Pebble bed reactors

The problems at Fukushima got me thinking about pebble bed reactors, described in the WIRED magazine article below. It would be very sweet to have a reactor that self-regulates its temperature in event of catastrophe. Here is a recent NYTimes article.

WIRED: ... Beneath its cavernous main room are the 100 tons of steel, graphite, and hydraulic gear known as HTR-10 (i.e., high-temperature reactor, 10 megawatt). The plant's output is underwhelming; at full power - first achieved in January - it would barely fulfill the needs of a town of 4,000 people. But what's inside HTR-10, which until now has never been visited by a Western journalist, makes it the most interesting reactor in the world.

In the air-conditioned chill of the visitors' area, a grad student runs through the basics. Instead of the white-hot fuel rods that fire the heart of a conventional reactor, HTR-10 is powered by 27,000 billiards-sized graphite balls packed with tiny flecks of uranium. Instead of superhot water - intensely corrosive and highly radioactive - the core is bathed in inert helium. The gas can reach much higher temperatures without bursting pipes, which means a third more energy pushing the turbine. No water means no nasty steam, and no billion-dollar pressure dome to contain it in the event of a leak. And with the fuel sealed inside layers of graphite and impermeable silicon carbide - designed to last 1 million years - there's no steaming pool for spent fuel rods. Depleted balls can go straight into lead-lined steel bins in the basement.

... The key trick is a phenomenon known as Doppler broadening - the hotter atoms get, the more they spread apart, making it harder for an incoming neutron to strike a nucleus. In the dense core of a conventional reactor, the effect is marginal. But HTR-10's carefully designed geometry, low fuel density, and small size make for a very different story. In the event of a catastrophic cooling-system failure, instead of skyrocketing into a bad movie plot, the core temperature climbs to only about 1,600 degrees Celsius - comfortably below the balls' 2,000-plus-degree melting point - and then falls. This temperature ceiling makes HTR-10 what engineers privately call walk-away safe. As in, you can walk away from any situation and go have a pizza.

... If Wu's pebble-bed "thing" is, well, hot, it's because Chinergy's product is tailor-made for the world's fastest-growing energy market: a modular design that snaps together like Legos. Despite some attempts at standardization, the latest generation of big nukes are still custom-built onsite. By contrast, production versions of INET's reactor will be barely a fifth their size and power, and built from standardized components that can be mass-produced, shipped by road or rail, and assembled quickly. Moreover, multiple reactors can be daisy-chained around one or more turbines, all monitored from a single control room. In other words, Tsinghua's power plants can do the two things that matter most amid China's explosive growth: get where they're needed and get big, fast.

See also here. Unfortunately I believe the S. African project has run out of money.

... In the event of a complete shutdown of helium flow in the pebble bed, the temperature would rise at most to 2,900°F, a level well below the thermal limit of the graphite pebbles. At the higher temperature, the more plentiful uranium-238 nuclei absorb more neutrons (due to an effect called Doppler broadening) and the reactor output decreases, lowering the reactor temperature until an equilibrium is reached. The reactor heat is transferred passively by radiation, conduction, and natural convection to the steel reactor vessel, which is designed to reject the heat without human intervention.