dnd 5th – Are the fire damage also radiant?

The spell specifies the damage in the last sentence of the first paragraph.

The creature takes 2d6 Fire damage in case of failure of a save, or half of its damage if successful.

So, moving the sphere into a creature and forcing a saving throw against the damage remains fire damage

If you push the sphere into a creature, that creature
must make the saving throw against the
damage, and the sphere stops moving this turn.