Chances are that the nuclear engineers on site don't know what level of meltdown has occurred in the reactor and the inspection and measurement sensors will have been destroyed. The series of events need to be published especially when the timing if the addition of the sea water/boron salts were added, presumably before the hydrogen explosion as this may have triggered the reactor to start producing hydrogen which then migrated its way through the reactor pressure vessel by the process of of hydrogen embrittlement. The sea water when cooling down the nuclear reaction may have caused the uranium hydride to start liberating the hydrogen at these lower temperatures only for the reactor to then heat up again.

As the reactor geometry has become distorted (due to heat and pressure) and the zirconium cans seem to have burst this may have led to puddles of enriched uranium forming on the bottom of the reactor which is then subject to some very complex phase/chemical/nuclear/neutron interactions. i.e. a lack of a moderator maybe keeping things cooler i.e. stopping fast neutrons from becoming slower thermal neutrons with high degrees of neutron radiation passing through the reactor pressure vessel making it very difficult for any human beings to be able to get near the reactor. Poisoning the reactor with the boron salts may indeed have the reverse effect to the desired one simply because no one knows the extent of the reactor fuel puddling and the extent of the partial melt leading to an uncontrollable reactor pile. No one will know the extent of the reactor fuel can alignment geometries. This is why they are using words such as 'assuming a melt down has occurred'

The reality is that the engineers at this plant are not in control simply because they cannot even determine what they are meant to be controlling yet alone have the assets in place to control the reactor.

http://www.youtube.com/watch?v=6UZuNI7Yk2k

They might be lucky with this one but I wouldn't count on it. It should also be remembered that these nuclear engineers had the bright idea of building chains of nuclear reactors on the sea shore (historically subject to Tsunamis) next to a major fault line (historically subject to earthquakes). I think you could say that they lacked some foresight of what could go wrong in the very first place.