I'm surprised the reactor needs a moderator, I would assume that a liquid fuel reactor could be controlled far better by controlling flow into and out of the critical mass. It's all at normal pressures, just hot (temperature and radiation wise), so just bog standard plumbing practices for handling fluid levels should work.

Being able to scramble the reactor by just dumping the contents through a set of diverters into separate flat bottom chambers to cool and eventually freeze is an awesome safety feature.