Comparison of sodium zirconium phosphate and Synroc matrices for immobilization of high-level waste Page: 4 of 4
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Calcination at 700 C led to essentially fully crystalline NZP. The densities of cold-pressed
pellets that were sintered at -1100*C reached 90% of theoretical values without taking special
In sintering studies, substitutions of rare earths (surrogates for actinides) into NZP were
attempted. Limited solid solubility of rare earths and molybdenum on the order of 0.2 formula
units appeared to exist. However, it was difficult to draw definite conclusions because of sodium
volatilization and the very fine grain size, which precluded SEM microanalysis of the NZP
grains. From XRD, it was clear that monazite and xenotime phases were present after the solid
solubility limits had been attained. Table 1 shows the densities, porosities and phase
assemblages of samples made by hot isostatically pressing of NZP with different types of
TABLE I. Densities, Porosities, and Phases Present in Hot Isostatically Pressed Preparations
Sample Composition Waste Ions (g/cm3) (%) Phases
Na0 SCs0 Sr. 1Zr(PO4) Cs, Sr 3.23 3.6 NZP + tr ZP
NZP* - 3.09 3.6 NZP + tr ZP
NZP + 20 wt% PW-4b Purex 3.23 1.5 NZP + M + ZP
Na 5YZrGd0 (PO4) Actinides (simulated) 3.76 3.3 NZP + X + ZP
Na 5Ca0 5Gd0 5Zr 5(PO4)3 Actinides (simulated) 3.43 3.8 NZP + M + ZP
M= monazite; X= xenotime; ZP = ZrP207; tr = traces
* theoretical density = 3.19 g/cm3.
The solid solution strategies were simple replacement of Na with Cs, 2Na for Sr; substitution of
Y for Na and a coupled substitution of Gd for Zr; and a coupled substitution of Ca for Na and Gd
for Zr. The appearance of monazite and xenotime was not unexpected from the above
substitutions for rare earths in NZP done for the preliminary sintering studies, and both monazite
and xenotime are durable phases in water. The sodium leach rates for all preparations were
typically 0.01 to 0.1 g/m2/d except when Purex waste (PW-4b) was simply added to NZP, for
which the leach rate was ~1 g/m2/d. The release into solution of other elements from the hot
pressed samples was generally lower than that of sodium. However, the weight losses of these
samples were typically 10 to 20 wt% in the first 7 days, corresponding to bulk leach rates of
-1 g/m2/d. Moreover the weight losses in 28 day experiments were less than those in the 7 day
experiments, indicating that substantial reprecipitation and hydroxylation were taking place. On
the basis of the current work, it is clear that dense NZP-based waste ceramics have chemical
durabilities within an order of magnitude of Synroc.
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Zyryanov, V. N. & Vance, E. R. Comparison of sodium zirconium phosphate and Synroc matrices for immobilization of high-level waste, article, December 31, 1996; Illinois. (digital.library.unt.edu/ark:/67531/metadc677465/m1/4/: accessed November 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.