Safety of a pebble bed reactor.

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vanesch
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I heard and read several times that a pebble bed reactor is "safer" than a PWR or a BWR. However, I fail to figure out how. After all, light-water reactors also have negative temperature coefficients, which seem to be the "invention" that makes the pebble bed reactor "inherently safe". But I fail to see what's so fundamentally different between this type of reactor, and the old graphite-gas cooled reactors by the UK and the French, except that now, the reactor core is made out of pebbles instead of blocks...
 

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Astronuc
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Well, I had a nuclear reactor physics prof who specialized in HTGRs and his comment was, there could be an accident, e.g. LOCA, and one could scram the reactor, then go to lunch, order a nice bottle of wine and take one's time to figure out how to resolve the problem because the core wouldn't melt, and it could maintain its integrity even without forced cooling during the decay heat period. I don't imagine the nuclear regulators would take such a relaxed and casual attitude, but there is a certain level of confidence that the core materials can handle high temperatures without loosing integrity and releasing fission products to the environment.
 
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Well, I had a nuclear reactor physics prof who specialized in HTGRs and his comment was, there could be an accident, e.g. LOCA, and one could scram the reactor, then go to lunch, order a nice bottle of wine...
I always thought this was the russian way to handle a reactor accident (just using vodka instead of wine....)

seriously, what about one other favourite worst-case scenario: suicide-terrorist-flies-747-sized-aircraft-into-reactor ? would it provide a safety advantage in this case too ?
 
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Astronuc
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seriously, what about one other favourite worst-case scenario: suicide-terrorist-flies-747-sized-aircraft-into-reactor ? would it provide a safety advantage in this case too ?
Western plants are designed with containment structures surrounding the pressure vessel and major components of the primary circuit. These have been tested against aircraft impact, and the modern plants are designed with that as a consideration.
 

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