Japanese officials say they have activated a water decontamination system in a key step toward shutting down reactors at the crippled Fukushima nuclear plant.

The Tokyo Electric Power Company and government officials said they began using the decontaminated water to cool the cores of the number one, two and three reactors Monday afternoon. They hope to begin full-scale operation of the system soon.

TEPCO described the activation of the system as a vital step in its plan to achieve a safe shutdown of the reactors by the end of the year. The company is running out of places to store an estimated 100,000 tons of radioactive water that has been poured into the reactors since their regular cooling systems were disabled by a tsunami on March 11.

The new system will allow operators to keep recycling the same water through the reactors while emptying contaminated water from the basements and tunnels of the reactor buildings. That will clear the way for other repair work.

The breakthrough, which follows setbacks that delayed the activation of the system for a week, also relieves concerns that radioactive water will overflow the existing reservoirs and spill into the ocean.

The reactors at the plant have been leaking radiation into the atmosphere and ocean since the March 11 earthquake and tsunami, which left about 2,300 people dead or missing.