Actinide Co-Conversion by Internal Gelation
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0986-OO09-03
Actinide Co-Conversion by Internal Gelation Anne-Charlotte Robisson, Jacques Dauby, Corinne Dumont-Shintu, Estelle Machon, and Stéphane Grandjean CEA Marcoule, Nuclear Energy Division, DRCP/SCPS/LCA, BP 17171, Bagnols-sur-Cèze, 30207, France ABSTRACT Suitable microstructures and homogenous microspheres of actinide compounds are of interest for future nuclear fuel or transmutation target concepts to prevent the generation and dispersal of actinide powder. Sol-gel routes are being investigated as one of the possible solutions for producing these compounds. Preliminary work is described involving internal gelation to synthesize mixed compounds including minor actinides, particularly mixed actinide or mixed actinide-inert element compounds. A parameter study is discussed to highlight the importance of the initial broth composition for obtaining gel microspheres without major defects (cracks, craters, etc…). In particular, conditions are defined to produce gel beads from Zr(IV)/Y(III)/Ce(III) or Zr(IV)/An(III) systems. After gelation, the heat treatment of these microspheres is described for the purpose of better understanding the formation of cracks after calcination and verifying the effective synthesis of an oxide solid-solution. INTRODUCTION In the context of both the advanced fuel cycle (Generation IV program) and the transmutation of long-lived radionuclides (minor actinides) new concepts are required for the design and fabrication of fuels and targets. Several options are being considered, notably mixed actinide phases in the form of small beads (microspheres about 500 µm in diameter). Mixed actinide phases can be either pure actinide-based compounds (e.g. containing U, Pu and minor actinides) or oxides formed with an inert element (e.g. Zr). Sol-gel processes meet the requirements for producing these specific mixed phases. Development work is in progress in the Atalante laboratory complex on applying the well known internal gelation process, initially developed by KEMA laboratory in the 1960s on uranyl solutions [1], to actinides at different oxidation states. In this process, condensation reactions are initiated by hexamethylenetetramine (HMTA) and urea. When heated in acidic media, HMTA decomposes into ammonia causing the pH to rise and resulting in the following actinide hydrolysis and condensation reactions. HMTA decomposition: (CH2)6N4 + 4 H+ + 6 H2O ⇔ 4 NH4+ + 6 CH2O Hydrolysis: Me4+ + x H2O ⇔ Me(OH)x4-x + x H+
(1) (2)
These studies were then extended to the fabrication of actinide oxides, carbides or nitrides by internal gelation of U(VI) and/or Pu(IV) solutions; they were the subject of many publications between 1960 and 1990 [2 - 4]. In the context of fabricating future nuclear fuels or transmutation targets, this method has several advantages including easy implementation, minimization of contaminated dust particles, and potential process improvements [5]. In particular, internal gelation of minor actinides could be a very promising avenue of research. We investigated the
production of
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