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Destruction of long-lived radioactive waste

Report by the ENS Expert Group "Transmutation "

The Group = Jean Ravier (France), chairman;Per-Erik Ahlstrom, SNS, Sweden; Dr Leo Baetsl‚, BNS, Belgium; Harm Gruppelaar, NNS, Netherlands; Kevin Hesketh, BNES, UK; Dr. Heinz Kusters, KTG, Germany; Pedro-Adolfo Landeyro, SNI, Italy; Prof. Meir Segev, INS, Israel.

All activities associated with human civilization consume naturally occurring resources and produce waste materials. Whereas the influence of animal life is confined to its natural habitat, that of all human activities, especially industrial activities, has strongly influenced the biosphere and atmosphere particularly in densely populated areas such as Europe.
Since the industrial revolution, man has made increasing use of fossil energy and mineral resources which have been depleted and to a certain extent been transformed into waste products polluting the human environment and suspected of exerting a detrimental impact on global climate.
Age-old historical trends, now accentuated by ecological concern, have boosted the pressure for a reduction of waste output and of its noxiousness. Reduction and recycling of wastes are one of the most recent trends in resource management and have been pioneered by the nuclear community since the early days of nuclear power. Undoubtedly because of the small volumes of waste it generated, nuclear industry has been a leader in pursuing "containment and disposal" rather than the more common "dilution and dispersal" strategy.
From this point of view, the nuclear establishment, aware of its social and long-term responsibility, has always carefully considered the future management of irradiated fuel discharged annually from some 400 commercial nuclear power plants now in operation worldwide including ~120 GWe nuclear electric capacity operational in Western Europe and 45 GWe operational in the ex-USSR and East European countries. Various means of management types are considered for each of the main irradiated fuel constituents discharged from LWRs - uranium, plutonium, actinides and fission products:

uranium, constitutes about 96% of the fuel unloaded from commercial power reactors. In the case of light water reactors, the most widespread type of reactor in Europe and in the world, the spent fuel on discharge contains still 0.90% enriched in the fissile isotope 235 whereas natural uranium contains only 0.7% of this isotope;
plutonium, constitutes about 1% of the weight of discharged fuel, it is a fissile material which can be used as fuel in present and future commercial reactors;
minor actinides constitute about 0.1% of the weight of discharged fuel. They consist of about 50 % neptunium, 47 % americium and 3 % curium, which are very radiotoxic;
fission products (iodine, technetium, neodymium, zirconium, molybdenum, cerium, cesium, ruthenium, palladium, etc.) constitute about 2.9% of the weight of discharged fuel. At the present stage of our knowledge and our technological capacity, they are considered as the final waste form of nuclear power production, unless a specific use is found for the non-radioactive platinum metals.

For illustration, a typical 1000-MWe PWR unit operating at 75 % load factor generates about 21 tons of spent fuel at a burn-up of 43 GWd/t and this contains about: 20t of enriched U; 230kg of Pu; 23 kg of minor actinides; 750 kg of fission products.
Until recently Partitioning and Transmutation has mainly considered the minor actinides but it is now recognized that to make an impact on the long-term radiotoxicity and on the actual residual risk from disposed wastes it would be necessary to develop appropriate means for incineration of Pu, minor actinides and some long-lived fission products.
It may be useful to the non-specialist to make a few key points at this stage:

The 21 tHM (contained inside 42 PWR fuel elements with a total volume of about 11 m3) will have produced an electric energy of about 6.6TWh (6.6 billion kWh) at a burnup of 43 GWd/tHM, which is the present industrial state of the art. This same energy output corresponds to the burning of 2 million tons of coal in a conventional power plant giving rise to 120 000 t of ashes, 5.4 million tons of CO2 and 50 000 t of SO2.
All the physical reactions which produce energy take place within the fuel sub-assembly itself, enclosed inside a strong metallic cladding. Thus all the waste resulting from these reactions remains confined during the discharge of the spent fuel, within the cladding of the discharged sub-assembly. If this sub-assembly is considered as waste, its volume is 10 000 times smaller than the wastes from coal.
Radioactive decay, which is a natural physical process, takes place in fission products, minor actinides and plutonium in the same way as it occurs in the uranium of the earth's crust and other naturally occurring radioactive elements. As a result, the inventory of the radioactive products contained in irradiated fuel is not fixed for ever: these products will gradually disappear by natural disintegration after periods of time specific to the isotope of interest (for example, half lives are 18.1 years for the curium isotope 244, 24 000 years for the plutonium isotope 239, 2.14 million years for neptunium). It must be pointed out that the products resulting from the decay of these isotopes are themselves radioactive and that a long series of radioactive decay processes must follow one another in order to finally produce a stable element. Irradiated fuel management must therefore take into account a very long-term evolution of the residual radioactivity.
In view of the above, there is no need to take urgent and irrevocable measures regarding the four kinds of products contained in the sub-assemblies when these are discharged from the reactor. on the contrary, a few years interim storage in a pond is liable to be favorable from the technical point of view as it will enable advantage to be taken of the rapid decay of radioactive nuclides during the first few years following discharge. Interim storage of the typical LWR fuel for 40 years will decrease the radioactivity by a factor of 10 as compared to only one year of interim storage. on the other hand, however, technical considerations and the need to maintain the support of public opinion lead us to recommend that it should be the generation which used the nuclear-generated electricity that should resolve the issue of the long-term management of the resulting waste.

