It's impossible to hide a radiation leak because anyone with a Geiger Counter will be able to detect it straight away. Geiger Counters are widely available at universities, hospitals, through the military and many other organisations.
The current levels being measured at the actual nuclear site where radiation levels are highest is currently equivalent to the dose received during a chest x-ray. So negligible.
The reactors at risk were built in 1971 (using 1960's knowledge) so feature a design that is not used on newer reactors. The main problem is that even though the fuel rods have been withdrawn from the reactor there is still sufficient heat being produced by the natural radioactive decay of the fuel materials that a constant circulation of coolant is needed to prevent overheating. Coolant circulation has stopped due to failure of the pumps and backup systems so the coolant around the rods has turned to very high pressure, superheated steam (and some hydrogen) and had to be deliberately vented which caused an explosion (but no damage to the actual reactors). If the fuel rods get too hot there is a small chance that they may melt and fall into the bottom of the currently undamaged reactor containers and cause a breech (by melting through the steel) which may then actually cause a genuine contamination problem. Engineers are rigging functioning pumps and using seawater to prevent the meltdown. Seawater was a last resort because it will mean that the reactors cannot be used again.
My opinion only:
Prognosis - risk of a wide spread or dangerous contamination is very low but worth taking precautions over.
How to avoid in the future? - phase out obsolete power stations and replace with newer and safer versions. Nuclear power is still the only real way of providing our ever growing energy needs whilst having a minimal effect on the environment. The Sun and entire core of this planet is one huge molten radioactive fusion reaction anyway and radiation is always around us naturally. We need to get over our paranoia about radioactivity and nuclear power and harness natures energy gift to us and stop burning as much non-renewable and polluting fossil fuels.