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.

Comments (12)

Posted October 04, 2023 by Raghavendra Rao Hebbani
Nice clear explanation. A diagram and isotope equation would have greatly enhanced the explanation. Thanks
Posted September 17, 2023 by mish
So basically if I understand correctly. The new fuel rods are installed with fresh pellets. When they are installed, they are installed with the control rods inside. Then they start bombarding the fuel rods with neutrons. But because the control rod are there, they absorb all those neutrons. Once the control rods are lifted, the newtons start to cause fission in the fuel rods. Is this correct?
Posted May 19, 2023 by Gus Jones
Thanks, I always wondered about the assembly of the pellets and rods, why they don't start a chain reaction by themselves and the actual start-up of a reactor.
Posted May 18, 2023 by Richard
Thank you, very simple and clear explanation.
Posted September 05, 2022 by Chris Klug
Re: Adam. A Califonrium-252 source undergoes what is called spontaneous fission, which results in the release of neutrons with each decay. This is always occurring, even as the source is placed into the reactor core, but the control rods absorb the neutrons. As the control rods are removed, fewer neutrons from the startup source are absorbed by the control rods and more neutrons are able to start fission reactions in the Uranium-235 in the fuel rods, bringing the reactor up to power. The neutrons emitted by Cf-252 are actually very close in energy to those normally emitted by U-235 fission, so they work well for startup.
Posted January 04, 2022 by Shon Mardani
Thank you Adam, please confirm that there is NO Atomic bomb and free the humanity once and for all.
Posted September 21, 2021 by Ellen Morton
Hi Adam! Thank you for the feedback - we will take it into consideration for future articles.
Posted September 19, 2021 by Adam
I wish this article explained the startup process better. "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." How are they inserted? When the control rod starts being removed, does the initial reaction suddenly begin because the startup source is physically close to the uranium? If not, what triggered the initial firing?
Posted September 19, 2021 by Adam
I wish this article explained the startup process better. "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." How are they inserted? When the control rod starts being removed, does the initial reaction suddenly begin because the startup source is physically close to the uranium? If not, what triggered the initial firing?
Posted March 18, 2021 by James Patrick Chaburka
I enjoy the tech side of startup, as I was in the navy on a nuke boat. Submarines, on a boat' everyone had to learn the process of nuke physics to get there fish, and then some. Good times.also hard times
Posted October 29, 2020 by Steve Kovacic
I love Nuclear Energy, Love to read every news on that subject. Find it very interesting. Thank you very much.
Posted August 12, 2020 by William Jensen
As a former nuclear utility employee & still part of the industry as a service provider, I'd like to thank you for taking the time to put this out there.

Leave a Reply

Please read our Comment Guidelines.

*
*
*

For real-time updates, follow us on Twitter

Follow Blog via Email

Enter your email address to follow this blog and receive notifications of new posts by email.

Opt out from these emails

Check out our new Facebook page