I keep running across the assertion that nuclear waste can be used as nuclear fuel. In several cases it is asserted that it is only regulations that prevent this use. One went so far as to say "[W]hat’s the difference between nuclear fuel and nuclear waste. The scientific answer is really nothing, there is no difference. The only difference is a technological one. Nuclear fuel is radioactive material we know how to burn as fuel and radioactive waste is nuclear material we don’t know how to burn as fuel but there is no other real, physical difference between fuel and waste. It’s a technological difference only."
The quote in the previous paragraph is simply false. As will be explained below, nuclear waste, no matter what reasonable definition you use, contains a significant amount of material that cannot be used in a nuclear reactor. For the rest of this I'll address the more reasonable versions of the assertion.It is helpful to think of two types of fuel, active fuel and potential fuel. Active fuel undergoes a reaction that releases large amounts of energy, allowing the reactor to generate power. Potential fuel undergoes a reaction (or reactions) that convert it to an active fuel. In our day to day world only active fuel is thought of as fuel. Most current nuclear reactors use U-235 as active fuel and in normal operation almost all of it is used up. At the same time a small amount of the U-238, acting as potential fuel, is converted to Pu-239 that then acts as active fuel. All of the uranium originally put into the reactor is fuel but the vast majority is potential fuel and only a small amount of that either starts as or can be converted to active fuel in current reactors. It is sometimes said that the only difference between nuclear fuel and nuclear waste is the type of reactor it is used in. If by "fuel" we mean active fuel, which is the only kind we are familiar with in normal life, this is simply wrong. It is possible to build reactors where virtually all of the potential fuel is converted to active fuel but those reactors are complex and expensive and they require a large amount of complex processing to get anywhere near this point. It should also be noted that these future reactors can NOT be operated using only what we now call waste (more detail below). New, active fuel is needed.
The first problem is that the term "nuclear waste" means entirely different things to different people. For this discussion we'll be considering "spent nuclear fuel". This is a small fraction of the radioactive material produced in a reactor and is the only really problematic material short of a nuclear accident.
For this discussion we can consider most nuclear fuel as just a mixture of two isotopes of uranium. The majority is U-238, about 95% of it. It is potential fuel so it cannot take part in the power generating nuclear reactions. The rest, about 5%, is U-235. This is the active fuel that produces the desired energy in the reactor.
When the spent nuclear fuel is removed most of the U-235 has been used up, it is now <1% of the material. It is converted to what are called fission byproducts. This consists of a wide range of material, some quite radiologically hazardous, but for considerations of use as fuel it is worthless. Some of it is worse than useless. It will absorb neutrons, suppressing the desired nuclear reactions. It is truly waste. A small amount of the U-238 has been converted to plutonium, mostly Pu-239 with a smaller amount of Pu-240 and other isotopes. In total about 1% is plutonium. Some of the uranium is also converted to other uranium isotopes.
The claim that spent fuel is reusable is based on reprocessing. The idea is that by a variety of processes the spent fuel can be separated into components and then placed back into the reactors that it came from. Often the claim is made with numbers attached. Most commonly I've seen it as something like this paraphrased example: "Only around 5% of the energy available in nuclear fuel is extracted in our current reactors. 90% of rest could be extracted if the fuel were reprocessed".
The 5% number should sound familiar, it is the original amount of U-235. The other 95% is based on the idea that the U-238 can also be used as a nuclear fuel.
This is almost correct if we are careful when we talk about what kind of fuel we mean and, more importantly realize that it requires a type of reactor that is different than essentially all of the ones currently in use. The fact these issues are almost never mentioned when the claim is made is what makes the claim so misleading.
To understand this better a bit more background is required. All of the reactors in this discussion are fission reactors. Nuclear fission occurs when a neutron is absorbed by a nucleus and that nucleus splits, roughly in half, and often emits a number of neutrons. There are many isotopes that will undergo fission but the important subset for this discussion are those that emit enough neutrons when they fission that a chain reaction is possible. These are called fissile materials. There are only a few such isotopes and three of them are most important for nuclear power: U-233, U-235 and, Pu-239. Of these only U-235 exists in any noticeable amount in nature. But there isn't much, it is just 0.7% of natural uranium. The remaining 99.3%, is essentially all U-238.
Most power reactors use uranium that has been enriched in U-235 to about 5%. This is done by separating out almost pure U-238, increasing the proportion of U-235. The separated material is known as depleted uranium. Enrichment is done so that the neutrons released by the fission are likely enough to reach another U-235 nucleus and cause a sustained reaction. This results in the 95% U-238 mentioned above. As mentioned above, the operation of the reactor results in some Pu-239 being created, some of it used as active fuel but about 1% of the the spent fuel is plutonium.
The transformation from the potential fuel, U-238, to Pu-239, is a multi-step process.
U-238 + neutron → U-239
U-239 decays to Np-239 (half-life of about 25 minutes)
Np-239 decays to Pu-239 (half-life of about 2.5 days)
Near the end of the useful life of the nuclear fuel a significant fraction of the energy comes from the fission of Pu-239. It is possible to design a reactor to optimize the production of Pu-239. These are called "breeder" reactors and they produce more active fuel than the active fuel they consume. Breeder reactors sound too good to be true. Producing more fuel than is consumed doesn't seem possible, but it is. That's one of the problems with this entire topic. So much of what happens is so far outside of the intuition developed in our day to day world that we are often lead to conclude things that aren't true.
As an interesting aside, the oft mentioned Thorium reactor is also a breeder. Here Th-232 is a potential fuel that goes through a similar (but slower) multi-step reaction to produce the active fuel U-233, the third fissile isotope.
Current reprocessing of spent fuel produces, among other things, MOX fuel. These are Mixed OXides of uranium and plutonium, current reactors can use this material as a minority of its fuel. Almost all of this is produced by combining plutonium from reprocessing with depleted uranium. Depleted uranium is used because the uranium obtained from reprocessing (RepU) contains various impurities that impede the nuclear reactions and other negative consequences that are beyond the scope here. This is in direct contradiction to the common assertion that this MOX fuel is composed mostly of the reprocessed material and results in more of the potential fuel being used.
In conclusion, there is a lot of misinformation about nuclear power and so called "nuclear waste" much of that misinformation greatly exaggerates the dangers and problem. But a significant amount is being spread by nuclear proponents. Whereas it is possible to process spent nuclear fuel and extract virtually all of the nuclear energy available in the uranium fuel (both active and potential) this is far more complex and expensive than is implied by proponents, it would require reactors that are significantly different than those in use today, and it would not use all of material as fuel.
