Management of Commercially Generated Radioactive Waste - U.S. ...

P. 10
appropriate host phases. The main criteria are similarity of chemical parameters, particu-
larly the ionic radius and the charge. Other parameters such as polarizability and
d-orbital interactions will have a lesser effect in determining the amount of substitution.
Thus one can use a table of ionic radii to predict possible substitutions, remembering that
charge balance must be maintained by a coupled substitution of another element whenever
necessary.
P.1.3.2 Ionic Radii
Table P.1.6 lists the ionic radii of the important nuclear waste elements and of the
elements present in minerals which are most likely to be substituted. Usually, complete
substitution may occur if the ionic radii differ by no more than 15%. Limited substitution
may occur if the radii difference is larger, or a new compound may be induced to form. This
compound may be isostructural with the host phase or may have a distinctly different struc-
ture. If the phase is isostructural, then stability properties of the new phase may be
similar to that of the host, or certainly be close enough to warrant further investigation.
TABLE P.1.6 Selected Ionic Radii(a)
Ion CN(b) Ionic Radius (A) Ion CN Ionic Radius (A)
Cs +
Sr 2+
1 +
X 1.81 Na
VI 1.02
VIII 1.25 IX 1.32
11- VI 2.20 K 1 + VI 1.38
15+ VI 0.95 IX 1.55
Tc 4+
Tc 7+
VI 0.65 Ca 2 + VI 1.00
VI 0.56 VIII 1.12
La 3+ VIII 1.16 Ba 2 + VI 1.36
VIII 1.42
3 +
Dy
4 +
Ce
4 +
U
6 +
U
4 +
Np
Pu3 +
4 +
Pu
3 +
Am
Am 4
3 +
Cm
VIII 1.03 C1 1 - VI 1.81
VIII 0.97 Br 1 - VI 1.96
VIII 1.00 Y3+ VIII 1.02
II 0.45 Zr 4 _ VIII 0.84
VIII 0.98 Ti 4+ VI 0.61
VI 1.00 Th 4 VIII 1.04
VIII 0.96 Mn3+(HS)(c) VI 0.65
VI 1.00 Fe3+(HS) VI 0.65
VIII 0.95 Cr 3
VI 0.62
VI 0.98 Ce 3 + VIII 1.11
(a) After Shannon and Prewitt (1969).
(b) CN = coordination number.
(c) HS = high spin.