You also get neutrons from D-D reactions (which produce 1 neutron about half the time). Even if the plasma isn’t hot enough to get a lot of D-D reactions, it will likely produce some to make up the shortfall.
Edit: and I completely forgot (lol) that when DD doesn’t produce a neutron it produces a tritium. So, that will also help.
You also get neutrons from D-D reactions (which produce 1 neutron about half the time). Even if the plasma isn’t hot enough to get a lot of D-D reactions, it will likely produce some to make up the shortfall.
Edit: and I completely forgot (lol) that when DD doesn’t produce a neutron it produces a tritium. So, that will also help.