suffered extensive damage. three of them are in some stage of melting down, and nearly everyone who works at the plant, some 800 people now have evacuated leaving just a few dozen workers to try to cool down those overheating reactors. cnn international correspondent stan grant has the latest from tokyo. >> reporter: the nuclear emergency here in japan continues to move into uncharted waters. there has been so many developments and so much confusion about those developments, there was a hydrogen explosion in reactor number three earlier this day. that has raised speculation that it may have damaged the vessel which houses the fuel rods. there was also a spark in radiation around about the same time. it was measuring the highest reading yet recorded way above the previous readings. and that was inside the plant itself. the officials saying that reading would be dangerous to human health. then they're also dealing with a fire in reactor number four.