Comment by NervousRing
5 days ago
1. We don't have a perfect understanding of plasma dynamics and how they'll react to different conditions. Predicting plasma instabilities before they mess with your reactor remains a big challenge for our computation capabilities.
2. Yeah, material science is also a big one. When you are working with the magnetic forces typical in a modern fusion reactor, your materials undergo a lot of mechanical stress. The "first wall" that has to bear the brunt of the nuclear reactions becomes radioactive. Some plasma ions invariably go off trajectory and we have a "diverter" to prevent them from hurting the reactor but that reduces the temperature.
3. Our reactors aren't efficient enough. Everyone taking about "q" value means the energy they put into creating the reaction to get the plasma to fuse. It's called q-plasma which is a misleading metric. The true breakthrough will be sustained q-total, which will be the ratio of the total energy you get out over the total energy you put in. Nobody in the industry likes to talk about it, because we are decades away from reaching this.
4. Modern designs are becoming extremely expensive. The most serious design right now is being funded not by a state of a country but by the biggest countries in the planet.
5. Someone help me here I've ran out of points
5. The fuel is notoriously difficult to contain. There will be leaks, and even small ones can spoil the reaction and tip your device into unpredictability. Also, the fuel has a tendency to infiltrate metals and embrittle them.
5. Working big pieces of steel and making large process plants is not cheap or easy.
6. Volumetric power density sucks compared to fission reactors. This leads to #4, since for a given power output the reactor is an order of magnitude larger or even worse. Designs with some hope of evading this showstopper may be possible, but they are rare on the ground, and the tokamak doesn't appear to be one of them.