At least some nuclear power plants where designed to withstand a plane crash. E.g. the one in Doel (Belgium) [1] (dutch link).

[1] http://fanc.fgov.be/GED/00000000/2900/2954.pdf

WTC Twin Towers were also "built to withstand a plane crash." But obviously not those planes that have hit them:

http://www.telegraph.co.uk/news/1340225/Twin-towers-built-to...

And the initial design was 30 floors less.

The Fukushima plant had a wall to protect it from tsunami and the motors to keep the pumps working when there the electricity fails. The whole design however didn't cover that particular wave:

http://www.dailymail.co.uk/news/article-1388629/Japan-tsunam...

The tsunamis like that happened there "only once in a thousand years" so nobody (who made the executive decisions) expected that one.

Still there were the stone markers in Japan, many hundreds of years old, with the marking "Do not build your homes below this point!"

http://www.nytimes.com/2011/04/21/world/asia/21stones.html

"As time passes, people inevitably forget."

If a terrorist flies a plane into a nuclear plant, the only thing that will be uninhabitable will be the plane.

Neither will it do that flying int nuclear plant.