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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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