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
increase the proportion of U-235
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?