Can a fusion reactor like ITER maintain it's fusion reaction and generate net output electrical power continuously; so long long as fuel is supplied, or does the reactor need to be shut down periodically and the reaction stopped so that it can be cooled down? If there is a cool down period for a reactor could anyone please enlighten me as to how long it is versus it's active operating time. Fusion power will help Mankind in the outer reaches of space were sunlight is too sparse for energy generation, or fission products are too scarce, but if a reactor has a cool down time it's energy output will be intermittent.
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$\begingroup$ That could be said of an internal combustion engine, but multiple cylinders firing at different times during a full cycle smooths out the power production. A flywheel helps a lot. The same principles can easily be applied to a fusion generator that operates in on/off cycles (e.g., laser fusion) $\endgroup$– S. McGrewCommented May 9, 2020 at 23:27
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For most existing designs, magnetic fusion reactors are supposed to operate continuously, without need to stop for cooling down. For the D-T fusion reaction (the easiest one to achieve), most of the produced energy will be released from the fusion plasma in neutrons. In most reactor designs, thermal energy deposited by those fusion neutrons inside of the reactor wall (blanket) would be removed by active cooling and used to power a turbine and generate electricity. However, a fusion reactor would have to be stopped periodically, after operating for a ~1 year, for maintenance. The main reason for that is a high heat and neutron loading of the fusion reactor nuclear zone. The nuclear zone will be exposed to intense heat and neutron radiation, irradiation by ionized particles, and hard X-ray photons. This would severely limit the lifetime of nuclear components, for existing technology; replacing the nuclear components would require shutting down the reactor periodically. Note that ITER is not going to be a fusion reactor but rather a reactor-scale fusion experiment. It is planned to operate in pulses a few hundred seconds long.
answered May 10, 2020 at 0:17
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$\begingroup$ Thank you for the information. So they do not overheat during their operation because the walls of the reactor are made of heat resistant materials and are being actively cooled? And when you say "most existing designs" which ones in specific do you mean? $\endgroup$ Commented May 10, 2020 at 0:31
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$\begingroup$ Yes, actively cooled components have to be a part of a fusion reactor, again, for most designs but not for all. For ITER, see for example this article iter.org/mach/Blanket. When I say "continuous operation for most designs" I mean that there are various ideas for fusion energy, and some confined plasma configurations cannot be maintained continuously but only for a limited time period; still those have been considered for fusion energy. $\endgroup$ Commented May 10, 2020 at 0:40
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$\begingroup$ I suppose the question should then be how long would a typical confined plasma design usually operate for, a guessing not longer than a few minutes. between it's operating periods what is the machine doing? adding fuel and removing waste products, or re-initiating it's start up conditions? How long would the down time be between operating cycles, a few seconds, or minutes? $\endgroup$ Commented May 10, 2020 at 0:54
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$\begingroup$ Are we talking about ITER now, or about a design of a fusion reactor? We can easily find out the details for ITER, their operating plan is open to the public. For a reactor, as I said, a typical design would imply continuous operation for about a year. Fuel would be added and waste products removed continuously, during reactor operation. After operating for ~1 year, the shutdown time (opening the reactor chamber) would probably take days or weeks. $\endgroup$ Commented May 10, 2020 at 1:02
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1$\begingroup$ It is certainly feasible from the physics point of view. However there are issues in technology, engineering, and economics of it. There is no question whether you can make it work, in principle; but there is a question whether it will be practical, i.e., competitive with other energy sources. $\endgroup$ Commented May 10, 2020 at 1:29
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Can a fusion reactor like ITER maintain it's fusion reaction and generate net output electrical power continuously; so long long as fuel is supplied
It is worth pointing out that ITER is not the only sort of fusion devices, and there are several others that work in a pulsed fashion for one reason or another. A good example is the National Ignition Facility, NIF, which uses tiny fuel capsules that burn out in a few microseconds.
Many such devices consider this a feature. In order to remove heat from a continuous operation device like ITER, you need to have a "first wall" that is highly heat conductive, or the material will heat up faster than you can pull the heat back out with active cooling on the other side. However, this material also has to have several other particular qualities:
- it cannot interfere too strongly with the magnetic fields holding everything together
- it cannot be too susceptible to damage from neutrons, which will be released from the fusion reactions
- it has to be extremely strong mechanically in order to hold a vacuum in the center while being subjected to enormous magnetic fields
- we have to be able to afford it
There are many in the industry that suggest such material does not exist. There are any number of suggestions for such a material, but none have actually been built or tested. Furthermore, as the first wall is on the inside the reactor in toroidal machines like ITER, if it does require maintenance, you have to take the entire machine apart to get at it. And when you do, it's radioactive so that maintenance is expensive and time consuming.
In contrast, the pulsed systems like NIF do not require this combination of features. Simple stainless steel containers are often all you need. For this reason, when the container does begin to wear out, you simply throw it in the junk heap for a decade or so while it cools down and then sell it for scrap. Moreover, because this first wall is further from the reactions and mechanically isolated, you can design the system to allow direct access so you simply disconnect some tubes and wire and there it is, no need to take apart the entire system around it.
On the downside, no one's managed to get one of these pulsed machines to work. The closest we've come is about 1/3rd of the way to "ignition", and that's perhaps 100 times less energy than it needs to power itself, let alone your home. So in spite of the many mechanical issues in the ITER-like designs, they remain the favorite approach.
answered May 11, 2020 at 21:02
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$\begingroup$ Actually, it is not quite right that "for pulsed systems like NIF... simple stainless steel containers are often all you need". For ICF too, damage of the first wall is one of serious roadblocks on the way to a fusion reactor. That was looked at back in the 1980s, and the solution was found in the form of liquid metal walls. For magnetic confinement concepts liquid metal walls have been also looked at but they don't look as feasible due to the toroidal geometry (in most designs) and presence of strong magnetic fields. $\endgroup$ Commented May 12, 2020 at 0:24
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$\begingroup$ @MaximUmansky - due to the distance between the reaction and first wall, the inverse square law is working in your favor and, unlike magnetic designs, there is no need for the first wall to be close to the fuel. As a result, all production designs I have seen, notably LIFE and it's European counterpart, use steel first walls. I am aware of the liquid metal wall designs, but they are generally not considered worthwhile. $\endgroup$ Commented May 12, 2020 at 12:44
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$\begingroup$ @ Maury Markowitz Well, the LIFE design is a somewhat special case since it was mainly considered for hybrid fusion-fission operation. And still, according to the Wikipedia article and references therein, it uses more than simple stainless steel containers and does involve liquid metal there "The target chamber is a two-wall structure filled with liquid lithium or a lithium alloy between the walls.[53] The lithium captures neutrons from the reactions to breed tritium, and also acts as the primary coolant loop.[54]" $\endgroup$ Commented May 12, 2020 at 18:19
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$\begingroup$ Actually the hybrid design was only considered very briefly. I talked to some of the designers to understand why. Their commercial liason group found 100% rejection of any power plant design that contained any transuranics whatsoever. The lithium was a breeding blanket. BTW I wrote the article you're quoting :-) $\endgroup$ Commented May 13, 2020 at 12:05