From a nuclear expert:
Spent fuel in the pools in Units 3 and 4 is now uncovered. Unlike the releases from damaged fuel in the reactor cores of Units 1, 2, and 3, which were largely filtered by scrubbing in the containment suppression pools, releases of volatile fission products (Cs, I) from these pools have direct pathways to the environment.
Efforts to deliver water to these pools have proven to be very difficult, and fuel damage is occurring. The use of the evaporation of salt water as a heat sink over periods of more than a few days is not viable because the quantities of salt deposited as the water evaporates becomes large in volume and plugs the flow paths through the fuel, degrading heat removal. Fresh water supply is difficult to come by. It may be practical to bring fresh water by helicopter (this is being attempted), but the amounts needed imply a very large number of flights and radiation levels are extremely high above the pools making overflights hazardous. If radiation levels on the ground increase further, personnel access will become more challenging. Additional spent fuel is stored in pools in Units 5 and 6 and in a large centralized storage pool. A key issue is how to continue to make up water to these pools in the longer term, particularly if site access becomes more difficult or impossible.
In short, this accident is now significantly more severe than TMI. It resulted from a unique combination of failures to plant systems caused by the tsunami, and the broad destruction of infrastructure for water and electricity supply which would normally be reestablished within a day or two following a reactor accident.
My earlier belief that this would not lead to widespread radioactivity dispersal is based on the assumption that cooling to the spent fuel pools can be maintained. This is currently uncertain. Iodine-131 poses the most significant radiological risk to the surrounding populations, and access to potassium iodide or other iodine-rich foods would be prudent.
UPDATE: Commenter Zach Clayton pointed out something that I should have made very clear in this article–there is no iodine-131 in the spent fuel. It decayed away to harmless xenon-131 a long time ago. The xenon is also completely stable, and the krypton only contains krypton-85 which poses essentially no threat. The riskiest substance in spent nuclear fuel is likely the cesium, and potassium iodide pills will do little to counteract that threat. But cesium doesn’t “bioaccumulate” in the thyroid either.