But uranium isn’t the fuel for reactors, enhanced uranium is.
Coal is the fuel for coal-fired power plants.
Coal is obtained from the ground with no enhancement.
Nuclear fuel, obtained originally from the ground, is concentrated (enhanced).
So, a pile of nuclear fuel is more dangerous than a pile of coal, yes?
Yeah. So?
Ammonia is dangerous to humans. It is manufactured. Hydrogen and nitrogen, neither of which are lethal to humans, are its components.
Uranium mined out of the ground is not particularly dangerous. Once refined into a more purified form then yes, it has a greater danger associated with it.
There are thousands of compounds we make and industry produces all sorts of nasty stuff. Some are flat out lethal to humans.
Uranium is no different. Indeed there are some things industry produces that makes uranium seem downright mild. Our only real fear with uranium is bad guys get enough refined stuff to make a nuke.
Well, if the entire core isn’t submerged, you’re probably screwed. So, in that sense, you are right - you can’t pump in more water to a completely-full housing. However, you can increase the rate of water circulation, which will result in faster cooling. This is what people mean when they say “more water.”
You can change the amount of water, by changing its density. You do this by changing the temperature. Flow the water through quickly and it will be colder, because it is spending less time being heated by the core, therefore more dense. Flow the water through slowly and it heats up more, becoming less dense. A reactor designed this way will have a negative thermal coefficient - the hotter it gets, the less dense the water, the less moderation, and the less power will be generated.
The Chernobyl reactor used graphite for moderation, and the moderation from water would actually over-moderate the core. It therefore had a positive thermal coefficient - the hotter the reactor, the more power generated. This was a very, very bad design. No modern reactor would be built that way.
But you could do that with anything. There is no nuclear waste 3 feet away from me, but there is a smoke detector that may be giving me a minuscule dosage of radiation, therefore it’s more radioactive than nuclear waste?
Nice explanations, minor7flat5 and AndrewL.
Naturally occurring uranium is more than 99% U-238, which has a half life of 4.468 billion years. The remainder is primarily U-235, which has a half-life of 700 million years. With such long half-lives, neither isotope is particularly radioactive. You could hold pure U-238 and U-235 in your hand with no ill effect (so long as the latter is a subcritical mass). (Note that a critical mass of pure U-235 is a sphere with a mass of 52 kilograms, which corresponds to a solid sphere with a diameter of 17 cm.)
Commercial reactor-grade uranium is enriched to about 3-5% U-235. Again, so long as your “pile” of uranium was less than a subcritical mass, it is as safe as most any other heavy metal. (But I’d still want to wash my hands thoroughly after handling it, and I certainly wouldn’t want to ingest it.)
Your terminology is kind of amusing, since the first nuclear reactors were actually called nuclear “piles.” 
Are you more worried about criminals stealing your car or getting at the gold at Fort Knox?
The Americium-241 contained in smoke detectors, with its half-life of 432.2 years, is far more radioactive than the uranium used in nuclear fuel, and also more radioactive than many types of nuclear waste. On the other hand, a typical smoke detector only has about 0.29 micrograms of Am-241, so I wouldn’t really worry about it.
On the other hand, I wouldn’t eat it, either, nor would I gather up all the Am-241 present in hundreds of smoke detectors, like this idiot did.
Thanks for the link. That was an interesting read.
You’re welcome.
Here’s another link that you might like (caution: Large PDF): http://www.inl.gov/proving-the-principle/chapter_15.pdf
Goes into much more detail than Wiki.
Hey, minor7flat5, this is a bit off-topic, but related. The Navy uses nuclear reactors on warships. Warships tend to go into inhospitable places and people might shoot things at them. How does the Navy keep reactors safe in the event of real shooting, close-in combat? If a torpedo hits the ship, what happens?
The reactors are in the center of the ship. Between the hull and the reactor core, there are various tanks and voids, a layer of armored steel, and then the shielding starts: several inches of steel, lead, and polyethylene.
If a torpedo explodes in such a way as to damage the reactor, it probably also ripped open the engine rooms (the largest compartments in the ship) and the ship is doomed anyway—it has always been my belief that if there are reactor problems from battle damage, then there are likely far worse things going on outside.
I imagine that a violent explosion could possibly scram the reactors (drop the rods, shutting down the plant), causing loss of propulsion. That might be a problem in the heat of combat.
