Lanthanum and Neodymium Solubility in Simplified SiO
2
–B
2
O
3
–Na
2
O–Al
2
O
3
–
CaO High Level Waste Glass
Abdessamad Kidari,
‡,†
Jean-Luc Dussossoy,
§
Emmanuelle Brackx,
¶
Daniel Caurant,
k
Magali Magnin,
§
and Isabelle Bardez-Giboire
‡
‡
CEA, DEN, DTCD/SECM/LDMC – Marcoule, Bagnols-sur-Ce`ze F-30207, France
§
CEA, DEN, DTCD/SECM/LMPA – Marcoule, Bagnols-sur-Ce`ze F-30207, France
¶
CEA, DEN, DTEC/SGCS/LMAC – Marcoule, Bagnols-sur-Ce`ze F-30207, France
k
E
´
cole Nationale Supe´rieure de Chimie de Paris (Chimie ParisTech), Laboratoire de Chimie de la Matie`re Condense´e de Paris
(UMR CNRS 7574), Paris, France
Lanthanum and neodymium incorporation in simplified high
level waste glasses has been investigated for SiO
2
–B
2
O
3
–Na
2
O
–Al
2
O
3
–CaO compositions quenched from 1200°C, for varying
La/(La + Nd) (atomic) and increasing rare-earth oxides con-
tents. In this system and beyond the solubility limit, rare-earths
(RE) elements are reported to form apatite phases with the
general formula Ca
2
RE
8
(SiO
4
)
6
O
2
. In the current study, speci-
ation of these trivalent RE
3+
cations in both amorphous net-
work and crystal phases was determined from X-ray
diffraction, scanning electron microscopy, optical absorption at
10 K, Raman spectroscopy, and electron probe microanalysis.
It appeared that RE
2
O
3
solubility was higher for La-rich for-
mulations than for Nd-rich ones and that an increase in the RE
oxide content reduces the connectivity of the network building
units through formation of non-bridging oxygens at the expense
of the oxygen bridges. This depolymerization of the glass net-
work did not affect neodymium environment which consisted in
silicate tetrahedra. The composition of the apatite crystals was
found to be affected by the La/(La + Nd) of the parent glass
and deviation from the ideal composition (Ca
2
RE
8
(SiO
4
)
6
O
2
)
occurred in the neodymium end of the system. It thus appears
that both RE
2
O
3
solubility and crystal composition are
strongly dependent on the type and crystal chemistry of the RE
elements.
I. Introduction
T
HE French nuclear waste management strategy is direc-
ted toward reprocessing of spent nuclear fuel and parti-
tioning of uranium and plutonium. The resulting high-level
wastes comprise fission products and minor actinides, these
are immobilized in borosilicate glass matrices at the indus-
trial scale since 1978. Electricity production continues to
evolve toward more efficient and cost effective production
and waste management, an increase in the combustion rate
of UO
2
is projected in the near future and participates to this
objective.
1
The actual R7/T7 glass matrix was intended for a
maximum waste load of 18.5 wt% and provides optimal
thermal stability and long-term chemical durability. How-
ever, an increase in the combustion rate from 33 to 60 GW.
d/t would generate spent nuclear fuel, not only in higher
amounts but also with a different isotopic composition.
Higher lanthanides and minor-actinides fractions are
expected and may prove problematic as these elements are
difficult to immobilize and lead to higher thermal outputs
and subsequent crystallization.
2,3
Current research focuses on
potential immobilization matrices capable of vitrifying this
new waste composition and should provide enhanced thermal
stability, long-term chemical durability, and resistance to
self-induced alpha radiations damage.
Simplified waste glass formulations with high contents in
lanthanides have been extensively investigated in the SiO
2
–
B
2
O
3
–Na
2
O–Al
2
O
3
–CaO–ZrO
2
system. The structural role
for neodymium is of particular interest as this element is
present in the waste composition and is also used as a minor-
actinide surrogate. EXAFS spectroscopy of neodymium in
alumino-borosilicate glasses showed that Nd coordinates 7–8
non-bridging oxygens and that alkali and alkaline-earth
cations present in its second coordination sphere stabilize this
environment.
4,5
Devitrification in this system above the solu-
bility limit is well documented, it was shown to induce forma-
tion of RE containing crystalline phases Ca
2
RE
8
(SiO
4
)
6
O
2
(RE: rare-earth). Ease of crystallization and the extent of this
phenomenon were found to be affected by the calcium con-
tent and the type of RE element. Crystallization is minimal
and even suppressed for La, Eu, Gd, Er, Yb, and Lu, whereas
it is significant when the ionic radius for RE was close to that
of Ca
2+
(Sm, Nd, and Pr).
5,6
An experimental approach where several RE elements are
incorporated in a glass formulation should provide key infor-
mation on the changes affecting the amorphous network, the
solubility limit, and the crystal phase composition. These
aspects would then be monitored as a function of the RE
content and also the distribution between the RE elements.
However, combination of multiple elements renders spectro-
scopic and diffraction investigations difficult. In this article,
we investigate the solubility of trivalent lanthanides, lantha-
num, and neodymium, in a simplified glass formulation
belonging to the SiO
2
–B
2
O
3
–Na
2
O–Al
2
O
3
–CaO system. In
this compositional design, both La
2
O
3
and Nd
2
O
3
aim to
mirror a lanthanide-minor actinide combination in which
lanthanum represents the lanthanide family of elements,
whereas neodymium acts as a minor actinide surrogate,
americium in particular with which it shares close ionic
radius and an identical oxidation state. We assessed the
effects of different La/(La + Nd) values on the solubility of
the REs and the structure of the amorphous network, these
for glasses containing up to 22.5 wt% RE
2
O
3
. Also, the
structure and composition of the crystals formed when La
and Nd solubility in the melt is reached has been linked to
C. Jantzen—contributing editor
Manuscript No. 31058. Received February 09, 2012; approved April 21, 2012.
†
Author to whom correspondence should be addressed. e-mail:
abdessamad.kidari@cea.fr
2537
J. Am. Ceram. Soc., 95 [8] 2537–2544 (2012)
DOI: 10.1111/j.1551-2916.2012.05273.x
© 2012 The American Ceramic Society
J
ournal