>Many of the Generation III+ nuclear reactors have a core catcher. If the fuel cladding and reactor vessel systems and associated piping become molten, corium will fall into a core catcher which holds the molten material and has the ability to cool it.
"Integrity of the Reactor Vessel is protected by surrounding it with water in the event of a threat of core melting, and therefore no core catcher is required"
In other words "we don't need a core catcher because we promise to keep refilling the boiled-off water after a blackout." There are other reactor designs that can safely shut down without any human action.
According to this, https://www.nrc.gov/docs/ML1117/ML11171A340.pdf, the two design alternatives are "dry cavity" (aka "core catcher") and "wet cavity". But the dry/wet distinction is a little misleading as according to this paper, https://www.kns.org/files/pre_paper/37/17S-854%EC%9D%B4%EC%A..., both alternatives require cooling water. The dry cavity design relies on indirect cooling--the water contacts the sacrificial layer ("catcher")--whereas in the wet cavity design the water directly contacts the core material, which has still effectively been "caught" in the cavity beneath the reactor vessel.
I'm not sure what all the pros and cons are for each approach, except that the wet cavity design has a higher risk of a steam explosion because of the direct water contact. But this seems to be addressed by containment structures designed for higher pressures.
EDIT: The succinct comparison of each approach is in the introduction to the second paper: "Some plants adopted the 'dry cavity' to enhance the spreading of the core melt on the cavity floor as well as to remove the steam explosion risk, while other plants use pre-flooding strategy to make the 'wet cavity' in order to enhance the coolability after RPV failure or to reduce the RPV failure probability."
Wow, that's actually pretty cool