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TRANSMUTATION of AMERICIUM and CURIUM in a LANTHANIDE MATRIX COMPANY WIDE CW-123700-CONF-018 Revision 0

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TRANSMUTATION of AMERICIUM and CURIUM in a LANTHANIDE MATRIX COMPANY WIDE CW-123700-CONF-018 Revision 0 Conference Paper TRANSMUTATION OF AMERICIUM AND CURIUM IN A LANTHANIDE MATRIX COMPANY WIDE CW-123700-CONF-018 Revision 0 Prepared by Rédigé par Hyland Bronwyn Reviewed by Vérifié par Edwards Geoffrey W R Approved by Approuvé par Hyland Bronwyn 2012/07/04 2012/07/04 UNRESTRICTED ILLIMITÉ Atomic Energy of Énergie Atomique du Canada Limited Canada Limitée Chalk River, Ontario Chalk River (Ontario) Canada K0J 1J0 Canada K0J 1J0 Proceedings of Global 2011 Nagoya, Japan, December 11-15, 2011 UNRESTRICTED CW-123700-CONF-018 Transmutation of Americium and Curium in a Lanthanide Matrix B. HYLAND1, E.D COLLINS2, R. J. ELLIS2, G. DEL CUL2 and M. MAGILL1 1Chalk River Laboratories, Atomic Energy of Canada Limited, Canada, K0J 1J0 Tel: +1(613)584-9243 ex. 44707 , Fax: +1(613)584-8198 , Email: [email protected] 2Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee, U.S.A. Abstract – With world stockpiles of used nuclear fuel increasing, the long-term remediation of used fuel is a growing concern. Many of the transuranic (TRU) actinides in nuclear spent fuel produce decay heat for long durations, resulting in significant nuclear waste management challenges. Partitioning and transmutation of the spent fuel is one possibility to deal with the long-lived transuranic actinides. It is desirable to develop a partitioning and transmutation scheme that is as straightforward as possible for both the reprocessing step and the transmutation in reactor. This study looks at a reprocessing scheme that is simplified to keep the americium, curium and lanthanides together in one recycle stream. These actinides can be transmuted to fissionable isotopes and then to shorter-lived fission products in a CANDU reactor. Many of the design features of the CANDU reactor make it uniquely adaptable to actinide transmutation. The small, simple fuel bundle eases the fabrication and handling of active fuels. Online refuelling allows precise management of core reactivity and the high neutron economy of the CANDU reactor results in high TRU destruction to fissile-loading ratio. The transmutation scheme described in this paper places the americium/curium/lanthanide mixture, diluted with an inert matrix, into the centre pin of a CANFLEX fuel bundle. This centre pin can be removed and reinserted into a new fuel bundle for further irradiation to achieve higher total transmutation of the actinides. This scheme provides the dual benefit of transmuting long-lived actinides into shorter-lived fission products, while at the same time reducing the void reactivity coefficient of the reactor. I. INTRODUCTION americium transmutation schemes to study fuel that has decayed for a significant length of time after exit. This Management of spent nuclear fuel will continue to be study uses 10-year-cooled fuel, so that there is a a challenge in the future, as stockpiles increase worldwide. significant build-up of Am-241 to be transmuted. Many of the transuranic (TRU) nuclides are long-lived, Studies conducted previously [1-7] have and continue to produce heat for thousands of years. This examined scenarios in which americium has been long-term decay heat presents a challenge to the disposal separated from spent fuel, either by itself, with curium, or of spent nuclear fuel (SNF). Reducing the decay heat of with all of the TRU nuclides. This study looks at a spent nuclear fuel from LWRs will increase the capacity of simplified scheme in which the americium, curium and long-term geological disposal sites. Am-241, with a half lanthanides are all kept together in one stream. life of 432 years, is a major contributor to the long-term The CANDU reactor offers attractive solutions for decay heat. Its primary contribution is between 100 and effectively dealing with used nuclear fuel from a light 1000 years after the fuel exits the reactor. Am-241 is water reactor (LWR) fleet. Many of the design features of produced in the reactor during irradiation, but most of the the CANDU reactor make it uniquely adaptable to actinide Am-241 in SNF results from the decay of Pu-241 (half life transmutation as well as utilization of LWR used fuel with of 14 years) after the fuel has been discharged from the minimal reprocessing. The most significant feature is the reactor. Consequently, it is beneficial when investigating high neutron economy resulting from the heavy water CANDU is a registered trademark of Atomic Energy of Canada Limited (AECL). Proceedings of Global 2011 Nagoya, Japan, December 11-15, 2011 UNRESTRICTED CW-123700-CONF-018 moderator, which allows a high TRU transmutation