The use of nuclear energy for electric power has long been and remains a particularly controversial source of risk and concern. This is driven partly by perceptual factors and partly by the characteristics of the nuclear fuel cycle.
The cycle begins with, first, the extraction of natural uranium and, then, enrichment of the uranium and its concentration prior to manufacturing the fuel elements; risks of exposure to radioactive material of operating personnel are of prime concern. A nuclear reactor operates with exceedingly high energy density and inventory of radioactive substances (fission products). Multiple and diverse safety systems are provided for reactor control and decay heat removal. Their total failure is highly improbable but may lead to core damage (meltdown) and to release of parts of the core inventory, with long-lasting effects over a large geographic area. For state-of-the-art reactors, an 'extremely low frequency, potentially high consequence' risk profile is regarded as typical.
When the fuel in a reactor is spent, the fuel rods are consigned to a reprocessing plant, to interim storage or to a final repository. Any transport of spent fuel rods must be in proof casks, and the transport process itself is another source of risk as the probability that a cask will rupture in an accident that exceeds its design limit is not zero. In reprocessing, the risk is concentrated upon the release of radioactive substances to the environment, while in storage the isolation of the waste from the biosphere needs to be ensured for very long periods (several millennia).
Vast efforts have been made to minimize the probability of events causing harm to public health and the environment, particularly toward reducing or even preventing entirely major radioactive releases, in all stages of the cycle. Future reactors must thus be designed so that, even in the event of rare core-damaging events, off-site emergency measures are not necessary because impacts will not reach beyond the immediate neighborhood. In final storage, several safety barriers are combined in order to exclude groundwater contamination as far as is humanly possible.
Looking at the nuclear fuel cycle, all four risk management strategies are or could be invoked when regulating and managing the risks associated with each step of the cycle. Strategies belonging to the category of simple risks are in place when designing passive safety barriers that should withstand special pressure or thermal emissions. In addition, occupational safety measures for workers in the uranium mine and in other stations of the cycle can be grouped in this category.
Complex risk assessments are needed to model the impact of low-dose ionizing radiation on human health. These assessments include toxicological experiments using test animals and extrapolating the results to humans and extrapolating from high to low dose effects. Additional insights come from epidemiological data, for example, from the results of comparing cancer rates in populations exposed to higher doses of radiation through warfare or accidental exposure with populations that are only exposed to natural background radiation. Combing dose-response models with exposure requires extensive modeling, which relies on an intensive exchange among natural scientists, statisticians, physicians, exposure specialists, and behavioral experts.
Although modeling the effects of low dose radiation to human health results in some uncertainty (particularly addressing interindividual variability and stochastic effects), a resilience-based approach is clearly needed for assessing and managing ecological impacts of large nuclear accidents and the long-term effects of final disposal of radioactive waste for hundreds and thousands of years. Scientists understand in principle the reactions of ecosystems to higher levels of radiation but are far from predicting the concrete biological consequences of large releases of radionuclides in different ecosystems over long time periods. Experiences in contaminated areas such as Hartford in the USA and the neighborhood of Chernobyl seem to indicate that biodiversity is actually increased and biotopes recover faster than many ecologists had expected. At the same time, however, radiation-sensible plants and animals are replaced by others who are less sensible. There is still lots of ignorance and guesswork when it comes to assessing the long-term effects of releasing large quantities of radionuclides in the environment. In terms of management, a combination of risk minimization, ban on using resources from contaminated areas for human purposes, and even cleanup is normally advocated. In the USA, there is also a strong movement toward including major stakeholders and the local population in designing plans for decontamination and partial cleanup. These management strategies constitute a precautionary approach to managing risks. Since the consequences are not yet fully explored and unexpected surprises may occur, the main philosophy is to restrict exposure as much as possible and to avoid irreversible consequences even if these consequences may turn out to be more benign than expected.
Public involvement is even more important when addressing ambiguity. The still-ongoing public controversy does not derive so much from the complexity of nuclear energy, nor from uncertainty: most of the processes and phenomena are largely understood and agreed. Much of the debate in the past was directed toward further reducing uncertainty and explaining complexity rather than addressing the major problem: ambiguity. The controversy is fueled by several ambiguities due to different perceptions of and associations with the risk. To the opponents of nuclear energy, the problem is that a meltdown and catastrophic release cannot be ruled out (catastrophic aversion), and that spent fuel remains active for such a long time period (time aversion). The proponents of nuclear power, on the other hand, are convinced that there are no other reliable options with comparably minimal greenhouse gas emissions. Each position is based on values. Value differences color the interpretation of the data emanating from risk assessments (which is rarely disputed), as well as the way the risk is framed in the debate. Opponents accordingly judge the risks as being intolerable; to proponents, nuclear energy is an optimal means of supplying the world's electricity at minimal risk.
The controversy has led to considerable differences in the decisions of risk managers, even in Europe - where Germany has decided to phase out nuclear generation, France continues to rely on it, and Switzerland seeks the consent of its voters to replace an old reactor with a new one. There is certainly a need for a discursive strategy including different value positions and visions of the future. There is no guarantee for a consensus, but without such a discourse political decision makers will have hard problems to handle this controversy in a democratic and peaceful way.
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