1. magnetic confinement - ENEA - Fusione

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3. FUSION TECHNOLOGY
3.8 Liquid Metal Technology and
Hydrogen Effects on Materials
Weight change data were calculated and reported in a parabolic diagram. Two
different parabolic rate constants that had similar values for the two different
systems were obtained, indicating a self-limiting mechanism due to a diffusioncontrolled
step of the reacting species. In this case, the system studied can basically
be regarded as the oxidation of metals due to possibility of the presence of lithium.
Moreover, the chromium content in the alloy is sufficient to promote the formation
of a chromium oxide layer able to protect the metal from further oxidation. In any
case, the two kinetics are basically identical due to the water-production rate, which
is determined by temperature, and both the parabolic rate constants fall within the
range reported in the literature.
Oxidation proceeds with time from
areas under the pebbles in contact with
the metal surface (fig. 3.36), up to
almost the entire surface after 2000 h,
when an oxide layer not exceeding 2
µm anywhere is observed. Under the
pebbles, the metal surface shows cracks
with a maximum depth of about 4 µm
(fig 3.37). It is not clear whether the
mechanism proceeds via the oxygen
that enters the metal or whether the
chromium diffuses outward or, more
probably, both.
X-ray diffraction analysis (XRD)
showed the absence of lithium
compounds and the presence of only
oxides (Cr,Fe) 2 O 3 and (Fe,Cr) 3 O 4 , with
iron and chromium present as minor
elements in the first and second case,
respectively.
Fig. 3.36 – Scattered
electron image at 20X of
EUROFER97 specimen
exposed for 1000 h in
Li 2 TiO 3 pebble bed.
Fig. 3.37 – Backscattered
cross-section
image at 2500X.
Considering the thermodynamic conditions and the oxygen chemical potential, the
temperature of 600°C seems to be borderline for (Cr,Fe) 2 O 3 .
3.8.9 Li 2
TiO 3
pebble reprocessing; recovery of 6 Li as Li 2
CO 3
Lithium titanate is one of the most promising candidates for tritium breeding. The
temperature of tritium release from polycrystalline Li 2 TiO 3 ceramic pellets and
pebbles was found to be lower than from many other Li ceramics. This material also
shows good chemical stability in air and has acceptable mechanical strength.
A process for obtaining Li 2 CO 3 from Li 2 TiO 3 sintered pebbles by wet chemistry was
developed. This is considered useful in view of the recovery of the 6Li isotope from
lithium titanate breeder burned to its end of life in a fusion reactor. The process was
optimised with respect to the chemical attack of titanate by using an aqueous HNO 3
solution. The subsequent precipitation of lithium carbonate by Na 2 CO 3 produced a
powder with chemical and morphological characteristics suitable for its reexploitation
in the fabrication of Li 2 TiO 3 pebbles. Reprocessing was also planned to
adjust the 6 Li concentration to the desired value by using 6 Li-enriched LiOH*H 2 O
and to obtain its homogeneous distribution in the powder batch.
A specific procedure was used to add a number of small carbonate batches (each one
obtained from 40 g of starting pebbles) in order to produce a batch of about 400 g of