Chernobl was a poor design. Most contries had stopped using graphite moderated reactors back in the ‘50s as they are very unstable at low power levels.
TMI was an example of systems which worked and with modern computer systems probably never would have failed.
Japan failed due to lack of a reliable back-up power solution. Reactors require about 10MW just to run pumps and computers. The backup generators were destroyed by the wave and thus after the SCRAM, no power existed to run cooling pumps. Simply moving the generators 30ft above ground instead of 10ft below would have saved the plant.
Chernobyl was a RBMK reactor with graphite moderator and water cooling. This combination can result in unstable operating conditions, and did with Chernobyl. The MHR has a graphite moderator but uses inert helium as the coolant. It does not require any back up power, even after a complete loss of coolant accident.
I may be wrong, but I seem to remember that they were testing water systems in an unsafe manner, something to that effect as well.