Perhaps a lucky hit from an armor-piercing bomb or shell might make make it to the core—like those ones from WWII that were dropped on carriers that would pierce through so many decks before exploding.
The reactor will not (can not) nuke itself.
At worst you tear it open and make a radiological mess.
That is bad in its own right but no nuclear explosion would happen.
(This was mentioned earlier, roughly [not a direct quote], by Chronos but I am losing track of who said what in which thread)
The U.S. Navy tries to balance robustness (keeping the reactor operating during so-called “tactical situations”) with safety.
Years ago, when the USS Thresher was lost (due to flooding in the engineroom because of a break in a pipe that connected to sea), reactor safety was such a high priority that the reactor was immediately scrammed (shut down). This proved to be a problem, as the sub then had no propulsion with which to try to drive to the surface, and no procedure for quickly restarting the reactor.
Along with many other changes made to submarine design (i.e. the SUBSAFE program), changes were made to operating procedures, including NOT shutting down the reactor during a flooding situation, and fast recovery startup procedures.
So far as battle damage is concerned, a reactor pressure vessel is much more robust than the submarine hull itself. As minor7flat5 states, any battle damage sufficient to damage the reactor is probably enough to sink the submarine.
If a submarine were to flood and sink, there are many passive systems to ensure reactor safety (i.e. to minimize damage to the environment from the doomed sub). For example, the control rods are held out of the core by electromagnets. If all electrical power is lost (as would be the case during such a casualty), the rods are automatically driven back into the core by gravity and powerful springs. This would shut the reactor down.
Cooling of the residual decay heat would be effected by the seawater filling the submarine.
The Navy has conducted periodic monitoring of both of the U.S. nuclear submarines that were lost (USS Thresher in 1963 and USS Scorpion in 1968), and has not detected any significant impact to the environment due to radioactivity from the subs. The reactor fuel assemblies are still reportedly intact in both cases.
Incidentally, the cause of the sinking of USS Scorpion has not been definitively determined, but the resulting debris field are consistent with a torpedo explosion (most likely from accidental detonation of one of Scorpion’s own torpedoes).
Note that neither submarine was lost due to a malfunction of its nuclear reactor.
The cores are well shielded.
But nothing is forever.
What happens when normal deterioration breaches the core?
No environmental impact today does not imply no impact tomorrow.
Radioactive materials, including nuclear waste, either have long half-lives or short half-lives. The shorter the half-life, the more radioactive the substance, but also the shorter period of time it persists in the environment. The primary constituent of the fuel in nuclear reactors (U-235) has a half-life of 700 million years, which means that it’s not actually all that radioactive.
Many of the daughter nuclides produced in the nuclear reaction have much shorter half-lives. The most radioactive of these (with half-lives of decades or less) will have already decayed in the time since the subs sank in the 1960s. So the worst stuff is already gone.
Of more concern are radioactive materials with half-lives in between these extremes. Nevertheless, with the most radioactive materials already having decayed, I do not think there is much danger from the cores in their current location, but I cannot attest to this definitively, as the long-term fate of reactor cores on the bottom of the ocean is outside of my area of expertise.
I do know that there is a lot of radioactive material (including from naturally occurring sources) already in the world’s oceans, but it is not of much concern, because the oceans are large, and one “solution to pollution is dilution.” On the other hand, we also no longer intentionally dispose of nuclear waste at sea, and above-ground (and below-water) nuclear bomb tests are no longer conducted.
Fukushima Nuclear Accident – a simple and accurate explanation
It’s 5000 words, but that’s what’s required just to get to simple.
Here’s actually a decent “layperson” explanation of how nuclear reactors work:
http://zidbits.com/2011/04/04/how-do-nuclear-power-plants-work/
~TJ 
I have long wondered why they don’t put a concrete ‘‘waffle’’ under the reactor core. The melted core would distribute itself into small pockets. Maybe add some boron or graphite. Isn’t the problem of a melt down that the fuel that had been in rods melts and gathers in a blob?
How would the “concrete waffle” not melt? If the core is hot enough to melt through the floor of the containment structure, I don’t see how concrete is going to put up any more of a challenge than butter.
Incidentally, what is the floor of the containment vessel made up of? I’d assume several meters of concrete, but also steel or what?