Hello everyone and everybody!
As ITER recently celebrated some landmark with the usual spiel of "unlimited clean energy for free", I want to share my disappointment about tokamaks and nuclear fusion
========== Tritium from external sources ==========
The only conceivable reaction in a tokamak in foreseeable future is D-T fusion, but our environment contains no tritium in useful amount, because the production events are rare and T decays quickly.T is presently produced in uranium reactors
, where excess neutrons irradiate Li, or in Candu reactors, D. This can't be upscaled:
- 1 U or Pu fission provides less than 1 extra neutron, but 200MeV energy
- 1 neutron hitting Li or D produces less than 1 T
- Consuming the T in a fusion reactor produces <20MeV energy
- The fusion reactor would add <10% power to the necessary fission reactors. Useless.
========== Tritium regeneration ==========
Alternately, a fusion reactor would breed its own tritium
. Neutron irradiation of Li is considered. The public hears about this, but nothing more.
Alas, T breeding is difficult and has drawbacks:
- Consuming 1 T in a fusion produces 1 neutron
- 1 neutron hitting an Li produces at best 1 T
- There are losses
========== Neutron multiplication ==========
All considered (all possible?) processes seek to multiply the neutrons
to regenerate T. A 14MeV fusion neutron breaks a heavy atom like Pb to release several neutrons of lesser energy. Though:
- Papers report a computed 1.15 breeding factor only. Other design constraints, which abund at tokamaks, may very well drop the factor below 1.
- Neutron multiplication is polluting! At identical produced energy, the waste from Pb destruction is as much radioactive as fission is.
While the public may discover these drawbacks, (a part of) the fusion community knows them well
and investigated two decades ago the bombardment of heavy atoms by fusion neutrons and the resulting pollution.
I should come back with figures about the pollution.
Marc Schaefer, aka Enthalpy