Energy too cheap to meter

That was the promise of nuclear power in the 1950s

at least according to Luis Strauss chairman of the Atomic Energy Commission

That promise has not yet come to pass

but with some incredible new technologies, perhaps it still could.

The question is "should it?"

Energy isn't scarce. It's everywhere. Seriously, literally all mass is energy

The trick is getting at it.

Burn coal and you liberate a tiny bit of the energy locked in its chemical bonds

That's easy and cheap to do but the energy you get is pathetic per kilogram of coal

and worse per ton of carbon dioxide released into the atmosphere

At the other end of the spectrum is the energy released when particles of matter and antimatter are brought together

They annihilate each other releasing a hundred percent of the energy contained

Sounds great except that antimatter is incredibly difficult to create and store

in between breaking chemical bonds and Matter-antimatter annihilation we have nuclear energy

the strong nuclear force holding nuclei together contains an enormous amount of energy

The Sun is powered that way

Releasing a mere 0.4 percent of the mass of hydrogen nuclei as it fuses them into helium

But that's enough to power the Sun for 10 billion years

Practical fusion power stations are a holy grail of energy production, but are still a long way off

Until then our only viable source of nuclear energy is fission

Which means breaking very heavy nuclei into more stable small parts

If we want to convert mass into energy, fission gives us the most bang for our buck

Unfortunately, that bang can be literal. Use of nuclear energy may risk the proliferation of nuclear weaponry

and there's also the problem of nuclear waste and the specter of horrible accidents

This last one was painted in terrifying detail in the recent dramatization of the Chernobyl disaster

Nuclear reactors sounds scary because the disasters are pretty epic

However, the reality is that far far more people die from straight up air pollution due to coal-fired power plants

than ever died in nuclear reactor accidents

In fact, the radioactivity around coal fire plants is also higher due to the traced but completely uncontained

radioactive products of coal burning

But the most compelling attraction is that nuclear power doesn't directly produce carbon emissions

In fact nuclear power may be our most sure path to reducing carbon emissions and halting climate change

But can we do nuclear power safely enough?

There are modern ideas including the much hyped Thorium Reactor

that suggest maybe we can

Before we can understand those, we'll need to review how nuclear reactors work

Every fission reactor exploits the same phenomenon

Certain very large nuclei like uranium and plutonium can split into smaller nuclei when hit by a single neutron

When these nuclei split, they release energy and fast-moving neutrons

Those new neutrons can smash into nearby nuclei breaking them up and releasing more neutrons

If you have enough of these heavy nuclei, if you exceed what we call critical mass

then neutrons produced in every fission trigger at least one more fission

That's a chain reaction, a domino effect

That can be a runaway chain reaction in which each split nucleus causes more or other nuclei to split

Resulting in an explosive release of energy. That would be an atomic bomb

But if you can regulate the process,

make sure that each nucleus splitting causes on average only one other nucleus to break

Then the reaction can be controlled

It can be made to produce a steady amount of heat that is used to power a turbine often just by boiling water

The most common commercial power plants use uranium fuel, in particular the isotope

Uranium-235

Uranium-235 has 92 protons and 143 neutrons

it makes up less than 1 percent of naturally occurring uranium

which is mostly uranium 238 with three extra neutrons

U-235 is useful because it's highly fissile

which means it has a high probability of intercepting a stray Neutron and splitting

It's fissile in the presence of the fast-moving neutrons created by its own fission

But it's many times more fissile if those neutrons are first slowed to become so-called

Thermal neutrons

On the other hand the more stable Uranium 238 is only fissile to fast-moving neutrons

and not at all to slow neutrons

In fact, is much more likely to absorb slow-moving neutrons

The cheapest way to do commercial fission is to take advantage of

uranium-235's high fissibility to these thermal neutrons

To sustain thermal fission in uranium you need to enrich it by a few percent

relative to U-238 so that more neutrons get created and fewer get absorbed

You also have to slow down those neutrons into the sweet spot for splitting U-235

To do this, thermal reactors use some sort of moderator

The most common moderator is plain old water

Because hydrogen nuclei in H2O are around the same mass as neutrons

they absorb a lot of momentum in Neutron collisions and

Conveniently that same water can also work as a coolant. It takes heat away from the uranium fuel

