Depending on the scale and reactor design, we have really good examples of run away fusion reactions. Run away reactions are easy, controlled ones are hard.
And whilst I won’t doubt that if fusion ever becomes commercially viable the reactors would be walk away safe it doesn’t mean that you don’t need to account for that in your design.
That is a run away fission reaction that ignites a short lived fusion reaction. We don't even talk about neutron populations or k factors in fusion because there is no avalanche effect possible.
And whilst I won’t doubt that if fusion ever becomes commercially viable the reactors would be walk away safe it doesn’t mean that you don’t need to account for that in your design.