Why pebble-bed reactors are the go
Zhang Zuoyi, the project's 42-year-old director, explains why. 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.
"In a conventional reactor emergency, you have only seconds to make the right decision," Zhang notes. "With HTR-10, it's days, even weeks - as much time as we could ever need to fix a problem."
This unusual margin of safety isn't merely theoretical. INET's engineers have already done what would be unthinkable in a conventional reactor: switched off HTR-10's helium coolant and let the reactor cool down all by itself. Indeed, Zhang plans a show-stopping repeat performance at an international conference of reactor physicists in Beijing in September. "We think our kind of test may be required in the market someday," he adds.