* Plant to use "co-precipitation" process
* Agreement on plant follows visit from French delegation
* Cost of equipment undisclosed
* Accord comes at sensitive time for Areva, Lauvergeon
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By Christian Plumb and Yoko Kubota
PARIS/TOKYO, April 19 (Reuters) - French reactor maker Areva , stepping up its role in Japan's clean-up from its worst-ever nuclear disaster, will provide Tokyo Electric Power (TEPCO) with equipment to clean up radioactive water.
The decontamination plant, which uses a process called "co-precipitation" to isolate and remove radioactive elements, will be delivered after Areva specialists studied the crippled reactor at close range over the last three weeks.
Areva Chief Executive Anne Lauvergeon landed in Japan in late March, part of a wider French delegation that flew out to help TEPCO bring the crippled Fukushima nuclear plant under control. France is Europe's top generator of nuclear power, from which it gets nearly 80 percent of its energy needs.
At a news conference in Tokyo, Lauvergeon said TEPCO was hoping for the water treatment to start before the end of May.
"The situation is a great challenge...every day, every hour is vital," she said. "We will do everything we can."
TEPCO said over the weekend that it hoped to take an initial step of cooling the reactors and spent fuel at the crippled Fukushima Daiichi plant within the next three months before bringing it to a cold shutdown within six to nine months.
The Japanese power company has started removing highly contaminated water from one of the reactors, a key step to repair the cooling system.
Areva said other processes could be used in parallel with the co-precipitation treatment plant, which was "the most suited to the present emergency" but could be supplemented by other medium and longer term moves.
Asked about the cost of the decontamination plant, Lauvergeon, one of France's best known female executives, would not provide a specific figure, saying this was not the time to negotiate costs.
"The Japanese government is also supporting this," she said. "I trust that there are no concerns over the issue of payment."
The Japanese crisis comes at a sensitive time for Areva, which has been trying to market next-generation reactors designed to withstand tsunamis, earthquakes or even plane crashes.
Lauvergeon's reappointment to a second term as CEO is in question both because of cost overruns in building the first of the so-called EPR plants and because of rocky relations with state-run power company EDF. (Additional reporting by Dan Magnowski in Tokyo; Editing by Nina Sovich and Jon Loades-Carter)