Still, it is very unlikely to become packed tightly enough to reach what is known as critical mass and start a chain reaction. The plutonium would qualify as weapons grade only if a large quantity was packed together.
It takes a lot of advanced, precision engineering to create a nuclear bomb. It involves a carefully controlled type of explosion that simply couldn't happen by accident (otherwise a lot more countries would have nukes). There's no chance of a "mushroom cloud" here in the slightest.
That doesn't mean explosions aren't possible. The fuel rods and spent fuel are extremely hot and remain so for a long time. They have to be constantly cooled or they will start melting through the bottom of the reactor. When really hot metal suddenly comes in contact with cool concrete, with concrete would explode taking harmful radioactive material with it and polluting the area.
This is really the scenario they're desperately trying to prevent. The more loss of containment of the nuclear materials that occur, the more dire and lasting the consequences become.