We have several systems in place, as Mr. Pereira mentioned. There is a system that will inject what we call a poison, which stops the reactivity in the reactor core. There are also, as you mentioned, several emergency systems that will carry more water to cool down the core.
Generally, there are several systems to prevent serious accidents, as you mentioned. There are always various sources of radioactivity in a nuclear system and we need to have systems ready for every emergency.
A third very important system, since we want to ensure no radioactivity can escape the generating station itself, is one that is able to suck out all the air surrounding a reactor. This system extracts all radioactive substances and carries them to a building that was specifically built for such a situation.