Since the Fukushima Daiichi catastrophe more than two years ago, we have been reading of a mysterious "black substance" that tends to show up in roadsides and other random places and shows very high levels of radioactive emission. Believe it or not, nobody until now had studied it with anything else than a geiger counter.
In an interview at Fairewinds (Dr. Gundersen's site), Prof. Marco Kaltofen, President of the Boston Chemical Data Corp., explains that his team took samples to analyze, detecting high amounts of cesium isotopes but also of radium-226, a product of uranium disintegration. They do not have the means to detect the uranium directly but they strongly suspect its presence in the "black substance":
... this particle contains not only fission waste products from the reactor but very likely contains a concentrated unburned nuclear fuel. And that’s unusual. This sample had by far the highest level of uranium daughters that we’ve seen in a dust or soil sample. We’re actually seeing material that might well have come from inside a failed fuel assembly.
Gundersen interprets it as being clear evidence of the containment being breached in the explosions of March 2011.
There is a high risk of ingestion, especially among children and agricultural workers.