The story...

If the hijackers wanted to kill the maximum number of people, then why didn't they fly into a nuclear power plant?

Our take...

We can think of at least four reasons...

#1: it looks to us like the attacks were more about symbolism than killing "the maximum number of people" (WTC = the US economy, Pentagon = the military, maybe Flight 93's target was something Government-related like the White House or Congress). What do you think has more symbolic and propaganda value in the Middle East: “we destroyed the Pentagon” or “we destroyed Indian Point”?

#2: nuclear reactor buildings are small, close to the ground, and tend to be surrounded by many other buildings. They're far more difficult to hit than the 9/11 targets.

#3: even if a plane did hit the building, it's not guaranteed to break through the containment structure. A study undertaken for the Nuclear Energy Institute took the example of a direct hit by a plane fuselage and 9,500 pound engine, and found that:

For the models representing all types of U.S. containment buildings, no parts of the engine, the fuselage, the wings or the jet fuel entered the containment buildings. The containment structure was not breached, despite some crushing and spalling (chipping of material at the impact point) of the concrete.

Evaluation of the models representing both types of used fuel pools determined that the stainless steel pool liner ensures there would be no loss of pool cooling water even though some crushing and cracking of the concrete occurred at the point of impact. Because the used fuel pools were not breached, there would be no release of radioactivity to the environment.

For the analyzed dry fuel storage facilities, the steel canister containing the used fuel assemblies was not breached. Because the dry storage structure was not breached, there would be no release of radioactivity to the environment.

For the analyzed used fuel transportation container, the container was not breached, so there would be no release of radioactivity to the environment.
http://www.nmcco.com/education/facts/security/crash_analysis.htm

Of course that study is undertaken for the nuclear industry, and so some people will say that maybe it's been "fixed", but studies prior to 9/11 also revealed that concrete was stronger than you might think.

In 1988 Sandia National Laboratories in USA demonstrated the unequal distribution of energy absorption that occurs when an aircraft impacts a massive, hardened target. The test involved a rocket-propelled F4 Phantom jet (about 27 tonnes, with both engines close together in the fuselage) hitting a 3.7m thick slab of concrete at 765 km/h. This was to see whether a proposed Japanese nuclear power plant could withstand the impact of a heavy aircraft. It showed how most of the collision energy goes into the destruction of the aircraft itself - about 96% of the aircraft's kinetic energy went into the its destruction and some penetration of the concrete, while the remaining 4% was dissipated in accelerating the 700-tonne slab. The maximum penetration of the concrete in this experiment was 60 mm, but comparison with fixed reactor containment needs to take account of the 4% of energy absorbed in moving the slab.
http://www.uic.com.au/nip14.htm

This still leaves some questions. Are US power plants protected by something equivalent to “a 3.7m thick slab of concrete”, for example? We don’t know.

There’s also the issue that a hijacked commercial plane would be considerably heavier, perhaps 10 times the weight of the test jet. How much of a difference that makes depends on where the plane hits, though (if one engine and wing smash into the ground, then their mass is irrelevant). And with the jet managing only 60mm penetration of a 3.7m slab, there’s a clear possibility that larger impacts would also be survived.

#4: Even if there was a containment breach, that doesn't imply a massive death toll. Chernobyl is reported to have killed 30 people "during the accident or within a few months", for instance ( www.chernobyl.co.uk/health.html ). The same site suggests 2,500 deaths over the long term, caused by increased cancers, for instance, but are terrorists really looking to produce more thyroid cancers in local children, say? We don't think so: they want something spectacular that will make the papers tomorrow, not speculative documentaries on the event's possible contribution to leukemia or whatever, ten years later.