Does nuclear waste last millions of years?


Prior to reading this article, it is recommended that you read our basic definitions for discussing nuclear science. In this article we refer to ‘used nuclear fuel’ and to ‘nuclear waste’. For our current discussion these are both the same thing. To get a deeper understanding of nuclear power in general, see our general nuclear article.

Nuclear waste is made of many isotopes

Nuclear waste is not a homogeneous substance, it is composed of many elements that emit radiation for different lengths of time. When a nucleus emits radiation, it changes into a nucleus that is slightly different. Over time, radioactive materials transform into other materials, eventually reaching a state in which they are no longer radioactive. For more information on this process, see our explanation of ‘decay chain‘.

The majority of the material in spent nuclear fuel is a relatively stable form of uranium called uranium 238 (U-238). It has a half life of over four billion years, so it will be around for a long time. The next largest fraction of material is unspent uranium 235 (U-235) and plutonium fuel with half lives of 700 million years and 24 thousand years respectively. These materials are do not change substantially in character except on geological time scales. That is, they are not going away very quickly if we just wait.

Different stages of danger

The elements that are extremely radioactively dangerous in freshly spent nuclear fuel are those with relatively short half lives. A short half-life would be on the order of seconds to weeks. These will decay away to negligible amounts over periods of days to months after they are taken of the reactor thanks to their short half lives. Before this time has passed however, they cause the emission of very large amounts of radiation and thermal heat.

After a few months the fuel can be reprocessed more safely. In places like Japan, fuel at this stage is reprocessed. Most of the remaining U-235 and plutonium can be reused in new fuel. The most major issue is actually dealing with medium half-life materials that decay on a scale of decades. Most of these substances are not useful in new fuel. If we chose to let them decay away to being negligible it would be necessary to wait tens to hundreds of years. There are other options to consider however.

The material that would not be useful in new fuel can be stored more easily since its volume is much smaller than the initial spent fuel. It is important to note however that this smaller volume could still be very dangerous if mishandled. Some of the materials within this part of used fuel are useful in medicine as tracers or for radiation therapy. Some of these materials may also be useful in future fast neutron reactors for power production. Additionally some of this material could be broken down in smaller reactors designed for this task. Alternatively, these elements can be changed into more stable materials using small particle accelerators. In this last case we would pay for our peace of mind with the energy cost of running the accelerators.

Answer: This is a very misleading myth, and a popular one

See our section on half life for an explanation of why the longest-lived substances are less dangerous radioactively than the short-lived. It is incredibly misleading to say that nuclear waste lasts for millions of years. Some of these materials certainly would last that long, but what is our metric for what is dangerous?

Compare radioactivity to uranium ore

It is impossible to ask for zero radioactivity because our world and our universe has notable low level radioactivity going on constantly. We would be comparing to a theoretical zero-radiation ideal that does not exist. A better measure of radioactivity is to compare our waste to the radioactivity of natural uranium ore dug up from the ground. After all, this exists in nature and it is the original source of our materials. Using this metric, we can estimate that in general it will take a few hundred years for our current stockpiles of used fuel to be at around the same level of activity as natural uranium ore. In short, the nuclear fuel can return to being as radioactively dangerous as natural uranium ore in few hundred years, rather than millions.

Waste can still be dangerous!

It is important to note that waste in this form, even after having decayed to a more safe level of radioactivity, can still present notable dangers to humans. Firstly this waste would have a complex chemical profile that would make it difficult to store and/or deal with in any permanent fashion. It is possible to deal with this, but it remains a notable difficulty that should not be understated. Another important factor is that used fuel in this dense form may also have an internal gaseous pressure of many atmospheres. This could introduce difficulties in storage considering that the waste may break apart or emit gases. Keeping this contained may be an additional engineering difficulty that again should not be trivialized.

Being unwilling to leave nuclear waste for future generations does not preclude us from considering nuclear development. There are nuclear generator technologies you can support that produce less and/or more easily processed waste. Technologies also currently exist for reprocessing existing waste left to us by previous generations. Many of these reprocessing technologies are still being researched and show great promise. These include pyroprocessing, liquid thorium fuels, SILEX laser seperation, etc.

It is important you let your government know what you think of as responsible management of used nuclear fuel in an intelligent and productive manner. One of the primary issues is of course keeping the nuclear materials from being used in bombs.

This article is part of our nuclear myth and fact project.

Ben Harack

I'm an aspiring omnologist who is fascinated by humanity's potential.

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