Preventing melt down to where it's needed

Which is to drive a turbine either directly or via a secondary loop of water

I just described very very crudely. The principles behind the light water thermal reactor

These are the most common because they're cheapest but let's talk about the problems. First, the safety

Every major disaster has been with a thermal reactor due to a cooling failure in Three Mile Island

the water escaped a jammed hatch in Chernobyl water boiled increasing the neutron count and in Fukushima a

Tsunami knocked out the water pumps

The common issue is that water cooling requires active effort to maintain and so is prone to disruption

Modern light water thermal reactors addressed the failures of the past and repeats of these disasters are very unlikely

But unforeseen failures are still possible especially due to human error

Even the smartest nuclear engineer can have a perma Simpson moment

One way around the coolant issues is to use molten metals or molten salts

these can be liquid over a very large range of temperatures reducing the chance of accidental boiling and

They allow the system to be operated at much higher temperatures which increases efficiency and at much lower pressure than water

the high pressures required for water-cooled reactors add a lot of complexity and size and

potential to explode

Perhaps the worst downside of the common modern reactor is the waste

They use only around 1% of the uranium extracted from the ground the U-235

Some of the U-238 gets converted to fissile plutonium by absorbing neutrons

But most of it is either unused or converted to heavier non fissile elements. These are the so-called

Transuranic Actinides

Elements heavier than uranium on the actinide sequence of the periodic table

They are very radioactive and have half-lives of tens of thousands of years

That means they're dangerous on geological timescales, and there is literally no place on earth

We can guarantee that containment vessels will be safe against earthquakes, volcanic activity or eventual crushing by Ice Age glaciers

A possible solution to this nightmare waste disposal issue is to try to burn all of the heavy nuclei as fuel

One way to do this is to use fast neutrons a fast reactor doesn't try to slow down the neutrons that means

U-238 can split along with U-235 and along with any

Actinide that happens to be produced by Neutron absorption

The waste products of a fast reactor are the fission products much smaller nuclei than the actinides

some of these are

Incredibly nasty like cesium 137, but they have half-lives of centuries not tens of millennia

So safe storage is at least plausible

Fast-neutron reactors get to be smaller than their slow thermal cousins because they don't need a neutron moderator

That makes them ideal for things like submarines

The issue with these guys is that you need much more enriched fuel

The U-235 content needs to be over 20%

several times higher than in a thermal reactor

And that's just because the overall fission rate is much lower

per fast neutron compared to slow neutrons that

Enrichment is expensive and so after abundant natural uranium deposits were discovered and fuel became cheap

commercial interests opted for the thermal reactor even though it wastes 99% of the fuel and leads to

eons of looming environmental catastrophe

Fast reactors also have the advantage that they can create or breed their own fuel

Although fast neutrons don't keep nuclei as easily

When they do hit they liberate more free neutrons than when a slow Neutron causes fission

Typically 2 to 3 neutrons per split

That means you have one Neutron to contribute to the fission chain reaction

and at least one more to be absorbed by a non fissile element to turn it into something fissile

Element that can do this is called fertile

For example uranium-238 is fertile because it can absorb a neutron and be transformed into plutonium-239

A typical breeder reactor includes a reactor core burning highly enriched uranium or plutonium

Surrounded by a blanket of fertile material that cycles into the core as it becomes fissile

Thermal and fast reactors have different advantages and disadvantages

Regarding nuclear proliferation the waste of a thermal reactor isn't fissile

But it could be bred into fissile material

The ultimate waste products of a fast breeder reactor are not dangerous in this way

But the intermediate products include weapons-grade plutonium

Which you definitely don't want in the wrong hands some of the advantages of both of these reactor types can be achieved by

switching to a completely different fuel

thorium

That's the thorium reactor

Thorium is another actinide two spaces lighter on the periodic table compared to uranium

It's not naturally fissile

But it is fertile upon absorption of a neutron it decays into protactinium-233

and then into uranium-233

And U-233 is nicely fissile. It's even better than U-235 and plutonium-239

Because it absorbs fewer neutrons

Which means better neutron economy and more importantly on average uranium-233

Produces slightly more than two neutrons per split even when it's split by a slow-moving neutron

That means it's possible to breed new uranium-233 from thorium in a thermal reactor

You don't need a fast reactor

There are different ways to build a thorium reactor, but perhaps most promising is the liquid fluoride thorium reactor or LFTR

