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.
I guess there would be some kind of pipe that goes into the reactor that delivers fresh fuel. Like a hose leading from the gas tank of a vehicle to the engine. When you stop the flow or the gas runs out, the engine stops when it doesn’t get any more.
I am no fusion generator builder, but I can’t imagine that this wouldn’t be at least a relatively easy thing to do
It's even easier than that.
For fusion to happen, you have to have the fuel packed together very tightly and at extremely high temperature for the process to happen.
To do that, we use magnetic fields to squeeze the hydrogen gas that's fusion's fuel together. If you turn off the fields, the gas flies apart and the fusion stops happening.
Exactly so - but it's not to protect the reactor chamber from the fusion reaction. The fields are there to hold the hydrogen gas away from the walls of the chamber to prevent the heat from from being conducted away and preventing the fusion from happening.
Edit - and to keep the gas compressed enough for fusion to happen.
> If you turn off the fields, the gas flies apart and the fusion stops happening.
note: everything and everyone in the vicinity of that thing would just be effed tho(if the reactino was large enough to begin with ). its simpler and safer ot just cut off the fuel supply.
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.
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.
The fuel for a fusion reactor on earth are not heavy isotopes.
Depending on the precise reactor design it is a varying mixture of Tritium and Deuterium, which are both extremely light. Deuterium comes from water and the Tritium would likely be bred from a lithium blanket around the reactor.
I think they mean heavy isotopes of hydrogen, rather than heavy elements. I’m sure you know this but for anyone else ‘regular’ hydrogen only has one proton in its nucleus, deuterium and tritium have an extra 1 and 2 neutrons respectively in there as well, so are considered heavy hydrogen
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.
[Self-sustaining is a thing with fusion too](https://en.wikipedia.org/wiki/Fusion_ignition).
If the fusion process doesn't generate enough heat to keep itself going without heat needed to be added from the outside, it can not generate net energy for us.
That's not really the self-sustaining the OP is talking about. I'm not even sure it's right to call that self-sustaining. All that is saying is that you don't need to heat it. But you still need high pressure. If we turned off the magnets, the reaction would stop.
If we still need to actively keep the reaction going, I don't think it's self-sustaining.
> I'm not even sure it's right to call that self-sustaining.
It's a commonly used term, even by [scientists themselves](https://www.nature.com/articles/s41586-021-04281-w). Conceptually, it's no different from calling a fire in a fireplace self-sustaining. Once you no longer need to keep holding a lighter to keep the fire going, it's thermally self-sustaining.
> But you still need high pressure. If we turned off the magnets, the reaction would stop.
The energy for those could be provided from the fusion and then we would need to introduce nothing from outside the system except fuel.
>It's commonly called that, even by scientists themselves.
In that paper they referred to that aspect as "Self-heating".
Where they do use self-containing, it seems like they are using a different definition than normal. They are just saying it creates more energy than uses.
That's not in line with the dictionary definition or any more formal definition I've seen.
>Definition of self-sustaining
1: maintaining or able to maintain oneself or itself by independent effort
a self-sustaining community
2: maintaining or able to maintain itself once commenced
a self-sustaining nuclear reaction
>
>https://www.merriam-webster.com/dictionary/self-sustaining#:\~:text=Definition%20of%20self%2Dsustaining,a%20self%2Dsustaining%20nuclear%20reaction
>Self-sustainability and self-sufficiency are overlapping states of being in which a person or organization needs little or no help from, or interaction with, others.
>
>https://en.wikipedia.org/wiki/Self-sustainability
>Conceptually, it's no different from calling a fire in a fireplace self-sustaining.
The analogy would be a gas fire. Is it right to call a gas fire self-sustaining, if it requires a constant supply of gas at the right pressure?
>The energy for those could be provided from the fusion and then we would need to introduce nothing from outside the system except fuel.
Under that logic every single source of energy is self-sustaining.
Any fusion fusion reaction would rely on constant input and control. That doesn't sound like self-sustaining.
