Assuming Japan doesn't build any new nuclear reactors by 2035, it could face rising economic costs and risks to its energy security, the International Energy Agency says.

IEA Executive Director Maria van der Hoeven outlined the effects of two different scenarios for post-Fukushima Japan: "low nuclear," reflecting a major scale down of nuclear power, compared with a central scenario in which there would not be a reduction of nuclear power.

"Our analysis at the low nuclear case found out that it would carry large economic costs for Japan, raising energy security concerns, and increased emissions compared to the new policies scenario," or the IEA's central scenario, van der Hoeven told reporters in Tokyo, Japanese news agency Kyodo reports.

Platts calculates 11 of Japan's 54 nuclear reactors are now operating. That represents a combined capacity of 9.864 gigawatts, or 20 percent of the country's total installed nuclear power capacity of 48.96 gigawatts.

Some of the reactors are shut down because of a pre-Fukushima government regulation requiring nuclear power plants to conduct scheduled maintenance every 13 months. But because of additional stress test conditions imposed by the government in July in response to the Fukushima disaster, it is unlikely that those reactors will receive permission to open any time soon, thus leading to the possibility of Japan losing its nuclear output completely by April or May 2012, Platts says.

Japan's basic energy plan prior to the Fukushima disaster called for nuclear power to account for 53 percent of all electricity generated in the country by 2030. Before Fukushima it totaled about 30 percent.

Data released this week by Japan's Federation of Electric Power Companies already reflects the country's rising oil and gas consumption associated with reduced nuclear output.

In October, Japan's nuclear utilization rate fell to a record low of 18.5 percent. That same month, major Japanese power utilities consumed nearly six times more oil and 31.7 percent more liquefied natural gas, compared with October 2010.

If nuclear power output drops to zero next year, van der Hoeven said, IEA estimates Japan will have to spend $3 billion each month on additional oil and liquefied natural gas next year.

IEA estimates that annual gas imports in the resource-poor country could reach $80 billion in 2035 under its low nuclear scenario, compared with $66 billion under IEA's central scenario. IEA also says that Japan would have 50 million more tons of carbon emissions by 2035 under the low nuclear scenario.