Does a fusion reactor have a cool-down period or can it operate continuously 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.
 A: 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.
A: 
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.
