How do you restart a nuclear reactor?

Nuclear reactors are regularly stopped for repairs and refueling, and in emergencies can perform a SCRAM to bring fission to a screeching halt. (Or at least try to).

So how do you start it again? If you remove the control rods from whatever “neutron poison” you are using will fission start right back up, or do you have to first fling a neutron at your uranium/plutonium pellets? How long does is take from a reactor being shut down to producing power as compared to a coal or natural gas plant?

The fuel itself is radioactive, which means every now and then one of its atoms will spontaneously fission, releasing one or more neutrons.

If you want to build a bomb, you assemble a bunch of radioactive material into a very compact shape so that some of the neutrons from spontaneous fission events tend to hit other atom and cause them to fission as well, releasing even more neutrons and cracking even more atoms. The whole chain reaction happens very rapidly, so that you get a massive release of energy that blows the lump of fuel apart and flattens your city. You need to assemble that very-compact-shape extremely rapidly; if you’re too slow, the shape will melt/vaporize/blow itself apart with just a small fraction of its energy before it has a chance to really release its full potential. That’s why bombs either involve a slug of fuel that gets blasted into a mating hole in another slug of material (gun type), or a sphere of fuel that gets compressed from all sides by exquisitely coordinated blasts of high-explosives (implosion type).

If you want to build a nuclear reactor, you assemble a bunch of radioactive material into a somewhat less compact shape. You still want neutrons from spontaneous fission events to hit other fuel atoms and make them fission, but not at such a ridiculous rate as a bomb, and you also want to be able to control that chain reaction - increase its rate, decrease it, or shut it down altogether. So your slightly-less-compact-shape also has room for you to insert control rods that absorb a lot of those neutrons, keeping the rate of reaction under control. Your slightly-less-compact-shape also needs room to circulate coolant that extracts heat from the nuclear material; afterall, that’s the whole point, to extract thermal energy and use it to heat water so you can make steam and drive a turbine/alternator.

Spontaneous fission events are happening all the time in the fuel. So you pull the control rods out, and your chain reaction starts building up: instead of being absorbed by the control rods, the neutrons from spontaneous fission events start splitting other atoms of fuel, and those fission events spit out more neutrons that shatter still other atoms of fuel, all at an accelerating rate. Insert the rods, and the chain reaction starts dying down. Somewhere in there is a middle ground, where atoms are fissioning at a steady rate, and you have a power plant operating at full power.

So is uranium radioactive enough that holding a piece of it in your hand is dangerous because it naturally has neutrons bouncing around?

I guess what I don’t get is: does just having a bunch of uranium sitting around together cause fission to start and keep going, or do you need something to trigger it?

Unstable isotopes are constantly decomposing. Some faster than others. Put a mass of them together, and they stimulate more reactions.

This is the basis for dating things by carbon 14 content or other elements.

Define dangerous. If you picked up a uranium fuel pellet that hadn’t been in a reactor yet and held it in your hand, you probably won’t suffer any harm. I’d recommend washing your hands afterward however, although that’s more for any possible chemical toxicity. There are enough natural fission events going on in the uranium to start a chain reaction, but it doesn’t take much to start when the conditions are set up properly. One thing that a lot of people are missing is that creating the chain reaction is not just a matter of having enough uranium. A reactor core also has moderator material which slows down the neutrons and makes them more likely to hit a uranium atom and trigger fission, and may also have neutron reflecting material to bounce neutrons that escape the core back in. In most nuclear power plants the uranium by itself is insufficient to sustain a chain reaction, but once set up in the proper geometry with the rest of the core a chain reaction will start spontaneously.

Yes. Marie Curie learned that the hard way.

Nuclear fuel is refined from uranium ore to increase the concentration of radioisotopes. So a chunk of uranium ore is always spitting off neutrons here and there, but won’t start a chain reaction.

Having enough radioactive uranium in a small enough volume will cause the neutrons to initiate further fission events which spit out more neutrons and a fission chain reaction will occur spontaneously. (This is called a critical mass.) Refined fuel is necessary for this to happen. To make a fission bomb (the kind used in WWII), you need to refine the fuel even further. This level of refinement isn’t used for nuclear power plants.

