I think you're trolling. Instrumentation matters very little here, if at all.
The laws of physics involved are known and they do not change. Such accidents were being modeled since the seventies, TMI even produced a nice case study to validate the models.
For a given core configuration, where starting parameters (water level, temps, pressures) are known, one can find out if and when the core melts. The process does not take days, for a reactor as big as those at Fukushima. The calculation can be done literally on the back of a napkin by anyone in possession of freshman year physics. So many MW of heat, so much fuel, so many tons of water, so much steel in the vessel that could melt. Easy.
There is no need for further measurements.
Good crisis management is the uncanny ability to make good decisions without having all the data. Take a pilot in a stalled fighter jet with an engine flameout, at low altitude. Does he try to determine the cause of his engine flameout and whether a recovery might be possible?
Smart pilots pull the ejector handle and generally live to find out the results of the post-accident inquiry. Brave pilots try something, anything. Sometimes it works. Bad pilots get confused and flustered trying to decide what to do. Bad pilots end up dead.