Why, after a uranium fuel rod reaches the end of its fuel cycle (typically 3 years)....?

Question

does most of its energy come from the fissioning of plutonium?

Answer 1

The short answer is that most of the uranium 235 is used up and the result is more plutonium (produced from the U-238) than U-235.

For more details, check out the first two paragraphs of:
http://www.chemcases.com/nuclear/nc-13.htm

and the section title "Plutonium: a fission energy source" in:
http://www.uic.com.au/nip18.htm

Note that at the end of the fuel cycle, though you have more plutonium than U-235, you have much less of both than your starting quantity of U-235, which is why you've reached the end of the fuel cycle.

The process by which Plutonium is created in the reactor (from the U-238) is well covered in:
http://www.fas.org/nuke/intro/nuke/plutonium.htm

For some of the other products of nuclear decay:
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radser.html