Japan – As I understand it the situation in reactors 1, 2 and 3 is that all three reactor cores are or have burned their way through the containment vessel. The situation at reactor 4 is not as bad. As in the core has not actually melted down.
The main problem with reactor 4 is the building is sinking 15 cm per year at one end plus the 36 cm at the initial earthquake. At the top of each reactor there is a cooling pool to store the used / spent cores from the reactor.
To add to the problems, 300 cubic meters of highly radioactive water eroding all the foundations of reactors 1, 2, 3 and 4 every day.
The plant’s operator, Tokyo Electric Power (Tepco), said radiation near the bottom of the tank measured 1,800 millisieverts an hour ” high enough to kill an exposed person in four hours. 18 times higher than before and no one a TEPCO can tell us why.
On the Fukushima site there are 600,000 spent fuel rods residing in 6 cooling ponds on the top of reactors, and one at ground level. These fuel rods are ‘hot’ as in highly radioactive. Lots of lethal alpha particles and neutrons. If the fuel rods are exposed to air, then the stack of fuel rods will start burning Chernobyl style.
So far the Japanese have not attempted to move the spent cores to a safer location. This they need to do especially reactor 4 which is in danger collapsing due to gravity and groundwater undermining the building.
TEPCO has attempted to build a impermeable barrier between the plant and the sea, to prevent this water from reaching the sea.
In my Humble opinion this is a very serious mistake. The impermeable barrier will turn the ground under all the reactors into a highly unstable swamp waiting for the next earthquake to destabilise reactor 4 building even further. Think the Mexico earthquake here.
The fact that the Japanese / TEPCO have not attempted to move the fuel rods implies that they have been storing radioactive cores at all the other reactors, in order to save money.
In order to stop the burning cores in reactors 1, 2 and 3 the Japanese / TEPCO need to dig under the the reactors and build a massive bed of boron and lead to dilute and slow down the neutrons driving the nuclear reaction. BUT once again the impermeable barrier is completed this would mean that the radioactive water would need to be pumped out further undermining the reactor building.
This is what the Russians did under the concrete containment floor of Chernobyl to to extinguish the core should it burn its way through the containment floor. In the event this was not needed because the Russians kept dumping Lead and Boron into the reactor to if not extinguish the core, at least retard its neutron producing ability.
The Russian ‘bio robots’ a euphemism for human beings to dumped enough Lead and Boron into the burning core that when it burned through the bottom of the reactor floor it hit a 6 foot layer of sand which helped dilute the core and cool it enough to let it solidify into the so called elephant’s foot of radioactive glass that it is now.