Of the 63 reactors under construction worldwide, with a generating capacity of 71 gigawatts, 29 are in China, the IEA said in a report published Thursday.
About 420 reactors are in operation globally, and generation levels are set to reach a record high this year, the IEA said.
The average age of nuclear reactors in advanced economies, where 70% are concentrated, has exceeded 36 years, twice the average in emerging market and developing countries.
Most old reactors will need to be decommissioned within the next 10 years or renovated to extend their useful life by another 10 to 20 years, the agency said. Nuclear power's share of the total power generation of advanced economies fell to 17% in 2023 from a peak of 24% in 2001.
IEA Executive Director Fatih Birol told Nikkei that in addition to facing political uncertainty, the nuclear industry in advanced economies doesn't "perform well."
Meanwhile, China's nuclear reactors have been operating for an average of just 9 years.
Of the 52 reactors that have started construction since 2017, 48 were designed by China or Russia, the IEA said.
Russia also holds a 40% share of the production of enriched uranium, the fuel for nuclear power plants. It put temporary restrictions on exports to the U.S. in November.
"Highly concentrated markets for nuclear technologies, as well as for uranium production and enrichment, represent a risk factor for the future and underscore the need for greater diversity in supply chains," the report said.
Nuclear power projects in Europe and the U.S. are eight years behind schedule on average, with construction costs more than doubling. The recently built Units 3 and 4 at Plant Vogtle in Georgia -- the first new reactors in the U.S. in about 30 years -- were planned to have energy costs of $5,600 per kilowatt, but the figure rose to $14,700 as of 2023.
Small modular reactors (SMRs) have been drawn attention as way forward from some, including Amazon, which has announced a significant investment in the field. Proponents say SMRs have shorter construction times and lower costs than existing large-scale plants.
If governments follow their current energy plans, the IEA estimated that SMRs, which will enter commercial operation for the first time around 2030, will have a total generating capacity of 120 GW by 2050, with more than 1,000 reactors in operation. Investment is expected to reach $25 billion in 2030, five times the current level, and $650 billion in 2050.
U.S. tech giants see nuclear energy as solution to the soaring energy demand of artificial intelligence data centers. "Widespread deployment of SMRs could accelerate the shift in market leadership," the IMF says, noting that 60% of new construction starts to 2050 use designs from the U.S. or Europe based on announced plans.