200 TBq, or about 2 kilograms of plutonium. Approximately 92% of environmental Pu is attributed to atmospheric testing.

Relatively small amounts of plutonium result from accidents. About 15 kilograms (90 TBq) of plutonium was released from the reactor at Chernobyl. The contribution from military sources is even smaller. For example, the aircraft accident involving nuclear weapons near Thule, Greenland, in 1968 released about 0.9 TBq of alpha activity from plutonium.

The rate of plutonium deposition in the environment has varied substantially over the past fifty years. The largest rates were during the period of atmospheric testing in the 1950s and early 1960s. Releases from reprocessing facilities reached a maximum estimated rate of 70 TBq per year during the mid 1970s, but are currently at about 0.1 TBq per year as a result of improved facilities and procedures. A major concern for reprocessing and storage facilities is the potential catastrophic loss of contain-ment and a high, localized release of material.

Global distribution of plutonium is inhomogeneous. Concentrations are high at mid-latitude zones of each hemisphere and are highest in the northern hemisphere, where most atmospheric nuclear tests were conducted. Maximum ²³⁹Pu activities (70 Bq-80 Bq per square meter ) appear at 35°-45°- north latitude. Activities of the isotope are about 15 Bq per square meter at mid latitudes of the southern hemisphere and are 1 Bq-10 Bq per square meter near the equator and poles.

Behavior of Environmental Plutonium

Studies show that Pu exists primarily as an oxide in land deposits and in ocean sediments. Behavior in the environment is strongly dependent on the physical and chemical conditions of both the material and the medium. Important properties of the oxide are particle size and solubility. Plutonium appears in water as ionic species of Pu(IV) and Pu(VI).

When plutonium is released into the atmosphere, its behavior depends on the particle size and the dispersal mechanism. Stratospheric aerosols formed by nuclear testing and satellite burnup distribute globally over a period of years; material released by accidents typically deposit locally within minutes or hours. Airborne redistribution of potentially dispersible particles with geometric diameters less than 10 µm is unlikely because such small particles readily adhere to surfaces of large soil particles.

Processes for translocation and redistribution of environmental plutonium are both mechanical and chemical. Vertical transport in soil is slow compared to lateral redistribution by processes such as cultivation and erosion by wind and water currents. Other mobilization mechanisms, including biological transport, depend on the solubility of the plutonium in water. In water, distribution constants, that is the fraction of the total Pu dissolved, are in the 10^-4 -10^-5 range, showing that plutonium is an insoluble solid with a solubility similar to that of glass (SiO₂).

Chemical uptake by biological systems occurs via several pathways. The plutonium fraction transferred by root uptake of plants ranges from 10^-3 to 10^-5. Particle inhalation by grazing animals is of negligible concern compared to their gastrointestinal uptake. The fraction of ingested plutonium absorbed is approximately 10^-4, and the combination of the plant uptake and animal ingestion from plant sources shows that the fraction of deposited Pu translocated to herbivores is 10^-7 to 10^-9. Behavior in marine and fresh water systems is similar; however, Pu concentrations in edible species (e.g., fish and crustaceans) in seawater vary from 30 to 3000 times that of edible species in fresh water.

Human Effects of Environmental Plutonium

As with animals, incorporation of plutonium by humans also occurs primarily by inhalation and ingestion. Inhalation is of concern only in instances of accidental release where local concentrations are high for a short time. Studies of human populations give uptake fractions of 10^-4 to 10^-6 for ingested Pu.

The effects of plutonium on human health and longevity are the primary concern. Biologically, Pu is classified as a radiotoxin. The risk of cancer death is estimated to increase by 0.2% (2 in 1,000) for a person who breathes highly contaminated air (0.1 µg of respirable oxide per cubic meter) for one hour. Homogeneous dispersal of one kilogram of oxide in a typical municipal water supply would result in a Pu concentration of about 1 nanogram of Pu per liter. The increased risk of cancer death for a person who drinks two liters of that water per day for seventy years is estimated to be 0.01% (1 in 10,000).

Relevant Studies

Although the mission of TA-55 does not include studies on environmental Pu, work being conducted under the Plutonium Repackaging Program is directly related to hazard assessments for incidents involving both facilities and weapons. Unlike Pu metal, PuO₂ is a powdered material with potential for environmental dispersal. Studies are underway to measure the size distributions for oxides from different sources and define the mass fractions of dispersible (< 10 µm geometric diameter) and respirable (< 3 µm geometric diameter) particles. Results show that the dispersible and respirable fractions of the oxide vary by a factor of about 10⁴ depending on the method of preparation. A credible assessment of the dispersal hazard is possible only if the oxide source is considered.

This article is based on "Plutonium in the Environment," LA-UR-96-1261, by John Haschke, in the Safety Series Document, "Safe Handling and Storage of Plutonium," Chapter 5, International Atomic Energy Agency, April 1996.

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