It's not a case of them having no idea but that the instrumentation effort necessary to map out where water was relative to neutron flux sources in the corium w

It's not a case of them having no idea but that the instrumentation effort necessary to map out where water was relative to neutron flux sources in the corium would be hugely expensive and serve no practical benefit to safety. So unless some EU grant lets a uni team spend a few million euro getting equipment in there to come to be a clearer understanding that results in the same overall answer of whether or not it's a new safety risk ("no"), it's a curiosity. Managing neutron flux in a deliberately designed structure via water immersion is what nuclear operators do every day on water-moderated reactors. But the core is no longer in a designed state, it's a combination of portions of the fuel mixed with all the other metals and bits of concrete from the lower portion of the reactor. The people monitoring know basically what it's doing, but they give considered answers like ""Based on predictive estimates, it is expected that in the future there will be an increase in the neutron flux density, which will be determined by the process of moisture loss" -- Ukraine's Institute For Safety Problems Of Nuclear Power Plants