The technology could also take decades to deploy in Australia as no small modular reactors had been built successfully using a mass-production model.
The Australian Conservation Foundation issued the warnings on Thursday in a report examining the nuclear technology and its potential costs in Australia.
It comes days after Nationals leader Matt Canavan reiterated his call to remove Australia's ban on nuclear power and as the federal government considers rules for data centre developments.
The report, Phantom Energy, analysed the progress of small modular reactor technology, data centre electricity demand in Australia, and figures from sources including the CSIRO, Australian Energy Market Operator (AEMO), and the Nuclear Energy Agency.
Despite calls to unlock the technology, the study found no commercial reactors had been built using the modular manufacturing methods designed to lower their cost.
Only two small plants described as small modular reactors had been built in Russia and China, and both had suffered long delays, cost overruns, and did not use modular construction.
Another five small modular nuclear projects were under way in China, Russia, Uzbekistan and Canada, the report said, but others had been abandoned in Argentina, France and the US.
The nuclear technology was not ready to meet rising energy demand from data centres, Australian Conservation Foundation campaigner Mia Pepper said, and its future was unclear.
"Small modular nuclear reactors remain unavailable and prohibitively expensive," she said.
"They remain a theoretical answer to a very real and rapidly growing challenge: decarbonising the electricity system while meeting increasing demand from electrification, industry, and data-intensive technologies."
More than 160 data centres operate in Australia, with another 225 developments planned or proposed.
Using forecasts from the CSIRO and AEMO, the report calculated that powering Australia's data centres using small modular reactors would cost between $8.5 billion and $12.2 billion a year.
By comparison, large-scale solar projects would cost between $780 million and $2 billion a year, and onshore wind developments would cost between $1.9 billion and $3.1 billion.
The federal government is considering rules for data centre developments, including standards for their power supply and water use, with draft legislation expected before the end of 2026.
In an interview with the ABC on Monday, however, Senator Canavan said the government should remove Australia's prohibition on nuclear power to meet data centre demands.
"Right now while we ban nuclear energy, we have de facto bans on coal and gas, it's almost impossible to meet the enormous requirements of data centres, certainly without giving people confidence that there can be more energy after than before," he said.
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