 Brazil currently gets about 4% of its electricity from nuclear |
Luiz Inacio Lula da Silva said hundreds of millions of extra dollars would be made available for the project over the next eight years.
Work on the third reactor for uranium enrichment stopped in the 1980s over security fears and lack of funds.
Brazil's has two operating nuclear reactors - Angra 1 and Angra 2.
"If money was lacking, it won't be lacking now," President Lula said during a visit to a navy research centre in Sao Paulo.
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"I've made a commitment to provide the necessary funds so we can complete that project.
"We will complete Angra 3, and if necessary, we'll go on to build more (nuclear reactors) because it is clean energy and now proven to be safe."
President Lula added that Brazil could now afford the luxury of becoming one of the few countries in the world capable of uranium enrichment.
He also said that building a nuclear submarine could take about eight years.
Energy shortage warning
President Lula's comments come days after the country's energy commission recommended restarting the third reactor, near the coastal city of Angra dos Reis.
Brazil's Angra 1 and Angra 2 - located in the same region - have an installed capacity of about 2,000 megawatts, and Angra 3 would increase capacity to 3,000 megawatts.
Angra 3 would require an investment of about $3.7bn (£1.85bn) with construction due to be completed by 2013, Energy and Mines Minister Nelson Hubner said last month.
Brazil spent some $800m (£400m) on parts for Angra 3 in the 1980s before construction was halted.
Brazil, which is heavily dependent on hydro-electricity, could face energy shortages in a couple of years if generating capacity is not increased, analysts say.
A severe drought in 2001 led the authorities to introduce energy rationing.
