Passive Cooling in Nuclear Reactors

Yes ; In pool type fast breeder reactors, normal heat removal is through secondary sodium circuit with sodium pump circulating the coolant.

When these pumps are not available , reactor will be shut down and decay heat of nuclear fission will be removed by natural circulation (no pump or rotating part or active part) and hence passive cooling; Passive refers to absence of active parts

With regards

If I recall my reactor physics correctly, in a Westinghouse 4-loop pressurized water reactor, with the steam generators situated higher than the core, it's possible to achieve up to 10% power on natural circulation.

If I remember correctly, they eventually shut off the circulating pumps after the three mile island incident. Their reasoning was the large pumps were actually creating more heat than they were removing during the final cooling stages, the analogy they used at the time was the model T circulating system.

I haven't checked recently, however a article I read several years back stated, the French were either investigating or incorporating encapsulating there fuel in ceramics. Supposedly even a complete loss of cooling would not result in a catastrophic melt down.

The previous post mentioned partial power being available from reactors without pumps running. I believe that some nuclear subs are designed that way, they can run at reduced power without the noise of the pumps.

As far as thermal circulation I can personally attest to it. 20 years ago I bought a dodge with a 440 B engines. Basically a parts car, however I drove it a few weeks before I disassemble it. They heater never worked, and the engine temperature would rise to about 200° in and settle back down to about 180 degrees. When I eventually disassembled the engine I discovered that water pump impeller had completely rusted away, I was cooling with a model T circulating system.