Radioactive footprints

"A grad student in a lab down the hall attempted to snap freeze a tube containing radioactive protein. Some of the liquid nitrogen leaked into the tube, and it exploded. Shortly thereafter, a series of radioactive footprints (from at least two people) was discovered that trailed down the hall and into a common room. The entire hallway and freezer area for ten labs were blocked for days while cleanup crews worked. Months later, this single spot of radiation remains, somehow impervious to countoff."