European Nuclear Society
e-news Issue 14 Autumn 2006
http://www.euronuclear.org/e-news/e-news-14/hlw.htm

International Nuclear Energy Academy

HLW disposal: Status and Trends

An International Nuclear Energy Academy Statement by

Bertrand Barré

Assisted by Dan Meneley, Dave Rossin and Jorge Spitalnik

Introduction

High Level Waste disposal is viewed by many as Nuclear Power’s Achilles Heel, and some people are even convinced it constitutes an insoluble problem. It is certainly a question about which the perception of the specialists, aware of the progress achieved in the last two decades, differs deeply from the perception of the public-at-large and the media. This paper attempts to bridge part of this gap by providing up-to-date information on the status of HLW disposal across the world.

Each country having its own classification of radioactive waste, we shall adopt the simplest. A radioactive substance is a substance which contains radioactive nuclei in amount or concentration high enough to motivate radiation protection measures. A radioactive waste is a radioactive substance resulting from a process of human activity and which has no foreseen use in the present technical and economic context: it must be disposed of without harming people and environment. We shall distinguish only three broad categories: Low level waste LLW, intermediate level waste with long lived isotopes LL-ILW and high level waste HLW.

LLW constitutes the bulk of the radioactive waste in volume and in mass, but it contains only a small fraction of the total waste radioactivity. The origin of LLW is quite diverse: nuclear power, medicine, research, industry, etc. Many countries have licensed operating LLW disposal sites, usually surface storage sites which accept conditioned (immobilized) waste packages with such specifications as to insure that within two or three centuries, given the short radioactive period of most isotopes, the radioactivity of the disposal site will be of the same order of magnitude as the natural background radioactivity.

LL-ILW and HLW originate almost exclusively from nuclear reactors and their fuel cycle facilities, as well as the defense facilities of those countries which developed nuclear weapons. Though quite limited in volume, they constitute the bulk of the waste radioactivity. For those countries with no weapons activities and which do not reprocess their spent fuel, all their HLW and LL-ILW is inside their spent fuel assemblies which constitute for them the ultimate waste. We shall now focus only on those two categories of waste.

Containment, Storage, Disposal, Transmutation

For all the fear it inspires, radiation has two precious characteristics:

1. It is easy to detect at levels far below the detection threshold of any noxious substance (one can detect a single disintegration when one cannot detect a given chemical unless billions of molecules are present);

2. When detected, it is easy to protect oneself from radiation by a combination of three ways: keeping distance, limiting exposure time and providing shielding.

The problem of radioactive waste disposal is therefore only a problem of containment: making sure the radioactive species will stay where they were located, or that the migration time from their original site to the biosphere will be long enough for the radioactivity to have decayed much below present acceptable limits.

The problem is exactly the same for the containment of the radioactive elements within a nuclear reactor, but in the case of HLW the volumic activity is far smaller, while the containment time must be far longer. The solution, therefore, is basically the same: containment by multiple imbedded barriers. The first barrier is the matrix which contains the radioactive elements, then there is the waste packaging, and then additional barriers are added, according to the chosen disposal method.

The basic choice is between long term surface (or subsurface) storage and deep geological disposal. Transmutation of the longest lived elements might in the future be a preliminary to either method.

In surface storage – sometimes called interim storage – the conditioned waste packages are stored in engineered facilities for a given period of time, it being clearly stated that they will be retrieved from the facility at the end of the specified period. The facility may be located at ground level (surface facility) or shallowly buried (subsurface facility) in order to improve its physical protection against external aggression. Both surface and subsurface storage facilities must be kept under full surveillance and monitoring during the specified period, and one must demonstrate that the waste package can actually be retrieved if the decision is made to do so. Interim storage provides a satisfactory medium term solution, but it still leaves to our successors the burden of implementing a permanent disposal solution.

