Let’s imagine that you own a nuclear power plant. You are in a pretty good position these days because, compared to a coal-fired power plant, your operation is very green. You are producing lots of reliable electricity without releasing any carbon dioxide into the atmosphere. You don’t release any other pollutants, like sulfur or mercury, into the atmosphere either. And you also are not creating huge quantities of coal ash, which at many power plants ends up being stored in immense lagoons. These lagoons raise lots of questions in terms of long-term stability, especially during floods.

So as a nuclear power plant owner, you are feeling good. There is, in fact, only one fly in your ointment at the moment. That fly is called high-level nuclear waste, and it appears every time you need to refuel your reactor. In fact, the fly grows a little bigger with each refueling. Let’s take a look at how this high level nuclear waste works.

First a little background. The goal of a nuclear reactor is to create heat so that it can produce steam that drives a generator. The nuclear fuel in the reactor creates the heat through nuclear fission. The fuel consists of a mixture of Uranium-235 (about 4%) mixed with Uranium-238 and other elements.

The U-235 is the actual fuel for the reactor. Each fuel pellet containing the U-235 is very small (about the size of your little toe), but it contains the equivalent heat of perhaps a ton of coal. A typical reactor contains several tons of these pellets arranged in rods that fill the reactor core.

The U-235 sitting in the core undergoes a fission process as described here. In the process, it creates a tremendous amount of heat. As described on this page:

Each rod is full of pellets of uranium oxide. An atom of U-235 fissions when it absorbs a neutron. The fission produces two fission fragments and other particles that fly off at high velocity. When they stop the kinetic energy is converted to heat – 10 million times as much heat as is produced by burning an atom of the carbon in coal.

In a nuclear bomb this heat is all released in less than a second to create a gigantic explosion. In a nuclear reactor the heat is released gradually over the course of a year or two to boil water that drives electric generators.

At the end of that year or two, the reactor shuts down for a month for refueling and the fly in the ointment appears. Between one quarter and one third of the fuel in the core is removed and replaced with fresh fuel rods. Several tons of highly radioactive material – also known spent fuel or as high-level nuclear waste – now has to be managed.

The spent fuel is still quite hot – so hot that it must be stored in a spent fuel pool. It’s like a gigantic swimming pool 50 feet deep. The water is there to absorb the heat. The depth of the water protects people against the radioactivity of the fuel and also provides a margin of safety.

After several years in the pool, the spent fuel has cooled enough to be manageable outside of a spent fuel pool. However, it is still highly radioactive and dangerous. In fact, it will be dangerously radioactive for thousands and thousands of years. It also contains enough U-235 and plutonium to be interesting to terrorists who might like to build their own bombs. Therefore you can’t leave it lying around.

In the ideal case, one of two things would happen. One possibility is recycling. A recycling facility could reprocess the spent fuel to extract the usable U-235 and the plutonium. It could then create new fuel, and do so economically. However, in the United States we do not recycle nuclear fuel:

The other option would be to move the now-cooled spent fuel into a permanent storage facility that can keep it safely away from people for thousands and thousands of years. The Yucca Mountain facility in Nevada was the proposed site for such long-term storage, but that idea has been abandoned.

So, right now, nuclear power plants store their cooled spent fuel in containers called dry casks. These are typically strong steel cylinders that are then surrounded in concrete. The combination of steel and concrete provides a secure container and protection from the radioactivity of the spent fuel inside the cask. These casks are stored on concrete pads outside the reactor facility, and will remain there until the United States comes up with a better long-term solution for high-level nuclear waste.

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