The operator of Japan's crippled Fukushima nuclear plant said Monday it has switched on a key decontamination system that cleans radiation-tainted water used to cool the site's damaged reactors. Last week, Tokyo Electric Power (TEPCO) said it had discovered a defect in its Advanced Liquid Processing System (ALPS) and switched it off for repairs. The embattled firm said two of three lines that clean the toxic water were running again as of Monday afternoon. A third line remained offline while workers tried to fix a filter defect which had prevented proper decontamination. The problems meant supposedly purified water still had a large amount of strontium -- produced during nuclear reactions -- which accumulates in the bones and can cause several types of cancer in humans. Company officials said there was no immediate safety risk, but added that the poorly-filtered water would have to be cleaned again. It was not the first time the utility has switched off the system, which has been hit by a series of glitches since trial operations began a year ago. TEPCO is struggling to handle a huge -- and growing -- volume of contaminated water at the tsunami-damaged plant. There are about 436,000 cubic metres of contaminated water stored at the site in about 1,200 purpose-built tanks. Many experts say that at some point the water will have to be released into the sea after being scoured of the most harmful contaminants. They say it will pose a negligible risk to marine life or people, but local fishermen and neighbouring countries are fiercely opposed.