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|User Info||Something To Contemplate This Weekend; entered at 2018-07-06 22:03:02|
Registered: 2010-02-12 Bremerton, WA
Good thought experiment. But - there are a variety of factors that will determine what the ultimate outcome will be. [This is where my nuclear engineering, courtesy the US Navy, comes into play...]|
If we assume that grams of enriched U235 are added slowly - like slower than 1 gram per second - we have the potential of a criticality accident eventually. This is where a reactor goes 'super-critical' briefly - generates a lot of heat and radiation - and the 'pile' probably gets destroyed - and it might get 'distributed' like a small Conventional bomb. It will not go super critical where the assembly acts like a nuclear bomb.
The US had a criticality accident in 1961 - when workers on an Army reactor created a 'prompt critical' event - and the 3 workers were killed.
[ https://en.wikipedia.org/wiki/SL-1 ]
Japan has had two criticality accidents at their research facility (Tokaimura) - 2 workers killed in 1999. Too much fissile material in a small space allows a sudden output of power...high enough to destroy stuff, irradiate people near by...but it doesn't explode like an atom bomb. However - the criticality event at Tokaimura is the likely outcome of Karl's thought experiment based on bringing more and more U235 together - but not at 'near instantaneous' speeds.
For Karl's thought experiment - the level of criticality would also depend on the side walls of the box and the bottom...whether they would act as a 'reflector' to 'bounce' neutrons back into the box - or if they didn't reflect neutrons back. As grams are added - the 'pile' would approach criticality - where k(effective) approaches unity - and maybe goes slightly above it. The pile starts having more and more fissions; k-eff of 1 - the power stays constant. k-eff>1 - it ramps up and up and up - each 'generation' - more fissions than the previous. All the fissions release heat. The uranium will get hotter - the 'pile' expands....more and more radiation is released...and the heat might destroy the box sides if they are not resistant to extremely high temperatures.
OTOH - if you wanted to make this a bomb - you would create 2 'piles' of subcritical mass - and bring them together extremely rapidly - [like the bomb used in Hiroshima - August 6, 1945, which had 141 pounds of highly enriched Uranium.] If you tried to bring them together but the speed was not great enough - the pieces would start heating up - and the pieces might not fit together - and the likelihood of a getting a high yield nuclear explosion is much lower. [Plutonium bombs can't be built to explode in the same way - bringing 2 pieces together rapidly to create a super-critical nuclear yield explosion can't be done practically - the required speeds are too great; Plutonium bombs use 'implosion' to squeeze a sub-critical mass to a higher density that makes it super-critical.]
AND - for those not yet bored by looking at the science of reactors - consider that the earth has evidence of a natural reactor - one that operated about 2 billion years ago - in Gabon Africa. (Google Okla natural reactor for more details.) When uranium mined from an area 'rich' in uranium in the Okla region, the uranium had a much lower than expected content of U235. (Normally - natural uranium - only 0.7% is U235, the rest is U238. What was found in that region was MUCH lower...)
AND- with the engineering discussion over- the idea of a government doing something absurdly stupid that will result in a high level of danger to people - the government must be stopped. Somehow. It must be stopped - or people near the stupidity must get far far away...while warning people that this is a dangerous situation and they too should try to pitch in and stop the government - or they too should flee the area.