The benefits of fluoridation

One of the cleverest things about LFTRs is that they work at atmospheric pressure. This changes the economics of nuclear power. In a light-water reactor, the type most commonly deployed at the moment, the cooling water is under extremely high pressure. As a consequence, light-water reactors need to be sheathed in steel pressure vessels and housed in fortress-like containment buildings in case their cooling systems fail and radioactive steam is released. An LFTR needs none of these.

Thorium is also easier to prepare than its rivals. Only 0.7% of natural uranium is the fissionable isotope ²³⁵U. The rest is ²³⁸U, which is heavier because it has three more neutrons, and does not undergo fission because of the stability these neutrons bring. This is why uranium has to be enriched by the complicated process of centrifugation. Plutonium is made by bombarding ²³⁸U with neutrons in a manner similar to the conversion of thorium into ²³³U. In its case, however, this requires a separate reactor from the one the plutonium is eventually burned in. By contrast thorium, once extracted from its ore, is reactor-ready.

It does, it is true, need a seed of uranium or plutonium to provide neutrons to start the ball rolling. Once enough of it has been converted into ²³³U, though, the process becomes self-sustaining, with neutrons from the fission of ²³³U transmuting sufficient thorium to replace the ²³³U as it is consumed. The seed material then becomes superfluous and can, because the fuel is liquid, be flushed out of the reactor along with the fission products generated when ²³³U atoms split up. Similarly, more thorium fluoride can be bled in as needed. The consequence is that thorium reactors can run non-stop for years, unlike light-water reactors. These have to be shut down every 18 months to replace batches of fuel rods.

Bombs away?

Thorium has other advantages, too. Even the waste products of LFTRs are less hazardous than those of a light-water reactor. There is less than a hundredth of the quantity and its radioactivity falls to safe levels within centuries, instead of the tens of millennia for light-water waste.

Paradoxically, though, given thorium’s history, it is the difficulty of weaponising thorium which many see (as it were) as its killer app in civil power stations. One or two ²³³U bombs were tested in the Nevada desert during the 1950s and, perhaps ominously, another was detonated by India in the late 1990s. But if the American experience is anything to go by, such bombs are temperamental and susceptible to premature detonation because the intense gamma radiation ²³³U produces fries the triggering circuitry and makes handling the weapons hazardous. The American effort was abandoned after the Nevada tests.

The gamma-ray problem is created by a quirk of the process that turns thorium into ²³³U. A small amount takes a different path and ends up as radioactive thallium—which is very radioactive indeed. Its gamma rays are so powerful that they can penetrate concrete a metre thick. Extracting, smelting and machining material containing even trace amounts of it is beyond the scope of all but a handful of national weapons laboratories. Rogue nations interested in an atom bomb are thus likely to leave thorium reactors well alone when there is so much poorly policed plutonium scattered around the world. So a technology abandoned because it could not be turned into weapons may now, in part for that very reason, be about to resurface.

Date: 2015-12-24; view: 964