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EVALUATION of THORIUM BASED NUCLEAR FUEL Chemical Aspects

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EVALUATION of THORIUM BASED NUCLEAR FUEL Chemical Aspects JULY 1995 ECN-R--95-007 A / energy innovation EVALUATION OF THORIUM BASED NUCLEAR FUEL Chemical Aspects R.J.M. KONINGS PJ.A.M.BLANKENVOORDE E.H.P.CORDFÜNKE K. BAKKER VOL 2 7 K \ 0 The Netherlands Energy Research Foundation ECN Het Energieonderzoek Centrum Nederland (ECN) is is the leading institute in the Netherlands for energy het centrale instituut voor onderzoek op energie- research. ECN carries out basic and applied research gebied in Nederland. ECN verricht fundamenteel en in the fields of nuclear energy, fossil fuels, renewable toegepast onderzoek op het gebied van kernenergie, energy sources, policy studies, environmental aspects fossiele-energiedragers, duurzame energie, beleids- ofenergy supply and the development and application studies, milieuaspecten van de energievoorziening en of new materials. de ontwikkeling en toepassing van nieuwe materialen. ECN employs more than 800 staff. Contracts are Bij ECN zijn ruim 800 medewerkers werkzaam. De obtained from the government and from national and opdrachten worden verkregen van de overheid en van foreign organizations and industries. organisaties en industrieën uit binnen- en buitenland. ECN's research results are published in a number of De resultaten van het ECN-onderzoek worden neer- report series, each series serving a different public, gelegd in diverse rapportenseries, bestemd voor ver- from contractors to the international scientific world. schillende doelgroepen, van opdrachtgevers tot de internationale wetenschappelijke wereld. The R-series is for research reports that make the results of ECN research available to the international De R-serie is de serie research-rapporten die de technical and scientific world. resultaten van ECN-onderzoek toegankelijk maken voor de internationale technisch-wetenschappelijke wereld. Netherlands Energy Research Foundation ECN Energieonderzoek Centrum Nederland P.O. Box 1 Postbus l NL-1755ZG Petten 1755 ZG Petten the Netherlands Telefoon : (02246) 49 49 Telephone : +31 2246 49 49 Fax :(02246)44 80 Fax : +31 2246 44 80 Dit rapport is te verkrijgen door het overmaken van This report is available on remittance of Dfl. 35 to: f35,-- op girorekening 3977703 ten name van: ECN, Facility Services, ECN, Faciliteiten Petten, the Netherlands te Petten Postbank account No. 3977703. onder vermelding van het rapportnummer. Please quote the report number. © Netherlands Energy Research Foundation ECN © Energieonderzoek Centrum Nederland JULY 1995 ECN-R--95-007 KS001973299 R: Fl DE008367595 EVALUATION OF THORIUM BASED NUCLEAR FUEL Chemical Aspects R.J.M. KONINGS P.J.A.M. BLANKENVOORDE E.H.P.CORDFÜNKE K. BAKKER Abstract This report describes the chemical aspects of a thorium-based fuel cycle. rIt is part of a series devoted to the study of thorium-based fuel as a means to achieve a considerable reduction of the radiotoxicity of the waste from nuclear power production. Therefore special emphasis is placed on fuel (re-)fabrication and fuel reprocessing in the present work. i Keywords thorium fuel cycle ECN-R-95-007 CONTENTS 1. INTRODUCTION 5 2. OCCURRENCE 7 3. ORE PROCESSING 9 4. RADIOTOXICITY 10 5. PHYSICO-CHEMICAL PROPERTIES 11 5.1 ThO2 11 5.2 (Th,U)O2 16 5.3 (Th,Pu)O2 21 6. FUEL FABRICATION 22 6.1 Thoria and mixed thoria/uraida for LWRs 22 6.2 Thoria coated particles for HTGRs 23 7. IRRADIATION PERFORMANCE 25 8. FUEL REPROCESSING, THE THOREX PROCESS 27 8.1 Head-end treatment and dissolution of the fuel 27 8.2 The extraction of uranium and/or thorium 28 8.3 Separation of thorium and uranium 30 8.4 Development and status 31 9. FUEL REPROCESSING, PYROCHEMICAL TECHNIQUES 33 10.FUELREFABRICATION 34 11. WASTE MANAGEMENT 35 12. CONCLUSIONS AND RECOMMENDATIONS 36 REFERENCES 37 APPENDIX A. ABREVIATIONS AND DEFINITIONS 41 A.I Abreviations 41 A.2 Definitions 41 APPENDIX B. THE THERMAL PROPERTIES OF THO2(G) 42 REFERENCES 43 ECN-R-95-007 Evaluation of thorium-based nuclear fuel. Chemical aspects ECN-R-95-007 1. INTRODUCTION The major part of the nuclear power is being generated in reactors which are operated in the uranium cycle, where 235U is the fissile material and 238U is the matrix. A major disadvantage of the use of •uranium fuel is the production of transuranium elements through neutron capture in 238U and in the products of this process. These elements, in particular pluto- nium, neptunium and americium, contribute significantly to the long-term radiotoxicity of the high-level waste for which underground burial is being considered in most countries. Presently, partitioning and transmutation of the above mentioned transuranium elements is attracting much attention as it might be a means to achieve a considerable reduction of the radiotoxicity of the waste from nuclear power production. A different approach to this waste problem is the use of thorium-based fuel as reactors