Sealed with concrete and flooded with water after World War II, the Congolese mines produced the uranium used for Americas World War II atomic bomb project. But in the last few years, thousands of Congolese miners have begun working the mine again. They are not looking for uranium, but more easily salable minerals like copper and cobalt. But there appears to be a market for the uranium ore. Someone in the vicinity of the mines is refining tons of dirt (ore) containing uranium to produce yellowcake (uranium oxide.) To do this, the dirt is crushed and bathed in sulfuric acid, dried and filtered to produce a yellowish powder called yellowcake. The yellowcake must then be processed further, using anhydrous hydrogen fluoride and fluorine gas, to form Uranium Hexafluoride. This stuff, while not very radioactive, is nasty and must be kept in air tight containers. If Uranium Hexafluoride gets in contact with water, if forms poisonous and corrosive gasses. Uranium Hexafluoride is used to obtain the highly radioactive Uranium 238 material that is used in nuclear power plants and nuclear bombs. Yellowcake is not very radioactive, a bit more so than the granite commonly used as a building material (and not considered a health hazard.) However, because yellowcake is a powder, it can easily be inhaled. This is dangerous in the long term. Even workers in uranium mines developed higher rates of cancer years later because they inhaled the dirt containing uranium.
The Democratic Republic of the Congo's long dormant uranium mine is operating again, illegally. It is feared that the illegal mining, refining and export of minerals may include the production of radioactive material suitable for a dirty bomb (explosives surrounded by radioactive material to contaminate a large area and cause panic.)
The problem with mining and refining operations in Congo is that they can take place outside the view of anyone who might oppose such operations. Such illegal operations flourish in Congo because there are buyers. Since there is a market (an illegal one) for radioactive materials, there's the possibility that some one is refining the uranium ore sufficiently to produce radioactive material suitable for a dirty bomb, or worse.