Solving the wonderful mystery of a nuclear startup

One of the many questions we receive from community members is about the startup process of a nuclear reaction in our plants. For those of us who aren’t nuclear operators or engineers, it can seem like a mysterious process. To help shed some light on this mystery, here’s a highly simplified explanation for how it works.

First, let’s talk about how the reaction process works for a commercial nuclear reactor. A neutron collides with a Uranium-235 atom. This extra neutron creates unstable Uranium-236 isotopes, which split almost instantly. This splitting produces heat, which is converted into energy for power, and it produces two neutrons which continue the process, called a chain reaction. We call this process of splitting atoms fission. A reactor core is designed to be inherently stable while operating, where a feedback loop lowers the power of the core as temperatures increase, providing stability. Additionally, the reactor core is carefully controlled, reactors have control rods, which can be inserted into the reactor to absorb neutrons to slow the reaction.

The essential ingredient in the process is the fuel itself. Nuclear plants use uranium as their fuel source. It’s a naturally-occurring element that is mined from the earth, enriched and converted into a dense, compact fuel pellet about the size of a pencil eraser. Were you to visit a fuel fabrication facility in the United States, it would resemble any other manufacturing facility, as workers move fuel from one stage in the process to another.

At this point, you might wonder why these pellets do not cause a reaction. The answer is that a sustained nuclear reaction requires the right kind of conditions. There must be a certain amount of fuel. For the reaction to be sustained, there must be enough uranium atoms for a continuous reaction. That’s one of the reasons someone in a fuel fabrication facility can safely touch individual fuel pellets or even fuel rods during the manufacturing process. Simply, it takes a considerable amount of fuel to achieve a sustained nuclear reaction like what’s achieved inside one of our site’s nuclear reactors.

Once there is enough fuel to cause a reaction, a plant that has never operated before and has all new fuel would need something to start the neutrons firing. This is caused by a startup neutron source, such as Californium-252 or Plutonium-238. These metallic elements are inserted into the reactor with new nuclear fuel. As control rods are lifted from the reactor core, neutrons from the neutron source start to fire, and because there is enough fuel for a sustained reaction, we start the fission process, which sustains a plant for an entire 18 to 24 months of operation.

Operating nuclear plants only change out approximately one-third of their fuel. Because of that, they don’t require a startup neutron source like brand new reactors. Instead they use source neutrons for starting the reactor back up. These primarily come from three different processes: intrinsic source neutrons, photoneutrons and transuranic source neutrons. Though different, essentially, each of these processes results in the release of neutrons to start a nuclear reaction.

Achieving and sustaining a nuclear reaction is a complex process. It requires the proper amount of fuel as well as a means of setting the first neutron off. Most importantly, it requires highly skilled operators to safely control startup as well as extremely knowledgeable reactor engineering and technical specialists to offer support. The result: a safe and sustained nuclear reaction, which provides the carbon-free energy to power our homes and businesses.