The results obtained for uranium clearly show the interest of a systematic analysis of the biological effects of low-level chronic exposure on the major functions of living organisms. The main conclusions are as follows:
- Certain major physiological functions of living organisms such as breathing, behaviour or feeding, are modified very precociously and at very low exposure levels.
- More belated responses are observed for major functions such as reproduction when the exposure periods become significant in comparison with the lifetime of the organism considered. These responses are observed as from a threshold level. From an operational perspective, the knowledge of this threshold level marks the transition between the no-effect domain and the toxicity domain.
- A model has been used to simulate the demographic impact on animal populations based on the observed effects on the major functions of individual organisms.
The biological effects of uranium alone are increased in case of simultaneous exposure to another toxic metal, cadmium. On the contrary, they may decrease in the presence of other elements such as selenium, a trace element essential for life.
The results obtained with the laboratory rat show that the effects of chronic exposure cannot be extrapolated from knowledge of the effects of acute exposure. The main conclusions drawn from the experiments conducted are the following:
- The accumulation and excretion rates of uranium in a chronic exposure situation vary according to exposure time.
- They differ, quantitatively and qualitatively, from the acute exposure models.
- The organs affected by chronic exposure are different from those affected by acute exposure.
- Some of these organs display functional anomalies that are biological effects not associated with the development of cancers, namely modifications of behaviour and sleep, and effects on the metabolism of xenobiotics (pollutants, medicine, etc.).
The results obtained contradict the paradigm of the radiological protection system, at least as regards the model of a rat contaminated with uranium via ingestion. Chronic exposures through internal contamination yielded unexpected results in terms of organs affected and biological effects. However, it has not been demonstrated whether these biological effects have an impact on health and lead to the development of pathologies. Likewise, it remains to be determined whether the data obtained with an experimental model can be directly extrapolated to humans and other radionuclides.
The results acquired for the 'environmental' aspect are indispensable for determining the exposure level for which the protection of ecosystems is ensured under chronic exposure conditions. This knowledge provides the basis for implementing an environmental radiological protection system. The present and future nuclear context increases the challenge associated with environmental radiological protection, since most European Union member states are or will soon be confronted with the implementation of new nuclear facilities (e.g., EPR), the expansion or continuation of nuclear programmes including decommissioning, the implementation of waste disposal sites, or the management of old uranium mining sites after exploitation.
The data obtained for the 'health' aspect show uncertainties in the models for risk management after internal contamination. These uncertainties must be clearly identified, quantified and integrated in the radiological protection system. However, it does not seem necessary to question this system, since it has been designed to serve as an 'envelope' system covering a large number of situations with a sufficient safety margin. Moreover, it is the best system available to us at the moment. On the other hand, this system can be improved by integrating new data obtained from research. This research will need to be conducted with other models (including man) and other radionuclides so as to complement the studies in progress.