The various management strategies for spent fuel

The management of irradiated fuel should ensure that the biosphere is protected under economically acceptable conditions without entailing unfavorable short-term consequences and the public must be convinced of the effectiveness of the methods. Since the spent fuel contains very long-lived radionuclides some protection is required for at least 100000 years. Two means are possible:

we can wait for the natural decay of the radioactive elements by isolating them physically from the biosphere by installing successive barriers at a suitable depth in the ground. This strategy leads to deep geological disposal;
we can make use of nuclear reactions that will transmute the very long-lived wastes into less radioactive or shorter-lived products.

Whatever the solution chosen for highly radioactive wastes, deep geological repository disposal will always be necessary. There exists a volume of longlived and intermediate level waste arising from irradiated structural materials and losses during the various operations of the nuclear fuel cycle that is 20 times greater than that of the high-level waste. Tests are in progress to try to reduce the volume of these wastes, but there remains a lower threshold below which we cannot reasonably go. For humankind, the risks of a waste storage site depend on its radiotoxicity and the possibility of transfer to the biosphere. This transfer can occur after failure of the barriers and subsequent migration of the elements into the surrounding geosphere. International studies (Pagis, PACOMA) have shown that these phenomena are very slow, so that no activity would be noticeable for at least 400 000 years, the effects would be very slight, that is to some four orders of magnitude below the average level of natural radioactivity (2 msv/year) and the limits recommended for radiological protection (1 msv/year). Uncertainties regarding the transfer mechanisms, however, as well as the fact that we cannot totally dismiss the possibility of the waste coming into contact with the biosphere following a geological upheaval or accidental intrusion foster the distrust of the public. These uncertainties can only be approached by a statistical analysis of the possible threats. Different irradiated fuel management approaches can be envisaged:

Deep geological disposal of irradiated fuel without reprocessing. The fuel is encapsulated after some interim storage time period varying from 10 years as planned for in the US to 40 years as planned for in Sweden to allow sufficient decay of the residual power. This solution may be the least expensive and requires the least handling. on the other hand, it implies some waste of energy, the formation of what are in fact uranium and plutonium mines.
The alternative strategy of reprocessing of the spent fuel followed by deep geological disposal of wastes has been chosen by France, UK, Japan and other countries. Uranium and plutonium are quantitatively separated from the other nuclides with yields ranging from 99.7 to 99.9%. The recovered uranium is re-enriched and recycled in LWRs thus reducing the need for fresh uranium ore. The plutonium can be used in LWRs (or even better in Fast Reactors) as Mixed oxide fuel. Fabrication of MoX fuel implies that the reprocessing has taken place shortly prior to fuel fabrication in order to keep the radiation levels in the fabrication plant as low as possible. By storing separated plutonium, Am-241 will grow-in from decaying Pu-241 . According to this approach the considerable energy content of the spent fuel is put to work in LWRs or Fast Reactors. The minor actinides and highly radioactive fission products are embedded in glass and are to be placed at the proper time into deep geologically sealed repositories. Their radiotoxicity decreases with a factor of 10 to 100 in 10000 years. While the recycling of plutonium in LWRs decreases the growth rate of plutonium stocks, only the use of Fast Reactors specially designed to burn plutonium can decrease the plutonium inventory of spent fuel.
Advanced Reprocessing involves the separation, not only of uranium and plutonium, but also that of the so-called "Minor Actinides" (neptunium, americium, curium) and some long-lived fission products into single element or element-group packages with similar nuclear and/or chemical properties. This way suitable solutions can be designed to improve conditioning or to set up transmutation scenarios. Transmutation of Pu and minor actinides will reduce the radiotoxic potential of high-level waste but has little effect on the release rate of the radioactivity to the environment since the very low solubility of the actinides is the controlling transfer factor to the biosphere. Further R&D is required to investigate all the aspects of this way of waste management in order to be able to truly assess its benefits or consequences for the fuel cycle. Among the problems to be solved are the partitioning of hazardous materials with a high efficency and their subsequent transmutation.

