[ RadSafe ] Article: A fresh look at glass -- implications for radioactive waste disposal
crispy_bird at yahoo.com
Mon Jun 5 13:05:56 CDT 2006
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A fresh look at glass
1 June 2006
Physicists and engineers in the US have discovered for
the first time that glass can completely return to its
original state after being bombarded with high-energy
electrons. The result shows that the glassy state can
be extremely stable thermodynamically, contrary to
what was previously thought. As well as being
fundamentally important, the work could have
implications for storing nuclear waste (Phys. Rev.
Lett. 96 205506).
Understanding the nature of the glassy state remains
one of the last great unsolved mysteries in
condensed-matter physics. Glasses are notoriously
difficult to study because, like liquids, they lack
long-range order. They are formed when a molten
material is cooled so quickly that the atoms do not
have time to arrange themselves into an ordered
crystalline lattice that has a lower energy and is
ADF images of glass sample. (a) Before the damage, (b)
after 2 min of electron-beam damage; (c)(d) images
recorded after 30 sec and 2 min of recovery with beam
turned off (image: (Phys. Rev. Lett. 96 205506)**
Some glasses can last for thousands of years. This,
and the fact that glass can effectively absorb
radiation, has led scientists to believe that they
could be used to store certain types of nuclear waste.
However, silicate-based glass (the most common type of
glass) can easily be damaged by ion, electron and
ultraviolet beams. Moreover, this damage is thought to
(a) Line scans of the ADF intensities across the
damaged area in the sample [the dashed line in figure
1(a)] obtained from images before and after damage
[images 1(a) and (b)] and during recovery; (b) the
difference between line scans from figures 1(a)and
Image: (Phys. Rev. Lett. 96 205506)**
To investigate this further, Andre Mkhoyan and
colleagues at Cornell University studied the effects
of high-energy electrons on thin films of an
alumino-silicate glass containing calcium oxide. This
glass comprises alumina (amorphous Al2O3) and silica
The researchers began by focusing a 100 KeV electron
beam from a scanning transmission electron microscope
for two minutes onto an area of the sample measuring
about 6 square nanometres. Next, they monitored the
damage caused, in real time, by looking at how the
chemical composition of the glass changed using
techniques called electron energy loss spectroscopy
(EELS) and "annular dark field" (ADF) imaging. They
then turned the beam off.
After about two minutes, Mkhoyan and co-workers retook
EEL spectra and ADF images from the glass to determine
its composition. They found that it was the same as at
the beginning of the experiment, which shows that the
glass had completely "recovered" (figures 1 & 2).
According to the team, the results suggest that the
glass is very stable thermodynamically, which was
unexpected given its disordered atomic structure.
The researchers say that the stability of such glasses
could have implications for storing radioactive
materials. "Our experiments suggest that damage from
beta-emitting nuclear waste sealed inside this
material would 'self heal'", states Mkhoyan. "These
investigations may even be highly relevant for the
energy future of the US since the National Academy of
Sciences recommended vitrification in 1994 as the
preferred method for the long-term storage of
plutonium." However, he adds that more work still
needs to be done to study the effects of other types
of radiation in these materials.
The team now plans to study the limits of recovery in
aluminosilicate glasses as a function of composition
and search for new stable glass systems.
"You get a lot more authority when the workforce doesn't think it's amateur hour on the top floor."
GEN. MICHAEL V. HAYDEN, President Bush's nominee for C.I.A. director.
John Jacobus, MS
Certified Health Physicist
e-mail: crispy_bird at yahoo.com
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