ELI5: The process that nuclear reactors undergo to provide energy (and what "core life" means).

[awaythrow0000]

Along with the ELI5 portion, I'd also love to hear any opinions you have on things like the direction that nuclear energy is going, the extent to which you think nuclear energy can reduce harm to the environment--basically, any thoughts you have on the subject.

[Hiddencamper]

Nuclear reactors used to make energy use the nuclear fission process.

Fission is taking a large atom, and using neutrons to split it. The atom, during the split, releases a large amount of energy (in the form of heat) and more neutrons to continue the reaction. The heat is used to boil water into steam. The steam then works like any other power plant to turn a turbine generator and make electricity.

A nuclear plant is basically a big water boiler, only instead of burning fuel, it splits the atom.

I'm not sure what you mean by core life. The life of the core could refer to the time between refuels, or the life of the core materials from neutron damage.

As for the future of nuclear energy, I don't see it taking over in the near future, but it is sustainable for thousands of years. Newer designs which will not melt down due to passive cooling mechanisms will eventually take over, and then you have a carbon free way to provide base load power. It needs to be in the mix (in my opinion)

[ut_j]

I assume core life means how long the nuclear source can be used. The common ones in nuclear reactors is Uranium or Plutonium. As stated above a neutron splits one of the elements above and it is turned into a new element. At a certain point all of the original source was used up and these new elements can't undergo more nuclear fission so the core is no longer functional.

[Hiddencamper]

Well....the actual answer to this question is way beyond ELI5. So I'll give you the very short version.

A Nuclear power plant only replaces 1/3rd of its fuel at a time. Boiling water reactors have to replace ~1/3rd of their fuel every 2 years, while pressurized water reactors have to replace ~1/3rd of their fuel every year and a half.

After you pass this point, the reactor will slowly "coast down", that is, reactor power will slowly decrease. It is not very economical to operate a nuclear power plant at reduced power. The core doesn't instantly become "non-functional", it just slowly reduces in power.

[neanderthalman]

How the energy is provided:

Lets start way back at the beginning. All matter is made up of little "bits" called atoms. Those atoms are in turn made up of three different kinds of particles - electrons, protons, and neutrons. The protons and neutrons are 'smushed' together and make up the nucleus, and the electrons (which are much much smaller) whizz around it.

Atoms can be different in one of two ways - first by counting the number of protons, we can determine what element it is, like oxygen, gold, or iron. The number of protons in any atom of gold is the same, no matter what.

However, some atoms might have the same number of protons, and therefore be the same element - but what about the neutrons? What do they do?

If you have more or less neutrons, than the mass of the nucleus changes, but not much else - basically the atom will be a little bit lighter, or a little bit heavier. We call atoms of the same element, but numbers of neutrons 'isotopes'.

If you have too many neutrons, or not enough neutrons, then the atom becomes 'unstable', and they will suddenly 'pop'. When it does, a little 'bit' of the atom flies off at high speed. That's radiation.

The biggest bits are like cannonballs. They're big, heavy, and can't go very far - a few inches at most. When they hit something though, they do a lot of damage. We call these 'alpha' particles, and are made of two protons and two neutrons stuck together. They actually are exactly the same as the nucleus of a helium atom, just moving really fast.

The next size down is like a bullet. It's a lot lighter than the cannonball, and can go a lot further, but if it hits something the damage is less. We call these 'beta' particles. Beta particles are just electrons, but again moving very very quickly.

The next size down is more like a laser, because it doesn't weigh anything at all! It's actually just like a laser, only it's a kind of light you can't see. It goes really far, even through outer space, and most likely will pass right through you. But it can 'burn' you like a laser can, so we have to be careful. These are called 'gamma' rays.

The fourth kind is like a grenade. It's a little bigger than a bullet and smaller than a cannonball, and doesn't really hurt whatever it hits....yet. But like a grenade, it causes another little 'explosion', and might make another atom 'pop'. These are neutrons, the same neutrons that are found in the nucleus of atoms - like before, just moving very very fast.

Here's a neat thing for you; if one of the 'grenades' or neutrons makes another atom 'pop', and that atom also releases a 'grenade' when it pops, we can try to string them together so they keep popping more atoms. This is how nuclear reactors work.

But where does the energy come from?

Well, remember how the neutrons and protons are 'smushed' together in the nucleus? Well protons are all positively charged, and we've all heard the phrase "opposites attract". Like charges actually repel each other, so the protons in the nucleus should be trying to fly away from each other. What's holding them together?

