Comment by cestith

It contains a different mix of elements and isotopes than spent solid fuel rods from light water reactors.

The reactor creates new fissile material while it runs, then fissions that. The new fissile material is recycled into the reactor. Even actinides can largely be recycled into the fuel stream. The fuel and coolant being a liquid mixture allows a lot of chemical processing and returning parts of the initial waste back into the reactor more fully.

The waste that's actually handled for storage tends to have a lower proportion of fission byproducts that carry their own radioactivity, but some traces of highly active sources. I've read cesium, strontium, iodine, xenon, krypton, barium, and various noble metals are the bulk of the waste. Some of that cesium will be cesium-137. Some of the strontium will be strontium-90.

Solid rods from a light water reactor are not even nearly completely spent before they become too degraded for producing electricity. They contain cesium-137 and strontium-90 too but along with uranium-235, plutonium-239, americium-241, neptunium237, and curium at the time they're ready for storage. Small parts of this can be reprocessed rather than stored, but often for nuclear weapons. Having solid rods also makes it more difficult to separate elements.

MSTR waste is dangerous, but for about three hundred years. The radiation involved tends to be largely gamma, but from a very small portion of the waste rather than the bulk of the mass. Light water reactor waste can be dangerous for tens of thousands of years, producing alpha, beta, and gamma from most of the mass of the waste. The alpha and beta radiation will last tens of millennia.