Cool article. Reading up on history of this stuff, this was a problem that was hard to understand even in the 40-50s. This caused a lot of issues where people irradiated themselves by pouring the wrong stuff down the wrong drain, or leaning over and acting as a neutron deflector at the wrong time. If you are interested in this stuff, wikipedia has a list of criticality accidents.
I don't think people weren't aware of the danger involved, eve in the 40s/50s. Especially as the experiments mentioned in the article were performed especially to get better data about the criticality state. If they had known nothing at all the spheres would have started the chain reaction much earlier, or not at all...
IAEA has quite a lot of reports on criticality accidents, just search their library[]. It's almost always sloppy experimenters not following their rules to get their work done faster and "easier". So, even if I don't get to work with fissible(?) material, it's a good lesson to learn for everyone working with dangerous machinery.
Most of their reading material is very good, and it's all free downloads!
http://en.wikipedia.org/wiki/Criticality_accident