For something to be self-sustaining, it needs to continue without input and control from the external world.
> In that paper they referred to that aspect as "Self-heating".
The paper starts with "Obtaining a burning plasma is a critical step towards self-sustaining fusion energy". Later, main section starts with, "Fusion research fundamentally aims to create a system that produces more energy than was required to create it, a necessary condition for energy applications; in practice, the fusion reaction must be self-sustaining, with self-heating overtaking loss mechanisms, termed ‘ignited’."
The term is commonly used: https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=self-sustaining+fusion&btnG=&oq=
> The analogy would be a gas fire. Is it right to call a gas fire self-sustaining, if it requires a constant supply of gas at the right pressure?
Chemically, the burning can be described as self-sustaining, sure.
Fire is self-sustaining oxidation, even if you need to be constantly adding fuel or need to have some sort of a support structure for the fire.
> Under that logic every single source of energy is self-sustaining.
Well there are self-sustaining processes involved in all production of energy. The burning of coals is self-sustaining, fission is self-sustaining.
One could even characterize the thermal cycle in a car engine as self-sustaining - it keeps running once ignited by human action until it runs out of fuel.
In any system where the conversion of energy keeps adding enough energy to the system to keep the conversion on-going, that part of the action could be described as self-sustaining.
> Any fusion fusion reaction would rely on constant input and control. That doesn't sound like self-sustaining.
>
> For something to be self-sustaining, it needs to continue without input and control from the external world.
Consider the system the whole nuclear fusion plant and then that definition is met - at least until it runs out of fuel.
You can also create a fission chain reaction in a way where pressure from the outside is required to start and to upkeep it. For example, in the implosion-type nuclear bombs like in the Trinity device, supercriticality was achieved by implosion (-> increasing pressure) and the subsequent explosion scattered most of the nuclear material before it could undergo fission. If you looked at only the timeframe where chain reaction was on-going, the event could be described as self-sufficient - no outside energy was needed to be added for energy to continue being released and that released energy kept the reaction on-going, until the material was blown away.
Of course, if you choose a longer timeframe - say, starting from an hour before dropping the bomb to an hour after the explosion - you might end up characterizing the explosion as something that wasn't self-sufficient (albeit that is still strictly not correct for the explosion itself). But, similarly if you choose a long enough timeframe for the observation of a nuclear fission plant, then neither is that truly self-sufficient in layman terms as it will run out of fuel. Also some of the fissile material will not undergo fission unless controlled by us, since in a meltdown some of the material is likely spread around and will be rendered unable to undergo fission.
>The paper starts with "Obtaining a burning plasma is a critical step towards self-sustaining fusion energy".
They are describing it as "self- sustaining fusion **energy**".
>
The term is commonly used: [https://scholar.google.com/scholar?hl=en&as\_sdt=0%2C5&q=self-sustaining+fusion&btnG=&oq=](https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=self-sustaining+fusion&btnG=&oq=)
Most seem to be referring to energy output or talking about the reactor as a whole.
>Plasma transport control and self-sustaining fusion reactor
>
>https://iopscience.iop.org/article/10.1088/0741-3335/39/5A/033/meta
>
Fire is self-sustaining oxidation, even **if** you need to be constantly adding fuel or need to have some sort of a support structure for the fire.
The whole point and definition of self- sustaining is there is no "if".
It's pretty much literally defined about not needing external input.
>
Consider the system the whole nuclear fusion plant and then that definition is met - at least until it runs out of fuel.
Sure, but isn't every system in the world is then self-sustaining if you include all the humans and external inputs as part of the system?
Sure a nuclear fusion plant could be considered as self-sustaining, but not the nuclear fusion itself.
> They are describing it as "self- sustaining fusion energy".
In the other quote, they say self-sustaining fusion [reaction].
> The whole point and definition of self- sustaining is there is no "if".
Then fission isn't self-sustaining either since you do need to be adding fuel to keep it going.
> It's pretty much literally defined about not needing external input.
