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This Afterpop is brought to you 
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Hello everyone, and welcome to 

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the Afterpop, where we strip 
down nuclear ideas until they 

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are as clear as mud. 
Well, hopefully clearer than 

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that. 
Today we're tackling a big 

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question. 
What is reactor physics? 

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Now you might be thinking 
physics sounds hard. 

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Equations, symbols, maybe some 
late night math homework. 

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But reactor physics isn't just a
stack of equations. 

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It's about understanding how 
tiny particles smaller than an 

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atom can create enough energy to
light up entire cities. 

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And don't worry, I'm not about 
to hand out a test. 

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We're going to explain it like 
you're in science class, but 

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maybe a fun one. 
So first, reactor physics. 

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Our previous episode, we talked 
to Iquan Caleb about his journey

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at University of Michigan 
studying reactor physics. 

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So we wanted to create this 
afterpop to dive in. 

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So what is reactor physics? 
Imagine you've got a giant pot 

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of popcorn kernels. 
Each kernel can pop, but it 

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needs a spark of heat to get 
things started. 

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Now picture one popcorn 
exploding and bumping into its 

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neighbors, making them pop too. 
That's kind of what's happening 

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in reactor physics. 
Reactor physics is the study of 

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how neutrons, tiny neutral 
particles inside the atom, move 

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around, hit other atoms, and 
sometimes even cause them to 

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split apart. 
That splitting apart is called 

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fission, and when fission 
happens, it releases more 

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neutrons, kind of like popcorn 
bumping into its neighbors 

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energy in the form of heat. 
The same way popcorn popping 

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releases steam and smaller 
atomic pieces called fission 

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fragments. 
I don't really have an analogy 

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for fission fragments. 
Maybe those are like the burnt 

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crispy pieces of the popcorn 
that you leave at the bottom of 

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the bag. 
Now, reactor physics is about 

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predicting and controlling all 
of that. 

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It's the math and the science 
that answers. 

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How many neutrons will be 
released? 

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Where will they go? 
Will they keep the chain 

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reaction going or will it fizzle
out? 

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If chemistry is about mixing 
ingredients to bake cookies, 

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reactor physics is about making 
sure the oven doesn't burn them.

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Let's start with the neutron 
life story. 

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We like to call this the neutron
life cycle. 

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Let's walk through it step by 
step, like a storybook. 

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Step one birth. 
A neutron is born when uranium 

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or plutonium splits. 
Step 2. 

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Fast childhood. 
At first, this neutron zips 

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around at super high speeds, too
fast to actually be useful. 

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Step 3. 
Slowing down. 

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In most reactors, we want 
neutrons to calm down so they're

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easier to control. 
This happens in a moderator, 

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often made of water. 
Step 4. 

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Adulthood, AKA useful work. 
Once slowed down, the neutron 

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might hit another uranium atom 
and split it. 

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That's the chain reaction and 
Step 5, old age AKA absorption. 

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Eventually neutrons either 
escape, get absorbed by 

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materials that don't split, or 
are captured by control rods. 

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Understanding this life cycle is
the heart of reactor physics. 

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If too many neutrons escape or 
die off, the chain reaction 

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stops. 
If too many are born and keep 

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multiplying, the chain reaction 
could speed up dangerously. 

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Reactor physics finds this 
balance. 

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So let's talk about the balance.
The balancing act is also known 

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as criticality. 
Reactor physics uses the ideas 

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of criticalities. 
Sub critical, meaning not enough

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neutrons. 
The reaction doesn't chain and 

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the fire quote UN quote fizzles 
out. 

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Critical, just the right number 
of neutrons, steady controlled 

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power. 
This is kind of like the 

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Goldilocks Zone. 
Then there's supercritical. 

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This is too many neutrons. 
This is where things can get out

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of control. 
Overheated. 

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Here's an easy way to picture 
it. 

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Imagine a campfire. 
If you have just one spark, it 

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might not light. 
If you add a few sticks, it 

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grows. 
But dump a whole crispy 

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Christmas tree on it and you'll 
have chaos. 

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Reactor physicists designed the 
campfire of neutrons so it stays

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in that Goldilocks zone. 
Not too small, not too wild, but

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just right. 
So let's look at some of the 

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tools of the trade. 
What do reactor physicists 

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actually do? 
Reactor physics courses at 

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places like the University of 
Michigan, MIT, and NC State show

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that students learn neutron 
transport equations. 

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Fancy math that predicts how 
neutrons move? 

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Think of it like weather 
forecasting but for particles. 

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Two diffusion models. 
A simpler version saying on 

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average neutron spread heat like
from a campfire. 3. 

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Criticality calculations using 
computer codes to see if a 

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reactor design will sustain a 
chain reaction. 4 Reactor 

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kinetics. 
Studying what happens when power

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changes quickly, like how fast 
you can press the gas pedal 

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without stalling the car. 5. 
Fuel behavior, How uranium 

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pellets inside rods can shape, 
heat or even crack over years. 6

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control systems. 
How rods, coolants and geometry 

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keep everything balanced. 
It's a mix of math, computer 

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modeling, and hands on 
experiments from working with 

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reactor simulators to watching 
how neutrons behave in research 

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reactors. 
So why does it all matter? 

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Reactor physics makes sure that 
reactors run safely, never too 

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fast or unstable, helps design 
new types of reactors like small

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modular reactors or fusion 
prototypes, predicts how much 

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fuel is needed and how long it 
will last, and guides how spent 

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fuel is handled safely. 
Basically, without reactor 

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physics, nuclear engineers would
be guessing. 

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And when you're dealing with 
atoms that can release millions 

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of times more energy than a 
candle flame, guessing is not an

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option. 
In our episode with Umich alum 

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Ikwan Kaleb, he used tea kettles
and Jango blocks in his Tik Toks

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to show analogies of what 
happens in reactor physics. 

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Reactors are just the fancy tea 
kettles making steam to spin 

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turbines. 
And Jenga blocks are like chain 

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reactions, carefully pulling and
stacking blocks. 

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One action sets off many others.
Reactor physics teaches you the 

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rules of the Jenga tower so it 
doesn't topple over. 

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So what is reactor physics? 
It's the science of keeping the 

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chain reaction balanced, safe, 
and useful. 

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It's popcorn that doesn't burn, 
a fire that doesn't fizzle, and 

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a tea kettle that powers entire 
cities. 

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And it's a career path, one that
people like Iqwan Kaleb are 

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using not just to power homes, 
but to inspire the next 

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generation in classrooms and 
across the world. 

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Thanks again for listening to 
this episode of Naked Nuclear. 

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We hope you learned a little 
something about reactor physics.

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Thanks again to our episode 
sponsor, Nuclear Talent Scout. 

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Until next time, stay curious.