They are having valve problems. One of the possible reasons is that it may be turning into a solid plastic.
If so, that could be one of the best outcomes. As long as it does not blow up before the process completes.
Yes, as of recent the third possibility mentioned by officials is that it will Turn into plastic and not explode.
Provided the plastic doesn't need significantly more space than the source material, of course. We all know what happens when you try freezing a sealed bottle filled with water.
Turning into plastic is an exothermic reaction. That's what's heating the tank.