Gordon Research Conference on: NUCLEAR WASTE and ENERGY Prague, 1996

 

Igor N.Beckman

 

NUCLEAR WASTE RISK ASSESMENT AND MANAGEMENT

 

            Taking into account the widespread use of nuclear energy for electricity generation and its potential for heat production, it is clear that sound, safe and efficient radioactive waste management is a necessary component of the nuclear industry. As in the field of nuclear waster, the activities in nuclear safety are geared both to information collection, analysis and exchange, expert assistance, advice and services in specific situations and work on establishing international norms. Activities included technical guidance to the countries of former Soviet Union on minimizing radioactive wastes from nuclear fuel cycle facilities; a development of quality assurance and control for radioactive waste packages and advanced technologies for processing radioactive wasters, and for sitting, design, construction, operation, closure and post closure of radioactive waste disposal facilities. A new policy, regulation and planning for decommissioning large unclear facilities was proposed.

            In the Moscow State University progress has been made in these research programs: radiochemical aspects of the development of nuclear power, safety of nuclear installations, radioactive waste management, integrated systems for separation of radionuclides, marine radioactivity monitoring, ets. Our efforts on the nuclear fuel cycle facilities while minimising their environmental and health impact. Radioactive waster management was sponsored through co-ordinated research programmes on the use of inorganic fibrous adsorbents and membrane extractors to concentrate radionuclide in waste streams in to solid materials and on waste treatment and immobilisation technologies.

            This report describes the application of risk methodology to solve the problem of radioactive waste management. The most attention will be paid efforts that should be undertaken for the reduction of radiation sources. The work concentration mainly on the radon risk estimation and classification, requirements for limiting radiation exposure due to radionuclides from nuclear waste and the optimum radionuclide risk reduction processes.

            The rapid increase in the utilisation of risk assessment, since the presentation in 1983 of an analytical paradigm, has raised numerous and difficult scientific and policy issues. This report describe insights as risk assessment has moved from relatively simple default approaches toward addressing the complexities of exposure scenarios and radiochemical toxicities. Risk assessment must improve to assist in decision-making despite limitations of available information and human understanding. Mathematical modelling, pharmacokinetics, mexanism of toxicity-these must be pathways to greater clarity and conciseness, not just complexity and technical sophistication. It is this context that makes risk assessment such a challenging multidisciplinary field. The report demonstrate that developing a flow from information-gathering to analysis to discussion to decision is still a challenge, even within the risk assessment/management/communication fields. The  languages of mathematics and human speech must come together to facilitate discussion and decision-making if risk assessment and risk management are to achieve the success that society desires from them.

            In 1983 , the National Academy of Science (USA) released its risk assessment/risk management (RA/RM) “paradigm”. Risk assessment was defined as “...the characterization of the potential adverse health effects of human exposures to environmental hazards”. The item “risk” has been addressed in terms of the estimated mortality rate. The paradigm has presented society with a scheme, or a conceptually common framework, for addressing many risky situation. The human health risk assessment paradigm has served to evaluate hazards poses by exposure to developmental and reproductive toxicants, mutagens, carcinogens, and systemic toxicants. Most recently, this paradigm has been modified to evaluate potential impacts on fish, wildlife, endangered species, and selected flora due to chemicals or other stressors. The goal of any risk assessment is to estimate the like hood of an adverse effect on humans, domestic animals, wildlife, or ecological systems from possible exposures to chemical or physical agents. An integrated health/ecological risk assessment framework consist of the four component: Hazard Identification, Exposure Assessment, Exposure-Response Assessment, and Risk Characterization.

The method developed here provides a quantitative, objective measure of ecological risk for natural populations exposed to mixtures of chemical contaminants and radionuclides. It is founded on generally accepted risk assessment concepts: use of toxic units to assess the joint toxic effects of mixtures and expression of ecological risk as a relationship between toxicological end points and estimated environmental concentrations.

            For radiation protection purposes the stochastic effects are expressed as a lifetime risk of early death by cancer or of a serious genetic effects. To calculate the total risk of an exposed population it is necessary to use models that extrapolate the information based on a limited period in the lives of individuals to the total lifetimes. In general, two different models are used: the absolute risk model and the relative risk model. In the relative model the age sensitivity has a large effect than in the absolute model. For a number of cancers it has been found that relative projection models describe the data better than absolute model. The effect per unit of dose is found to be high dose rates than at low dose rates, which means that a linear extrapolation from the high-dose levels to the low-dose levels overestimates the effect in the low-dose region.

            In this report we will outline how nuclear waste research in the future may fit into a framework often used in investigations of radioactive substances that pollute the environmental. This framework is the source-risk chain, which consists of five stages:

            source—dispersion—exposure—dose—risk

Radionuclide migration in the geosphere is considered here in the framework of the disposal of nuclear waste in the ground. The safety of a repository should be judged in terms of the total health risk, defined as the product of the probability of occurrence of the event by the probability of health effect given the dose resulting from the event. Such a philosophy is perhaps acceptable for “minor” effects, not for “catastrophic” ones. The present regulatory prescriptions of a geologic repository are bases on four criteria: a minimum canister life; a maximum radionuclide release rate from the engineered barriers (also called the “near field”); a maximum radionuclide arrival at the accessible environment; a minimum value of the groundwater travel time from the repository to the accessible environment, determined in natural conditions prior to the construction of the repository. The migration of radionuclides in the geosphere is governed by three major mechanisms: advection with the flowing water, if any; diffusion, molecular or thermal; reaction/interaction with the fluid constituents or the medium.

This report will review the relative role of each process both in the near field and the far field. A series of scenario will be defined, covering a broad spectrum of conditions: present conditions possibly modified by the presence of the waste; most likely changes that will occur if the evolution of the system continues at the same rate as in the past; accident situation due to natural phenomena; accidental situation due to human action. The potential and limitations of a decision theory framework to assess priorities in research, priorities in data collection, confidence in the validity of the performance assessment results will be discussed.