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PROFESSOR: So I got much more than one request

to do some stuff on nuclear materials,

and I think it's just about the right time.

That you guys know enough about radiation

interacting with matter and everything,

and stopping power, and processing,

to actually make sense of nuclear materials and radiation

damage.

And this is my whole theme, so happy

to come talk to you guys about this

and show you why I think it's interesting.

Because it all goes--

this slide kind of gets onto it.

It starts off with the single-atom atomic defects

that make up the basic building blocks of damage

and ends up with things that break in nuclear reactors

under radiation.

And so to understand the whole thing,

you've got to know everything from the single atoms

on the sort of femtosecond scale,

all the way up to the engineering scale

where things evolve over years or even decades.

So we'll be talking--

first, probably today, we're going

to go over a material science primer.

So who here has had any courses in material science?

No one.

That's good because I'm assuming that there is a--

see, no one knows anything here.

I know there's a couple material scientists in the class,

and I'll apologize ahead of time if it's a bit of a review.

But we'll be going mostly through what are materials

and what are the defects that change their material

properties, and how do they behave.

That'll take us through about today.

So then tomorrow, we can see how radiation causes those defects

and actually changes material properties.

So there's a whole laundry list of different ways

that materials fail, and most folks

are concerned with all of these--

everything from simple overload, which means you

stress something too much and it just breaks,

to all the different forms of corrosion.

That's a whole field in itself.

And then there's the things that just

we have to worry about because they're only activated

with radiation damage.

And in this case, this isn't quite ionization by radiation,

but it's actual radiation slamming into nuclei

and moving atoms out of their place.

And we've got one figure that we had recently

in a paper that sums up the entire multi-scale picture

of radiation damage, from the femtosecond to, let's say,

the megasecond scale.

Or I think it's more than that.

Maybe gigasecond would be the right word for that.

And all the way down from the angstrom to the meter scale.

And I want to walk you through sort of a lens scale

by lens scale depiction of radiation damage.

It all starts with knocking atoms out of place.

We've mentioned this a little bit

when we talked about nuclear stopping power,

and this is where it actually comes into play.

Sometimes an incoming neutron or photon or ion

can displace an atom from its original site,

and we call that a physical-- it's a displacement.

And then that atom comes off with quite a bit

of kinetic energy and can knock into a whole bunch

of other atoms.

Now this loss of the solid crystalline structure,

you can't really tell what the original structure looked like,

right?

It actually comprises a very small, localized zone

of melting called a thermal spike.

If you think about, all these atoms

are vibrating at fractions of an eV--

at thermal energies, like the thermal neutrons

we talked about in the reactor.

Then you hit them with an MeV neutron.

They might transfer 100 keV of energy.

And a bunch of these atoms will then

be moving about at, let's say, a few hundred eV.

That's way beyond liquid temperature.

So actually, it's been theorized that there's

a little pocket of atoms around three to five nanometers wide

that reaches, like, 10,000 Kelvin for a very, very

short amount of time--

less than a picosecond.

Because almost instantly, those atoms

knock into the ones around them, and this

is how the process of heat transfer occurs.

And so, very quickly, you get what's

called the quench, where most of those atoms

very quickly knock into other ones,

slowing down, finding their equilibrium positions again,

but not every one.

You can see there's a few places where the atoms are still

out of their original location.

And it's those residual defects that actually

comprise radiation damage.

And as those defects build up, they start to move.

They can diffuse.

They can be transported ballistically

by more radiation damage.

They can move by all sorts of different mechanisms

and eventually find each other, forming what's called clusters.

So a bunch of those missing atoms could find each other

and make a hole, which we call a void.

A bunch of the extra atoms shoved in

between the other ones can form things

called interstitial clusters.

We say interstitial because it's like in the space

in between where you'd normally find some atoms.

So let's say you had a whole bunch of those missing atoms

come together, forming a void.

This is an actual Transmission Electron Microscope,

or TEM, image of a void--

pockets of vacuum in materials.

Notice anything interesting about its shape?

AUDIENCE: It's, like, rounded.

PROFESSOR: It's rounded, but what's most striking to me

is it isn't actually round.