In this design both thorium and uranium-233 are bonded with fluorine and

dissolved in a molten fluoride salt beryllium or lithium fluoride

the fission in the uranium produces heat and neutrons to sustain fission and

to breed more uranium from the thorium

The uranium and thorium can either be mixed together or separated with a thorium blanket surrounding the uranium core

In either case the molten salt containing the uranium

Also transports heat out of the core to secondary circuits that ultimately power turbine

The actual fusion only happens in the reactor core

Because that's where the moderator

Slows down the neutrons to make fission much more likely in this case

The moderator is a lattice of graphite channels through which the fluid flows

Graphite is particularly great because it slows neutrons without absorbing them

When the fluid is away from the graphite neutrons speed up which means fission slows down

Because it's in liquid form

The fuel can be quickly drained from the reactor in emergencies a plug with a low melting temperature

Will melt if the core gets too hot or if power supplying a cooling fan goes out

The fuel then drains to a tank where fission is impossible

in addition built the right way, the liquid fuel becomes less fissile as temperature increases

That's because at high temperatures thorium is increasingly good at absorbing neutrons

So not enough neutrons are left to continue to fission

this whole setup is a great example of

Passive or walkaway safety meaning that in the event of an emergency

Even if every mechanical or human mechanism failed the reactor would simply power down

Another compelling advantage of the lifter and molten salt reactors in general is that they can be small because they don't need giant

structures to handle the high pressure water

in fact

It was for use in submarines and aircraft that molten coolant reactors were first conceived

But now this compactness and modularity

means they could be inserted into the current electrical grid to replace coal or natural gas plants or you know on a

Lunar or Martian settlement or a starship that same modularity

Poses perhaps the biggest risk if small thorium reactors became widespread

They'd become less easy to regulate and monitor

we'd want to be very careful that the reactor design leaves the weaponizable U-233

completely inaccessible without

enormous effort

Nuclear power is a possible solution to our dire energy and climate challenges

The question is do we need it or can we meet those challenges with renewables like wind and solar?

assuming significant advances in battery tech

I don't know the answer and I'd love to hear your opinions

What I do know is that we face an enormous hurdle in our progression as a technological species one

Which may take all of the ingenuity we can master

We should think very carefully about whether the power of the atom is necessary to survive and thrive

Into the next technological stage and send us to greater distances and further futures in space-time

In a recent episode we talked about how black holes influence the galaxies they formed in often by killing them

Let's see what you had to say

Steve C comments that this whole black hole killing star formation thing seems like a negative feedback

Interaction more gas equals more active black hole equals more outward radiation and wind equals less gas

Suitable for star formation or feeding a black hole like a thermostat at the center of a galaxy

Well put, Steve! That's exactly what it is

Chuck Ritter's Dorf asks, whether quasar jets and a preferred direction

Relative to the galaxy as a whole and how does this influence their effect on the galaxy?

so full blown quasars the most luminous of accreting black holes or active galactic nuclei are

Typically in bowl-like elliptical galaxies. So orientation isn't as meaningful as in a spiral galaxy.

weaker active galactic nuclei tend to live in spiral galaxies for example Seyfert galaxies

and they can have their Jets pointing in any direction at right angles to the disk or even straight into the disk

But in general when such a jet is first launched

It tends to balloon out and spread its energy through a good fraction of the galaxy

So orientation isn't so important. Proghead777 asks, whether the central supermassive black holes gravitational

Influence is extended by frame dragging

Well, the answer is yes, but not very far

Frame dragging is the dragging of the fabric of space around a rotating massive object in the case of a black hole

it's most obvious effect is that it changes how closely an objects can orbit the black hole in a stable way.

If objects orbit in the same direction as a black hole's rotation that can be stable much closer in

but if they orbit in the opposite direction

then the stability limit is further out this results in a black hole shadow a

Blank region in the middle of the accretion disk that depends on black hole spin

It's one way we'll be able to measure that spin

But this definitely doesn't extend more than 10 or so times the black hole event horizon

Oppie asks why NASA isn't dedicating more resources to investigating all West themed planets in elliptical galaxies

I don't know, Oppie. I just don't know.

Probably the same evil conspiracy

That got Firefly cancelled after one season

Why do the powers that be fear space cowboys so much?