By sticking neutron-absorbing stuff, like graphite or boron, in the middle of the reaction, you can moderate it and prevent it from blowing up. If the reaction gets too hot, stick in more control rods and absorb some neutrons, which will cool it off a bit. If you need more power, take some control rods out.

Marie Curie was working with radium. Radium is vastly more radioactive than even enriched uranium.

The moderators are not there to ‘prevent it from blowing up’. The moderator is required for the chain reaction to happen. In a typical nuclear reactor the neutrons emitted by fission events are traveling so quickly that they tend to leave the core before they can be absorbed by another uranium atom. Moderators slow the neutrons down and make them more likely to be absorbed. If you remove the moderator, the reaction can’t self-sustain and stops. It is possible to build a reactor without neutron moderators, but it takes much more uranium and a more dense packing of the fuel to get the chain reaction to happen.

Graphite is a moderator, although not one used much in western reactors. Water performs the moderation in most modern reactors. Boron isn’t a moderator, it’s a neutron absorber which is used to kill the chain reaction in an emergency.

You can still buy uranium jewelry/pottery. Such things can still be found today and they are not terribly hazardous.

Nuclear bombs require a nuclear initiator to do their thing. Just mushing the fuel together doesn’t start things fast enough to go boom so a neutron source helps kickstart the reaction.

Interestingly there are natural reactors in existence (places where there is enough uranium naturally to sustain a low-level reaction).

To restart a reactor, you remove both of the neutorn poisons:

You lift the control rods slowly, starting reactions in the bottom of the reactor (or, rarely, drop the control rods out the bottom, but most rods go in the top. When you need to SCRAM and have no power, you want gravity on YOUR side.)

You also slowly filter the boron neutron poison out of the reactor feedawter as it circulates around, slowly lowering the PPM to the point where the reaction is producing the desired amount of heat => steam => electricity.

There are done slowly so you can watch for any abnormalities in plant startup and potentially have a relatively small amount of heat to dump if you see problems. Putting the pedal to the metal then finding out your turbine has a bad vibration, one of your feedwater pumps is going wonky, or the thermal shock had cracked one of your steam generators would be bad news. This is ESPECIALLY so when you’re coming back online from a sudden and unexpected trip which puts a lot of stress on various components of the plant

ETA: Uranium 235 will not spontaneously react, and you could hold a lump of it in your hand and get less dose than holding some lantern mantles or taking a commercial airline flight. Enriched Unranium 238, however, will react and is used as nuclear fuel.


You’ve got those reversed. U-235 is the isotope that will support a chain reaction. Natural uranium is mostly U-238 with a little U-235. Enriched uranium has the portion of U-235 increased to the point were it can be used as fuel.

That depends on what brand of reactor you are using.

Some are push-start, some are pull-start…

Seriously, key thing is “critical mass”. In a bomb, this is a small sphere ( about 10Kg?) of the mainly more unstable isotope of uranium (u238, IIRC) or plutonium, and as mentioned above, must be assembled quickly.

In powerplant reactors, the material is more spread out with coolant and moderating material between them - so a larger mass is needed, hence the cylindrical design that seems to be about 20 or 30 feet on a side, of tubes containing fuel rods in a bath of coolant…

If you remove the moderating material, the number of neuron collisions that split nearby uranium atoms (and create heat) increases. The more collisions, the more heat, the more neutrons released for the next collisions. If you remove coolant, that heat doesn’t go anywhere until the fuel rods begin to crack, break, melt, burn, whatever. But you keep making more heat meanwhile…

All of these threads seem to go the same way:

[li]OP asks a question.[/li][li]Partially correct information gets posted.[/li][li]Partially incorrect information gets posted.[/li][li]Completely correct information gets posted.[/li][li]Completely incorrect information gets posted, often right after the correct information was posted.[/li][li]Incorrect conclusion is drawn.[/li][li]Rinse and repeat. :rolleyes:[/li][/ol]
In any event:

You can certainly hold U-238 (the most common uranium isotope, with a half-life of 4.468 billion years) or a subcritical mass of U-235 (with a half-life of 700 million years) in your hand with no ill effects (though I’d wash my hands afterward, and I certainly wouldn’t ingest it). Due to their long half-lives, they are only slightly radioactive. Both isotopes decay primarily by alpha decay.

