>The limited efficiency of tritium production from lithium-6 might require obtaining some from fission reactors to top up the fuel cycle and keep fusion reactors operating.
Since nobody has yet demonstrated tritium breeding, a citation is actually not needed.
There exist purely theoretical models where neutron multiplication is used so that the single fast neutron generated in a D+T reaction can breed more than a single tritium atom when it hits li-6 (which would be clearly insufficient to have a self fueling rector).
It's all a complex mess of absorption cross sections and neutron spectra, dependent on lithium blanket geometries, coolant and structural material parasitic interactions. The tritium breeding efficiency today sits at exactly 0% and until that changes, concerns that it may never reach the required 110% are at least somewhat warranted.
From the talks I've been to the targeted performance of blankets is 120% and simulations predict this should be possible without prohibitively thick blankets.
ITER's TBM will be doing important work to test theory.
It isn't clear to me that external sources of tritium will be necessary. Between Li-6 fission and Be neutron multiplication there is theoretically plenty of neutrons to go around even though they aren't easy to capture.
Viability of blanket breeding hasn't been demonstrated to work yet, but my original point is that it hasn't been demonstrated to not work. I see now the "might" in your statement.
Sure, by the "it hasn't been demonstrated to not work" standard the whole field should generate cheap electricity tomorrow. Just that it's so unimaginably, fiendishly hard to make it work, perhaps the most challenging technical problem in history. It might never work, for interesting values of never.
My point was that fueling is in itself a very difficult and complex problem that has yet to be solved, far from the "it runs on seawater" popular tropes.
Citation needed.