* GDF Suez will now unveil plans for nuclear
* Two other reactors to close in 2015 (Adds quotes, background)
BRUSSELS, July 4 Belgium's cabinet postponed the planned closure of one of its oldest nuclear reactors by a decade on Wednesday over concerns the country may not be able to generate enough alternative energy.
The Belgian cabinet approved a plan to keep the 1975 Tihange 1 reactor, owned by GDF Suez unit Electrabel, open until 2025, 10 years longer than planned.
However, it overturned a proposal to delay by one year the planned 2015 closure of Electrabel's two 1975 reactors at Doel in northern Belgium.
"What is important is to guarantee security of supply for the country, and that at the best price, that was my mission," Melchior Wathelet, state secretary in charge of energy, told Reuters.
"Today I want to say that Belgium has a vision in regards to electricity and it's true that was missing for a number of years."
The agreement now has to be voted upon by Belgium's parliament, however that is seen as largely a formality after cabinet's approval.
A report prepared for the Belgian government earlier this year showed the country was at risk of electricity shortages if the three oldest reactors were taken off the grid as planned in 2015.
The Fukushima nuclear disaster in Japan in March 2011 prompted several nations to spur plans to exit the sector.
Belgium has long considered a complete exit, but the move depends on it having enough alternative sources of energy in time.
Around 57 percent of the country's electricity supply came from nuclear reactors in 2011.
Belgium's decision clears the way for GDF Suez to reveal its new nuclear strategy, although a spokeswoman for Electrabel said the company needed more information.
"This decision ... does not contain enough precision, or the necessary clarity to allow us to take a decision on the future of our nuclear activities in Belgium," the spokeswoman said.
Nuclear power took up 10 percent of GDF Suez's total energy production of 465 TWh last year - down from 13 percent in 2010.
That compared with 55 percent in gas and 13 percent of hydro power as it brought gas-fired power plants, dams and wind farms on line in Latin America and Asia.
GDF Suez's goal to keep its nuclear power share unchanged in its energy mix until 2030 is being challenged partly by its absence in key nuclear projects, dominated by EDF and Areva, Westinghouse or ZAO Atomstroyexport.
GDF Suez's shares fell nearly 5 percent when Belgium agreed last year to carry on with its nuclear exit plans, however they were broadly unchanged after the cabinet decision. (Reporting by Ben Deighton in Brussels and Benjamin Mallet in Paris, editing by Robert-Jan Bartunek and Jason Neely)