Leaching of Vitrified High-Level Radioactive Waste
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LEACHING OF VITRIFIED HIGH-LEVEL RADIOACTIVE WASTE * A.R.HALL, A.HOUGH and J.A.C.MARPLES Chemistry Division, AERE Harwell, Didcot,
Oxon OXIl ORA,
U.K.
INTRODUCTION The work reported here is
part of a study of the leaching of individual
elements from waste glasses.
The first 0
of the major constituents at 30 C, glass UK189;
part of the paper reports on leaching 0
and leaching of the alkalis at 20 C from
the second part describes leaching of Sr, Cs,
actinides from glasses UK189 and UK209.
Tc and various
Compositions are given in Table 1.
TABLE 1 COMPOSITIONS OF GLASSES UK189 AND UK209 (wt%) Oxide UK 189 Glass formers Si0 2 41.51 B2 0 3 21.87 Na20 7.68 Li 2 0 3.69 Waste from reprocessing Magnox Reactor Fuel M 6.23 5.03 03 Fe20 3 2.68 Cr 2 0 3 0.55 NiO 0.36 ZnO 0.44 U3 0 4 0.06 FPox (full spectrum of 9.90 inactive isotopes)
UK 209 50.88 11.12 8.30 3.99 6.34 5.11 2.73 0.56 0.36 0.44 0.06 10.08
The constituents of a glass may be classified according to the way in which they are leached:
(i)
Major glass network elements which are leached at a
constant rate, suggesting removal from the surface only (e.g. silicon): those elements which are leached by ion exchange with protons (or H3 0+ (ii) groups)
in the leachant with Atime dependence,
the surface by a diffusion mechanism. typify this class.
suggesting removal from beneath
The alkalis and other network modifiers
The process results in
a hydrated surface layer depleted
in elements of this class, often called the gel layer:
(iii)
elements which
are leached at a rate slower than the main glass network and thus become concentrated in the gel layer.
Fe, Al and some actinides appear to fall into
this class. DOF/RW/82.037. *This work has been commissioned jointly by the UK Department of the Environment and the Commission of the European Communities as part of their Radioactive Waste Management Research Programmes. The results may be used in the formulation of UK Government policy but at this stage they do not necessarily represent such policy.
84 LEACHING OF THE MAJOR CONSTITUENTS Experiments have been carried out in
an effort to understand the effect of
(a) pH and (b) the onset of saturation effects due to accumulated dissolution products in the leachate, addition,
on the leaching of the main glass constituents.
In
leach rates have been measured for certain of the fission products.
This gives a guide as to which elements fall into which of the three classes listed above. Experimental.
2
Samples of 12cm
surface area were cut from annealed castings, Final polishing was carried out dry, using 24m diamond
ground and polished.
powder on paper laps so as to provide a fresh surface untouched by water. Leachate analysis was made by emission spectroscopy. initial
leachant, which was analysed continuously for the first intermittently for up to I hour. made by sealing samples in temperature-controlled
few minutes and then
Longer tests of up to 3 months duration were
Teflon vessels containing the leachant placed in a
shaking bath.
very dilute
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