Optimization of Heat-Liberating Batches for ASH Residue Stabilization
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13,7
19,6
3,5
2,6
6,4
2,4
1,3
MgO
Losses by
1,3
calcination 17,2
The radionuclides composition of ash residue is shown in Table 2. 143 Mat. Res. Soc. Symp. Proc. Vol. 556
© 1999
Materials Research Society
Table 2. The radiochemical composition of ash residue. Radioniclides content, Bq/kg 33 08 suIm beta sun suiml37 Cs Co Sr 1 Ba 1 Ag -34U by 37
beta by alpha
1 cS
'OSr
1,4*10O
7,7*
3,5*10
6,4*104
7,6*108
1,5*10'
8,1*10'
1,8*10'
1,7*10
238U
3,2*10'
23 8
2,5
p'u
2
9
Pu
104 1,5* 10
The purpose of experiments consists in the choice of a heat liberating batch (reductionoxidation mixture of components) that not only provides enough heat liberation but also produces during reactions products consistent with a glass matrix which is optimal for radioactive waste immobilisation. In order to optimise the process a following scheme was applied: a)the choice of components of heat liberating batch, b)conduction of exothermic reactions, c)process parameters control (temperature, process duration, carry over of components and radionuclides), d)quality control of final product (structure, water stability etc.)
At a first stage experiments were carried out by using as energy source (heating batch) thermites - mixtures of Fe 2 0 3 and Al, W0 3 and Al, NiO and Al. Application of these compositions required preliminary rigorous preparations of reacting composition (thermite and ash residue) concerning the necessity to compact the mixture and dry it. The content of radioactive ash in the obtained solid was not higher than 16 - 24 wt.%. The sintering process continues during a few second and considerable carry over of radionuclides was monitored (up to 80% of Cs-137). A new heat liberating batch on the base of red lead and silicon-calcium was able to incorporate about 30 wt.% of ash residue into a vitreous material [I ]. In order to obtain this material additives in form of glass forming batches (about 33 wt.%) were used as well as additive heating of containers was required. In order to improve the process of stabilisation of ash residue the heat liberating batches were optimised by taking into account necessary requirements: * to increase the ratio content of ash residue in the final product; * to improve properties like water durability, thermal durability, mechanical strength; * to enhance ecological safety of technological process - e.g. to decrease the radionuclides carry over. By means of thermodynamic analysis, provided by a computer software ASTRA.4 (designed for simulations of chemical and phase equilibrium) a heat liberating batch was chosen with empirical formula: Ca3.23 -Si 7 25 -Fe0.94-K2 02 -Mnl.4 3-0 18.73 -A12 52-Nao. 70-Mgo. 71Cu0.09PI. 44-S0 .1o. The reactions between components of this batch occur with generation of a large quantity of energy: the equilibrium temperature reaches 1600 - 1700 'C. Different variants of weight ratio (heat liberating batch)/ (ash residue) were tested: 50/50, 44/56, 40/60. The stabilisation process was carried out in the experimental unit in atmospheric
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