European Nuclear Society
e-news Issue 42 Autumn 2013
http://www.euronuclear.org/e-news/e-news-42/hsc.htm

ENS High Scientific Council Position Paper¹

Irradiated Fuel and Waste Management: the Achille’s heel of the nuclear industry?

 
The management and final disposal of irradiated fuel and nuclear waste is often presented by the media and perceived by the public as being an unsolved problem that restricts the future of nuclear energy. However, the nuclear industry focused on this problem very early on and has developed proven technical solutions. 

Technical solutions and political advances

Whereas large volumes of short-lived radioactive waste are already handled by the nuclear industry in surface storage facilities, the management method for high activity, long-lived waste has not yet been decided in detail and is still being studied in all countries that generate nuclear power. Scientific knowledge is continually progressing, technical solutions are emerging, and all within a context in which science and technology interact strongly with social and economic issues. Many technical advances have been made during the last twenty years in fields as varied as partitioning, transmutation, waste conditioning, storage and underground disposal.

The main principles of nuclear waste management

Reducing the dangers of waste, decreasing its volume, partitioning the waste into homogeneous categories are principles that are familiar to domestic waste management. They also apply to nuclear waste. Within a closed fuel cycle waste management - from its production to its final destination - looks like a chain of which treatment-recycling, conditioning, storage and disposal of the final waste are the main links. With the open cycle option, the first link is absent.

Recycling: the first link inf the irradiated fuel management chain

The first option is, therefore, to close the fuel cycle. This option has a very important influence on the nature of the waste produced, as well as on its subsequent management.

The various options available – the direct storage of spent fuel or the specific conditioning of separated actinides - have been studied worldwide. These options may offer distinct advantages for the nuclear industry in general, but as far as waste management is concerned, the closed fuel cycle is clearly more favourable because it offers the possibility of considerably reducing the radiotoxic inventory, and of putting the waste into a stable and safe form. Recently, the economy of this option has been further reinforced by the increase in the price of natural uranium, which provides a powerful incentive to save on fissile matter.

In the long run, the probable development of fourth generation nuclear power plants will make the closed fuel cycle more widely implemented. With these new plants, one can hope to further reduce the waste toxicity by transmuting actinides. Indeed, the nature of the final nuclear waste (by definition non-recyclable) depends on the nuclear technology being used: final waste in 30 years may well be different from what it is today. For example, it may be possible to further reduce the radio-toxicity of vitrified waste by eliminating some radionuclides (e.g. minor actinides) from the inventory. The exclusion of these radionuclides from the waste would also reduce the exothermicity of the waste, making subsequent waste management simpler. This objective of producing cleaner and cooler waste is the main driving factor behind research into partitioning and transmutation. However, in order to gain a substantial benefit in terms of radio-toxicity, one needs to transmute the separated radionuclides. The technology already exists for recycling plutonium, but research is still needed to make the recycling of other radionuclides viable on an industrial scale.  

Transmute, recycle: where is the limit?

It is already clear that fission products are not readily transmutable, neither with current nuclear reactors nor with the fast reactors envisaged for the future. Whatever the nuclear system used, fission products will, therefore, remain present in the final nuclear waste. The issue of actinides is less certain since the future of these radionuclides depends on the nuclear reactors available, as well as on the chosen fuel cycle policy. With current reactors plutonium can be recycled under a MOX form, but minor actinides tend to accumulate in the waste. Fast reactors might offer the possibility of transmuting these radionuclides, but this transmutation will remain slow and difficult. Moreover, putting minor actinides in the fresh fuel complicates both fuel fabrication and reactor operation, and it is doubtful whether those responsible for operating the facilities would be very enthusiastic about taking on this burden. The actinides are very insoluble and immobile in geological media and could be stored in an underground repository without compromising its safety. The only important advantage for the transmutation of actinides is the reduction in the waste thermal loading and toxicity, with the associated reduction in the size and cost of the repository.

The two first links in the irradiated fuel management chain, fuel processing and waste conditioning, work well together and are already being implemented in some countries (e.g. France, at the La Hague facility, or Japan at Rokkasho-Mura). Once the waste has been conditioned in canisters one still has to solve the problem of deciding what to do with the latter. Thanks to their chemical and mechanical stability, the present forms of conditioning (concrete, metallic compacted waste, glass) are well adapted for storage and eventually for the underground disposal of long-lived waste. These links in the waste management chain are coherent and this coherence will be maintained with the development of fourth generation fast neutron nuclear systems because future reactors will require a closed fuel cycle with spent fuel treatment processes that will complete rather than replace the existing one.

