The Toxicity of Plutonium, Americium and Curium: A Report Prepared Under Contract for the Commission of the European Communities Within Its Research and Development Programme on Plutonium Recycling in Light Water Reactors

By J. C. Nenot and J. W. Stather

The Toxicity of Plutonium, Americium and Curium provides a biological basis for an assessment of the radiological health problems resulting from human exposure to americium, curium, and plutonium. This book discusses the method of injection of the chelating agent DTPA for removing soluble forms of americium, curium, and plutonium. Organized into 10 chapters, this book begins with an overview of the biological effects in man that are attributed to exposure to actinides. This text then examines the uses of americium and curium in transmission scanning studies in tissues, in smoke detectors, and in neutron sources. Other chapters consider the metabolism or effects of plutonium in humans that can be employed to predict the potential consequences of human exposure. This book discusses as well routes of entry of radioactive materials into the body by inhalation, by ingestion, through cuts, or by absorption through the intact skin. The final chapter deals with the assessment of the radiological health problems resulting from the use of mixed oxide fuels of uranium and plutonium in light water reactors. This book is a valuable resource for scientists.

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 22, 2013
• ISBN: 9781483182100

Book preview

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Chapter 1

INTRODUCTION

Publisher Summary

This chapter describes the current knowledge on the metabolism and biological effects of actinides in animals and man, from which an assessment of radiological health problems can be made. To assess the consequences of human exposure to the actinides, firstly, it is necessary to identify their routes of entry into the body, secondly to understand the factors influencing their distribution and retention in tissues, thirdly to determine the tissues at risk, and fourthly, to define dose-response relationships for the critical tissues. No detrimental biological effects in man can be unequivocally attributed to exposure to actinides. Extensive animal studies have shown that the biological effects occur predominantly at the point of entry in regional lymphatic tissue, draining the sites of deposition and in the skeleton and liver, following deposition in these organs from the blood. In the event of accidental contamination of humans by actinides, therapeutic procedures may be used in an attempt to increase their rate of elimination from the body. The chapter discusses the current developments for treating intakes of actinides.

The Commission of the European Communities has initiated a programme to evaluate the merits of using plutonium in light water reactor fuels. As part of this programme, it is necessary to consider the implications of handling materials containing increased quantities of plutonium, americium and curium. This report provides a synthesis of current knowledge on the metabolism and biological effects of these actinides in animals and man, from which an assessment of rad.i ological health problems can be made. Relevant data are included from the many recent studies that have been conducted both in the Member States of the European Community and elsewhere.

In order to assess the consequences of human, exposure to these actinides it is necessary first to identify their routes of entry into the body, second to understand the factors influencing their distribution and retention in tissues, third to determine the tissues at risk and fourth to define dose-response relationships for the critical tissues.

There is only a limited amount of information on the metabolism of these actinides in humans although some data are available on the distribution of plutonium in human autopsy samples. Published studies on the behaviour of actinides in man. involve exposure to unknown physico-chemical forms in most cases, often at unspecified times, and in many cases chelating agents have been used which may have influenced their metabolism. Animal studies are therefore necessary to elucidate those factors influencing the metabolism of actinides in the body. A major limitation to the value of many animal studies, however, is that the amounts of actinides used have been greatly in excess of those levels likely tc be encountered in cases of human exposure. Since the behaviour of actinides in the body is influenced by the mass deposited, studies on animals exposed to relatively low doses have been considered wherever possible.

No detrimental biological effects in man can be unequivocably attributed to exposure to actinides. Extensive animal studies have shown that biological effects occur predominantly at the point of entry (lungs or wound site) in regional lymphatic tissue draining the sites of deposition and in the skeleton and liver following deposition in these organs from the blood. Effects have generally been observed at levels of activity in tissues much greater than those equivalent to maximum permissible body burdens in man,

Depending upon the radiation dose to tissues both early and late somatic damage could be anticipated in exposed individuals. Early somatic effects are assumed to require a threshold ose before any damage occurs and are unlikely except as a result of a massive intake following a major accident. The main late somatic effect is expected to be cancer, although life-shortening may also occur as a result of non-specific radiation effects. Cancer induction is assumed to be linearly related to the close with no threshold. The histological types of radiation-induced cancer that occur in experimental animals often differ from those commonly seen in man and there are species differences in radiosensitivity. Only human data have therefore been used for calculating risk coefficients for radiation induced cancer in the lung, bone, bone marrow, liver and gastrointestinal tract. These estimates of risk have been based mainly on the results of epidemiological studies on humans exposed to external radiation but some information is also available on humans exposed to incorporated alpha-emitters.

Radiation damage to the germ cells can result in spontaneous abortion or hereditary disease. Hereditary effects may therefore be expected to occur in the descendants of exposed individuals. No evidence of genetic damage resulting from the incorporation of actinides in the gonads has been demonstrated either in man or in animal studies. Estimates of risk coefficients for radiation induced hereditary disease have been extrapolated from studies on animals exposed to external radiation.