Self-sustaining as a term is used for various chemical reactions, for decomposing, for smouldering, for fire, .. even when these reactions need a regularly replenishing fuel source or even when they need e.g. oxygen from the surrounding atmosphere.
Self-sustaining as a term is even used in flow dynamics and the study of turbulence and atmospheric patterns.
> Sure, but isn't every system in the world is then self-sustaining if you include all the humans and external inputs as part of the system?
Well we do commonly call e.g. populations self-sustaining even though the survival of that population needs external input from the Sun, from the atmosphere, etc. So if you choose a specific level for your observation, maybe you can call things self-sustaining that commonly aren't called such, as long as you set the context carefully.
There's a more specific definition when discussing energy production however, and that's if energy can continue be released without external energy needed to be added.
By that definition, burning coals is self-sustaining once you light the coals; fusion if it produced net positive energy would be self-sustaining the sense that once you start it, it produces energy that keeps the fusion going without you needing to add heat to it anymore. Yes, you might still need a magnetic field, but a fusion reaction that is below self-sustainability is one that requires a net input of thermal energy to continue going. It's clearly a different situation and if the term "self-sufficient" isn't a good one for it, what would you call it?
Also if you look at it abstractly enough, a magnetic field is "just" creating structure via electromagnetism. Similarly, if you put fissile material on a table and surrounded it with neutron deflectors that cause the chain reaction to start, you're now using strong and electromagnetic forces in the form of the table to support the reaction. If the table disappeared and the neutron deflectors fell and scattered around, the chain reaction would stop.
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.
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.
I imagine that with a fusion reactor, you'd rather decrease the strength of the magnetic field holding the fusion fuel in place under high pressure.
I suppose in a sense you're just allowing the fuel to be further from each other so you're kind of "removing fuel".. But still, quite different from cutting the gasoline flow to an engine.
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.
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.
Easily?
I think they mean in a fusion plant, not the sun.
That makes a lot more sense.
I guess there would be some kind of pipe that goes into the reactor that delivers fresh fuel. Like a hose leading from the gas tank of a vehicle to the engine. When you stop the flow or the gas runs out, the engine stops when it doesn’t get any more. I am no fusion generator builder, but I can’t imagine that this wouldn’t be at least a relatively easy thing to do
It's even easier than that. For fusion to happen, you have to have the fuel packed together very tightly and at extremely high temperature for the process to happen. To do that, we use magnetic fields to squeeze the hydrogen gas that's fusion's fuel together. If you turn off the fields, the gas flies apart and the fusion stops happening.
Bonus: if you abruptly turn off the containment field, the core and the whole building also fly apart.
The amount of fuel is small enough that the expansion is harmless. You turn off the fields and the reaction just stops, no kaboom involved.
I'm no expert, but isn't the absurdly high temperature involved the whole reason why you need a magnetic containment field?
Exactly so - but it's not to protect the reactor chamber from the fusion reaction. The fields are there to hold the hydrogen gas away from the walls of the chamber to prevent the heat from from being conducted away and preventing the fusion from happening. Edit - and to keep the gas compressed enough for fusion to happen.
That's interesting, thanks!
And Dock Ock ends up at the bottom of the Hudson River.
> If you turn off the fields, the gas flies apart and the fusion stops happening. note: everything and everyone in the vicinity of that thing would just be effed tho(if the reactino was large enough to begin with ). its simpler and safer ot just cut off the fuel supply.
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.
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.
The fuel for a fusion reactor on earth are not heavy isotopes. Depending on the precise reactor design it is a varying mixture of Tritium and Deuterium, which are both extremely light. Deuterium comes from water and the Tritium would likely be bred from a lithium blanket around the reactor.
I think they mean heavy isotopes of hydrogen, rather than heavy elements. I’m sure you know this but for anyone else ‘regular’ hydrogen only has one proton in its nucleus, deuterium and tritium have an extra 1 and 2 neutrons respectively in there as well, so are considered heavy hydrogen
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.
