Quake-driven nuclear problems: Seawater plus uranium equals _?_

Adding to my list of questions on the effects of the Sendai Earthquake on the Fukashima Dai-ichi plants.

What's the effect of injecting seawater in the Fukushima BWRs, other than its obvious corrosivity?

In typical boilers, any time mineral-laden water is used, a mineral scale develops at the interface between steam and water. It's most noticeable at the waterline, but also happens below the surface where bubbles form. Thousands of gallons of seawater have been boiling into vapor the last couple of days inside the primary containment vessels of Units 1 and 3, leaving hundreds of pounds chlorides (about three percent by weight of seawater) behind.

If mineral scale is building up in the primary containment vessels, given that this hard substance is not a good conductor of heat, it would make the cooling of the wrecked fuel rods more problematic. I know the melting point of salt is well below the melting point of uranium but mineral scale could aggregate onto the now-oxidized zirconium alloy of the ruptured cladding.

So: is mineral buildup playing into recent information about what is happening at Units 1 and 3, the first to have their secondary containment structures blow open? According to this report the cores inside the primary containment structures at Units 1 and 3 aren't cooling as they should be, given the seawater being injected by fire hose:

"However, by Monday night there were reports that efforts to continue cooling Units 1 and 3 might be running into problems." 

 I understand that core cooling at 1 and 3 are not of the same urgency as dealing with the latest crises: Unit 2 (possible breach in primary containment, maybe around the wetwell) and Unit 4 (possible fuel fire in a spent fuel storage area, due to uncovering of stored fuel assemblies as water leaked out of the pool and was not replaced.)

Link to Google satellite imagery: here