The Shidaowan plant, a demonstration high-temperature, gas-cooled reactor with a pebble-bed module (HTR-PM), went into commercial operation last December. Shidaowan’s twin 100-MW units house tiny uranium capsules encased in graphite shells about the size of billiard balls (dubbed “pebbles”), which make the energy density of the fuel much lower than in a traditional nuclear reactor with fuel rods. In the pebble design, the nuclear fission reaction occurs more slowly than in conventional reactors,...