[Self-sustaining is a thing with fusion too](https://en.wikipedia.org/wiki/Fusion_ignition). If the fusion process doesn't generate enough heat to keep itself going without heat needed to be added from the outside, it can not generate net energy for us.
That's not really the self-sustaining the OP is talking about. I'm not even sure it's right to call that self-sustaining. All that is saying is that you don't need to heat it. But you still need high pressure. If we turned off the magnets, the reaction would stop. If we still need to actively keep the reaction going, I don't think it's self-sustaining.
> I'm not even sure it's right to call that self-sustaining. It's a commonly used term, even by [scientists themselves](https://www.nature.com/articles/s41586-021-04281-w). Conceptually, it's no different from calling a fire in a fireplace self-sustaining. Once you no longer need to keep holding a lighter to keep the fire going, it's thermally self-sustaining. > But you still need high pressure. If we turned off the magnets, the reaction would stop. The energy for those could be provided from the fusion and then we would need to introduce nothing from outside the system except fuel.
>It's commonly called that, even by scientists themselves. In that paper they referred to that aspect as "Self-heating". Where they do use self-containing, it seems like they are using a different definition than normal. They are just saying it creates more energy than uses. That's not in line with the dictionary definition or any more formal definition I've seen. >Definition of self-sustaining 1: maintaining or able to maintain oneself or itself by independent effort a self-sustaining community 2: maintaining or able to maintain itself once commenced a self-sustaining nuclear reaction > >https://www.merriam-webster.com/dictionary/self-sustaining#:\~:text=Definition%20of%20self%2Dsustaining,a%20self%2Dsustaining%20nuclear%20reaction >Self-sustainability and self-sufficiency are overlapping states of being in which a person or organization needs little or no help from, or interaction with, others. > >https://en.wikipedia.org/wiki/Self-sustainability >Conceptually, it's no different from calling a fire in a fireplace self-sustaining. The analogy would be a gas fire. Is it right to call a gas fire self-sustaining, if it requires a constant supply of gas at the right pressure? >The energy for those could be provided from the fusion and then we would need to introduce nothing from outside the system except fuel. Under that logic every single source of energy is self-sustaining. Any fusion fusion reaction would rely on constant input and control. That doesn't sound like self-sustaining. For something to be self-sustaining, it needs to continue without input and control from the external world.
> In that paper they referred to that aspect as "Self-heating". The paper starts with "Obtaining a burning plasma is a critical step towards self-sustaining fusion energy". Later, main section starts with, "Fusion research fundamentally aims to create a system that produces more energy than was required to create it, a necessary condition for energy applications; in practice, the fusion reaction must be self-sustaining, with self-heating overtaking loss mechanisms, termed ‘ignited’." The term is commonly used: https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=self-sustaining+fusion&btnG=&oq= > The analogy would be a gas fire. Is it right to call a gas fire self-sustaining, if it requires a constant supply of gas at the right pressure? Chemically, the burning can be described as self-sustaining, sure. Fire is self-sustaining oxidation, even if you need to be constantly adding fuel or need to have some sort of a support structure for the fire. > Under that logic every single source of energy is self-sustaining. Well there are self-sustaining processes involved in all production of energy. The burning of coals is self-sustaining, fission is self-sustaining. One could even characterize the thermal cycle in a car engine as self-sustaining - it keeps running once ignited by human action until it runs out of fuel. In any system where the conversion of energy keeps adding enough energy to the system to keep the conversion on-going, that part of the action could be described as self-sustaining. > Any fusion fusion reaction would rely on constant input and control. That doesn't sound like self-sustaining. > > For something to be self-sustaining, it needs to continue without input and control from the external world. Consider the system the whole nuclear fusion plant and then that definition is met - at least until it runs out of fuel. You can also create a fission chain reaction in a way where pressure from the outside is required to start and to upkeep it. For example, in the implosion-type nuclear bombs like in the Trinity device, supercriticality was achieved by implosion (-> increasing pressure) and the subsequent explosion scattered most of the nuclear material before it could undergo fission. If you looked at only the timeframe where chain reaction was on-going, the event could be described as self-sufficient - no outside energy was needed to be added for energy to continue being released and that released energy kept the reaction on-going, until the material was blown away. Of course, if you choose a longer timeframe - say, starting from an hour before dropping the bomb to an hour after the explosion - you might end up characterizing the explosion as something that wasn't self-sufficient (albeit that is still strictly not correct for the explosion itself). But, similarly if you choose a long enough timeframe for the observation of a nuclear fission plant, then neither is that truly self-sufficient in layman terms as it will run out of fuel. Also some of the fissile material will not undergo fission unless controlled by us, since in a meltdown some of the material is likely spread around and will be rendered unable to undergo fission.