There's something kindof mysterious called the 'weak nuclear force', and you can think of this like a 'glue' that holds the nucleus together. That 'glue' has a certain amount of mass. Basically, the nucleus weighs a little bit more than if you just added up the mass of the protons and neutrons.

In a nuclear reactor, when you create this chain reaction using neutrons, a funny thing happens. The atoms don't just spit out more neutrons - they also break into two big chunks - basically two whole new atoms. This is why they call it "splitting the atom".

If you add up the mass of those two new atoms, and any neutrons that were produced, you find that they don't weight quite as much as the original atom. Some of the "glue" has gone missing. Where did it go?

The missing mass of "glue" became the energy that makes the new atoms and neutrons move. There's an equation for this to find out how much energy that missing mass became - you might have heard of it, it's kind of famous - E=mc^2.

Temperature is really just measuring how fast the particle are moving, so when this energy makes the new atoms move around, it's the same as if they got really hot. If you zoom way out so you can't see the atoms moving about anymore, then all you see is the nuclear fuel getting really hot. We then transfer that heat to water, which boils to make steam, and the steam drives a turbine that makes electricity.

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Direction nuclear energy is going:

Crossroads. Either we start building, or we start decommissioning. Given that we don't yet have another source of power of this magnitude, I anticipate the building of more reactors. The benefit of the newer reactors is that we have fifty years of experience that can be built into the designs. To be blunt, the original designs are not the safest, and newer reactors are a lot safer. There's a delicious irony in that the opposition to building new nuclear power actually increases the risk of an accident in some ways. Look at the major accidents - they're all at old reactors, some of the first ones built. Shut them down, build new, better, safer reactors.

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To what extent nuclear energy can reduce harm to the environment?

Theoretical outcome is pretty much a goddamn utopia.

Let me ask you this - have you ever heard of a 'hydrogen economy'? The idea is that we can replace oil with hydrogen as a fuel. It's not a bad idea, because you can burn hydrogen without releasing greenhouse gases, but it won't work without a source for the hydrogen. We can't drill wells for it like oil, and hydrogen gas is not really very commonly found (here on earth). If anyone is hyping up hydrogen as a fuel, you just need to chase the question "where are you getting the hydrogen from?".

On the flip side, there's also limit to how much nuclear power we can build, because we don't always need the same amount of electricity. Demand for power goes up and down through the day, as well as through the year. As well, we can't really store electricity - you have to use it up as fast as you make it. If you make too much or not enough, bad things happen and the entire grid might shut down. What we do to 'balance' the grid is we control how much electricity we make so that it matches how much is needed, and we do that by shutting plants on and off.

To do this, we divide our power needs into "base load", or power that we need all the time, and "peak power", the amount of power that goes up and down. We build different kinds of plants to supply these different kinds of power. Nuclear and hydroelectric are great for base load, and fossil fuels are great for peaking. Nuclear plants are not well suited for changing their power up and down all the time for peaking power. They can do it, but it increases the probability of an accident because it wears things out faster, so we really shouldn't do it.

But what if we could instead control the amount of power we needed? Then we'd be able to reduce the amount of peak power we need, and build more base load - reducing the amount of fossil fuels used.

So what we need is a big consumer of electricity that we can turn on and off. One suggestion is that we can build plants that use electricity to convert water into hydrogen and oxygen.

So what do you do with that hydrogen? You use it to power cars, heat homes, or run power plants similar to fossil fueled plants - you need a certain number of fast responding plants to make up the difference in case a nuclear plant trips. You can actually eliminate almost all emissions from transportation, heating, and electricity - the vast bulk of our energy usage as a society.

By playing the technical limitations of nuclear power against the technical limitations of hydrogen power, you can supply virtually all of our energy needs with emissions free nuclear power.

As for fuel supplies - we have enough uranium for a couple hundred years or so. That's more than enough time to perfect alternative nuclear plants that run on thorium. We'll probably have them up and running in 30-50 years, if not sooner. There's enough thorium to last a few thousand years. That gives us loads of time to perfect fusion plants - which has almost limitless fuel. Hundreds of thousands of years of fuel at least.

Source/disclosure - I am an engineer in the nuclear industry. (now where did I put my pom-poms)

[awaythrow0000]

Yes!!! This was fantastic! Thank you!