Sites containing around half of Russia's weapons-usable uranium and plutonium still await security upgrades.
Cesium remains the most widely dispersed isotope while concerns remain over long-term contamination from strontium and plutonium.
In one significant respect U-233 is better than uranium-235 and plutonium-239, because of its higher neutron yield per neutron absorbed.
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But first Iran must freeze uranium work at today's level and, if talks progress, suspend its uranium and plutonium effort.
This week they said that Iran would suspend its uranium enrichment and plutonium reprocessing activities, in return for European concessions over trade.
And unlike America, Russia, Britain, France and China, the five official nuclear powers, it refuses to cap uranium and plutonium production for military uses.
For their part, North Korean officials point gleefully to South Korea's recent admission to having experimented with both uranium and plutonium in the past.
But had all this been in place when it was first caught out in 2003, its uranium and plutonium work might have been stopped before it really got going.
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In the talks leading up to the July deal, America had encouraged India to cap voluntarily its production of fissile material highly-enriched uranium and plutonium for bombs as the official nuclear five have done.
Officials trying to stem nuclear proliferation used to congratulate themselves that only a very few of the known incidences of trafficking involved the sort of uranium and plutonium useful for a bomb.
Ironically, the accident happened only one day before a ship carrying nuclear fuel made up of mixed uranium and plutonium docked in Japan, surrounded by coast guard vessels and greeted by demonstrators.
The Europeans had hoped that Iran might be persuaded to bring a permanent halt to all uranium and plutonium work if it received the right inducements, including trade and other, less proliferation-prone nuclear technologies.
Over the years since its main enrichment plant at Natanz was discovered, Iran has volunteered little information to inspectors, belatedly owning up, including to past illicit uranium and plutonium work, only when presented with hard evidence.
Extrapolating from the amounts of bomb-grade uranium and plutonium each is thought to possess, India may have up to 95 nuclear weapons and Pakistan up to about 50 (see table), though possibly both have far fewer.
According to papers filed with the Blue Ribbon Commission, the current research involves faster and simpler processes for separating uranium and plutonium from other fission products in spent fuel and from minor elements like americium and curium.
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Given a start with some other fissile material (U-233, U-235 or Pu-239) as a driver, a breeding cycle similar to but more efficient than that with U-238 and plutonium (in normal, slow neutron reactors) can be set up. (The driver fuels provide all the neutrons initially, but are progressively supplemented by U-233 as it forms from the thorium.) However, there are also features of the neutron economy which counter this advantage.
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We are working with European allies to make clear to the Iranian regime that it must give up its uranium enrichment program and any plutonium reprocessing, and end its support for terror.
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There is, moreover, little agreement over the price worth paying to induce Mr Kim to take the inspectors back and put his plutonium and uranium under lock and key.
From a public health perspective, the most important isotopes are short-lived isotopes of iodine (like Iodine-131), Cesium-137, Strontium-90, and possibly Plutonium-239.
North Korea protested the resolution by saying it would enrich uranium and weaponize plutonium, according to KCNA, the state-run North Korean news agency.
Iran also began building an actual weapons reactor, a 40 MW heavy water reactor at Arak, that would take natural uranium, and produce plutonium.
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"Japan accepts regular and irregular inspections from the IAEA and makes public how it handles and uses plutonium, which proves that Japan makes a peaceful use of it, " he said.
The material that went into Fat Man, the plutonium bomb detonated over Nagasaki, was made at Hanford, which at its peak consisted of nine nuclear reactors and five plutonium reprocessing plants.
That move prompted Britain, France and Germany to discontinue negotiations aimed at persuading the Iranians to abandon for good their ambitions to enrich uranium and make plutonium (another potential bomb ingredient).
Yet suspicions have mounted as Iran has invested in expensive technologies for enriching uranium and making plutonium (both possible bomb ingredients) before having a civilian nuclear-power industry that can make peaceful use of them.
Still, in a September letter to the United Nations, North Korea announced that Pyongyang's"reprocessing of spent fuel rods is at its final phase and extracted plutonium is being weaponized, " state media reported at the time.
The site produced plutonium for the bomb dropped on the Japanese city Nagasaki and continued producing plutonium for the US nuclear arsenal for years.
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That will evoke knowing smiles from Iran and North Korea, both of whom have been found secretly dabbling with plutonium and enriched uranium.
The fuel in No. 3 is a blend of plutonium and reprocessed uranium, referred to as MOX (for mixed oxide) and manufactured by the French nuclear company AREVA. MOX fuel rods are also less stable than plutonium-free rods.
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Yet their rivalry is fuelling the fastest, most dangerous build-up of bomb-usable plutonium and uranium anywhere.
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