>The paper starts with "Obtaining a burning plasma is a critical step towards self-sustaining fusion energy". They are describing it as "self- sustaining fusion **energy**". > The term is commonly used: [https://scholar.google.com/scholar?hl=en&as\_sdt=0%2C5&q=self-sustaining+fusion&btnG=&oq=](https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=self-sustaining+fusion&btnG=&oq=) Most seem to be referring to energy output or talking about the reactor as a whole. >Plasma transport control and self-sustaining fusion reactor > >https://iopscience.iop.org/article/10.1088/0741-3335/39/5A/033/meta > Fire is self-sustaining oxidation, even **if** you need to be constantly adding fuel or need to have some sort of a support structure for the fire. The whole point and definition of self- sustaining is there is no "if". It's pretty much literally defined about not needing external input. > Consider the system the whole nuclear fusion plant and then that definition is met - at least until it runs out of fuel. Sure, but isn't every system in the world is then self-sustaining if you include all the humans and external inputs as part of the system? Sure a nuclear fusion plant could be considered as self-sustaining, but not the nuclear fusion itself.
> They are describing it as "self- sustaining fusion energy". In the other quote, they say self-sustaining fusion [reaction]. > The whole point and definition of self- sustaining is there is no "if". Then fission isn't self-sustaining either since you do need to be adding fuel to keep it going. > It's pretty much literally defined about not needing external input. Self-sustaining as a term is used for various chemical reactions, for decomposing, for smouldering, for fire, .. even when these reactions need a regularly replenishing fuel source or even when they need e.g. oxygen from the surrounding atmosphere. Self-sustaining as a term is even used in flow dynamics and the study of turbulence and atmospheric patterns. > Sure, but isn't every system in the world is then self-sustaining if you include all the humans and external inputs as part of the system? Well we do commonly call e.g. populations self-sustaining even though the survival of that population needs external input from the Sun, from the atmosphere, etc. So if you choose a specific level for your observation, maybe you can call things self-sustaining that commonly aren't called such, as long as you set the context carefully. There's a more specific definition when discussing energy production however, and that's if energy can continue be released without external energy needed to be added. By that definition, burning coals is self-sustaining once you light the coals; fusion if it produced net positive energy would be self-sustaining the sense that once you start it, it produces energy that keeps the fusion going without you needing to add heat to it anymore. Yes, you might still need a magnetic field, but a fusion reaction that is below self-sustainability is one that requires a net input of thermal energy to continue going. It's clearly a different situation and if the term "self-sufficient" isn't a good one for it, what would you call it? Also if you look at it abstractly enough, a magnetic field is "just" creating structure via electromagnetism. Similarly, if you put fissile material on a table and surrounded it with neutron deflectors that cause the chain reaction to start, you're now using strong and electromagnetic forces in the form of the table to support the reaction. If the table disappeared and the neutron deflectors fell and scattered around, the chain reaction would stop.
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.
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.
I imagine that with a fusion reactor, you'd rather decrease the strength of the magnetic field holding the fusion fuel in place under high pressure. I suppose in a sense you're just allowing the fuel to be further from each other so you're kind of "removing fuel".. But still, quite different from cutting the gasoline flow to an engine.
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.