On Thursday, China and Russia signed on to the West's latest attempt to defuse the nuclear standoff with Iran. The incentive package that's now on the table includes a light-water reactor for civilian use. A similar deal had been worked out with North Korea in 1994, but the construction of that light-water reactor was officially terminated on Wednesday. Why do we like to offer this kind of power plant to nuke-hungry nations?
Because it's the most "proliferation-resistant" nuclear reactor on the market. There are two ways to make nuclear weapons. You can either take fresh reactor fuel and enrich it—as many fear the Iranians plan to do with all their centrifuges—or you can reprocess used-up reactor fuel into weapons-grade plutonium. The latter option becomes much more difficult and expensive when you're using a light-water reactor.
Light-water reactors are designed for commercial use and can run for years at a time on a single batch of fuel. ("Light water" refers to ordinary H2O; "heavy water" has a higher percentage of deuterium atoms, i.e. hydrogen atoms with an extra neutron.) That long burn fills out the plutonium by-product with other isotopes that make it less useful for nuclear weapons. If you shut down a light-water reactor early—after a few months, for example—you'd waste a huge amount of money.
Furthermore, it would be very easy to tell when the Iranians or North Koreans shut down their light-water reactors. To extract the fuel rods, you have to lift off a giant lid at the top of the reactor and take them out all at once. Weapons inspectors love this feature because it requires a large-scale operation that's almost impossible to conceal.
So-called "research reactors" tend to be both harder to inspect and more efficient at producing weapons-usable plutonium than the light-water variety. The North Koreans say they've already pulled some plutonium from their gas-cooled research reactor at Yongbyon. You can run a gas-cooled reactor for a few months at a time and then pull out the fuel rods for reprocessing without making a big deal of it.
We might offer Iran a Canadian-style, heavy-water reactor, which can also be used for non-military purposes. The only problem is that its design allows for the replacement of just a few fuel rods at a time. That makes reprocessing the spent fuel rods on the sly a much simpler operation.
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Explainer thanks Peter Hayes and David von Hippel of the Nautilus Institute.