LONDON  - Japan will transfer all the highly enriched uranium and separated plutonium held at the country’s fast critical assembly facility to the United States, the two countries said in a joint statement on Monday.

“This effort involves the elimination of hundreds of kilograms of nuclear material, furthering our mutual goal of minimizing stocks of highly enriched uranium and separated plutonium worldwide, which will help prevent unauthorized actors, criminals, or terrorists from acquiring such materials,” according to the White House. “This material, once securely transported to the United States, will be sent to a secure facility and fully converted into less sensitive forms.”

The announcement was timed to coincide with the Nuclear Security Summit in the Netherlands, attended by more than 50 world leaders, including US President Barack Obama and Chinese President Xi Jinping.

The transfer is mostly symbolic but it comes after China complained last month Japan is stockpiling nuclear materials that could be used in a future bomb-making programme (“Japan to let US assume control of nuclear cache” New York Times, March 23). It underscores US determination to resist the spread of dangerous technologies as much as possible and reinforce the non-proliferation regime.

The United States continues to press countries with civilian reactors to forego enrichment and reprocessing, and instead rely on international processing facilities operated by existing nuclear weapons states and other reliable suppliers in Europe.

The proliferation risks posed by nuclear programmes in Iran, North Korea, Israel, Pakistan and India are well known. But dozens of other countries are also accumulating enormous quantities of fissile materials as part of civilian energy programmes. Experts fear these stocks could be used to assemble a nuclear bomb quickly, if a government decided it needed a nuclear deterrent, or stolen by terrorists to produce a radiological weapon (a so-called “dirty bomb”).

“As of January 2013, the global stockpile of highly enriched uranium is estimated to be about 1,390 tonnes,” according to the International Panel on Fissile Materials, an independent group of arms control and non-proliferation experts from both nuclear weapon and non-nuclear weapon states founded in 2006. “The global stockpile of separated plutonium is about 490 tonnes, of which about 260 tonnes is the material in civilian custody,” the panel estimates.

Meanwhile, Italy and Belgium said on Monday they had moved excess nuclear materials to the United States for disposal or downgrading under the terms of past agreements, at the start of a nuclear summit in the Netherlands.

Italy said about 20kg of highly enriched uranium and separated plutonium had been moved from its territory. Belgium said a “significant amount” of the same materials had also gone. Both statements were countersigned by the United States.

The material will be stored in the United States until it is disposed of or made into low-enriched uranium for civilian purposes, the statements added.

Proliferation experts are concerned about poor security around some civilian stockpiles, as well as the practice of shipping large quantities of spent nuclear fuel and plutonium between producing states and countries with reprocessing facilities.

There is no evidence any of this material has been diverted for military purposes. But just a few kilograms of plutonium or highly enriched uranium is all that is needed to make an atomic weapon, and even less is needed for a dirty bomb, so civilian stockpiles could potentially be used to make thousands of weapons.

Less than 1 percent of the uranium ore occurring naturally is the fissile isotope uranium-235 (U-235). The rest is the more stable isotope uranium-238 (U-238) which is no use for power production or bomb-making.

Some reactor designs use uranium in its natural state, but most need the proportion of U-235 to be raised in order to work effectively. Before being manufactured into reactor fuel, natural uranium is therefore enriched by gaseous diffusion or ultra-fast centrifuges arranged in cascades.

Both methods exploit the small mass difference to separate some of the lighter U-235 from the heavier U-238. By repeating the process enough times, the concentration of U-235 can be raised to any desired level.

For civilian power reactors, the U-235 content is typically raised to between 3 percent and 5 percent (low enriched uranium, LEU).

But some research reactors and those producing medical isotopes require the proportion of U-235 to be increased to 20 percent or more (highly enriched uranium, HEU). Uranium for nuclear weapons is enriched to 90 percent.

The enrichment process is essentially the same whether LEU is being produced for civilian power reactors or weapons-grade uranium is being produced for bombs.

“The development of atomic energy for peaceful purposes and the development of atomic energy for bombs are in much of their course interchangeable and interdependent,” according to a US government report in 1946.

Because of the large volume of material that must be handled in the early stages, enriching uranium from 1 percent to 2 percent requires lots of centrifuges. But as the proportion of U-235 rises, the volume of material that has to be handled declines, and further enrichment becomes simpler.

“By producing LEU of 3-5 percent enrichment, much of the separative work (approximately 70-88 percent) necessary for getting to weapons grade is already done,” according to experts from Los Alamos National Laboratory and Stanford University’s Center for International Security and Cooperation.

“Only a fraction of the cascades used for a full-scale commercial enrichment facility would need to be diverted or constructed to produce sufficient weapons-grade HEU for a few bombs per year,” they explain (“Nuclear non-proliferation” in “Fundamentals of materials for energy and environmental sustainability” 2012).

According to the International Panel on Fissile Materials, 12 countries are known to operate enrichment facilities: Russia, the United States, France, the United Kingdom,Germany, the Netherlands, Japan, Argentina, Brazil, India, Pakistan and Iran. North Korea is also believed to have an operational enrichment plant.

Counter-proliferation experts, especially from the United States, have sought to discourage countries from enriching their own uranium. But many countries with civilian power programmes, including Iran, claim enrichment is one of their rights under the Non-Proliferation Treaty (NPT) and want to enrich their own fuel for security reasons.

Once uranium has been enriched, it is usually manufactured into cylindrical pellets 5-12 millimetres in diameter. The pellets are then inserted into zirconium-alloy tubes about 1-4 metres long known as fuel rods or pins. Multiple fuel rods are bundled together in a fuel assembly that is inserted into the reactor.

In a typical once-through or open fuel cycle reactor, the fuel is used just once. But when the spent fuel is removed from the reactor, only around 5 percent of the fissile material has been used up.

It is more efficient to run the reactor this way, in terms of optimising its performance, but means 95 percent of the fissile material inside the fuel rods goes unused, which is enormously wasteful.

It also generates enormous volumes of highly radioactive waste that must be stored safely. Spent fuel assemblies are generally stored in giant cooling ponds as the remaining fissile material continues to decay and give off enormous amounts of heat that must be removed to prevent damage to the fuel assemblies.

To resolve some of these problems, France, Japan and Russia have developed closed fuel cycles in which spent fuel is sent to be reprocessed. Unused fissile materialsare reclaimed, fashioned into new fuel rods, and run through the reactor again.

The problem is that the radioactive decay of uranium results in a host of different products, many of which are themselves radioactive. Some isotopes are short-lived, like strontium-92 and iodine-128, but others like plutonium-239 are much longer-lasting and ideal for making atomic weapons (“The nuclear fuel cycle: from ore to waste” British Nuclear Fuels, 1996).

Reprocessing operations separate these different materials and return them to the client, including the highly toxic and bomb-capable plutonium-239.

The first reprocessing facilities were built to separate plutonium for weapons. But today Britain, France and Russia all reprocess reactor fuel for civilian purposes, including fuel from Japan.

However, the United States decided in 1977 it would defer reprocessing indefinitely. President Jimmy Carter stated a moratorium was necessary to reduce the serious threat of proliferation and set an example for other countries. As a result, all US spent fuel remains in storage at each plant where it was used.

Reprocessing makes much more efficient use of the world’s finite stocks of usable uranium. If a large proportion of global electricity demand is to be met from nuclear reactors in future to help avert global warming, reprocessing may be essential.

Separation of plutonium is, however, a major proliferation risk. Ordinary reactor-grade plutonium is not ideal for making weapons. But both India and the United States have successfully detonated nuclear devices made from reactor-grade plutonium reclaimed from civilian reactors.