mr brightside wrote:What will have become of the reactor's control rods, is the Boron only effective for as long as the reactor remains physically intact? Assuming for a moment that No1 has suffered a full core meltdown, at what point will the control rods have become ineffective?
The control rods and all the metal from the fuel cladding, tubes and structure that makes up the fuel assembly will have melted to form a lava-like mess known as corium. Some or all of it will have melted its way through the bottom of the reactor pressure vessel and fallen into the bottom of the dry-well and torus. It might go even further into the soil below but will probably not have got far before solidifying, rather than heading towards Argentina.
Each boron nucleus only absorbs one neutron so it soon gets 'used up' and has to be replaced with fresh boron. Some of the water that has been poured into the reactors has had boron added and may have done some good, depending on where it went.
There is a theoretical possibility that re-criticality can occur in the corium and there has been much discussion about the iodine 131 (half-life 8 days) measurements that, if high, might indicate continued chain reaction proceeding. The uncertainties about the reliability of the data and the various possibilities for iodine getting concentrated mean that the debate continues but there are many folk who understand these things who acknowledge the likelihood that some fission reaction continued for some periods after the reactors were shut down.
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