“liquified” - became a liquid. No implication of whether it remained that way after that occurrence.
Since liquifaction of a core requires humungous amounts of energy, and the melt readily dissolves all sorts of foreign materials, (pressure vessel, concrete, etc.) maintaining the liquid state is highly improbable. So is maintaining the core in a single, compact mass, which is required for criticality. The energy to achieve and sustain liquifaction has three possible sources:
The likelihood of maintaining a core in the liquid state for an extended period of time is slim to none.
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Perhaps their main point was the amount of liquidity that occurred. Normal Corium is not 100 % liquid. It retains some solid within the melt mix. So it is partially solid and partially liquid. It can be quantified by how liquid or solid that its physical components are overall. Obviously the more liquid, the less the density. The more solid, the greater the density. More liquid means it should travel farther and faster then more solid. My guess is Kaku stated something he was not supposed to state. Probably national security/classified material that he read on a private wire service. Probably coming out of Japan or through the NRC. Need some more FOIA requests.