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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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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