But those were uninformed opinions ~ the physicists had long known the probability of that was very small so they forged ahead with the engineering part.
What you have here is something UNEXPECTED by the current hot-nukes guys. All they're doing is running a large volume of water (into a nuclear pile) and doing that repeatedly! Or, maybe they're just dumping it into the ocean. Do you actually know what these yahoos are doing?
Has anyone done it quite this way before? Isn't there a reason nuclear power plants use fuel that's packed in discrete amounts in precisely measured containers?
Yes. And they are not yahoos. They are good people doing the best they can in a difficult situation in a land that was devastated by a natural disaster. They are doing what the article said, using light water to remove heat from a material that still has a decay heat load.
And THAT IS ALL we are dealing with here. Decay heat. The materials generating the decay heat are in a geometry that is not precisely known. That makes heat transfer a difficult proposition, and modeling of the heat transfer process even more difficult. As to the temperature rise, it is explainable with much simpler and more probable mechanisms than fusion reactions. My guess (speculation, which is something generally frowned upon, but since you asked, I'll give it an honest try) is that something shifted in the cooling mass, a heat removal pathway that was previously open because blocked by debris, or perhaps vapor pressure built up enough to prevent coolant flow through an area previously receiving coolant. You see this happen all the time in other, more familiar examples. Just yesterday evening the logs in our fireplace shifted positions a little as they burned down, causing a slight increase in the radiant heat outflow. Until someone can get a visual inspection of the damaged cores, we won't known precisely what the geometry of the materials really is.