Nuclear power plants generate heat by bombarding "fissile" atoms (such as uranium-235) with neutrons. When the atoms are split, more neutrons are released, which then split other uranium atoms in a sustained "chain reaction." The heat energy released by this process is used to boil water to produce steam to turn a turbine which produces electricity.
The process is inherently dangerous, inherently dirty, and inherently very profitable -- if you don't have to pay the additional costs of: Accidents, cancers from routine releases, or the safe storage of the nuclear waste stream for thousands upon thousands of years. However, somebody will have to pay all these costs. Catastrophic, globally-impacting accidents are inevitable, as the world has seen four times in the last three decades: Three times in Fukushima and once in Chernobyl, with many "near-misses," many of which have been hidden from public scrutiny, and countless less serious -- but still very serious -- smaller accidents.
Since the day in January that San Onofre sprung a leak, the station's owner/operators, Southern California Edison, have tried to come up with a plan to "safely" restart the reactor. Of course, they can never be operated safely, because many more things than just the steam generators can break catastrophically at a nuclear power plant, but ignoring that fact, as SCE does, SCE wants to at least run one of the reactors -- not the one that sprung a leak, that one's trashed -- at partial power, so they can make some money and, perhaps just as important to them, avoid an investigation by the CPUC of what went wrong (the CPUC has been so supportive of SCE's "need" to make a profit from San Onofre, that perhaps this was not a major concern, but it is always possible that an investigation would reveal something).
Full story: San Onofre doesn't have to become our Fukushima by Ace Hoffman