The advantage of a fast neutron reactor is that the fuel is broken down into a much lower element then with a typical slow reactor used in the United States.
This means for slow reactor water level changes, there will be no noticeable lag but for rapid changes the lag will be greater.
So when the Unit 4 power plant operators stalled their low and slow reactor, it exploded and burned.
The nuclear reactions in modern slow reactors have a negative temperature coefficient.
If there are problems about natural resources, we shall turn to synthetics; if there are problems about fossil fuels, we shall move from slow reactors to fast breeders and from fission to fusion.
For a slow reactor, the fuel can be any of the above, with graphite serving as the moderator.