The major safety issue regarding nuclear reactors lies in how to cool them efficiently, as they continue to produce residual heat even after shutdown. Gas-cooled reactors discharge surplus heat and don’t need additional safety systems like water-cooled reactors do. The HTR-10 was subject to a test of its intrinsic safety in September 2004 when, as an experiment, it was shut down with no cooling.
Fuel temperature reached less than 1600 C and there was no failure.
“Using the existing operating HTR-10 reactor at the Institute of Nuclear and New Energy Technology of Tsinghua University in Beijing, we have already done what would be unthinkable in a conventional reactor—we switched off the helium coolant and successfully let the reactor cool down by itself,” said Wu.
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More on pebble bed reactors:
There have been some articles that are critical of the design and safety of pebble bed reactors and claim the demise of pebble bed reactors.
Here is the rebuttal of the criticism of pebble bed reactors. Dr Albert Koster, PBMR (Pty) senior consultant, nuclear safety, replies directly to criticisms of the PBMR reactor.
....In respect of the pebble bed modular reactor, we put all technology options in our generation planning process only once they are proven, and the PBMR must still be proven."