# Nuclear reactors: cure my ignorance **URL:** \ **Category:** Factual Questions\ **Created:** [March 12, 2011, 5:16pm UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424 "2011-03-12T17:16:20Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [March 12, 2011, 9:43pm UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/21 "2011-03-12T21:43:13Z")
> [@Whack-a-Mole](#): > > You can hold uranium in your hands with no ill effect. Indeed it is still possible to buy jewelry or pottery made from uranium (has a nice, yellow color). > > There are some natural “reactors” in the world where a low-level nuclear reaction occurs. Mostly though uranium is mined much like any other metal and does not require some special protective efforts. 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? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [March 12, 2011, 9:49pm UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/22 "2011-03-12T21:49:14Z")
> [@Musicat](#): > > 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. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 12, 2011, 9:57pm UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/23 "2011-03-12T21:57:40Z")
> [@Whack-a-Mole](#): > > Ok…I can see that. > > But you have a reactor with long fuel rods. > > I presume they all need cooling, top to bottom. > > So you pump in water to keep things in check. > > Now you want to control the rate of reaction to control the power being generated. > > If the fuel assemblies are already immersed in water how does “more water” change anything? > > It was said above that rods are not used to throttle reactor output so to me that leaves the water to do it. > > I am not arguing here, I am just laying out my confusion on the subject. Obviously the power plants work. I am just wondering at how they do it. > > ETA: An answer was given above while I wrote this but still not seeing how water is everywhere in the core to keep things cool and can be increased or decreased. Either you have water cooling or an element is uncovered. Seems a binary choice. Water or no water. You can’t have “more” water between two points. It is there or it isn’t. 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.” ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [March 12, 2011, 10:55pm UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/24 "2011-03-12T22:55:31Z")
> [@Whack-a-Mole](#): > > ETA: An answer was given above while I wrote this but still not seeing how water is everywhere in the core to keep things cool and can be increased or decreased. Either you have water cooling or an element is uncovered. Seems a binary choice. Water or no water. You can’t have “more” water between two points. It is there or it isn’t. 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. ---
**Author:** ![Fubaya](https://avatars.discourse-cdn.com/v4/letter/f/c2a13f/32.png) [@Fubaya](https://boards.straightdope.com/u/Fubaya)\ **Post date:** [March 13, 2011, 2:28am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/25 "2011-03-13T02:28:36Z")
> [@billfish678](#): > > Well, given that coal ash is allowed to float through the air willy nilly while radioactive waste is treated like nuns would treat used porn magazines, I think the fact the comparison of the effects of the two substances, even though they are contained differently, is quite fair. 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? ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [March 13, 2011, 4:11am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/26 "2011-03-13T04:11:53Z")
Nice explanations, **minor7flat5** and **AndrewL**. > [@Musicat](#): > > But uranium isn’t the fuel for reactors, enhanced uranium is. > > 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? 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](http://en.wikipedia.org/wiki/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.” 😉 ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [March 13, 2011, 4:26am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/27 "2011-03-13T04:26:32Z")
> [@Fubaya](#): > > 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? Are you more worried about criminals stealing your car or getting at the gold at Fort Knox? ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [March 13, 2011, 4:27am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/28 "2011-03-13T04:27:32Z")
> [@Fubaya](#): > > …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? 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](http://www.dangerouslaboratories.org/radscout.html). ---
**Author:** ![levdrakon](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@levdrakon](https://boards.straightdope.com/u/levdrakon)\ **Post date:** [March 13, 2011, 4:34am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/29 "2011-03-13T04:34:13Z")
> [@beowulff](#): > > BTW, I have a somewhat perverse fascination with these scenarios. In the early days of reactor design, the cores were not engineered with the many of the fail-safe (or at least fail-less-catastrophically) features of current reactors. One experimental reactor, the [SL1](http://en.wikipedia.org/wiki/SL-1), failed (for reasons that will never be know precisely, but probably because of really bad design and human error) in such a way that it produced many, many times it’s rated power in a few thousandths of a second. This basically turned it into a “steam bomb” - it overheated so fast that it shot the reactor core out of the housing like a ball out of a canon. Thanks for the link. That was an interesting read. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 13, 2011, 4:41am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/30 "2011-03-13T04:41:49Z")
> [@levdrakon](#): > > 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](http://www.inl.gov/proving-the-principle/chapter_15.pdf) Goes into much more detail than Wiki. ---
**Author:** ![Drum\_God](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drum_god/32/44_2.png) [@Drum\_God](https://boards.straightdope.com/u/Drum_God)\ **Post date:** [March 13, 2011, 4:51am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/31 "2011-03-13T04:51:08Z")
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? ---
**Author:** ![minor7flat5](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/minor7flat5/32/258_2.png) [@minor7flat5](https://boards.straightdope.com/u/minor7flat5)\ **Post date:** [March 13, 2011, 5:07am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/32 "2011-03-13T05:07:12Z")
> [@Drum\_God](#): > > 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. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [March 13, 2011, 5:36am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/33 "2011-03-13T05:36:39Z")
> [@Drum\_God](#): > > 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 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) ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [March 13, 2011, 5:56am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/34 "2011-03-13T05:56:46Z")
> [@Drum\_God](#): > > 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? > [@minor7flat5](#): > > 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. 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. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [March 13, 2011, 6:42am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/35 "2011-03-13T06:42:25Z")
> [@robby](#): > > 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. 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. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [March 13, 2011, 3:57pm UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/36 "2011-03-13T15:57:28Z")
> [@Whack-a-Mole](#): > > 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](http://en.wikipedia.org/wiki/Half-life) 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. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [March 13, 2011, 4:26pm UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/37 "2011-03-13T16:26:52Z")
[Fukushima Nuclear Accident – a simple and accurate explanation](http://theenergycollective.com/barrybrook/53461/fukushima-nuclear-accident-simple-and-accurate-explanation) It’s 5000 words, but that’s what’s required just to get to simple. ---
**Author:** ![TrapperJ](https://avatars.discourse-cdn.com/v4/letter/t/9d8465/32.png) [@TrapperJ](https://boards.straightdope.com/u/TrapperJ)\ **Post date:** [April 4, 2011, 11:42pm UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/38 "2011-04-04T23:42:30Z")
> [@DCnDC](#): > > I know (I think) that in the simplest, most basic terms a nuclear reactor is essentially just a big steam engine. However that’s about all I know (and I could be wrong). Here’s actually a decent “layperson” explanation of how nuclear reactors work: [http://zidbits.com/2011/04/04/how-do-nuclear-power-plants-work/](http://zidbits.com/2011/04/04/how-do-nuclear-power-plants-work/) ~TJ 🆒 ---
**Author:** ![thelabdude](https://avatars.discourse-cdn.com/v4/letter/t/dc4da7/32.png) [@thelabdude](https://boards.straightdope.com/u/thelabdude)\ **Post date:** [April 5, 2011, 2:41am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/39 "2011-04-05T02:41:22Z")
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? ---
**Author:** ![Erdosain](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/erdosain/32/7284_2.png) [@Erdosain](https://boards.straightdope.com/u/Erdosain)\ **Post date:** [April 5, 2011, 4:11am UTC](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424/40 "2011-04-05T04:11:38Z")
> [@thelabdude](#): > > 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? [Previous page](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424.md?page=1) [Next page](https://boards.straightdope.com/t/nuclear-reactors-cure-my-ignorance/574424.md?page=3)