One of the great problems right now is working out where the fuel in the damaged cores and the ponds is, and in what condition the fuel is in. We can take for granted that the fuel which was in units 1, 2 and 3 has been damaged by overheating. But the state of the fuel in the ponds was a bit more of a mystery to us.
After clearing the rubbish out of the pond at unit three it has been possible to inspect the pond, the pond is frankly in a bit of a mess. But the fuel seems to have escaped serious damage. Photographs have been taken of the fuel racks in the pond and it does not look like there has been been any dire melting or explosions in the pond.
I have seen that some samples have been taken from the pond at unit four to allow them to be examined (these were samples of unused fuel which were being stored in the pond at the time of the accident). The work so far suggests that the fuel in the pond is in good condition. This suggests strongly that no nuclear explosion occurred in the pond.
The other great question is the state of the reactors. I saw something interesting recently, it is a sensing system based on cosmic rays (muons). This looks to me like a good method for finding the fuel inside the damaged reactors without having to get up close and personal with the stricken reactors.
Another thing which needs to be done is for society to recover from the accident, I have seen some advice from the IAEA on the subject of remediation of the contaminated land (outside the nuclear reactor park). This document might be of interest to some of my readers. It includes a discussion of the cleaning of different types of areas which include farmland. As I predicted it does include the question of deep ploughing the land.