[ RadSafe ] Article: NMR finds holes in nuclear waste storage

[John Jacobus]

>From http://physicsweb.org/articles/news/11/1/5

NMR finds holes in nuclear waste storage
10 January 2007

A ceramic material touted for its potential to store
radioactive waste is much less resilient to radiation
damage than previously thought. Physicists in the UK
used a high-resolution nuclear magnetic resonance
(NMR) technique to show that alpha-radiation causes
too much damage in zircon to ensure safety over long
timescales. They now claim the NMR technique will help
to assess the long-term durability of other potential
ceramics by providing a deeper, atomic-scale
understanding of damage events (Nature 445 190).

Integrating radioactive material into mineral-based
ceramics is a leading contender for the disposal of
nuclear waste. Some of these ceramics, such as
"zircon" (ZrSiO4), already occur naturally with
slowly-decaying radioactive isotopes incorporated into
their crystalline structure. Nevertheless, they have
remained intact over billions of years despite the
damage caused by the onslaught of high-energy alpha
particles produced in the decay process. 

Some scientists had hoped that zircon could withstand
much higher doses of the radioactive plutonium isotope
239Pu, which is found in spent nuclear fuel. The risk
is that increased exposure to alpha particles would
displace too many atoms and damage the crystalline
structure irrevocably. But this damage had been
difficult to measure and in the past scientists relied
on vague empirical calculations based on the
assessment of large defects to predict how long the
ceramics would last. 

Mineral physicists Ian Farnan and colleagues at the
University of Cambridge may now have the answer,
however. They used a technique called "magic-angle
spinning" NMR on zircon, showing that each
alpha-particle displaces up to 5000 atoms in the
crystal lattice, rather than the 1000 to 2000
estimated before. The technique enhances the
resolution of the NMR spectrum by spinning a sample at
high speeds and at a certain angle to the applied
magnetic field. This is the first time individual
damage events have been witnessed, and could put an
end to the "back of the envelope" calculations that
had prevented scientists from accurately determining a
material's lifespan. 

Unfortunately this means that zircon containing 10% of
239Pu (roughly the dose required for radioactive waste
storage) would break down after just 1400 years –
nowhere near the 250 000 years that regulation
dictates. Although the technique has ruled-out zircon,
it could pave the way for characterizing other
materials over long timescales. 

"The main issue with siting a nuclear waste repository
is that there are many uncertain factors," said
Farnan. "When you extrapolate these into the future
you get a very large uncertainty, which can make the
idea of a repository intractable. But we feel that by
working on the material itself, that's where you are
going to get the biggest effect."



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“We must face the fact that the United States is neither omnipotent or omniscient — that we are only 6 percent of the world’s population; that we cannot impose our will upon the other 94 percent of mankind; that we cannot right every wrong or reverse each adversity; and therefore there cannot be an American solution to every world problem.”
-- John F. Kennedy 

-- John
John Jacobus, MS
Certified Health Physicist
e-mail:  crispy_bird at yahoo.com


 
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