Rio Tinto expects to approve its final tranche of spending on iron ore expansions during 2016, and also believes uranium prices will eventually improve.

Speaking after Thursday's $US2.9 billion half-year underlying profit was announced, Rio's executive team indicated that it was likely to give the green light to construction of the Silvergrass mine in the Pilbara in 2016, which is essential for the company to reach its target export capacity of 360 million tonnes per year.

Silvergrass has been deferred three times in the past two years, and was previously expected to cost $US1 billion to build.

Rio has focused on expanding existing mines rather than spending on new ones over recent years, but chief financial officer Chris Lynch said that strategy could only last so long before new mines were needed.

"The processes are in place now and it is almost time for Silvergrass to come. It is well heralded, it is well prepared and I would expect we would see it [approved] somewhere in 2016," he said.

"We fully expect it will come in for the final approval processes and those types of things during 2016, I wouldn't be at all surprised if that wasn't the earlier part of 2016."

Approval of Silvergrass will likely attract criticism from Fortescue Metals Group chairman Andrew Forrest, who believes miners should not be spending money on growing iron ore production at a time when iron ore prices are weak.

Silvergrass should lift Rio's export capacity from 350 million tonnes per year to 360 million tonnes, and chief executive Sam Walsh said the mine's contribution would be more focused on maintaining the quality of Rio's blended iron ore products, rather than growing exports.

Meanwhile, Mr Walsh said Japanese nuclear power companies were continuing to buy uranium from Rio, despite all 50 of Japan's nuclear power stations being turned off since the Fukushima nuclear meltdown in March 2011.

Rio has exposure to uranium through assets in Namibia, Canada and Australia's Northern Territory, and Mr Walsh said depressed prices for the mineral would eventually improve.

"There will be an uptick in uranium prices, the question is when and how long until we expect that. Even with the cutbacks in Japan we are still receiving some orders for uranium from Japan," he said.

"It is showing the nuclear companies there are expecting that there will be a need to bring back nuclear reactors in Japan that meet all the safety standards and all of the changes that have come through. Meanwhile we are seeing in China, and elsewhere, commitment to further nuclear power stations and even in the UK there is a commitment to another nuclear power station."

A nuclear power station on Japan's south coast is expected to be the first to resume later this month, and has already had fuel loaded into it.