Lots of stupidity, no criminal intent. I don't think he should goto jail.
This is such a one-off case that it doesn't need swooping examples quite as much, but the same logic stands - if something can have terrible consequences, to the extent that the government should be preventing it, just telling people "please don't do this" likely isn't enough.
I'm not up to speed on the particulars of Swedish law, but I imagine it has similar rules about reckless endangerment. Your DUI example is a good one. But we don't even really need specific examples, because the fact is, certain crimes are still crimes, regardless of whether or not the perpetrator had intent to cause harm. A reckless disregard for the safety of others can produce results every bit as harmful as those stemming from specific criminal intent.
Let's put it this way: anyone savvy enough to obtain radioactive elements and attempt to cause nuclear reactions in his apartment is also smart enough to understand how dangerous those materials are to himself and to those around him.