See here
http://www.wise-uranium.org/eftokc.html
quote from article
"The amount of uranium contained in this tank must be strictly controlled to avoid criticality. These controls however failed, and 16 kg instead of the permitted 2.4 kg were poured into the tank. About 6 kg were already sufficient to initiate a criticality under these circumstances."
This is what happens when you have untrained people doing a highly technical job and decide to take 'obvious' shortcuts.
Bert