The destruction of plutonium and the minor actinides

Plutonium destruction

With the nuclear power stations currently in operation, a fraction of the plutonium separated in reprocessing plants is used as fuel in pressurized water reactors (PWRs). Good use is thus made of the fissile nature of this plutonium by replacing part (one third) of the enriched uranium oxide sub-assemblies by MoX sub-assemblies (Mixed oxide: mixed uranium and plutonium oxides) when reloading a part of these reactors.
In this way, the plutonium inventory jc stabilized as electricity is produced. During this successive recycling in conventional PWRs. however, an increasing proportion of the plutonium isotopes 238, 240 and 242 are formed, and these are neutron absorbers.
Because of this, successive recycling may be limited to just once or twice. This, however, slows down the growth of the plutonium inventory by 30 % compared to what it would be without the use of MoX fuel in PWRs. Moreover, in view of the time required for fuel fabrication, plant residence, and interim storage before reprocessing, the use of MoX fuel postpones by 15 to 30 years the time when the question of how to eliminate the plutonium by other means must be addressed. It must also be noted that experiments have shown that it should be possible to process recycled plutonium so as to increase the number of recycling stages.
Moreover studies are now in progress to design light water reactors with 100 % MoX fuel sub-assemblies.
The fast reactor technology required to control and adjust the plutonium inventory has already been developed, although our present limited operational experience should be extended: the fast breeder reactors that exist in the US (EBR2), in the UK (PFR), in France (Phenix, Superphenix), in Japan (Monju), in Kazakhstan (BN-350) and in Russia (BN-600). In this type of reactor, the chain reaction is maintained by a mixed uranium and plutonium oxide fuel. The evolution of the amount of plutonium depends on the balance of its production by U-238 neutron capture and its destruction by various nuclear reactions.
Depending on the proportion of the U238 isotope in the core, a fast reactor can therefore be a breeder, a regenerator or a plutonium burner. Moreover the neutronic characteristics of these reactors allow them to use plutonium containing a high proportion of the even-numbered isotopes and therefore to burn plutonium of various origins, notably plutonium which has already been recycled one or more times in a PWR.

Destruction of the minor actinides

The recycling of the minor actinides in fast reactors and in PWRs is possible, but in the case of the PWRs the unfavorable consequences for the fuel cycle (an increase of the neutron and gamma sources) are much greater. Moreover, a higher fuel enrichment is required. The fact that each isotope has different characteristics means that a different solution must be found for each. The first concern is to be able to separate them (it would be more effective to destroy the different minor actinides separately provided we have efficient separation methods at our disposal). With some minor modifications, present reprocessing plants could separate about 80% of the neptunium. High-level partitioning of neptunium, americium and curium is the objective of research programs in many countries (France, the EEC, Japan, the US) with a view to the development of efficient liquid and solid partitioning methods.
It is essential to note that the elimination of the minor actinides does not yield a radiological benefit unless the plutonium is also eliminated. In view of this fact, we have to consider long-term scenarios in which PWRs and fast reactors are both used for the recycling and incineration of the actinides.
New reactor developments using plutonium and minor actinides as fuel are required to cope with this problem. Design studies of actinide burner reactors have been made in Japan and in the US and ought to be pursued in Europe.

Destroying long-lived fission products

As fission products are not fissile, their transmutation into stable elements has to be carried out by specific nuclear reactions induced by particles such as neutrons, protons, photons or light nuclei. The probability of these reactions is very low: therefore very long irradiation time or very high fluxes must be used to obtain acceptable results.
Maybe it will be possible to burn fission products in moderated sub-assemblies in the outer core zone of a fast reactor. otherwise the designing of high-flux reactors in which actinides and fission products could be treated simultaneously has apparently come up against difficulties that cannot be easily overcome. The use of particle accelerators or systems coupling a proton accelerator with a sub-critical reactor are therefore now being envisaged. Research is in progress, particularly in the US and in Japan, but the technological uncertainties of such projects are considerable.

Conclusions

The partitioning and transmutation of minor actinides and fission products has been identified as being an alternative strategy for the long-term management of long-lived radioactive waste from power reactors. In order to be able to determine whether partitioning and transmutation can become a viable alternative to the largely proven existing strategies of direct disposal and reprocessing (with no partition of minor actinides and fission products), a considerable amount of investigation will be necessary.
In order to be able to define the best strategies for efficient use of the world's natural uranium resources and for the long-term management of long-lived radioactive waste, it is the duty of the scientific community in Europe to consider all the viable technical possibilities. This paper has identified the main areas where research into partitioning and transmutation are needed:

continuation of studies on deep geological repositories;
optimization of LWR core design, fuel and/or fuel cycle to minimize the total amount of long-lived radionuclides produced in irradiated fuel;
development of advanced reprocessing techniques with a view to waste incineration or chemical stabilization;
optimization of Fast Reactors with the objective of maximizing plutonium and actinide consumption;
feasibility studies of devices able to significantly reduce the inventories of long-lived radioactive products and to put them to use.

The nuclear power scientific community is studying these various avenues.
In France, advanced actinide reprocessing and subsequent incineration is being studied, notably in view of carrying out a possible scientific demonstration in Superphenix.
In the United States, a dry method of actinide partitioning is being studied in Argonne as well as incineration by means of accelerators.
In Japan, a vast project OMEGA will cover all the above strategies.
Research is being carried out worldwide on the most suitable management methods for the remaining radioactive waste which will eventually lead to final deep geological repository disposal of high-level fission product waste. International collaboration is being set up for all these research issues. It should thus be possible in the future to reduce even further the volume and toxicity of the final waste products remaining from nuclear power production.

Last reviewed: 3 February 2012