operated in a thorium fuel cycle will produce much less transura- nium elements per unit of fission energy. However, the alpha-toxicity of the waste from thorium-fueled reactors is still above the level of the ore of which the fuel is fabricated, as non-natural long-lived uranium isotopes (232U, 233U, 234U) are formed in significant amounts during irradiation. Because natural thorium, 232Th, is not a fissile nuclide, an external start- up material has to be added to a thorium-based fuel. This can be natural uranium, enriched in 235U, or plutonium from reprocessing of LWR fuel. During irradiation of thorium fuel 233U is formed as a result of neutron capture in 232Th. This 233U is a highly fissile nuclide which significantly contributes to the power production, like 239Pu in the uranium cycle. It can make-up material fuel fabrication reprocessing Reactor waste mining ore geological disposal Figure 1.1 Schematic diagram of a thorium-based fuel cycle which includes repro- cessing ECN-R-95-007 Evaluation of thorium-based nuclear fuel. Chemical aspects be recovered through reprocessing of thorium fuel and can subsequently be used as an internal start-up material. If there is a net production of 233U at the end of the cycle, the amount of the external start-up material to be added to the fresh fuel levels off to an equilibrium value after a number of cycles. In the ideal case, addition of external material is not necessary as the discharged fuel contains sufficient 233U for a subsequent fuel loading, and one speaks of a self-sustaining equilibrium thorium cycle. In such equilibrium scenario's reprocessing and (re)fabrication of mixed thorium- uranium oxide fuels are important issues. The present report describes the chemical aspects of thorium dioxide fuel relevant to the thorium-based fuel cycle. The objective is to give an overview of the present status and to identify the areas where further re- search is needed. Not taken into account are fuel cycles based on molten salt technology, for example the thermal Th/U breeder concept proposed by workers from CERN. This work is part of a series of reports describing the potentiality of the tho- rium fuel cycle for reducing the radiotoxicity of the nuclear waste generated by nuclear power production. ECN-R-95-007 2. OCCURRENCE In nature thorium is present in accessory minerals in (late) magmatic rocks such as leucocratic granites, pegmatites, carbonatites and hydrothennal veins. In these rocks it occurs as a major component in minerals like thorite (ThSiO^) and thorianite (TI1O2)) which can also contain significant amounts of uranium. However, the principal thorium ore that is currently used, is the rare-earths phosphate monazite (L11PO4) which normaly con- tains 5-30% ThO2- As these minerals are moderately resistant to weather- ing, they are often concentrated as detritial minerals in stream and beach sands. Especially unconsolidated coastal beach deposits are well known for their high enrichment in monazite. For example, the beach placers in the state of Kerala (India) have been one of the world's major sources of monazite since the beginning of this century. Thorium ores are found in many countries. A compilation of the thorium resources [1] in the WOCA countries (World Outside Centrally Planned Economies Area) is given in Table 2.1. No reliable data are available for Eastern Europe, USSR and People's Republic of China. Large "reason- ably assured resources" (RAR) are present in Brazil, India, Turkey and USA. These are deposits of known size, grade and configuration so that the amount of thorium that is recoverable by current methods, can be speci- fied. Large "estimated additional resources" (EAR) are reported for Brazil and, on a smaller scale, in Turkey, Egypt, Canada and the USA. Monazite Table 2.1 Resources of thorium in some WOCA countries (as Th) Country Reasonably assured Estimated additional (109 g) (109 g) Argentina 1 - Australia 19 — Brazil 606 700 Canada 45 128 Egypt 15 280 Finland - 60 Greenland 54 32 India 319 — Iran — 30 Korea 6 — Liberia 1 — Norway 132 132 Madagascar 2 20 Malaysia 18 - Malawi — 9 South Africa 18 — Turkey 380 500 Uruguay 1 2 USA 137 295 total 1754 2188 ECN-R-95-007 Evaluation of thorium-based nuclear fuel. Chemical aspects has also been found to be an important constituent of the beach sands of the North Sea in northern part of the Netherlands [2]. On the basis of radiometric research, the total amount of thorium is estimated to be about 200 106 g [3], which is small compared to the world resources. In the earth's crust thorium is estimated to be at least three times more abundant than uranium. However, the total WOCA uranium resources are much larger than the thorium resources given in table 2.1: RAR are 4412 109 g and EAR are 2182 109 g [4]. This difference is is mainly due to the fact that systematic exploration of thorium resources has seldomly been performed. In most cases the discoveries are a by-product of the uranium exploration.
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