Damage to Reactors:
- On March 11, 2011 the catastrophic 15-foot tsunami that hit Japan crippled the Fukushima nuclear plant, affecting three separate reactors that released contaminated radioactive water into the surrounding area.
A total of eleven reactors were shut down when the earthquake hit but the three Fukushima Daiichi plant reactors were flooded.
Twelve to thirteen back-up generators as well as the heat exchangers used to dump both reactor waste heat and heat from decay were destroyed.
At 7:00 pm Japan declared a Nuclear Emergency and around 9:00 pm the plant issued an evacuation for 2km, eventually the evacuation zone expanded to 20km from the plant.
Two weeks after, the reactors stabilized in the general sense and by July recycled water from a new treatment plant helped cool the reactors.
The next step was to prevent anymore contaminated water from escaping the reactors. 
On March 30, 2013 Tokyo Electric Power Company (TEPCO) announced that it would initiate a water decontamination system to remove over 60 radioactive chemicals from the waters of the three Fukushima nuclear power plant reactors damaged in 2011.
TEPCO allowed 10,400 cubic meter of “slightly” contaminated water to flow into the sea so more “highly” contaminated water could be removed and stored away from the reactor facilities. TEPCO reasoned that this was a necessary decision to allow for workers to help alleviate the rest of the contaminated water.
According to the Japan Atomic Energy Agency and other respected institutions 15PBq of radioactivity leaked into the sea since late march through april. 
Advanced Liquid Processing System (ALPS):
Through the Advanced Liquid Processing System or ALPS, TEPCO expected to run a trial to remove over 60 radioactive contaminants in the remaining water in February 2013. A product of EnergySolutions and Toshiba, TEPCO expects ALPS to remove contaminants out of 250 tons of water a day. ALPS removes all radioactive substances with the exception of tritium.
By 2015, TEPCO expects to increase storage capacity for contaminated water to 700,000 meters cubed. Since 2011, 230,000 meters cubed of water has been recovered from the reactors. 
Why ALPS is not enough?
But as much as ALPS does to reduce radioactive contaminants in the waters inside the reactors it still does not remove tritium. Tritium, a radioactive isotope of hydrogen, goes through a half life of 12 years and out of the 230,000 tons of radiation believed to be within the Fukushima plant, hundreds of trillions of becquerels of tritium exist.
Several experts believe that until the plant can guarantee that no tritium will escape into the sea, any so-called “treated” water should not be released.