FRONTLINE investigates the growing crisis of antibiotic resistance.
Cameron Todd Willingham - executed for the arson murder of his three daughters. Could this case change the death penalty debate?
FRONTLINE examines the unfolding chaos in Iraq: What went wrong? How did we get here? And what happens now?
FRONTLINE correspondent Martin Smith reports from Iraq on the miscalculations and mistakes behind the brutal rise of ISIS.
Get Our Newsletter
Standard nuclear reactors use uranium that has been enriched to 3% U-235.
When the uranium inside a reactor is bombarded with neutrons, the U-235 nuclei split apart, releasing energy and giving off 2 or 3 more neutrons per split. These neutrons go on to collide with other U-235 atoms and cause their nuclei to split, and so on. This process continues as a self-sustaining chain reaction.
(Technically speaking, the U-235 absorbs neutrons and transforms to the highly excitable U-236, whose nuclei readily split apart, emitting neutrons.)
What about all the U-238 that does not fission?
A U-238 atom can absorb a neutron and convert to Pu-239 (plutonium). This conversion happens less readily than the fission of U-235. But over time, the Pu-239 builds up in the reactor and starts to fission, contributing a significant amount of energy near the end of the fuel's life.