Fissile Yield
It appears his experiment was a success. He worked at CMU in Chemistry, specifically the area of supercooling things.
On a whim he decided that it would be interesting to supercool something radioactive. That way he could study the subsequent heating "in slow motion" atom by atom.
He needed something very stable but radioactive. The best option seemed to be Uranium Hexafloride gas. Florine combines with anything and is very stable so any defrosting would be very slow. The radioactive Uranium would be easy to track.
The experiment was expected to last several years so he kept adding more of the gas, cooling it and compressing it into a softball sized, very dense solid. The whole experiment was put in the lab closet as there was literally nothing to see.
He was very excited when he raised the temperature. The softball stayed a solid all the way up to room temperature! This made the measurement process much easier.
In April he built a measurement device in the corner of the lab. It measured the radioactive gas released in great detail. In May he found that the ball was ever-so-slowly evaporating. He estimated that the process would take about a year.
Bob got a summer sabbatical to work at CERN. He would receive daily reports from the experiment but was too busy to analyze them. But he could see that the ball was slowly shrinking exactly as predicted.
One day the daily report showed no results. The video stream showed that someone had opened the experiment and taken the sample. It seems a grad student needed some of the measurement hardware for another purpose and didn't know it was still an active experiment. The ball was just left on a lab table and eventually thrown out.
When Bob finally analyzed the data he discovered that the Uranium Hexafloride released from the ball only contained U238 atoms. The U235 remained in the slowly shrinking ball.
This seemed like a great breakthrough for Uranium enrichment. Now the centrifuges were no longer needed.
Unfortunately, as Bob pointed out, somewhere in the city there is a ball of Uranium that is slowly shrinking, slowly enriching to pure U235. And, at some point, it might reach a concentration sufficient for a fissile yield explosion ... of several city blocks.