If something breaks and the water in the core depressurizes, it will instantly flash to radioactive steam. One of the primary purposes of the containment building is to prevent the radioactive steam from escaping into the atmosphere. In almost any normal everyday situation, there's no risk whatsoever that water could separate into an incredibly dangerous combination of pure hydrogen gas and pure oxygen gas. It takes so much energy to break those covalent bonds holding the water molecules together because they can't be separated without consuming a huge amount of energy.