MANILA (Reuters) - The Philippines is looking into operating the country's only nuclear power plant, built four decades ago at more than $2 billion but never used, to ensure the long-term supply of clean and cheap electricity, its energy minister said.

The Southeast Asian country is joining more than two dozen other countries looking to add nuclear power to their energy mix, including neighbours Indonesia, Vietnam, Malaysia and Thailand.

Energy Secretary Alfonso Cusi said on Tuesday reviving the mothballed 620-megawatt nuclear plant in Bataan province, northwest of Manila, will require a $1 billion investment.

Nuclear generation is one of the options for the Philippines to meet its growing power needs, with annual electricity demand expected to rise by an average 5 percent until 2030, he said.

"We have to weigh all our options, with emphasis not just on meeting capacity requirements, but sustainability and environmental obligations as well," Cusi said, speaking at the opening of a three-day international conference on nuclear power in Manila.

Cusi will revive a government task force created in 2007 to study nuclear power as an alternative to imported fuel oil and coal, which currently provide more than half of the country's energy mix.

He said technical experts, including those from the International Atomic Energy Agency, have been invited to help the country identify the next steps and come up with a "well-informed" decision.

Cusi is not committing any timetable for the study, but he expects the move to reignite protests against the project, especially by environmentalists and the Catholic Church arguing restarting the plant is unsafe and expensive.

"We need to move away from fossil fuels like coal but nuclear energy is not safe and will also harm the people and environment," said Zaira Patricia Baniaga of the Philippine Movement for Climate Justice in a statement issued before the conference.

The late Philippine dictator Ferdinand Marcos order the plant built in 1976 in response to rising energy prices and it was finished in 1984.

The facility never started generating electricity after it was declared unsafe because it sits on a major earthquake fault line and lies near the Pinatubo volcano, which was dormant at that time.

Pinatubo's 1991 eruption had no effect on the Bataan plant, 70 km (45 miles) away, but the project was mothballed in the wake of the Chernobyl disaster in 1986.

A decade ago Manila looked into reopening the plant but the 2011 Fukushima nuclear incident renewed concerns about safety.

(Reporting by Enrico dela Cruz; Editing by Christian Schmollinger)