Japan has walls for tsunamis. Granted, they didn't expect a 10m high one so that totally broke all boundaries. They also practise tsunami and earthquake drills frequently
Also their buildings were designed to be bouncy so they don't fall over as easily. Some of the older ones did collapse though...
The nuclear plants were designed to be well-insured as well, but the earthquake was so bad that it cut off both the power supply and the back-up power supply to the cooling system. Reactors were also designed to shut down as soon as there was a quake, but even when it's not working nuclear reactors get really hot...
So if the cooling systems fail it's bad, very bad.
When the first reactor's cooling system failed they tried to help cool it by releasing (radioactive) steam, but when the explosion happened in part of the cooling system things got bad and stuffs started leaking
The reactor itself can't explode because it's not pressurised. They designed it so something disastrous like that couldn't happen.
Also, a second reactor is now in danger of meltdown~
The end result of this would be basically the entire world being scared shitless of using nuclear power ever again =X
Which is pretty bad imo because the world can only provide us with so many resources, so we'll need to find another good, cheap, 'safe' and clean power source soon.
Tbh I think nuclear would be great if you were sure you could contain it. It's sorta like a yandere. When things are working out all is right with the world, but when something stuffs up, you are deader than dead.