It is suggested that hotspots are formed inside the earth. Some of the power sources are supposed to be at the core. To illustrate the principle, here is a reference. Not all hotspots reach at the core, so it is suggested that they are formed by large to radioactive blocks inside the earth.
One thing that makes that process very unlikely is the crossover depth at 400 km depth (see image below). Magma above 400 km depth rises, but magma sinks below 400 km depth. It is therefore suggested that the deep power sources heat the rock at the core (2890 km deep), that rises to 400 km depth in solid state and arrives there 50 million years later, and there it melts and rises further. So it would melt 50 million years after it was heated, while the power source stays behind.
There is more: Hotspot trails show the volcanic eruptions that have occurred in the past due to those hotspots. Some of them even reach towards trenches (interpreted as subduction areas), like the Hawaii hotspot, as you can see as number 12 in the image below.
Hotspots and hotspot trails
However, a lot of the hotspot trails (like Canary and Kerguelen) do not stretch out all the way across an oceanic plate. That means that they have an origin: there was a time that these hotspots did not exist yet. So hotspots cannot be powered by radioactive blocks inside the earth. How is a radioactive block inside the earth supposed to form?
Conclusion: hotspots are not powered by radioactive blocks inside the earth.