Care to make a prediction based on the known evidence so far? Or is it all just unknowable at this time?
Well, lets see what evidence we have, maybe you then see my problem with making any proper predictions. I couldn't do it much better than my tea leaves oracle before launches.
- Reactor constructed in 1971, based on BWR/4 or BWR/5 reactor design in the USA.
- Cooling was already in a bad shape before Earthquake because of previous damage
- Tsunami damage photographs show all feed water pumps at the shore had been completely destroyed.
- Reactor was shutdown in emergency because of P-wave trigger, one control rod failed to deploy properly or was at least only indicated as such.
- External power to the reactor failed completely.
- Internal diesel generator power for the circulation pumps failed
- External mobile diesel generators on site could not be connected to the pumps
- Only working pumps had been battery driven circulation pumps, which are only working for some hours. Battery packs had been reported to have been replaced at least once during the night.
- Pressure control of the reactor was lost, suggesting run-away temperatures because of insufficient cooling.
- Automatic pressure vent was reported as failed, pressure reports very between 50% or 100% above normal, which equals to 1500 - 2000 psig / 100-140 atm.
- Radiation at the control building is reported to be 1000 times higher than normal, normal is 0.15 µSv per hour.
- Explosion was observed, but no visible flames. Roof and walls completely destroyed, steel skeleton still standing.
- Shock venting in the first instants of the explosion through the roof shows a condensation front, but no condensing steam, suggesting a supersonic propagation. So this was a hydrogen explosion and no steam explosion (which would be maximal sonic and release large amounts of steam)
- The needed large amounts of hydrogen for such an explosion can only form in a nuclear plant when the fuel element cladding becomes hot enough to oxidize exothermally with the cooling water, which starts at 1750K and quickly drives temperatures beyond the melting point of zirconium of 2150 K.
- ballooning and popping of the fuel elements already happens at lower temperatures of 1500K
- No fires are visible afterwards, but also not expected (no large graphite moderator as in Chernobyl, only little combustible material around, which could already be consumed while the dust cloud obscured the view)
- Radiation inside Unit 1 is measured as 106 mSv at the personal dosimeter of a worker. That is 700,000 times higher as the typical hourly dose at a working reactor.
- Cesium and Iodine are measured outside the plant. Cesium can come from damaged filters inside the reactor building, but Iodine usually only exists inside the fuel elements, suggesting that the fuel elements popped open and the primary circuit integrity is compromised or the controlled venting of steam from the reactor already contained evidence of popped fuel elements.
So, by these facts so far, my estimate of the current situation is, that the meltdown is already at least in the fourth phase. I can't tell that primary circuit integrity is compromised as long as there are alternative sources for iodine in the current situation. If the cooling pumps had already been insufficient to keep temperatures below 1500K until now, they will also be too weak for preventing the fifth stage of meltdown, and a debris bed of molten material will form.
Cooling the reactor by seawater is the best option, but I have doubts that without working cooling pumps and after the explosion damaged the building, that this operation will reduce temperatures below 1500K quickly enough. As long as the meltdown is just in the fourth or fifth stage, the cooling by seawater could prevent further damage and bring the reactor back under control, but the reactor is still a total loss.
If the sixth phase is not prevented by quickly cooling the melting fuel, the resulting steam explosions inside the pressure vessel could alone mean a bad day. if there are just small amounts of steam getting produced at a time, this could be vented in a controlled way, by releasing again more nuclear material.
There are two possible outcomes of the sixth phase: The cooling efforts succeed and the corium solidifies again inside the lower plenum of the reactor, meaning the reactor is a total economic loss, but the radioactivity release remains limited. Or it comes to a recritically event, because not enough control rod substance or radiation poison is in the lower plenum suppresses the nuclear reaction, resulting in a rapid increase in pressure and temperature, that would destroy the pressure vessel, vaporize large amounts of water regardless of the source (coolant or seawater) and spread radioactive debris over a large region, possibly over many countries.
The large amount of radiation on a nuclear plant worker suggests that this re-criticality could already be happening or the primary circuit integrity is already compromised and the radiation inside the reactor building but outside the containment structure is much higher as usual.
But that is not the only danger. The cold sea water on boiling hot reactor containment could also increase new pressure explosions and cause structural damage by thermal stress.
I expect at least a further local contamination as best case. The reactor will be a total loss. Worst case would be a massive steam explosion caused by a total meltdown.
This is not TMI again. TMI was a meltdown that was stopped in phase six, but, the important difference is that in TMI, the coolant pumps had never been completely lost, but deactivated because of cavitation, and coolant circulation was available after the next shift noticed that natural circulation was failing because of steam pockets in the coolant channels. Once the pumps had been activated again and coolant levels topped again, the temperatures dropped quickly and the situation was slowly getting back under control (about 3 days later the reactor was already in a known state).
In this case, the reactor has to be cooled from outside, until the electrical power to the pumps can be resumed. If the explosion damage prevents this, only external cooling is available, but a last resort action. If there is re-criticality in the core, this sea water cooling is even worse, it would be just an ave maria action, your last chance in a deadly situation to prevent much worse. The water could help keeping the amount of nuclear material leaving the core low, but the total meltdown would happen anyway and a lot of evil stuff would go into the upper atmosphere.
Any complaints about my assessment? I could also just rely on tea leaves.