HIGH LEVEL NUCLEAR WASTE DISPOSAL

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Triet Nguyen

Nov. 14, 1994

NE 161

Department of Nuclear Engineering, University of California, Berkeley, CA 94720-1730

Key words: High level waste, disposal.

Abstract

High level nuclear waste from both commercial reactors and defense industry presents a difficult problem to the scientific community as well as the public. The solutions to this problem are still debatable both technically and ethically.

Contents

Introduction

About 30 countries in the world now rely on nuclear power to produce a substantial part of their electricity. The nuclear generating capacity of Organization for Economic Cooperation and Development (OECD) member countries represents about 80 percent of the overall world capacity. A further significant increase of installed capacity is expected over the next decade. Nuclear power produces significant quantities of radioactive waste, including spent fuel and high level waste. The latter is produced by the chemical reprocessing of spent fuel when it is considered desirable to recover the remaining uranium and plutonium. Also, due to the end of the cold war, lots of weapon rated nuclear materials has to be disposed or used in some way for civilian purpose. Both spent fuel and high level defense nuclear waste present a difficult disposal problem which the scientific communities as well as the public have to deal with in the future since some of the radioactive components of these wastes have extremely long half-lives and the intense levels of radiation and heat emitted from them during the first 50 years after removal from the reactor or defense equipment are very dangerous.

Background

This paper deals with different methods of disposing high-level nuclear waste. There are two main ways in which high level nuclear waste is disposed, they are underground repositories and under ocean repositories. Consideration of the ethical aspect of disposing high level nuclear waste underground or under the ocean will also be discussed since this issue plays a major role in how these wastes are perceived by the public, thus it effects how to dispose them.

Solutions

There are many proposals for disposing high level nuclear wastes. However the most favored solution for the disposal of these wastes is isolating radioactive waste from man and biosphere for a period of time such that any possible subsequent release of radio nuclides from the waste repository will not result in undue radiation exposure. The basic idea behind this is to use stable geological environments that have retained their integrity for millions of years to provide a suitable isolation capacity for the long time periods require. The reason for relying on such geological environments are based on the following main considerations:

-Geological media is an entirely passive disposal system with no requirement for continuing human involvement for its safety. It can be abandoned after closure with no need for continuing surveillance or monitoring.

-The safety of the system is based on multiple barriers, both engineered and natural, the main one being the geological barrier itself.

One way of disposing high level nuclear waste materials which meets the above condition is the concept of disposing these wastes by burial in suitable geologic media beneath the deep ocean floor, which is called seabed disposal. Seabed disposal is different from sea dumping which does not involve isolation of low level radioactive waste within a geological strata.

In the seabed concept, a multibarrier system would be involved, including a suitable waste form such as glass and the use of corrosion resistant packages. A deep seabed sediment formation would be chosen in order to contain radio nuclides after the waste package fails through corrosion and the radio nuclides are released from the waste by leaching. Such sediments would be made of very fine grained particles with the ability to absorb and impede the movement of most waste radio nuclides. Sites in the ocean would have to be chosen on the basis of the characteristics of the seabed sediments. They would need to be free from erosion and located away from the edges of tectonic plates where seismic or volcanic movements could disrupt a repository and exposes the waste packages.

Disposal of long-lived radioactive waste in deep ocean sediments is assumed to be conceivable in isolated ocean regions under water at least 4,000 meters deep. Potential disposal sites must possess thick, weak, relatively homogenous sediments of very fine particles. The objective of proper emplacement is to implant waste packages beneath the sea floor in such a way that the barrier properties of sediments can isolate the radio nuclides for thousands of years. Many concepts have been proposed for emplacing radioactive waste under the ocean floor. Two representative examples of emplacement concepts are the emplacement of strings of waste canisters in drilled holes and the burial in the sea floor by free falling penetrators. Figures of this two method are shown.

Fig. 1 Representative Emplacement Methods. Top figure is the emplacement of strings of waste canisters in drilled holes. Bottom figure is the burial in the sea floor by free falling penetrators.

The design of waste packages suitable for seabed disposal is an important technical aspect. The waste package contains the waste form which can be high level waste from fuel reprocessing and spent fuel: barrier materials, the canister and any additional container. Barrier materials should be strong, tough , creep and corrosion resistance. The target of barrier materials is to ensure containment for at least 500 years. As for the waste canisters, they should incorporate pressure-resistant and corrosion-resistant construction to minimize the size of the waste packages.

Details about cost and risk consideration can be found in volume one of the "Feasibility of Disposal of High-Level Radioactive waste into the Seabed". In summary, the penetrator option appears to be less costly than the drilled emplacement by a factor equal to 2.5. Regardless of the adjustment made in design and cost estimates, seabed methods appear to have a small impact on the total cost of the electricity generated and compares favorably with the cost of alternative disposal methods.

The most widely proposed method of disposing high-level nuclear waste materials is to bury them in underground nuclear waste repository. An underground repository is designed so that it has the ability to permanently isolate high-level radioactive waste from the accessible environment for very long periods of time. The Nuclear Regulatory Commission set this time period to be 1,000 years and recommends consideration of performance for up to 100,000 years. On April 30, 1995, President Reagan decided to accept the Department of Energy's recommendation that defense and commercial high-level radioactive wastes be commingled in a commercial repository on the basis of the projected cost advantage and the fact that no compelling requirement for a defense only repository was found.

Ethical Aspects

Ethical considerations play a major role in decision regarding how high-level nuclear wastes are disposed and handled. Because of popular skepticism and misperception about this issue, the public should be educated so that this problem can be resolved peacefully. Ethical considerations not only can but must provide a mediating framework for the resolution of such a popularized political controversy. Implicit in moral objections to proposals for permanent nuclear waste disposal are concerns about three ethical principles: fairness to individuals, equitable protection among diverse social groups and informed consent through due process and participation. Controversies over the production and disposal of nuclear wastes have come to serve as a volatile instrument for debating divergent agendas for the future of out affluent technological society. Moral objections to nuclear waste disposal bases its ground on the argument that:

-Nuclear wastes pose a unique threat of harm because they contain actinides and a few fission products with half-lives of millions of years.

-During this long period, one expect that the three proposed barriers (waste form, container, geological medium) designed to prevent reentry of radioactive poisons into the biosphere to be breached.

-Such leakage can be expected to potentially create grave hazards for human populations in the distant future since likelihood or leakage increases over a long time: hence, radiation exposure is inevitable. (However, with proper design and siting this radiation exposure is likely to be small compared to that from typical uranium ore bodies.)

These moral assessments are based on the assumption that any error or accident entails catastrophic effect for unconsenting present and future generations. Nuclear technology is unique and incomparable precisely because there will be disastrous. A skeptical and distrustful public has yet to recognize and be persuaded by the fact the performance standards and licensing procedures required by EPA, NRC and DOE are not only achievable, both technically and economically, but more sufficient by ethical standards.

Conclusion

After examing two main methods of disposing high level nuclear waste materials, we see that there are both advantages and disadvantages. More research is needed to perfect each method. We also see that the public needs to be educated and informed about this issue so that there can be an understanding between the public, the government and the nuclear science community about this matter.

References

-(1993) "Dynamic Analysis and Design Considerations For High-Level Nuclear Waste Repositories." Auazi A Hossain, Editor. New York, New York 10017-2398.

-(1988) "The Feasibility of Disposal of High-Level Radioactive Waste Into the Seabed," G. de Marsily. Paris, France.

-(1986) "High-Level Nuclear Waste Disposal," Harry C. Burkholder, Editor. Richland, Washington.