Waste conditioning: the second link in the chain

The industrial processes used for the conditioning of nuclear waste are already mature and operational. Basic research has permitted a good understanding of the physico-chemical mechanisms at play during both the fabrication and the ageing of the conditioning matrixes, glass, concrete or bitumen. The safety study of waste management relies on this scientific knowledge of the long-term behaviour of the confinement matrixes. Suitable conditioning forms have been developed for all types of waste. Solutions of fission products and minor actinides, which possess by far the highest radio-toxicity, are vitrified at industrial scale facilities. The quality of the glass obtained is well proven. For instance, the R7T7 glass developed for the confinement of fission products from the processing of light water spent fuel has become a worldwide benchmark. There are probably more scientific articles written about this glass than on any other industrial glass!

Metallic waste from the spent fuel bundles is compacted and put into steel canisters that are identical to the ones used for glass casting. The radiological impact of this waste form in a geological repository would probably be very small. Technological waste associated with the exploitation of nuclear facilities is conditioned in concrete. Most of this waste is short-lived and of low or medium activity. In many countries, including France, this waste is already stored in dedicated surface or subsurface facilities. A wide spectrum of concrete formulae has been developed to fit the diversity of the waste to be conditioned, solid or liquid. These concrete packages are well characterised and sufficient knowledge of their alteration mechanisms enables their confinement properties to be guaranteed.

What to do with the ‘final’ waste?

Radioactivity possesses two important characteristics that can somewhat mitigate the fear that it provokes: firstly, it is easy to detect, even at very low levels. A unique disintegration can be detected, whereas billions and billions of molecules must be present in order to detect chemicals. Secondly, once detected, it is relatively easy to protect oneself from the radioactivity, by combining shielding, distance, limitation of the exposure time and radioactive decay. Therefore, the solution for storing final nuclear waste all boils down to confining the radionuclides in an isolated and shielded installation for a period of time that is long enough for radioactive decay to take place. This is the idea behind both storage and underground disposal facilities.

Interim storage: a temporary solution that gives flexibility for the management of waste

Whatever the fate envisaged for irradiated fuel it must first be stored temporarily. Countries that have chosen the open fuel cycle option must store the spent fuel before its disposal; the ones which have opted for the closed fuel cycle option must store it for a few years, in order to let it cool before it is processed. The vitrified final waste is then stored temporarily. In all cases, storage is a temporary solution which provides flexibility for the management of waste, because it allows the waste to cool down - which in turn decreases its thermal load - before its final storage at the disposal facility. This ultimately reduces the cost of the installation. However, the safety and security of storage facilities is less well assured than that of an underground disposal facility, because the former demand active maintenance and are more vulnerable to human intrusions than the latter. Even if economic arguments plead in favour of long-term interim storage, public policy would be well advised to limit the duration of this storage to a reasonable maximum, for example the time for which maintenance of the installation can reasonably be guaranteed.

Underground disposal: the third and final link in the chain

Last but not least, one has to find a final place for the final waste. The deep geological underground disposal option seems to be the only long-term solution that does not require continuous societal control. General consensus has been reached on this issue, under the aegis of the International Agency for Atomic Energy (IAEA) and of the Nuclear Energy Agency of OECD:  no better solution has been identified. Disposal in a geological repository will always be an expensive operation, hence the need for reducing the volume and the thermal power of the waste as much as possible. These two parameters largely determine the repository capacity, and therefore, its duration of exploitation and its cost. The processing of spent fuel is already a major step towards achieving the aforementioned reductions since it involves the removal of the uranium (which represents 90 % of the mass of the spent fuel) and the removal of the plutonium (which represents the major contribution to the total radio-toxicity of the waste).  The American Advanced Fuel Cycle Initiative (AFCI) is exemplary in this respect. After more than 20 years of effort, which led to the launch Yucca Mountain repository spent fuel disposal project, the US Department of Energy is reconsidering the optimisation of its use and the nature of the objects that will be disposed of there.

