Yellowcake

Yellowcake is not a delicious flour-based sweet. It is a step in the processing of raw uranium ores. While it's often called U3O8, it actually consists of multiple different uranium oxides.

Most yellowcake is further processed - first into UF6 to separate some uranium-235, a process known as uranium enrichment, and then into UO2, which is used in nuclear fuel.

How it's made
The first step to baking making yellowcake is "leaching" out the uranium, that is, separating it from the rock containing it.
 * Traditionally, the uranium ore was mined out of the ground, whether in an open pit or underground mine, crushed into powder, and dissolved in sulfuric acid. This mixture of acid and ore is then filtered to remove all the now barren rock and other unwanted materials.
 * Nowadays, more than half of the world's yellowcake is made by in-situ leaching (ISL), also called in-situ recovery (ISR) or solution mining. It's similar to the previously described process but instead of taking the uranium ore out of the ground, crushing it up, and dissolving it in sulfuric acid, people leave it in the ground, pump in water and certain chemicals depending on the local geology to dissolve the ore, and pump out the resulting solution.

In either case, the uranium then is stripped" (isolated) by either running the solution through a matrix of resin/polymer beads and then adding some sort of acid or treating it with a special kerosene-based solvent, depending on what sort of ore the uranium came from. Following that, they precipitate the uranium, i.e. gather the uranium in the liquid into a solid form, using certain chemicals, again depending on the ore the uranium came from. Finally, the resulting slurry is put in an oven to dry out. The stuff leftover at the end is yellowcake.

Appearance
Despite the name, yellowcake is not always yellow. The temperature it dried at can affect how much water still remains and what impurities are leftover, which both affect its color. It's possible for yellowcake to be more of an orange or even dark green that almost close to black, and even when it turns out yellow, it can be multiple different shades.

Hazards
Despite the delicious-sounding name, yellowcake is not for eating.

As a whole, yellowcake is low-risk. Most of the hazards come from the chemical processes for obtaining yellowcake and the chemical toxicity of the substance itself, rather than radiological effects (i.e. standing in a room with it is fine, but don't snort it or stuff some down your throat.) The radiation it emits is very mild; you'd get more radiation from being exposed to cosmic rays during a flight than you would from standing next to a barrel of yellowake. The radium in the waste left over from processing ore from underground and open pit mining into yellowcake is usually a bigger radiological hazard.

In addition, there is very little risk of the uranium reaching criticality, because the uranium is not enriched enough to create a self-sustaining nuclear reaction.

Yellowcake can be transported in regular ol' 200 liter/55 gallon steel drums in standard shipping containers, and there's no need to use radiation protection so long as it's kept in the drums and the drums are kept in the shipping containers. Those drums should also have holes drilled into them, so that pressure doesn't build up and cause the barrel to literally blow its lid. Compare it to the practices for transporting nuclear waste, which includes keeping it in water pools for a few months, putting it into concrete casks, and loading it onto specially-designed ships, or those for some of the really nasty fission products leftover from reprocessing, which is basically turned into molten glass, poured into a steel container that's welded shut afterwards, and then put into a cask.

Role in international affairs
Since yellowcake can be (and usually is) enriched into fuel for a (uranium-based) nuclear reactor, a state buying some is usually interpreted as evidence they're developing a nuclear energy program, and with that come concerns they might be trying to obtain material for nuclear weapons, much like with hiring nuclear scientists or building a uranium enrichment plant. Hence why it was such a big deal when, in the 1960s, it was discovered that Israel, which at the time was very hush-hush about where it got its uranium and whether they really had the ability to manufacturer nuclear weapons, had brokered a deal with France and Argentina to buy some yellowcake.

The yellowcake forgeries
In the early 2000s, Italy's intelligence agency, SISMI, uncovered some documents that showed how Saddam Hussein tried to buy yellowcake from SISMI passed the info to the British intelligence agency MI6, who then passed it to the United States. Based on these documents, the US accused Iraq of developing nuclear weapons and sent troops to the country under pretense of disarming its WMDs. President George W. Bush declared in his 2003 State of the Union address that "The British Government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa", a statement that has gone down in history as the infamous "sixteen words".

There was just one problem: these documents were forged.

In 2008, the United States discovered and removed 550 metric tons of yellowcake from Iraq's Tuwaitha Nuclear Research Center. Was this proof that the yellowcake forgeries weren't actually forgeries after all? Nope, this yellowcake was imported well before the dates described by the falsified documents. Not only did it predate the 2003 Iraq War that the forgeries spurred, but they also predated the 1991 Gulf War too.