By Geert De Clercq
PARIS (Reuters) - The next generation of nuclear reactors being developed in countries such as France, Russia, China and Japan may not be safer than those being built today, French nuclear safety watchdog IRSN said on Monday.
In a study of six future reactor designs being worked on by the U.S.-led "Generation IV International Forum", the IRSN said only the sodium-cooled fast reactor (SFR) model was far enough along in the development process to envisage building a prototype during the first half of this century.
But it could not say whether it would be safer than models currently being built for service.
"While it seems possible for SFR technology to guarantee a safety level at least equivalent to that targeted by generation III pressurised-water reactors, IRSN is unable to determine whether it could significantly exceed this level," it said.
The comments represent the first significant public criticism of the next-generation designs among nuclear safety officials of the 4G forum's 13 member countries, which also include Britain, South Korea and Canada.
The vast majority of the more than 400 so-called second-generation reactors operating worldwide today are pressurised-water reactors (PWR), built mostly between 1970 and 1990.
Utilities are also building a handful of third-generation reactors like Areva's EPR reactor and Westinghouse's AP1000, which use the same basic design as 2G reactors but offer more safety features.
The 4G forum focuses on radical new designs, such as cooling reactors with liquid metals instead of water, and operating at much higher temperatures.
Leading nuclear nations channel millions of dollars into 4G reactor research every year, but the 2011 Fukushima nuclear disaster, the renewable energy boom and cheap shale gas have raised questions about the project.
The IRSN said that while the SFR reactor - unlike PWRs - could operate at low pressure, its main disadvantage was that sodium was highly reactive with water and air.
The agency also questioned the degree to which the reactor would be able to burn up "actinides", among the most dangerous by-products of nuclear fission.
4G advocates argue that SFRs would reduce the amount of nuclear waste and the need for deep storage, but the IRSN said this feature offered "only a very slight advantage", which would not be the deciding factor in the choice of future reactors.
The IRSN said that one of the six 4G models, the Very High Temperature Reactor could bring significant safety improvements, but that the feasibility of that concept had yet to be determined.
(Editing by Pravin Char)