Readers ask: Does the life of the rods affect their radioactive capabilities? Like, if they were getting near the end of their effective life, are they more or less dangerous? Are the rods the only danger, or are there other components, like cooling tanks or hydrogen containers, that are also dangerous with radioactive meltdown? Once the reactor's control rods are between sections of fuel rods, how is fission slowed to where it is controllable? It seems the control rods aren't adequate to regain control of the fission.
Science answers: Spent fuel is more dangerous because it contains a mixture of fission products, some of which can be long-lived radioactive waste, and also plutonium which is highly toxic.
In a boiling water reactor like those at Fukushima, the same water that cools the reactor core also boils and drives the turbine to generate power. This means that some radioactive material gets into the turbine and condensers and pumps, but these are usually very short-lived radionuclides.
When the control rods are inserted into the core, they absorb neutrons and stop the fission chain reaction. But they do not stop spontaneous fissions in fuel rods from fuel and fission products. These spontaneous fissions are what keep the fuel rods hot even when the core is shut down.