The cost of recycling spent nuclear fuel has surged since Japan started using such fuel in 1999, Kyodo News learned Saturday, adding to questions about the economics of nuclear fuel reprocessing.

Japan already is struggling with the sustainability of nuclear power generation using recycled fuel called mixed oxide, or MOX, which has resulted in a stockpile of nearly 50 tons of plutonium, with only a few reactors currently using such fuel.

The MOX fuel is produced by reprocessing spent nuclear fuel and reusing extracted plutonium and uranium. Japanese power companies send their spent fuel to France for reprocessing.

According to data from the Finance Ministry and other sources, the price of one MOX fuel unit imported in 1999 by the predecessor company of Tokyo Electric Power Company Holdings Inc. was 230 million yen ($2 million).

The price of the recycle fuel Kansai Electric Power Co. bought in September this year exceeded 1 billion yen. While power companies do not disclose MOX fuel prices, sources familiar with nuclear fuel recycling business said the price includes the cost of transport, private security service and insurance.

With many nuclear plants shut down due to safety concerns raised by the 2011 Fukushima nuclear disaster, only three nuclear plants -- two Kansai Electric reactors and one Shikoku Electric Power Co. reactor -- currently use MOX fuel in so-called pluthermal power generation.

The pluthermal project is the only way for Japan to consume plutonium produced in the process of fuel recycling, and the country's plutonium stockpile has declined only slightly after the restart of those three reactors which can run on MOX fuel.

Japan's holding of plutonium, which can be used to make nuclear weapons, has caused international concern. Japan is the only non-nuclear weapons state in the world that is carrying on with a commercial spent fuel reprocessing program.