So you would expect a void or a bubble to be kind of spherical,

right?

That's the minimum energy configuration of most things.

Not so when you have a little pocket of vacuum.

It's where crystallinity comes into play.

And these voids can end up forming superstructures.

What curious thing do you notice here?

For this whole ensemble of voids.

Yeah?

AUDIENCE: It seems like they're all in line.

PROFESSOR: They are all in the same direction.

Kind of funny.

That's definitely not an accident, right?

That's not like they're randomly aligned.

There's a reason for this, that we'll

go into in a couple slides.

Yeah?

AUDIENCE: What's the size scale here?

PROFESSOR: The size scale?

I think these are on the order of 20 nanometers or so.

Yeah, I cropped these images just to get points across.

Let's see if it says in the older one.

Not quite.

Yeah, but these voids can get upwards of tens of nanometers.

As small as single atoms.

Yeah?

AUDIENCE: Sorry, what is this?

PROFESSOR: This is the accumulation

of radiation defects into what's called voids.

Yeah.

Don't worry, we'll go over it in more detail again.

And if you get little pockets of vacuum in your material,

you're not creating or destroying mass.

You're just moving it.

So those voids, where that mass was has to go somewhere else,

and you actually get things that swell

in the reactor on their own.

They don't change mass but they change volume.

They just kind of puff up like Swiss cheese, sometimes

upwards of 20% or 30% changes in diameter and length

for some tubing.

Now if you're depending on these fuel rods

being a certain space apart in a reactor

and they start to swell, squeezing out the coolant,

you lose the ability to cool the reactor.

Because then how can you get water around something

where the tubes have then swelled together?

There's lots of other bad things that can happen,

which we'll get into.

And so then that's the origin of void swelling.

From single missing atoms called vacancies,

they can cluster into voids which

then cause physical dimensional changes of materials

on the scale of centimeters to meters.

And that's why we say it's this full multi-scale picture

of radiation damage.

But to understand, what is damage,

you have to know what is an undamaged structure

to begin with.

So it doesn't make sense to say, how does a structure change,

if you don't know how it behaves.

So I want to give a very quick primer to material science.

And apologies to any material scientists in the room

because this is going to seem really basic,

but this is a very quick intro to this whole field.

I want to go over quickly, what is a crystalline solid?

A perfectly undamaged material would

be a set of atoms lined up in a very regular lattice

and of regular array, where you move over a certain distance

and you find another atom.

And this extends forever and ever and ever,

all the way out to when you reach the free surface.

And so this is what we would call an undamaged material.

A pristine, perfect, single crystal.

By crystal, I mean an arrangement

of atoms in a certain direction.

So notice here, all of the atoms are lined up in, let's say,

some cubic xyz way.

That's what we would call one crystal or one grain.

You'll hear both of those.

And you'll notice also that the arrangement of the atoms

tends to determine what the physical objects look like.

Or we like to say that form follows

structure in material science.

So for materials like pyrite, which

follows a simple cubic structure, that's the crystals

you pull out of the ground.

They mimic their atomic configurations

in physical centimeter-sized space.

For gold atoms, they adopt a slightly different structure.

It's still cubic but there is atoms

shoved into the cube faces.

It's what we call Face-Centered Cubic, or FCC.

And you start to see cube-looking structures

all over single crystals of gold.

Another one, gypsum.

It's got a very different type of structure called monoclinic,

where none of the sides of this parallelogram are the same

and there are some funny angles.

But if you look at the arrangement of the atoms

and the actual crystals of gypsum that grow,

you see a striking similarity, which I find pretty neat.

I also want to mention, what is the absence of structure

in material science?

We call that something that's amorphous.

Amorphous means without form.

So for example, crystalline indium phosphide

would have this regular structure like this.

You move over a certain distance,

you see another green atom, and so on and so on and so on.

In an amorphous material, it can still be a solid,

but there is no fixed distance between any certain types

of atoms.

And radiation can cause a lot of this amorphization

by knocking the atoms about and having them freeze

in random configurations.

This is one of the ways that radiation damage can

embrittle materials because--

well, we'll get into that.

So now let's talk about the defects

that can be created in a perfect crystal.

The simplest ones, we call point defects.