Nuclear Inspection Van was a vehicle used by inspectors from the Nuclear Regulatory Commission. The term "van" is a bit of a misnomer, as the vehicle was approximately the same size as a mobile home RV.
The van contained a simulated workstation which the inspectors used to evaluate the competence of nuclear power plant employees. When Homer had his turn in the simulator, the supervising inspector assured Homer that the simulator was an exact replica of his real-life workstation in the nuclear power plant and that nothing could go wrong. Even so, Homer managed to cause a meltdown that released massive amounts of radiation and melted a hole in the parking lot large enough to swallow up the van as well as temporarily irradiate Homer Simpson and leave him with a desire for the destruction of mankind before his watch beeped to notify him that his lunchbreak had arrived. This left the inspector completely baffled, because the van didn't have any nuclear material in it.