All reactors have negative temperature coefficients of reactivity, including LWRs (the water moderator boils away), SFRs (the core thermally expands), and MSRs

All reactors have negative temperature coefficients of reactivity, including LWRs (the water moderator boils away), SFRs (the core thermally expands), and MSRs (the fuel density decreases). The key to avoiding radiation leakage out of any nuclear plant is decay heat removal [1]. If you can passively remove decay heat, your reactor is very safe. MSRs do this easily, as do other reactors with low-pressure coolants (like sodium metal-cooled reactors). [1] https://whatisnuclear.com/decay-heat.html https://whatisnuclear.com/decay-heat.html