It's impossible to pipe Lithium into Solar Neutron Activators, I have tried many different gas pipes (conduits, pressurized, Extracell's gas buses, Gas Tank).
Putting a Gas Tank directly in the Solar Neutron Activator works.

the label of the gastank is the output of it, so it has to face the destination of the gas.
rotate (replace) the gas tank and all is fine

The input of the solar neutron activator is on the bottom

really? Didn't look like it, oh well.