Here’s my estimate of the situation. I’m no nuclear engineer but I know something…
We use data from JAIF:
And an annotated picture of a GE boiling water reactor from UCS:
Now, using the above:
- At least half of the fuel in all reactors has been dry for quite a long time. It seems likely that the dry part has melted.
- The containment vessel in unit 2 is broken since it doesn’t seem to be keeping pressure.
- In units 3 and/or 4 also the spent fuel in the storage pool has become dry and has probably been damaged.
- This means radiation releases now and in the future.
- We hope they can keep what’s under water now.
- We also hope that they can increase water levels where they are lacking.
- Less water means more fuel melting and more radiation.
- If one or multiple units’ cores melts completely (we don’t even know if this has happened), it could even melt itself through the bottom of the containment.
- Also fuel might have broken at the exposed top and the bits fallen to the bottom. If there’s partial water level in the core, they’re under water there though so that helps.
- Melt through could mean large radiation releases, depending on what happens.
- A STUK (Finnish Radiation Safety Center) expert has described the hypothetical melt through situation as something like a blob of lava with intense radiation where you can’t go nowhere near for a very long time.
- On the other hand, since it’s been many days since shutdown, the residual power of the fuel should be low and handling them should be easier and the risk of melting through the containment less.
EDIT: Here are some radiation level references that appeared to Wikipedia, to understand what the 2 millisieverts per hour at Fukushima gate at the moment mean, roughly taking a chest CT scan every 3-6 hours. Not healthy after a while anymore but not quickly lethal yet:
Single Dose Examples
Eating one banana: 0.0001 mSv
Dental radiography: 0.005 mSv
Average dose to people living within 16km of Three Mile Island accident: 0.08 mSv; maximum dose: 1 mSv
Mammogram: 3 mSv
Brain CT scan: 0.8–5 mSv
Chest CT scan: 6–18 mSv
Gastrointestinal series X-ray investigation: 14 mSv
Hourly Dose Examples
Highest recorded radiation at reactor 2, Fukushima I: 8 mSv/hr
Recorded radiation at Fukushima I on March 15–16: 3–10 mSv/hr
Highest recorded radiation at Fukushima I: Reactor No. 3 – 1000 mSv/hr briefly on March 16, 2011 
Highest recorded radiation at Fukushima I: Gate Area – 11 mSv/hr briefly on March 16, 2011 
Typical dose near Chernobyl reactor 4 and its fragments, shortly after explosion: ≈ 10 000–300 000 mSv/hr
Yearly Dose Examples
Living near a nuclear power station: 0.0001–0.01 mSv/year
Living near a coal power station: 0.0003 mSv/year
Cosmic radiation (from sky) at sea level: 0.24 mSv/year
Terrestrial radiation (from ground): 0.28 mSv/year
Natural radiation in the human body: 0.40 mSv/year
Typical individual’s natural background radiation: 2 mSv/year; 1.5 mSv/year for Australians, 3-6 mSv/year for Americans
New York-Tokyo flights for airline crew: 9 mSv/year
Radon in the average US home: 2 mSv/year
Smoking 1.5 packs/day: 13 mSv/year
Current average limit for nuclear workers: 20 mSv/year
Background radiation in parts of Iran, India and Europe: 50 mSv/year
Lowest clearly carcinogenic level: 100 mSv/year
Temporary Emergency Elevated limit for 50 Nuclear Technicians during Fukushima emergency: Increased from standard 100 msv to 250 mSv/year to combat nuclear emergency at the Fukushima Dai-Ichi Nuclear Power Stations 1 through 4t