Melting Simulated High-Level Liquid Waste With Addition of TiN and AlN
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ABSTRACT Calcined simulated high level liquid waste (HLLW) with a desired amount of TiN and
AIN mixture was heat-treated at 1673-1873 K. It was revealed that the mixture of TiN and AIN (the atomic ratio of Al to Ti is 1:9) caused the melting of the specimen at 1673 K and the
separation of the elements into two groups: alloy phase and oxide phase. The analysis of the oxide phase showed that the compounds in it could be divided into four phases, and that all
fission product elements formed the complex oxides with Ti and Al.
It is considered that Al
and Zr dissolution in each phase contribute to the melting of the oxide phase at 1673K. A 30-
days Soxhlet leach test showed that the chemical durability of the oxide phase as a waste form was superior to that of glass waste form[l].
INTRODUCTION In order to develop a simpler and more rational solidification method than vitrification we have been studying a high temperature method by which HLLW is dry-treated in a simple
process[2,3,4,5]. As shown in Fig. 1, this method consists of 4 processes. Water and nitric acid in HLLW first vaporize at 973 K in the calcination process. Cs and Rb then vaporize up to 1273K by heating calcined HLLW and would be recovered by a cold trap for the attenuation storage. Further heating with a desired amount of a reducing agent, a mixture of TiN and AIN causes reduction and melting of the sample. Elements with higher standard free energy of oxide formation (platinum metals and other transition metals) than the reducing agent are reduced and form the alloy phase. Alkaline earth elements, rare earth elements, Zr, actinides and the metal elements of the reducing agent form the oxide phase of complex oxides. With the suitable reducing agent and treatment temperature the alloy phase and the oxide phase melt in the process and the both phases are obtained separately after cooling. -20
Cs
NOx
Rb
r 77NAIN
TT 4CLI~iW=ý L:.......1 "-973K Fission products
Uranlum Acthiide elements Corrosionproducts
I+
.. .~TION -12735g
]
.E. T!V .... =j4 EDLiCTiFON
O'ID-IFICATIO6N7
1673 A 5-
Room temperhaure
RuRA,Pd4 Mo, TeSe, Corrosion products
Actinide elements Rare earth elements Zirconium Transuranium elements
(Metalphase)
(Oxide phase) Fig.1 The process of the super high temperature method.
449 Mat. Res. Soc. Symp. Proc. Vol. 6086 2000 Materials Research Society
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Previous studies using simulated calcined HLLW have elucidated that the alloy phase melts up to 1673K by alloying the refractory platinum metals or Mo with corrosion products (Fe,Cr,Ni)[6], while the oxide phase melted around 1873K by the formation of complex oxides of fission product(FP) elements and Ti with addition of only TiN[7,8] and could successfully melt at 1673K by adding the mixture of TiN and AIN with the suitable composition.[9]. In the present study, the obtained oxide phase was identified and the mechanism of decrease in melting temperature was discussed. Chemical durability of the oxide phase obtained from 30 days Soxhlet leach test was also described. EXPERIM
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