In the event of accidental contamination of humans by actinides, therapeutic procedures may be used in an attempt to increase their rate of elimination from the body. Current developments for treating intakes of actinides have therefore been discussed.

Chapter 2

PHYSICAL AND CHEMICAL PROPERTIES OF BIOLOGICAL IMPORTANCE

Publisher Summary

This chapter discusses physical and chemical properties of plutonium, americium, and curium of biological importance. Plutonium, americium, and curium are produced in both thermal and breeder reactors. The main civilian use of plutonium is in breeder reactors and it may also be used as fissionable material in thermal reactors. It has other uses in industry and medicine, such as for power sources and cardiac pacemakers. Americium and curium have a few uses although americium-241 has been used for transmission scanning studies in tissues, in neutron sources, in smoke detectors, and in α-active foils with the applications in static eliminators. The property of plutonium ions in solution to rapidly hydrolyze and form polymers at high concentrations is of particular biological importance. Plutonium is used in fuel mainly in the oxide form. The chemical separation procedures used in the reprocessing of fuel elements involve their dissolution in nitric acid and the subsequent separation of plutonium from uranium and other fission products by extraction in organic solvents. Actinides commonly enter the body by ingestion or inhalation as particles. Depending upon the source of the release, actinides may be taken in either as individual elements or in association with other active or inactive materials.

1 Introduction

Plutonium, americium and curium are produced in both thermal and breeder reactors. The main civilian use of plutonium is in breeder reactors and it may also be used as fissionable material in thermal reactors. It has other uses in industry and medicine such as for power sources and cardiac pacemakers. Americium and curium have few uses although americium-241 has been used for transmission scanning studies in tissues, in neutron sources, in smoke detectors, and in α-active foils with applications in static eliminators.

2 Plutonium

The chemistry of plutonium has been described by Katz and Seaborg (1957), Cleveland (1970) and Taylor (1973a). It is a silvery white metal which melts at 639.5°C and oxidises readily on warming in moist air. In finely divided form the metal may be pyrophoric. When plutonium metal is burnt in oxygen or when oxygen containing compounds such as Pu(IV) oxalate or Pu(IV) peroxide are heated in vacuo to about 1000°C plutonium dioxide is formed. Plutonium dioxide is a highly refractory material which melts at 2200-2400°C and is difficult to dissolve by normal methods.

There are 15 known isotopes of plutonium having atomic weights between 232 and 246. Of these only 236-243 are of any biological interest either as a result of their production in nuclear power programmes or because of other uses. Table 2.1 shows the main physical properties of these isotopes. The isotopes Pu-239 and Pu-241 are fissile and therefore of special interest for fuel in both thermal and breeder reactors. In 1975 the estimated production of plutonium in the Countries of the European Community was 3-0 tons and it was anticipated that this would rise to 5-7 tons by 1980 (Haijtink, 1976). Pu-239 and Pu-240 emit an L X-ray of uranium in 4% and 11% of disintegrations respectively with an energy of about 17 keV. These X-rays can penetrate a few centimetres of tissue thus allowing Pu-239 (+Pu-240)* to be detected in the lung or a wound site. The other α emitting isotopes of plutonium also emit L X-rays in varying amounts. Pu-238 is used as a heat source in thermo-electric power generators such as cardiac pacemakers and Pu-236 and Pu-237 are used in tracer studies. Because of its short half-life (5 hrs) and low β energy (0.6 MeV) plutonium-243 is of little radiological importance.

Table 2.1

Physical properties of the manor isotopes of Plutonium, Americium and Curium

E.C. – Electron capture

Reference:

a– In comparison with 1 μCi of plutonium-239 which is taken as 1

A– Harte (1976)

Plutonium can exist in solution mainly in four valence states: Pu(III), Pu(IV), Pu(V) and Pu(VI) and in some conditions as Pu(VII). The individual oxidation states can be stabilised by appropriate oxidising, reducing or complexing agents. In concentrated acidic solutions a number of oxidation-reduction reactions can occur leading to the formation of an equilibrium in which the different oxidation states can co-exist.

There is little information on the oxidation state of plutonium in biological systems. In neutral solutions the formation of the Pu(IV) state is favoured and biological fluids contain ligands and complexing entities that tend to stabilise the Pu(IV) state. Stable plutonium complexes can be formed with citrate, ascorbate, amino acids and proteins. The stability of these complexes decreases in the order Pu(IV) > Pu(III) > Pu(VI) > Pu(V). It is, therefore, probable that most if not all plutonium in biological systems is in the Pu(IV) state.

Of particular biological importance is the property of plutonium ions in solution to rapidly hydrolyse and form polymers at high concentrations. The tendency to hydrolyse decreases in the order Pu(IV) > Pu(VI) > Pu(III) > Pu(v). Hydrolysis of Pu(IV) can result in the formation of relatively insoluble polymers, a process which is only slowly reversible. The formation of plutonium polymers in the body leads to their phagocytic uptake by macrophages and other cells that can accumulate particulate material. Compounds of Pu(III) and Pu(VI) hydrolyse less rapidly at physiological pH and can potentially be more readily absorbed from the gastrointestinal tract, lung or other