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Currently spent reactor fuel is removed from the reactor and placed in a pool of deep water. This helps dissipate the greatest heat emminating from the bundles and acts as a shield for the radioactivity. The length of time spent in the pool differs between reactor designs and fuel composition but 10 years is the most often quoted figure.
After the spent fuel has cooled it is transfered to "dry storage". In the Canadian system, the bundles are placed in containers and then stored in concrete bunkers either on-site or in a central location. These methods of dry storage have been in use for decades and have proven safe and reliable.
The next step is often undecided. The United States has passed a bill that gives Yucca Mountain a mandate as a DGD repository. Canada will soon be debating a similar motion in Parliament. The debate is usually between proceeding with DGD or continuing a dry storage program.
There are many pros and cons in this debate. Many of the arguments centre around ethical and social issues as opposed to technical reasoning. I wrote a paper in University that explores these issues. It can be found here: Provisions for the disposal of nuclear waste.
Well it is burying it but in a carefully engineered and controlled environment. It's not as if they just dump this stuff underground!
The Canadian system uses many layers of engineered protection. These include:
- The fuel pellet itself. All harmful materials are contained in the solid ceramic fuel pellet.
-The fuel cladding. While in the reactor the pellets are surrounded by zircalloy which contains them into a fuel bundle.
-A "corrosion layer". Part of the container, this copper shell is resistant to corrosion should something go wrong and water corrupts the storage area.
-A "pressure layer". A stainless steel shell which provides the strength of the container and is also corrosion resistant.
-Bentonite clay. A near impermeable substance which is densely packed around the containers.
-Clay sand mixtures. Several layers of this are strategically placed in and around emplacement rooms.
-Concrete. Is used to build emplacement rooms deep underground.
-The Canadian shield. This biospheric layer is the stable ground that the site will be built on. Careful selection is nessecary to prevent water migration and other faults.
Analysis shows that these structures should have a minimum life of 10 000 years. Obviously this is not guranteed but it is very well researched. Also nature has provided us with a handy site that shows even if our engineering fails completely we still may be ok.
The Oklo site in Gabon, Africa contains the remains of natural fission reactors! At this site conditions were just right for a chain reaction to take place and continued for up to a million years. By definition this was exposed to massive amounts of flowing water directly in contact with the "fuel". Analysis shows that the plutonium produced never left the site where it was formed! A website is here.
Now does the fuel remain dangerous for hundreds of thousands of years? Not nessecarily. It will remain radioactive for some time but unlike toxic waste, which is toxic now and forever, radioactive waste decreases its radioactivity with time! In fact a person could spend an entire day working alongside a sepnt CANDU fuel bundle and not suffer any effects after only 500 years! Most of the "danger" would come if large amounts of long-lived plutonium isotopes were to be ingested by a human being within 10 000 years. A very, very unlikely scenario.