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Nuclear Waste Storage and Water
Name: Marlene
Status: educator
Grade: 6-8
Location: PA
Country: USA
Date: N/A
Question:
The issue of storing of nuclear waste was
the intro to our "Plate Tectonics" unit. Our text
stated that nuclear waste is placed in special
containers in underground rooms. It later said that
water could be a cause of container leakage.
Students asked how water would affect the containers
if they were in 'rooms' underground. Could you
describe these rooms?
Replies:
Hi Marlene,
I am not sure I understand your question. Your text says that nuclear
waste that is stored in special containers in rooms or caverns below
ground, is still subject to container leakage if exposed to water. Then
your students are asking how water would affect the containers if they
were stored as described.
I am puzzled at their question, since your text has already clearly stated
the answer, namely, that the containers will leak when exposed to
water.
These underground "rooms" are simply large salt mine caverns. No
longer used salt mines are a favorite storage venue since they have
extremely low humidity and since they contain no water, corrosion of
the waste containers is not possible. A good storage site is one that is
geologically stable, and which has very low humidity. The leakage
problem your text refers to, is clearly caused by corrosion of the metal
containers, and subsequent leakage of the waste. A wet or humid
environment will cause this corrosion.
Regards,
Bob Wilson
Thanks for your question, Marlene. The "rooms" are actually tunnels carved
in the material making up the underground repository. There are several
designs that have been considered for spent nuclear fuel repositories. If the
repository is buried deep underground in a very dry climate, not much water
will get into the tunnels, but it is not possible to guarantee that no water
at all can enter the tunnel from the surface. One way to keep water off the
containers for a much longer time is to cover the package with what is called
a drip shield made from a material that, like the waste containers, is very
impervious to water.
I am attaching to this email a write up on the Yucca Mountain repository that
the United States worked on for a number of years – it describes the repository
design, and on the last page, there is a very good diagram that shows what one
of the tunnels might have looked like, how the container would be positioned,
and how the drip shield would protect the container.
Currently, there is one high-level radioactive waste repository (not for spent
nuclear fuel, just for items like clothing contaminated with small amounts of
long-lived isotopes) in operation – the Waste Isolation Pilot Plant near
Carlsbad, New Mexico. Unlike Yucca Mountain, this repository is dug out of a
very deep salt bed – the fact that the bed is made of salt means essentially
no water has been in that location for hundreds of millions of years. After
waste containers are placed in the tunnels of the repository, the salt will
gradually shift and completely enclose the containers. Everything we know
about the salt bed leads us to believe that it will be there for many more
millions of years – much longer than it will take for the radioactive material
in the containers to decay out to stable isotopes. So no water will reach the
containers until long after all the radioactive isotopes are decayed out.
I hope this helps to answer your students’ questions. Please contact NEWTON
again if you and your students have any more questions.
Dr. Laural Briggs
Nuclear Engineering Division
Argonne National Laboratory
Marlene,
The 'underground' rooms that you mentioned are just that, rooms carved
into underground rock formations. A site known as Yucca Mountain was
planned to be the location for spent nuclear fuel (the preferred
term, instead of 'nuclear waste'). These rooms carved out of rock are
chosen for their isolation and geologic stability. When spent fuel is
stored in a repository like this, it is sealed in air tight casks. The
cask designs that would be used in a geologic repository rely on
various metals that are chosen to be stable, but when looking at even
the most corrosion resistant metals, corrosion may occur over a time
span of hundreds or thousands of years. Water can permeate through the
ground and speed corrosion, which may not matter in the short term,
but can be a concern for long term storage. The minerals in the rock
walls can actually change the corrosion of the metals as well.
People had hoped that sites like the one described in your text would
be created to store nuclear waste from around the country, but for a
number of reasons, such sites have not been put into operation. Today,
most waste is stored on-site where it was generated.
The current industry standard to handle spent nuclear fuel starts with
underwater storage. After being removed from the reactor, bare (not in
a container) spent fuel is moved to a spent fuel pool, usually for two
years. Corrosion is not a concern here because the water chemistry is
very carefully controlled. After about two years the spent fuel is
then moved to dry storage in concrete casks. These casks are usually
stored near the reactor that the fuel came from because of the
complexities of transporting the spent fuel.
For more information on the spent fuel pools, see:
http://www.nrc.gov/waste/spent-fuel-storage/pools.html
For more information on dry cases, see:
http://www.nrc.gov/waste/spent-fuel-storage/dry-cask-storage.html
This answer was prepared in consultation with Mark Niehus, an expert
in nuclear non-proliferation. Many thanks to Mark for his help!
Hope this helps,
Burr Zimmerman
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Update: June 2012
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