Nuclear Reactor's fuel

The recent Chernobyl HBO series commercial inspired this article on nuclear power reactors. We’ll briefly explain the physics of nuclear fission and what happens to the uranium within a reactor.

Common nuclear reactors generate power by bombarding uranium atoms with neutrons. This causes the uranium atoms to split into smaller pieces, releasing more neutrons in the process. These newly released neutrons then go on to split other uranium atoms, creating a chain reaction. This process is called nuclear fission (distinct from nuclear fusion, which powers stars). Because the combined mass of the resulting “pieces” is less than the original uranium atom, the missing mass is converted into energy, as described by Einstein’s famous equation (remember that even massless photons have energy!).

E² = m₀²c⁴ + (p c)²

This articlemove a bit behind the simple understanding that they split uranium atoms with neutrons to create energy. It explores the nuances of neutron types, uranium isotopes, and the challenges of nuclear waste. Most people know that nuclear reactors split uranium atoms with neutrons, releasing energy, and that this process is somehow dangerous. Let’s explore why. There are two types of neutrons: fast and slow (or thermal). Slow neutrons are, well, slower. When a fast neutron passes through a material like water, it can collide with atoms (hydrogen in water’s case) and lose momentum, becoming a slow neutron. Materials that slow down neutrons are called moderators. Water is a common moderator. Other examples include graphite (which we’ll discuss later), heavy water, and beryllium.

A sustainable nuclear reaction requires that each split uranium atom, on average, splits at least one more. Otherwise, the reaction dies out. Conversely, if each split atom causes many more splits, the reaction becomes uncontrollable, leading to a nuclear explosion.

Uranium has several isotopes, but the most important are uranium-235 and uranium-238. Natural uranium is mostly uranium-238 (99%), with only 1% uranium-235. These isotopes react differently to neutrons:

• Uranium-238 + slow neutron = absorbs neutron, becomes plutonium-239.
• Uranium-238 + fast neutron = can split (releasing energy) or absorb neutron (becoming plutonium-239).
• Uranium-235 + slow neutron = splits readily (releasing energy) with high neutron efficiency.
• Uranium-235 + fast neutron = can split, but less efficiently than with slow neutrons.

Generally, slow neutrons are much more effective at splitting atoms than fast neutrons. Because uranium enrichment is expensive, using uranium-235 with slow neutrons is currently the most cost-effective way to generate nuclear power.

However, the 1% concentration of uranium-235 in natural uranium is insufficient for a sustained chain reaction. It must be enriched to 3-5%. This means increasing the concentration of uranium-235 about fivefold.

Enriched uranium is placed in a moderator (like water) and bombarded with neutrons to initiate a chain reaction. The reaction’s heat boils water, which drives turbines to generate electricity.

This common approach has drawbacks:

• Uranium enrichment is expensive.
• Only uranium-235 (1% of natural uranium) is used.
• Uranium-238 (the remaining 99%) is converted into transuranic elements (including plutonium-239) when bombarded with slow neutrons. These elements are highly radioactive, mostly non-fissile (except plutonium-239), and have half-lives of thousands of years.

So, we extract uranium, use only 1% of it, and create highly dangerous, long-lasting radioactive waste. Ironically, unenriched uranium can be handled safely (unless ingested or inhaled), but after being “burned” in a reactor, it becomes incredibly dangerous. Ideally, we’d like to “burn” all the uranium and eliminate the long-lasting waste. Fast neutron reactors offer a potential solution. They use fast neutrons instead of slow neutrons. Fast neutrons can split uranium-238 (releasing energy) or convert it to plutonium-239. Plutonium-239 can also be split by fast neutrons, releasing energy. Thus, uranium-238 can either directly or indirectly produce energy in a fast neutron reactor.

Fast neutron reactors don’t need a moderator, allowing for smaller reactor designs, suitable for applications like submarine propulsion. So, we can have smaller, cheaper, more efficient reactors with significantly less long-lasting radioactive waste. Sounds great, right? Not so fast. Because fast neutrons are less efficient at splitting atoms, the fuel needs to be highly enriched (around 25% uranium-235 or plutonium-239) to start the reaction. This is very expensive and produces weapons-grade material. However, once the reaction starts, the fast neutrons also convert fertile uranium-238 into fissile plutonium-239, creating more fuel. This is the principle behind breeder reactors. Ultimately, fast neutron reactors can significantly reduce waste, and because they burn transuranic elements, the remaining waste has half-lives of hundreds of years, not thousands. The problem is that the fuel for fast neutron reactors can also be used to make nuclear weapons, and breeder reactors can produce plutonium-239, which is also weapons-grade. The choice is between conventional reactors that use only a small fraction of uranium and create long-lasting waste, or fast neutron reactors that produce less waste but use fuel that can be used for bombs. What would you choose?

The next article will discuss how current nuclear power plants manage depleted uranium.