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TOPICAL REPORTS

Burning of : A complementary waste management option?

Partitioning and transmutation of actinides and fission products may serve as an additional tool in waste management strategies

by L.H. Baetsle W'e orldwide nuclear electric capacity amounts The vitrified waste is stored in engineered to nearly 340 gigawatts-electric (GWe) and facilities awaiting final disposal in underground produces spent fuel roughly amounting to 9000 repositories. Disposal of vitrified high-level tonnes heavy (tHM) per year. Each tonne waste (HLW) containing the minor actinides, or of spent fuel contains about 10 kg of trans- spent fuel with and minor actinides, (TRU) actinides, of which 0.8 kg are are from the environmental point of view very minor actinides, and 30 kg of fission products, similar at least during the first millennia. Beyond including 4 kg that are long-lived nuclei (having 10 000 years — the technical lifetime of an half-lives greater than 30 years). underground repository — spent fuel with its full The fate of the spent fuel depends to a great load of plutonium becomes the dominant en- extent on the national fuel cycle policy. For one- vironmental hazard. half of the world's capacity of plants, the policy calls for reprocessing, plu- tonium recovery, vitrification of residues, and Interest in partitioning and transmutation disposal of wastes. Among the countries follow- ing this course are France, United Kingdom, Two decades ago the question was already Japan, Germany, Belgium, Switzerland, the raised: Can we avoid the long-term hazard as- Commonwealth of Independent States, and sociated with TRU actinides and long-lived fis- countries formerly in the Soviet alliance. Large sion products? Important research and develop- reprocessing facilities have been constructed and ment (R&D) programmes were run in the Euro- are in operation in France and the United King- pean Community and in the USA to investigate dom while large plants are under construction in this issue. The success of these efforts was very Japan and . limited. It soon became apparent that it was prac- The other half of the world's nuclear capacity tically impossible to eliminate all TRU actinides produces spent fuel as a waste product. The so- and that some fission products, e.g. - called "once-through cycle" is being pursued in 99, -135, and -129, are equally the , , Sweden, Spain, and important in the dose-to-man evaluation on the some other countries. The long-term storage of horizon of a million years. spent fuel in engineered facilities is the present However, there is renewed interest in par- trend. Storage is to be followed by disposal in titioning and transmutation (P&T), largely be- suitable geologic formations. cause of difficulties encountered throughout the The TRU actinides are building up at a rate world in finding suitable geologic formations in of about 90 tHM per year. Approximately 45 locations which are acceptable to the public. tHM will remain occluded in the spent fuel struc- In 1988, the Japanese Atomic Energy Com- tures, leaving about 45 tHM available; 92% as mission launched a very important and com- recycled plutonium and 8% as minor actinides prehensive R&D programme. (See the following (, , ) immobilized in article.) It is aimed at the elimination of long- vitrified waste. term hazards resulting from nuclear power prod- uction and at the optimal use of resources. This initiative aroused interest from some other major Dr Baetsle is Counsellor to the Chairman and General nuclear countries, for example France, to set up Management of the Study Centre for Nuclear Research research programmes to improve or adapt the (SCK/CEN) in Mol, Belgium. currently used PUREX process. The objective

32 IAEA BULLETIN, 3/1992 TOPICAL REPORTS was to reduce the plutonium content in HLW and be anticipated that this or a similar separation to eliminate minor actinides by installing new or technology will, if sufficiently funded, come up additional extraction steps. with a dependable process compatible with the The P&T strategy can only be implemented PUREX process. The partitioning step will by a fuel-cycle policy incorporating reprocess- produce single elements or groups of elements ing as a key step through which all major ac- that are the material sources for transmutation tinides (uranium, plutonium) are recycled, and processes (neptunium, americium, curium), or which is capable of isolating the minor actinides that may become strategic resources for the fu- and some long-lived fission products from the ture (technetium and elements). effluent stream in order to prepare them for the The ranking of fission products is not altered subsequent transmutation steps. by the role of geologic confinement. Technetium- 99 and iodine-129 remain high on the list of the fission products to be examined in a P&T option General strategies and schemes because of their mobility in the geosphere. As a conclusion, one may postulate that a The general strategy of introducing P&T as P&T strategy ought to provide as much radio- an alternative waste management option is based logical protection to humanity as geologic dis- on the radiological benefit which is expected posal does. The selection of nuclides to be sep- from such a venture. The selection of the ac- arated and transmuted, and to what extent they tinides and long-lived fission products which are have to be eliminated, will be determined by the beneficial to eliminate by transmutation depends trade-off between a decreasing confidence in the upon a number of technical factors, including merits of geologic disposal as the time hazard and decontamination factors, and the ef- becomes unimaginable on a human scale, and fect of geological confinement.* Long-lived fis- the increase in waste management costs from sion products are much less toxic than actinides improved reprocessing and P&T. on the basis of hazard indexes once -90 P&T is not an alternative long-term and caesium-137 have decayed, that is after waste management option. Rather, it is a com- about 600 years. (See tables on page 34 for a plementary technique to geologic disposal cap- ranking of actinides and fission products by their able of further decreasing the radiological im- hazard factors.) pact of the fuel cycle over the very long term. There are two ways to approach the separa- tion of minor actinides and long-lived fission products from reprocessing streams: by modify- Recycling of minor actinides ing the current processes in order to reroute the critical nuclides into a single solution, for ex- If partitioning were to be implemented in the ample high-level liquid waste, and use this as a large reprocessing units (La Hague, Sellafield, source for partitioning processes; and by exten- Rokkashomura, etc.) a total output of about 1700 sion of the conventional PUREX process to all kg of neptunium-237 and 1500 kg of americium minor actinides and long-lived fission products (including some kg of curium) would become in second generation reprocessing plants. available each year. This corresponds to roughly Prior to the implementation of one of these 44% of the total world output. schemes, it seems obvious to improve the According to the transmutation method used, separation yield of plutonium from HLW within the minor actinides would be conditioned as the presently running plants. Rerouting nuclei oxides or in specially equipped fuel into single product or waste streams is very im- fabrication facilities which have to be built for portant for neptunium which occurs in many that purpose. Depending on the type of recycling different process streams. (homogeneous or heterogeneous) the dedicated The implementation of a step for technetium fabrication capacity ought to be in the range of recovery is not only important in a P&T strategy 68 to 85 tonnes of mixed oxide (MOX) per year. but is also a means to reduce the uranium- These facilities would be closely associated with plutonium contamination. the reprocessing plant activities. There are a number of reagents for partition- The production of metal fuel is based on a ing of actinides, the most promising of which up pyrometallurgical refining process which is to now is known as CMPO, which can be used in presently at the bench-scale development stage conjunction with TBP in the so-called TRUEX in the USA and Japan. A total capacity of 80 process. While additional work is needed, it may tHM per year of dry reprocessing-fuel fabrica- tion would have to be built in order to treat the * Comprehensive technical details are available from the output of minor actinides from the USA and author. some other countries.

