By Paul Rincon
Science reporter, BBC News
Some anticipate a major expansion of nuclear power
The case for nuclear power as a low carbon energy source to replace fossil fuels has been challenged in a new report by Australian academics.
It suggests greenhouse emissions from the mining of uranium - on which nuclear power relies - are on the rise.
Availability of high-grade uranium ore is set to decline with time, it says, making the fuel less environmentally friendly and more costly to extract.
The findings appear in the journal Environmental Science & Technology.
A significant proportion of greenhouse emissions from nuclear power stem from the fuel supply stage, which includes uranium mining, milling, enrichment and fuel manufacturing.
Others sources of carbon include construction of the plant - including the manufacturing of steel and concrete materials - and decomissioning.
The authors based their analysis on historical records, contemporary financial and technical reports, and analyses of CO2 emissions.
Experts say it is the first such report to draw together such detailed information on the environmental costs incurred at this point in the nuclear energy chain.
The report is likely to come under close scrutiny at a time when governments around the world are considering the nuclear option to meet future energy demands and reduce greenhouse gas emissions.
Mining companies are likely to have to dig deeper for deposits
Lead author Gavin Mudd, from Monash University in Australia, told BBC News: "Yes, we can probably find new uranium deposits, but to me that's not the real issue. The real issue is: 'what are the environmental and sustainability costs?'
New uranium deposits are likely to be deeper underground and therefore more difficult to extract than at currently exploited sites, said Dr Mudd.
In addition, he said, the average grade of uranium ore - a measure of its uranium oxide content and a key economic factor in mining - is likely to fall. Getting uranium from lower-quality deposits involves digging up and refining more ore.
Transporting a greater amount of ore will in turn require more diesel-powered vehicles - a principal source of greenhouse emissions in uranium mining.
"The rate at which [the average grade of uranium ore] goes down depends on demand, technology, exploration and other factors. But, especially if there is going to be a nuclear resurgence, it will go down and that will entail a higher CO2 cost," Dr Mudd explained.
Overall, the report suggests that uranium mining could require more energy and water in future, releasing greenhouse gases in greater quantities.
Thierry Dujardin, deputy director for science and development at the Nuclear Energy Agency (NEA), said the analysis made an important contribution to clarifying the impact of nuclear energy on CO2 emissions.
"It is the beginning of the answer to a question I have raised in many fora, including within the agency," he told BBC News.
But Mr Dujardin said he did not fully agree with the authors' conclusions.
"Even in the worst case scenario for CO2 emissions, the impact of nuclear on greenhouse emissions is still very small compared with fossil fuels," he explained.
The NEA official admitted that lower grades of ore might have to be exploited in future, but he added that emissions from mining were only a small part of those produced in the nuclear supply chain as a whole.
He said he was also confident that entirely new deposits would be found as the industry stepped up its exploration effort.
The nuclear industry is carrying out research into recovering uranium from rocks used in the industrial production of phosphates. Various technologies based on solvent extraction can be used to get the element from phosphate rocks.
And in the longer term, some predict that so-called fast breeder reactor technology would increase by up to 50-fold the amount of energy extracted from uranium.