It was my impression that the 2000 C is a "magic" stop if you loose coolant, due to thermal broadening of the neutron spectrum. It would have to be 4000 C to be a danger to the carbide? That is why they are considered passively safe.
"There was a pebble bed reactor accident at Hamm-Uentrop West Germany nine days after the Chernobyl accident. On May 4 1986, a pebble became lodged in a feeder tube. Operators subsequently caused damage to the fuel during attempts to free the pebble. The West German government closed down the research program because they found the reactor design unsafe." Sounds like they shut it down due to panic after the Chernobyl accident. The problem doesn't sound like it has to do with the reactor design in general but rather that that specific reactor had under dimensioned feeder tubes.
I don't really understand why people don't like the battery swap solution, wouldn't prevent people from charging at home at the same time. The key social benefit would be that you can charge batteries during peak production. For the individual the benefit is that you can fill up the tank in a few minutes compared to the 20 or so hours you need for a full charge now (which means you solve the range problem). You also spread out the cost of a new battery so that people doesn't have to save up for a large investment in a new battery.
The THTR is/was at Hamm-Uentrop. After the incident they found damage to the pressure vessel made from reinforced concrete, probably due to excessive temperatures. There were several reasons to shut down the reactor, which was a prototype to show a commercial size pebble bed reactor. One was the damage to the reactor. Other where high costs (it was not economical - not a surprise for a prototype), the lack of further fuel supply (they needed special highly enriched Uranium and also the difficulty getting rid of the waste - the reprocessing planed was never started for cost and possible other reasons. So at least 3 good reasons to shut it down, even without an accident in Russia. The higher than expected friction and wear on the fuel was a more general problem with the pebble bed design.
There is some thermal self limiting of the fission reaction somewhere lower maybe 1000 C (it depends on the setting of the rood used for regulation and the fuel used) for the average temperature. However there would be still the decay heat from fission products and new breed fuel to decay. This heat is sufficient to raise the temperature even further even well above 2000 C. AFAIK the 2000 C was the design value for the fuel should withstand, but experience showed problems (e.g. emission of fission products) already earlier. So The safe limit would have to be lower. At somewhere around 3000 C it would expect start of catastrophic damage with softening / melting of the fuel.
For the battery swap solution, one problem is how to charge wear and tear of the battery. There is not that much experience. Another problem is that it would need a kind of standard type of battery used in different cars. This would also add weight and be less flexible in the form factor. One may choose to have something like 1 pack in a small car, 2 in a large one and maybe 4 in a bus - so some flexibility, but still limed. Also once a standard is set this would be set at a time when the technology is still evolving. Also having enough of the full battery packs at the right station could be a problem. So there could be still the problem of coming to the station and still need to wait for charging. AFIAK handling the batteries also would need special training for electrical safety. Some 300 V or more DC are not fun, as an arc would stay on and fuses are tricky.
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