How to help your friends actually understand nuclear fission

Many of us have a mental model of power generation based on combustion. The internal combustion engine powers cars, trucks and airplanes. Combustion turbines are also used to generate electricity at natural gas plants. All of these devices require burning fuel. Nuclear fission, the process we use at our nuclear power generation plants, does not involve combustion. We do not burn anything.  

Fission has more in common with the process of making sourdough bread than it does with combustion.  

Wait. What? Say that again!  

Fission has more in common with the process of making sourdough bread than it does with combustion.  

Let’s explore.


To make bread we start with a key ingredient. Yeast. Yeast is a fungus that reproduces when a parent molecule enlarges, matures, and then detaches. Some yeast reproduces when the parent cell divides in two, which is similar to nuclear fission. But none of this can happen until the yeast is introduced into the right environmental conditions.


Nuclear fission is similar. We start with Uranium-235 (U-235), which doesn’t really do anything on its own until it is introduced into the right environmental conditions and in the right amount. Uranium is a naturally occurring element found in rocks and sand all around the planet. We use U-235 because it is fragile; it is easy to break apart. In the right conditions, when it is struck by a neutron, U-235 will break in half, releasing heat as a byproduct and more neutrons. The breaking, or splitting, of an atom like U-235 is called fission.

Let’s continue with our bread making. We take a small package of yeast and change its environment – we place it in some warm water and add some sugar. That will “activate” the yeast or start


the process of growth. The conditions have to be somewhat precise – the water cannot be too hot or too cool; there needs to be enough sugar to feed the yeast, but not too much. Then, if you add some fuel – flour and let it sit for a while, the yeast grows and the dough rises, usually doubling in size.


Bread dough – water, sugar, flour and yeast – can be developed into various forms, but the science is essentially the same. To make sourdough, a longer fermentation is required and can be sustained for a long time. You simply let the starter dough sit and save a little in the refrigerator, taking it out every few days to warm up and refuel it with some more flour. You can keep a batch of sourdough going for years if you just stop and refuel now and then.

Nuclear fission isn’t much different. Nuclear fuel arrives at our nuclear power plants in advance of each refueling outage. When it arrives, it is similar to that initial package of yeast – it doesn’t really do anything until it is placed in the reactor. Once it is in the reactor and all the parameters for fission are precisely controlled, the fission process begins, and heat is produced. We then use that heat to boil water, make steam and then spin a turbine to produce electricity. Like sourdough bread, we can keep the nuclear fission process going for years, decades, as long as we stop and refuel every so often.  

Unlike combustion, which destroys the initial elements to produce heat and ash or gas, fission is a creative process which generates and regenerates over time with heat as a byproduct. Like good sourdough bread starter, if you maintain precise control and refuel regularly, the nuclear fission chain reaction can be sustained for a long time.  

Comments (2)

Posted August 12, 2023 by Jack Keeling
In fact the "fuel" multiplies because U-238 is converted to Plutonium Pu-239 by a side reaction during the fission process but is US policy to not recycle, but to throw this fuel away in those huge concrete storage casks. This prevents terrorist organizations from getting this "spent fuel" and making an atomic bomb with it.
Posted August 11, 2023 by Stjepan Kovacic
Well explained. easy to understand . I like learning about nuclear energy.

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