You're overlooking the other requirement of the blanket—that it captures fusion neutrons to breed tritium, and provides a self-sustaining, closed fuel cycle. Li

You're overlooking the other requirement of the blanket—that it captures fusion neutrons to breed tritium, and provides a self-sustaining, closed fuel cycle. Lithium is mandatory for a D+T reactor blanket, because of these reactions: https://en.wikipedia.org/wiki/Tritium#Lithium https://en.wikipedia.org/wiki/Tritium#Lithium (Do you have a link about that beryllium nuclear lightbulb rocket? It sounds interesting).