Skipjack wrote:I think what they are doing is to use the fusion reactor as a neutron source to keep a fission reaction going. It would allow to burn more of fission fuel, even burn nuclear waste.
I suppose such is possible. But fusion to generate fission seems problamatic. First any tokamak used as a neutron source does not avoid the tremendous cost associated with high intensity fusion with a tokamak. And it still leaves the tritium bottleneck (perhaps coveredbythe fission side neutrons anyway). I also suspect that an external neutron source to stimulate uranium fission may ease some of the criticality balancing act issues, but it would not alter the subsequent fission, fission decay product problems. You would still have the actinide series radioactive isotopes that is the main challenge for cooling , even after the fission is shut down. For the same energy output you would have the same radioactive decay products, the same dangerous heating after shut down that requires fail safe cooling methods- which failed catastrophically in three Japaneese reactors recently. You would also have the same quantity of long half life decay products that has to be stored for a 100,000 years. It would seem to incorperate the worst aspects of both fission and tokamak fusion without much gain otherwise.
Adding a neutron to U238 makes plotonium239. From there on it would be close to a typical plutonium fission reactor.
If you are achieving direct fission of U238 with a fast neutron, the decay products might be somewhat different I suppose, and I don't know the half life chariteristics of the decay isotopes, but I suspect the radiation issues would not be much improved.
Going in the other direction like I speculated on would seem to change the picture considerably. Only a small fission reactor would be needed to generate the required tritium, so the radioactive activation and radioactive decay isotopes from fission would be quantitatively less, possibly much less.
Thermal loading issues on the fission plant could be much less critical, and thus much safer. You might be dealing with only a few million (or less) Watts of heating that needs to be handled immediatly after shutdown instead of up to several hundred Megawatts of radioactive decay heating.
Either approach might lead to useful energy production, though at high dollar costs, while effectively prolonging the uranium availability by a factor of up to ~ 200X. Thorium fission might serve almost as well, without the uncertain and tremendously expensive tokamak aspect.
If you wanted a neutron source to drive aU238 fission, a smaller, more dense D-T or even D-D fusion reactor might serve better. The construction cost and maintenance cost would be less, and the neutron source could be somewhat closer to the uranium fuel. A DPF, FRC or Polywell might serve better in this regard. The fusion reactor does not need to breakeven, only provide useful amounts of neutrons without too much cost.
To error is human... and I'm very human.