I was thinking of using helium or some molten metal or salt. The spacecraft has multiple, but inert, fusion reactors strapped in a compartment. When it's time to deploy a landing craft, a robotic arm takes the reactor and mounts it to an engine assembly(Canadarm style).
I figured this would be a cool setup considering how hard it would be to build a new reactor from scratch and I also figured that the reactor could be used elsewhere(melting small comets, powering mining bases, manufacturing) and should be ejectable if needed. Plus I thought it would be too complex to swap out the whole heating chamber so I chose to swap the reactor instead.