 
Underground disposal: a simple and robust concept

The safety of the underground disposal relies on its capacity to confine radionuclides within an underground facility, until radioactive decay has brought their radio-toxicity down to an acceptable level (usually a level equivalent to that of a natural uranium deposit). The safety demonstration of such an installation will rely in fine on confidence that the installation will behave as foreseen. Studies have, therefore, been carried out to better understand the evolution of waste packages in an underground environment and the migration of radionuclides through the man-made and geological barriers that isolate them from the biosphere. Removing from the waste the long-lived radionuclides that contribute most to its long-term radio-toxicity could significantly shorten the time that the waste will remain dangerous. This could also reduce the scientific uncertainties associated with very long timescales. This option will only be available if and when one is able to separate and transmute minor actinides.

The concept of underground disposal is flexible. Initially designed with the idea of definitively and irreversibly getting rid of the waste, the underground disposal concept has evolved in all nuclear countries towards the concept of reversibility (which means being able to retrieve the waste from the repository after some time). That approach seems to have become a prerequisite for public acceptance of these installations. It contributes to modifying the image of underground repositories without overly changing their general conception.

Thanks to the efficiency and the redundancy of its barriers, underground disposal is also a robust concept. The radiological impact of deep geological waste disposal that evolves normally should remain very small, local and delayed. However, altered evolutionary scenarios within the repository, which are by definition unpredictable (especially those associated with human intrusions), can have a larger impact.

Strictly speaking, the safety of a deep geological repository cannot be demonstrated, because the very long timescales make direct experiments inaccessible. Therefore, the objective must be more modest: to show by means of partial experiments that the main physical and chemical phenomena at play are understood and mastered and, therefore, to validate the main elements of the modelling of the repository’s long-term evolution.  The study of natural and archaeological analogues contribute to this process of confidence building, showing that in sites like Oklo radionuclides have been confined over extremely long durations.

All national and international studies show that the impact of a repository on man and on the environment will remain negligible, even in the very long-term. In order to convince people of that fact, it will be necessary to build confidence with converging indications that show that all the possible events that are liable to affect the repository have been envisaged and are found to be within acceptable limits… in short, that the repository concept is a well mastered one. This confidence already exists among most specialists, but is not shared by the public. Furthermore, as long as citizens have their doubts politicians will tend to postpone taking decisions. The example of Finland and Sweden, two countries which decided democratically to build geological repositories for nuclear waste, shows that it is possible to overcome this obstacle.

Let us behave responsibly and try to be sensible

We inherited nuclear waste from our predecessors and we produce waste ourselves. We cannot pass on this burden to our children, that is why we must put all our efforts into minimizing final waste and managing it in accordance with the best possible safety conditions and by exploiting the best technologies available today (this is why the concept of ‘retrievability’ is so important). The technologies exist. Their implementation requires that a political decision be taken. Contrary to widespread public opinion, much progress has been made in the design of technically and socially acceptable nuclear waste repositories. Most of the experts agree on this, but the public and politicians are still reluctant to act and must be persuaded by an irreproachable process of confidence-building. One thing appears very clear to the authors: it would be an irresponsible attitude to store this waste for a long time, while waiting for a hypothetical scientific advance to occur. Science also has its limits!

As has been shown, even if one considers only the scientific and technical aspects, waste management is a multidisciplinary challenge: knowledge of reactor physics is needed because the nature of the waste produced depends on the reactors used. Advanced chemistry is necessary to determine which radionuclides are left in the waste after the fuel has been processed. Mastery of physico-chemistry and material science is essential for understanding waste conditioning and the long-term behaviour of waste. Knowledge of engineering is a prerequisite for appreciating the thermal phenomena that are at play inside the storage installations. Expertise in mining engineering is vital when designing an underground repository - as is that relating to earth sciences like geo-chemistry to understanding and predicting the long-term evolution of these installations. Finally, a high level of competence in radioprotection is an absolute must in order to evaluate accurately the impact of all these processes and installations on man and the environment.

Nuclear energy will continue developing worldwide, in spite of the Fukushima accident. Even in those European countries that have decided to phase-out nuclear energy there is a legacy of nuclear waste that must be dealt with. The scientific and technical expertise needed for waste management already exists. Management decisions must be taken. Now is the time for political courage. 

¹ Prepared by Bernard Bonin

© European Nuclear Society, 2013