First off, he did not build a reactor. That describes a specific piece of nuclear engineering technology that involves being able to create a self-sustaining fission reaction. He was nowhere even close to that.
This is just a case of a kid not knowing what he was doing randomly experimenting with low level radioactive materials. People like to pretend that he was a genius, but he is closer to a Darwin award candidate (for playing with beryllium). He tried to make an Am-241/Be neutron source but he never read up on the efficiency of a neutron source produced by alphas hitting beryllium. It is extremely inefficient (about 70 neutrons per 10^6 alphas). And the efficiency of activating Th-232 isn't high either (with a cross section of only about 7 barns). He never had decent results because each 1 microcurie smoke detector source would emit at maximum about 2.6 neutrons/s in random directions--if it was perfectly installed by an engineer or physicist. You can do the math but even with 50 smoke detectors the probability of activating thorium with one neutron and then being lucky later with another neutron to fission U-233 is a pipe dream even if he let the experiment run for years.
Eventually the government had to come in and remove all of the low level waste he was playing with.
‘When David’s Geiger counter began picking up radiation five doors from his mom’s house, he decided that he had “too much radioactive stuff in one place” and began to disassemble the reactor.’
http://www.dangerouslaboratories.org/radscout.html
So is this article bogus and why is his skin fried? Sure is not acne. I’m no physicist, just curious. He looks worse than I did after a month under a Varian Linear Accelerator for treatment reasons. My background is 30 years in Radiology, so I find this interesting.