eli5: once we are able to sustain a fusion reaction, it is supposed to be a self sustaining process (so I have read). How would we be able to “switch off ” this process, and wouldn’t a self sustaining process be an example of a perpetual process?
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A self-sustaining process taken literally, would be “perpetual motion”. In practice, it is a reaction which will continue until it runs out of reactants or is somehow interrupted.
A car engine requires the starter motor to get going, but once started it is “self-sustaining” until you turn it off.
It would still need fuel. Our sun for example is a giant fusion furnace that fuses hydrogen into helium. So we could easily stop the process by cutting off the fuel.
Fusion requires a very large pressure and temperature. If you release the pressure, the reaction stops. Also, fusion requires fuel and will eventually run out and stop if it unless it’s given more.
It’s much more difficult to stop the fission reactions our current nuclear power is based on. With continuous cooling and insertion of rods that slow down the reaction, we can keep it under control but without them, the fission reaction will keep fissioning faster and faster and become hot enough to melt through the ground and spread to the ground water.
You need fuel in the form of heavy isotopes that can be broken apart and fuse during the reaction. Once all the energy is released it is over. The sun seems self-sufficient but it just has a massive fuel source that will last it eons.
The idea is you need a LOT of energy to START a fusion reaction, but after it gets going you get incredible fuel efficiency. Only a very small pellet could provide gigawatts. The biggest hurdle we have in fusion is simply pumping enough energy into the fuel to get a good reaction started.
Think of it like this you need a match and perhaps firelighters to light a fire. They provide initial heat to get your main fuel burning. If the match and fire lighters go.out too soon the main fire can go out as its not generatting enough heat. However hopefully eventually the wood or other fuel will start to burn enough before the firelighters burn out. At that point provided you add more wood the fire is self sustaining the heat from one piece of wood provides the energy to get another piece burning and so on.
Fusion is the same. At present because of limits to our technology the reaction only happens for a short period because for example gaps or variations in the magnetic containment field in the reactor that allow too much energy to escape, or in sustaining the supply of Tritium. Eventually we will solve these issues and the reactions will continue as long as we kep adding hydrogen to the reactor. But a short time after we stop doing so te fusion will cease just like a campfire you don’t add wood too.
Fusion isn’t self sustaining. Fusion is when you push together to atomic nuclei so that they combine. To switch it off, you just stop pushing them together.
Maybe you mean fission. Fission can be self sustaining. That’s when large atoms decay giving off radiation, that radiation can cause decay in other atoms. To stop this, you need to be able to insert a material which can capture the radiation so that the radiation doesn’t cause that much additional decay. There are lots of other ways to create reactors and methods to control the level of fission.
A fission process will eventually stop when all the radioactive atoms have decayed to non-radioactive atoms.
Simple – you stop the flow of fuel. Think about a motor on a car. Once you turn the engine on, the engine stays on until you turn it off right? In that respect, it’s a “self-sustaining” process. But car engines need fuel. Cut off the fuel and the engine shuts down.
Fusion needs to generate enough heat to keep itself going without us needing to add outside heat to it, that’s true yes. Otherwise the energy production can not be net positive.
It’s quite similar to a fire in a fireplace being self-sustaining; it keeps burning without human intervention, but the process would be radically altered if you deconstructed the fireplace, or opened or closed the air vents, or if the fuel ran out, etc.
Though, while a fire in a fireplace needs oxygen that typically comes from the outside, the fusion reaction inside a fusion reactor doesn’t necessarily need anything else from the outside but some sort of means of maintaining high pressure. That pressure is typically reached with magnetic fields. And it of course needs fuel like a fireplace would.
Self sustaining just means that there is enough energy created by the process to start the process again and we can also take some energy out to power other stuff. A fire is self sustaining if it’s hot enough to catch another log on fire and also roast out marshmallows. Uranium fision is self sustaining because when a uranium 238 atom splits, it releases 2-3 more neutrons that can cause 2-3 other U238 atoms to split. Any self sustaining reaction can be stopped if we either take away the fuel, or change the conditions that cause the reaction to be self sustaining. A fire, if we just leave it alone, will eventually burn out. If we pour a bucket of water on it, it will stop. Uranium fission, we have control rods that can absorb those extra neutrons so more fission doesn’t happen (or we can wait until all the uranium is gone). With fusion, it will just be until all the hydrogen is gone, or if we reduce the pressure/temperature, fusion can’t happen.
The major advantage to fusion over fission is if we lose control over the reaction (nuclear meltdown) if the fusion reaction breaks free, the pressure drops and we just have really hot gas spewing out, but the reaction stops, whereas with fission, we have motlen uranium pouring out and continuing to undergo fission and get hotter because there’s no control rods to slow the reaction when the uranium escapes the reactor.
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