Backers of mini nuclear power stations have asked for billions of pounds of taxpayers’ money to build their first UK projects, according to an official document.

Advocates for small modular reactors (SMRs) argue they are more affordable and less risky than conventional large-scale nuclear plants, and therefore able to compete with the falling costs of windfarms and solar power.

But the nuclear industry’s claims that the mini plants would be a cheap option for producing low-carbon power appear to be undermined by the significant sums it has been asking of ministers.

Some firms have been calling for as much as £3.6bn to fund construction costs, according to a government-commissioned report, released under freedom of information rules. Companies also wanted up to £480m of public money to help steer their reactor designs through the regulatory approval process, which is a cost usually paid by nuclear companies.

The new generation of mini reactors will typically have a capacity of less than a tenth of Hinkley C being built in Somerset. Photograph: EDF Energy

Ten companies hoping to build the plants requested direct government funding, according to the briefing paper by the Expert Finance Working Group on Small Reactors. While the report named the companies involved in the mini nuclear projects, it did not specify who was asking for financial support.

David Lowry, a nuclear policy consultant who obtained the document, said: “SMRs are either old, discredited designs repackaged when companies see governments prepared to throw taxpayers’ subsidies to support them, or are exotic new technologies, with decades of research needed before they reach commercial maturity.”

The working group that drafted the report, and was appointed by the Department for Business, Energy and Industrial Strategy (BEIS), urged the government in August to put in place a framework to help bring the smaller plants to market.

The government has already offered £44m of funding for research and development of one group of SMRs, which typically have a capacity of less than a tenth of the Hinkley Point C nuclear plant being built in Somerset, or enough power for 600,000 homes.

Mini nuclear power stations are unlikely to supply clean energy to Britain’s homes and businesses any time soon. Of more than 30 British, US and Chinese companies that have expressed an interest in building one in the UK, the majority told the working group that their power stations would be ready to deployed in the 2030s.

The companies include UK firms such as Rolls-Royce, Sheffield Forgemasters and Atkins, along with China’s CNNC, US companies NuScale and Westinghouse, and France’s EDF Energy.

The working group found the firms’ cost estimates “varied significantly”, to the degree that some of the companies clearly had a “lack of understanding” of how British nuclear regulation works.

It also noted that some of the companies proposed using “non-standard fuels” rather than the conventional uranium used by today’s nuclear plants, which “may add cost to business models” because of new facilities to produce and later manage the spent fuel.

The firms told the group that the four main barriers they faced were finding and confirming sites, the cost of regulatory approval for their designs, a lack of state funding and unclear policy.

The government is expected to make announcements soon regarding the siting regime and regulatory approvals for SMRs, sources told the Guardian.

Tom Greatrex, the chief executive of the Nuclear Industry Association, said the reactors could “provide flexible, reliable nuclear power to complement other low-carbon sources of generating electricity … [and] a potential export opportunity”.

A spokesperson for the UK SMR consortium, which includes Rolls- Royce, said: “One of the compelling elements of our consortium’s power station is that, after the design is fully developed and licensed, no government funding is required to construct or operate it.”