This episode is sponsored by Wren, a website where you can  calculate your carbon footprint.

You can also sign up to  make a monthly contribution to offset your carbon footprint or  support rainforest protection projects. [ ♬ intro ]. Nuclear power plants have a bit of a bad rap for a whole host of reasons, from  meltdown accidents to toxic waste.   But while nuclear comes with massive problems, one  good thing is it produces few greenhouse gases, so it’s been suggested as a  tool to fight climate change. ~ So a way to get energy from splitting  atoms - from nuclear fission - that avoids some of these problems could be handy.

And that’s where thorium comes in. While nuclear power today  uses uranium as its fuel, it’s been suggested that thorium, which is three  times more abundant in nature, would be better. And while it’s not quite a slam-dunk,  they might be onto something.

In nuclear power plants, you generate tremendous  energy from splitting apart the nuclei - the bulky middle part - of atoms. Currently, most nuclear energy  is made by splitting apart a particular kind of uranium nucleus called U-235. When a U-235 nucleus splits, it releases  energy, and particles called neutrons.

Those neutrons can hit other U-235 nuclei and  make them split and release more neutrons. This is a chain reaction - a  reaction that is self-sustaining. Nuclear materials that can self-sustain  like this are called fissionable.

Thorium is a bit different, though.  Unlike U-235, the main kind of thorium, T-232, isn’t fissionable, but fertile. When T-232 is hit by a neutron, it turns  into T-233, and that’s fissionable. So the reaction makes its  own fuel as it goes along.

So yes, it’s fertile in the sense that it  makes its own little nuclear fuel babies. You use more of the fuel you put in, which  drastically reduces the amount of wasted fuel. Oh, and thorium-based reactions don’t produce as much of the really nasty waste  products that uranium-based ones do - the kinds that will still be  toxic in thousands of years.

That’s because the reaction chain from thorium has more steps than the chain from uranium,  so you get less of those final products. Also, most uranium in nature is in  a nearly-unusable form called U-238, and extracting the usable U-235 form -  that is, enriching the uranium - is hard. But thorium doesn’t have this problem: all thorium in nature is already in T-232 form.

That’s a lot of letters and numbers, but the upshot is that thorium can be dug up and put in a reactor with a  lot less effort than uranium, once you’ve built the right kind of reactor. And rethinking reactors is a good  idea, because the current way we design them has a few problems,  mainly due to the ways they’re kept cool. In fact, every major nuclear power  disaster has been due to a cooling failure.

And while accidents are extremely rare,  and safety has improved a lot over time, it’s always good to look for safer alternatives. See, in most modern nuclear reactors, the  uranium rods are submerged in pools of water, in a kind of design that was first  used to make power in the 1950s -- and has been with us ever since. ~ One purpose of the water here  is to cool the reactants, and if that fails you can get a meltdown,  like what happened at Fukushima or Chernobyl. Thorium lends itself to different kinds of  nuclear reactors that don’t have these problems, including one kind called  molten salt reactors, or MSRs.

Instead of using uranium rods in water, some of these reactors use thorium  dissolved in a molten mix of metals and other stuff, chemically known as salts. That sounds kind of extreme, but  it actually makes things safer. Since these molten materials  stay liquid even if you pump a bunch of heat into them, you can keep  them liquid at much lower pressures, meaning you can’t get steam  explosions like at Chernobyl.

And clever MSR reactor designs allow  you to make passive cooling mechanisms: reactions where temperatures getting too  high make the reaction shut itself down, or make the fuel automatically get ejected. But despite these benefits, we’re probably not going to switch from  uranium-based plants any time soon. Even though thorium-based reactor designs  may be almost as old as uranium ones, the idea isn’t fully developed.

There’s still a lot of research  & development that would need to go into them before we could  ever get proper energy from them. And there are some critics. Some scientists have questioned  how much benefit thorium reactors would bring in practice, once a bunch of  engineering problems have been considered.

It may not be as efficient and  waste-free as we’d like it to be. And it’s possible to come up with improved  reactor designs that use plain ol’ uranium, like a project announced in 2021 in  Wyoming that also involves molten salts. Though, again, those kinds of improved reactor  designs aren’t without their critics, too.

As for why uranium reactors are  so much more well-developed, unfortunately it’s because of nuclear weapons. Since uranium was much more  useful for making nuclear bombs, most early nuclear research went into, well, that. So uranium-based reactors got  a big head start research-wise.

Because of that, it was always more viable commercially  to open new uranium-based reactors, like a really high-stakes version  of the betamax-vs-VHS fight. So while people are working on the problem, if you want to start a new  nuclear power station now, you kind of have to make it uranium-based. Like with all stories about making  energy, whether anyone ends up building thorium-based reactors  is a difficult decision based as much on politics as on science and engineering.

If we can solve the engineering problems, they might help to fight against climate change. It’s up to us to decide if the  risks and downsides are worth it. [Sponsor Outro]. But we’ll need a lot of different approaches  working together to stop climate change.

Wren is a website where you  calculate your carbon footprint, then offset it by funding projects that  plant trees and protect rainforest. You’ll answer a few questions about your habits -- like what you eat and how you commute -- and   then you’ll get a personalized report  that estimates your carbon footprint. Wren will provide you suggestions for ways to lower your carbon footage and also allows you to offset your footprint by making a monthly contribution  towards climate projects.

Once done, you receive monthly  updates from the tree planting, rainforest protection, and other  projects you helped support. You can use the referral link in the description to protect 10 extra acres of  rainforest when you sign up! [ ♬ outro ♬ ].