And how long do they have to keep up this operation of emergency cooling? Some say 10 days, some say 100 days, that´s several months! What happens when the crew start dying? Who will replace them? What about the reactor 4 used fuel rods... how long do they have to keep trying to cool them? Forever? AFAIK they´ve not been in use for months...
So basically I don´t really see how there is even any hope that they can keep this under any kind of semblance of control for too long. What will happen to Tokyo when they decide to give up all efforts of damage control and just let the whole site burn?! Will they evacuate the whole city, nay the whole nation eventually? I think that is the real worst case scenario that few have even thought about, let alone said out loud. This might become 8 on the scale of 1 to 7. I really hope I´m wrong. Please, someone calm me down convincingly
Once they get situation under control (= make sure the rods are under water, and stay under water), the cooling can be arranged for as long as it's needed without big issues.
Despite what france says, the incident should be rated 4 or 5 on INES scale for now, with possibilities to rise to 6 depending on how things go from now on. Even if everything continues to go wrong, the type of reactors in question should alone make sure it won't reach 7 like Chernobyl did.
Examples from IAEA on the incidents:
7 - Chernobyl, Widespread health and enviromental effects
6 - Kyshtym, Significant release of radioactive material
5 - Windscale Pile, release of radioactive material, Three Mile Island, severe damage in reactor core
4 - Tokaimura, fatal overexposures of workers, Saint Laurent des Eaux, melting of one channel of fuel in reactor with no release outside the site
And the descriptions of relevant scales:
Level 6
Serious Accident
• Significant release of radioactive material likely to require implementation of planned countermeasures.
Level 5
Accident with Wider Consequences
• Limited release of radioactive material likely to require implementation of some planned countermeasures.
• Several deaths from radiation.
• Severe damage to reactor core.
• Release of large quantities of radioactive material within an installation with a high probability of significant public exposure. This could arise from a major criticality
accident or fire.
Level 4
Accident with Local Consequences
• Minor release of radioactive material unlikely to result in implementation of planned countermeasures other than local food controls.
• At least one death from radiation.
• Fuel melt or damage to fuel resulting in more than 0.1% release of core inventory.
• Release of significant quantities of radioactive material within an installation with a high probability of significant public exposure.
In case of Fukushima, it fullfills some of the scale 5 descriptions, like release of radioactive materials outside the actual site, however the amounts outside site have been minor so far, and possibly the "severe damage to reactor core" (currently unknown how severe the damages are)
On the other hand, it doesn't even fullfill the fatal exposure of at least one of scale 4 accident - only deaths so far are due other problems (one apparently fell from crane, the other one is not known but they both disappeared already on friday)
The scale 6, despite what french says, hasn't happened, and most likely won't happen.