rate available for transmutation rather than being parasitically relative to the fissile loading because more neutrons are absorbed in the moderator. Online refuelling also allows Cm-245 Heavier Cm-242 Cm-244 (n, γ) (n, γ) 162.8 d 18.10 y 8500y mass curium (n, f ) isotopes - 83% Am-242m Am-244m Am-241 141 y 26 m (n, γ) IT Am-243 - 432.2 y (n, γ) (n, γ) 7370 y (n, f ) Am-242 (n, f ) Am-244 16.02 h 10.1 h - -, EC EC (17%) Pu-238 Pu-239 Pu-240 Pu-241 Pu-242 Pu-243 (n, γ) (n, γ) (n, γ) (n, γ) (n, γ) 87.7 y 2.4E5 y 6563 y 14.4 y 3.7E5 y 5 h (n, f ) - (n, f ) - Figure 1. The transmutation pathways of Am-241. precise management of core reactivity, and further decay to Cm-244. Cm-244 has a relatively short half-life, increases the neutron economy relative to batch and alpha decays to Pu-240. Am-242m can also decay by refuelling. Lastly, the small and simple fuel bundle electron capture to Pu-242. The isotopes Cm-242, Cm- simplifies the fabrication and handling of active fuels. 244 and Pu-238 all have an impact on the decay heat of CANDU fuel bundles are short in length (49.52 cm) and the spent fuel. light in weight (~21 kg), consisting of either 37 pins or Am-242m, Cm-245, Pu-239, and Pu-241 are the 43 pins (CANFLEX® fuel) which simplifies the fissile isotopes. The other isotopes act as a poison, fabrication and handling of the bundles. These capturing neutrons and reducing the coolant void characteristics also enable a CANDU fuel bundle to reactivity (CVR) of the bundle (when located in the function as a target carrier with minimal or no design central element). change to the bundle. With regard to curium production, for schemes with relatively short irradiation time (a few years) the curium 1.1 Transmutation Pathways isotopes that are created are the lower mass, short-lived curium isotopes, Cm-242 and Cm-244. These isotopes The transmutation of Am-241 follows several pathways have half-lives on the same time scale as fission products, that affect the decay heat production of the spent fuel, and once produced in the used fuel the curium could be and result in the production of isotopes of curium and stored and decayed in a manner similar to fission plutonium. In the first step, a neutron captures onto products, and not put into long-term storage or further Am-241, creating Am-242 or Am-242m. transmuted. Several different pathways are available after the initial neutron capture. Am-242m has a high fission 1. REPROCESSING/RECYCLING SCHEME cross-section, so by this path the Am can be transmuted by fission. In the second pathway the Am-242 beta The proposed reprocessing/recycling scheme for TRU decays into Cm-242. The Cm-242 then alpha decays actinides is illustrated in Figure 2. The separations steps with a relatively short half-life (163 days), and some of would be simplified significantly by not requiring the the original americium will end up as Pu-238. The complex steps needed to separate the chemically similar, Am-242m can also neutron capture to Am-243, and a trivalent TRU actinides, americium and curium, from the second neutron capture creates Am-244 or Am-244m. trivalent lanthanide fission products. The overall scheme The Am-244 nuclides both have short half-lives and beta fits the definition of “modified open cycle” since ~10% to 20% of the americium-curium would eventually be CANFLEX® is a registered trademark of AECL and the Korea Atomic Energy Research Institute (KAERI). Proceedings of Global 2011 Nagoya, Japan, December 11-15, 2011 UNRESTRICTED CW-123700-CONF-018 disposed, along with all of the fission product elements, to the high level waste emplaced in a geologic repository. Separations U-Pu-Np MOX Fuel SNF LWRs Process “A” Product Blending Fabrication U-Pu-Np Am-Cm FPs “Burnable Poison” Separations Separations Am-Cm-Ln FPs Target Rod HWRs Process “B” Process “C” Fabrication All FPs + Other FPs HLW Solidification and Disposal Residual Am-Cm Figure 2. Reprocessing/Recycling Scheme Separations Process A (Figure 2) would likely be a hafnium contamination in this model of zirconia. The solvent extraction process using tri-normal butyl purpose of the zirconia is to dilute the Am/Cm/Ln phosphate as the extractant to recover the recycle fuel mixture, in order to investigate the effect of varying the components, uranium, plutonium, and neptunium. amount in the centre pin. The amount of Am/Cm/Ln in Several options for Separations Process B will be the centre pin was varied between 5% and 60% by considered. One option is to use the TRUEX process, volume. Other methods to vary the quantity of followed by a cation resin loading/ calcination process, Am/Cm/Ln could be employed, such as reducing the although it is possible that the TRUEX process could be density of the centre pin, or dilution using recycled omitted. In the latter version, the monovalent and uranium.
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