Pebble-bed reactors use a very different reactor core configuration from conventional pressurised water reactors. Instead of the fuel being packed into rods that are arranged in an array and surrounded by a liquid coolant, the fuel – which contains uranium enriched to lower degree than conventional nuclear fuel – is packed into spherical particles surrounded by layers of ceramic, and the coolant is helium under moderate pressure (around 30 atm). Originally developed in Germany in the 1960s to breed uranium from thorium fuel, they operate at a much higher temperature than PWRs – over 900°C at the reactor outlet compared with around 300°C for a PWR – and are often believed to be fundamentally safer than PWRs because they are immune to meltdown even in the event of total coolant loss.
A new commercial-scale pebble bed reactor called HTR-PM, with a thermal capacity of 200MW, is set to come online in China at the Shidao Bay Nuclear Power Plant (on the east coast of the body of water that separates China from the Korean peninsula) later this year. It is believed to be the first "Generation IV" reactor to enter service.
In a paper in the journal Joule, German chemist and reactor safety expert Rainer Moormann, a critic of pebble bed reactors since the early 2000s, urges caution over the operation of HTR-PM. Writing with Scott Kemp and Ju Li of MIT, Moorman said: “There is no reason for any kind of panic, but nuclear technology has risk in any case. A realistic understanding of those risks is essential, especially for operators, and so we urge caution and a spirit of scientific inquiry in the operation of HTR-PM.”
Moormann, Kemp and Li’s main concern is that because pebble bed reactors are regarded as intrinsically safe, HTR-PM has been built without a high-pressure, leak-tight containment structure to serve asbackup in case of accidental release of radioactive material, and it also does not have a redundant active cooling system.
“The absence of core meltdown accidents does not mean that a dangerous event is not possible," said Moormann. One issue that has arisen with prototype pebble bed reactors is that localised hotspots can form in the core and unexpectedly high levels of radioactive dust have formed. Moormann also believes that such reactors produce a higher volume of radioactive waste than conventional reactors, although proponents of the design argue that the waste remains locked within the layers of ceramic and graphite that form the fuel pebbles.
To reduce risk, the paper’s authors advise that precautionary steps should be taken. These should include continuous monitoring, the insulation of containment and cooling systems, and an extended start-up phase to allow the reactor to be observed and monitored as it comes up to operational temperature. They also advise against the current plan of storing fuel waste in above-ground canisters.
"There was already some controversy about pebble-bed HTGRs, but my impression was that many problems of them were not sufficiently published and thus not known to some of my colleagues," said Moormann. "I hope that the pros and cons will be broadly discussed."