Post-Japan, Is a New Type of Nuclear Reactor in the Future?

TWR-reactor

As the ongoing nuclear saga in Japan plays out, a spotlight is being thrown on the reactor technology at the heart of it. With an alternative, innovative reactor design, much of the disaster could've been averted.

The Fukushima Daiichi nuclear plant at the heart of Japan's radiation woes at the moment is a 40-year-old General Electric design of boiling water reactor (BWR). The particular sequence of natural disasters that befell Northern Japan, combined with the particular design flaws of the BWRs has resulted in all that fear and headline-grabbing worry. But could an alternative reactor design have withstood the assault of one of the largest recorded earthquakes and a horrific tsunami? Turns out the answer is yes.

The alternative, being pointed out by Freakonomics and Nathan Myhrvold's blog, is a kind of nuclear plant called a traveling wave reactor (TWR) that's been designed by TerraPower (which has Bill Gates as a key investor) and it's unique.

The Japanese BWR system relies on the hot reactor core made of enriched uranium super-heating low-pressure water into steam, which then pushes turbines and generators to produce electricity. Due to the quirks of the GE engineering, the reactor-"killing" control rods are thrust into the system from underneath, while spent fuel rods are hoisted out from the top of the reactor and are stored in a large tank (whose eerie blue glow you may have seen many times in older sci-fi movies) of water suspended many stories up in the building. The various subsystems in the design meant that in Japan the reactor wasn't cooled quickly enough due to failings in the power as the earthquake, then tsunami, removed the engineers' ability to intervene in the process. The default way the reactor works is to heat up, unless it's actively cooled, and its the over-heating, then the hydrogen explosions, which ruptured the reactor vessel and the cooling tank, exposing fuel to air and letting it heat up still more, which then made the situation much worse.

But in the traveling wave reactor proposed by TerraPower, everything is different. The company touts it as a "proliferation-resistant energy that produces significantly smaller amounts of nuclear waste than conventional nuclear reactors," implying that in normal operation it's better anyway. All it takes is an "initial start-up with a small amount of low-enriched material" and then the reactor "can run for decades on depleted uranium--currently a waste byproduct of the enrichment process" that's needed to make fuel for conventional reactors. According to TerraPower "an established fleet of TWRs could operate without enrichment for millennia."

The reactor works like this: A pile of depleted uranium is given a kick-start by a small chunk of enriched uranium--a highly volatile radioactive material that spits out neutrons. These neutrons travel into the depleted uranium, converting it into active heat-generating fuel (which heats water into steam, conventionally), and then this region activates the neighboring depleted uranium--the traveling wave of energy generation and new fuel-creation that gives the process its name, although the "burning region" actually stays still, and the fuel is slowly slid through the active zone.

Everything happens in an enclosed reactor that needn't be opened for many decades, and there's no need for a spent fuel pool as the spent fuel (now plutonium-239) is merely left in the reactor core behind the front of the traveling wave, where it cools by itself. There's no need for active cooling, and if there was an interruption in the plant's operation, then the reactor would quickly cool by itself. If a disastrous earthquake and tsunami hit a TWR in action, it's unlikely a super-hot reactor emergency and potential meltdown would occur.

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Read more coverage of the Japan earthquake.

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