The bomb seems to be a "gun" type device, where a projectile of fissionable material is rapidly propelled into another mass of the same material to produce a critical mass and an explosion. However, the device is said to work with plutonium, which fissions too rapidly to function in this type of bomb.
After the FBI declares the device to be "classified," the agent abandons it in the squad room instead of making sure that it is safeguarded.