This wasn’t a nuclear reactor like you have at power plants or whatever, it was a californium-252 subcritical neutron multiplier. Meaning that not only was there less that one-thirtieth of the (not weapons-grade) uranium that would be required to make a nuclear bomb, there wasn’t even enough to undergo a nuclear chain reaction. It couldn’t melt down or explode or anything like that, it didn’t need a cooling system or regulating system. It was just a few chunks of metal that sat there and were only dangerous if you stood too close to them for too long; being in the room above this device would have been as hazardous as being in a room built over a bowl of pudding. Less hazardous, in fact, since the pudding could attract disease-carrying pests.
There’s a method of testing to see what chemical elements are in tiny samples of material, and it’s called neutron activation analysis. Basically, you throw neutrons at the sample until the atoms in it have absorbed some of them. This will cause certain atoms in the sample to become slightly radioactive, and they’ll give off a characteristic amount of gamma rays. You can then use gamma ray detectors to measure the amount of these rays and to figure out what types of atoms were giving off the rays (this is called trace multi-elemental analysis). This is a very accurate method for testing very small amounts of substance, and is used for all sorts of things.
To do this, you need to have a source of neutrons. Californium-252 is a radioactive isotope that is good at giving off neutrons. It is also extremely expensive (having to be made artificially due to its scarcity in nature), with the Atomic Energy Commission selling it at over $60,000 for a milligram. If the uranium Kodak owned had been californium, it would have cost about a hundred billion dollars. And since californium-252 has a half-life of less than three years, they would have had to keep on buying more and more. Obviously, Kodak and other people who use neutron activation analysis will want to use as little californium as possible and to get the most use out of it that they can. That’s where the much less expensive (relatively speaking) uranium comes into play. The uranium is shaped into slabs and put around the californium, and when the neutrons from the californium hit the uranium it makes even more neutrons due to the uranium atoms being unstable and falling apart. Then all of these neutrons go flying along and hit the sample, and the detectors do their detecting of the gamma rays.