Isotopes of uranium do occasionally decay by spontaneous fission. This occasional spontaneous fission event is sufficient to initiate the chain reaction in a power reactor, so that all you need to do to start a power reactor is to remove the control rods. (You can’t count on this, however, in the nanoseconds available to start the chain reaction in a nuclear bomb, so you need a neutron “initiator” to guarantee that the reaction will proceed.)

You are confusing the control rod (which contains a material that absorbs neutrons) with the moderator (which slows the fast neutrons produced in a fission reaction down to a speed slow enough that the resulting so-called “thermal” neutrons can initiate a subsequent fission event). This makes your quoted statement exactly backwards. Instead, removing moderating material (usually water, but sometimes graphite) decreases the rate of reaction (which is what AndrewL correctly stated earlier in this thread).

P.S. I am sure to have made some misstatements in this post as well. (Isn’t that a message board rule when you comment on other posters’ misstatements?) :wink:

Precisely 100% incorrect information. The moderator slows down the neutrons enough to make capture of a neutron by a uranium atom more likely. Without the moderator, the required size of the core would be much, much larger. Furthermore in most reactor designs the coolant is the moderator, which gives some degree of safety in event of coolant loss. Furthermore most reactors have a negative thermal coefficient - the hotter the core gets, the less reactive it gets, both because the density of the moderator decreases, and the increasing thermal vibration of the uranium atoms decreases the chance of neutrons hitting the uranium atoms at the right speed to cause fission.

There’s only one thing you have to worry about: you can’t put too much water in a nuclear reactor. (Cite)

If that bothers you, consider the cost. What did you pay for this education?

I’m a former Engineer-qualified nuclear submarine officer. I have started up a nuclear reactor (two different designs) dozens of times. Even today, I could conduct a fast recovery startup of the S6G reactor I qualified on with my eyes closed.

I can assure you that I am not the one being educated in this particular topic here.

Instead, I (and AndrewL) am correcting some of the completely incorrect information posted here.

It’s all about adding positive reactivity.

In general, as others have said, rods are withdrawn, removing an impediment to neutron travel.

Reactors typically have a lot of energetic stuff going on in the first hours and even days after they are shut down (cf. my post here on decay heat), so there is a usually fairly substantial neutron population, so the reactor will start up fairly easily by just pulling the rods (slowly!).

If the plant has been shut down for many weeks, and if it is an older core, these factors may reduce the neutron population so low that the power level cannot be detected on instrumentation.
Then you need to follow an altered startup procedure: pull and wait. The reason for this is that “startup rate” is exponential, measured in “decades per minute”, or powers of 10 per minute. If you were to simply pull rods until the sleeping reactor came to life, the startup rate might very well be several decades per minute while the total wattage of the reactor was still less than a nightlight bulb—an unmeasurable power level. Then, by the time power was measurable on your instruments, it would be screaming up far faster than you could ever control it, and you would have an incident.

If pulling the rods does not add sufficient positive reactivity to wake the dormant reactor, then … I forget what happens—perhaps that’s end of life criteria. (I was a chem and rad tech guy, not a reactor operator—that’s the fellow with his hand on the control rod lever—and it has been 25 years).

Here’s a fairly detailed page written by a sub sailor describing the general reactor startup process, with some mention of the pull-and-wait startup.

And keep fighting that ignorance folks, the “moderator” in nuclear power terminology does not restrain the reaction; it is actually necessary to sustain the reaction (in a typical pressurized water reactor). Something else: a reactor that is critical is simply one that has a self sustaining reaction. Nothing bad about that.

In nuclear bombs, you actually don’t want very many spontaneous fissions. Plutonium that has too high a concentration of the isotope Pu-240 can’t be used in bombs because Pu-240 has too high a rate of spontaneous fission. The problem from a bomb designer’s viewpoint is to assemble a critical mass before it can blow itself apart again. Even explosive assembly or implosion is glacially slow on the timescale that nuclear reactions take place. If there are too many free neutrons about, the mass of fissile material will start reacting before it’s fully assembled, and release just enough energy to blow itself apart again- a sub-kiloton yield “fizzle”. To get a really big bang you have to assemble your critical mass as quickly as possible and then kick start the reaction with an initiator- something that releases an extra burst of neutrons on cue just when the fissile mass is at it’s most compact.