In deep geologic disposal, the stratum itself constitutes the ultimate barrier against the migration of the radioactive elements: once full, the disposal facility will be sealed and one does not intend to retrieve the waste packages. Such was the initial concept, a concept put forward by the US National Academy of Sciences as early as 1957, when asked by the Atomic Energy Commission. More and more, in order to facilitate public acceptance, the concept is being refined into “reversible” geological disposal. In a reversible geological disposal, waste packages are intended to stay, but the possibility to reverse the decision and retrieve them is kept open for a significant period of time, ranging from one to a few centuries. It is meant to be a definitive solution, the best which can be implemented today, but it does not preclude the possibility for our successors of finding an even better solution. For practical reasons, HLW will be held in a surface storage facility for a number of years before being sent to geological disposal. This allows all but the longer half-life radio-nuclides to decay, and thus the heat source itself is substantially cooled down.

During the first few centuries, most of the radioactivity of the waste comes from the fission products; thereafter, the longer lived actinides (uranium, neptunium, plutonium, americium and curium) take over. When the spent fuel is reprocessed, recovered uranium and plutonium remain in the nuclear cycle and only traces of them, together with the fission products and the “minor” actinides are vitrified to constitute HLW packages. The radioactivity of vitrified HLW decays much more rapidly than the radioactivity of the spent fuel. If one pushes the reprocessing one step further to recover the minor actinides (“partitioning”), curium could be conditioned to decay by itself while neptunium and americium could be fissioned in nuclear reactors into “ordinary” fission products (“transmutation”). The radioactivity of the resulting HLW packages would decay even faster, and the necessary containment time within the disposal facility would be reduced. This is called P&T, for partitioning and transmutation.

Implementing P&T would not eliminate the need for ultimate disposal, but it would alleviate some design constraints on the disposal facility. Partitioning has been developed at the laboratory scale, and significant results have recently been obtained. Transmutation has been demonstrated experimentally, but present Light Water Reactors would be poor transmuters. The high neutron fluxes inside the core of a Fast Neutron Reactor would be much more efficient. Furthermore, a metal-fuelled Fast Neutron Reactor with integral reprocessing and fabrication facilities promises both high P&T efficiency and very low levels of trace actinide materials in the waste stream. P&T is therefore a possible useful future sophistication of the basic two methods above described.

International Survey

As shown on the table below, which is not exhaustive, many advances were achieved throughout the world during the last two decades:

Almost all countries using nuclear power have studied geological disposal, through underground labs or “natural analogues”, and taken part in international round robin computer simulations. Main results show that glass and concrete, the most extensively studied matrices for HLW and LLW containment respectively, are durable. High integrity copper containers have also been developed for the geological disposal of spent fuel. If the proper site and the proper stratum are selected, the geological barrier is very efficient at preventing radioactive nuclides migration.

While no demonstration of the behavior of a geological disposal facility can be fully rigorous and definitive, given the timescales involved, there are now many converging indices that the mechanisms governing the disposal evolution in time are understood and mastered, and that those mechanisms will induce minimal environmental impacts.

Conclusion

We have inherited radioactive waste and we produce it every day. We cannot simply transmit the burden to our grandchildren. On the other hand, we must take into account a certain degree of public mistrust of scientists and engineers when the horizon spoken about exceeds a few centuries.

Contrary to widespread perception, a lot of progress has been accomplished in many countries towards achieving technically and socially acceptable HLW disposal. While there is no perfect consensus, the majority trend is to construct geological disposal sites, with some requirements for temporary reversibility.

Concerns about HLW management should not, therefore, prevent mankind from pursuing the development of nuclear power. Nuclear power and hydropower are today the only significant and reliable sources of baseload electricity which do not originate from fossil fuels and do not emit large amounts of gas that contribute to the greenhouse effect.

This is an Executive Statement of the International Nuclear Energy Academy. It represents the views of the author, but has been endorsed by the Executive Committee of the Academy as a contribution to the responsible development of civil nuclear energy.

© European Nuclear Society, 2006