IAEA BULLETIN, 3/1992 33 TOPICAL REPORTS

studied in Japan are two new concepts which aim Actinides in high-level waste at the nuclear incineration of actinides. ICRP-61 1/tHM 1 Am-241 3.3- 12.3 x1013 12 Approaches and alternatives 2 Am-243 1.8-2x10 3(1%) Pu-240 7.6-8x1011 11 In conclusion, P&T is becoming an addition- 4(1%) Pu-239 3.2-3.5x10 al tool in the overall waste management strategy 5 Np-237 4.7-6.4x1010 10 to reduce the radiological impact of actinides and 6 Cm-246 2.4-2.7x10 long-lived fission products. However, it is not a full alternative to geological disposal. Fission products Improved reprocessing can significantly Nuclide 1/tHM reduce the plutonium content in HLW. Some partitioning techniques for americium and 1 Sr-90 3.9 x 1012- 2.13x10" curium are promising and rerouting of neptunium 2 Cs-137 3.8 x1012- 3.66x10" 9 in conventional reprocessing is beneficial. 3 Tc-99 1.6 x10 A comprehensive scheme of partitioning, 4 Sn-126 6.6 x108 8 which includes all minor actinides and long- 5 1-129 5.8 x10 lived fission products, is still a remote aim and 6 Cs-135 1.3X108 7 needs intensive R&D efforts. 7 Zr-93 9.4 x10 Transmutation of separated nuclides into Notes Rankings are on the basis of criterion in ICRP-61 of the International Commission on Radiological fuel pins or irradiation targets is a very impor- Protection They are based on the volume of drinking required to dilute a mixture of radionuclides to acceptable drinking water limits The hazard rankings for actinides are as they would occur in HLW cooled for 200 tant step in the overall P&T strategy. This further to 1000 years needs a thorough technological and economical analysis in the frame of a comprehensive trans- Ranking of However, these facilities would not be con- mutation technology. long-lived fission nected to the conventional reprocessing activi- Transmutation is a time-consuming process products and ties. They would be an integral part of fast-reac- which requires reactors with high and/or ener- actinides by tor complexes for burning actinides with a dedi- getic fluxes. Such neutron fluxes are radiological hazard cated capacity depending on the reactor power of available in high-flux reactors and fast reactors. each site. The transmutation yield in tailor-made reactor types, such as the IFR or MABR, is relatively high. Nonetheless, the time period to destroy the Transmutation of minor actinides nuclides is very long (20-30 years) depending on cooling- and fuel-cycle times. Accelerator In principle it is possible to transmute the driven reactors provide irradiation facilities with minor actinides in existing nuclear plants. How- fluxes which are roughly one order of magnitude ever, the transmutation is slow and produces higher and could shorten the irradiation periods essentially heavier nuclides, and repeated reuse significantly. of spent fuel materials is difficult if homo- The economics of the P&T approach needs a geneous recycling is adopted. The heterogene- very thorough analysis. This is important since ous recycling of minor actinides in specially improved reprocessing, chemical partitioning, designed fuel elements has not yet been dev- target fabrication and fuel development, fast eloped sufficiently .Transmutation of americium reactor development, and recycle operations for is possible under certain conditions. minor-actinide fuel require the construction and The most efficient transmutation of minor operation of important cycle actinides occurs in fast neutron reactors capable facilities. They further require the development of transforming the actinides into fission of mature partitioning and transmutation tech- products. A very important point to note is the nologies applicable to industrial quantities. large inventory of minor actinides of a fast reac- These efforts have to be compared with the tor. This permits the transfer of almost the entire problems associated with public acceptance of annual output to an "incineration" reactor. How- geologic repositories and the delayed hazards ever, the net annual incineration throughput is from the migration of very long-lived nuclei in rather limited to approximately 5%. geological strata. Efforts to reduce costs for Specially designed reactors are under study geologic disposal ought to be the economic to burn plutonium and minor actinides produced motor behind P&T, and steps to minimize at a light-water reactor station. The Integral Fast human exposures should be the driving Reactor (IFR) concept developed in the USA force for decreasing the long-term radiological and the Burner Reactor (MABR) impact of nuclear electricity production. n

34 IAEA BULLETIN, 3/1992