In a brand new reactor with new fuel rod assemblies, how does the fission reaction get started?
Uranium nuclei don’t just split on their own if you get enough of it together, do they?
Actually, they do. The whole purpose of refining the uranium, making fuel bundles, adding moderator, etc, is to get enough uranium in proximity to itself that it does react. The moderator keeps it from reacting too quickly.
Sure they do.
U-235 has a spontaneous fission rate of 0.16 fissions/sec- kg.
This part is incorrect. The neutron moderator allows the reaction to take place by slowing neutrons down. Otherwise there would be little or no reaction.
It’s not a matter of “if you get enough of it together”. Even if you had one uranium atom sitting all by itself, it’d have a chance of spontaneously fissioning in some time span. If you hit it with a neutron, that just makes it more likely to fission. Get enough of it together, and the neutrons from one atom fissioning (however it happens, spontaneously or triggered) will cause on average more than one other atom to fission right away, and those will in turn cause more than one, and so on, in a chain reaction.
For a reactor the spontaneous rate is going to be enough that you will never notice any lag in the startup, not in human time terms. A nuclear weapon does need an external source, indeed a significant part of getting things right is to avoid any spontaneous neutrons wandering about and starting the chain reaction until the weapon is fully assembled. This significantly restricts the design geometry and choice of fissile material. Once it is assembled (which is done with an explosive) the neutron generator suffuses the compressed mass of fissile material with neutrons and away it goes. Yield drops dramatically if the assembly starts to fission too early due to a spontaneous neutron. There is always a chance this happens, but the design tries to reduce the probability.
I think, in practical terms, they start it up by pulling out the control rods?
One interesting aspect of a reactor startup is that if the reactor has been shut down for a long period of time, the actual neutron population is so low that it is below the detection range of the nuclear instruments.
This poses a bit of a challenge because reactor startup rate exhibits exponential growth (measured in “decades per minute”). Pulling the rods introduces positive reactivity, thereby encouraging reaction rate growth. The problem is that you can’t see what the rate is because the total power is a whisper of the lowest measurable amount, so if you just pull the rods recklessly, the startup rate might grow way higher than is safe. By the time the power is visible on the most sensitive instruments, it could be screaming upward at many orders of magnitude per minute.
The reactor operator has to perform a special kind of startup in these circumstances where rods are pulled in small increments with waiting periods in between.
I did a quick search and found this PDF slide deck that talks all about one particular reactor’s startup, with descriptions of the various source neutrons (boy, that is barely ringing a bell, but I studied this stuff 25 years ago, so it’s all but forgotten).
ETA: I knew I answered this one before. In that post there’s even a link to a good description of a sub reactor startup.
Thanks Minorflat. I was just coming in to say the same thing. IIRC you just do NOT yank the rods and get it going. You gotta be pretty careful at startup. I also seem to recall there was/is a special source of neutrons in a nuclear reactor (or at least some kinds) to make this startup process easier/safer.
Here’s a couple of posts by some random doper regarding this same common misconception.
(you wouldn’t imagine how rarely this topic comes up in casual conversation)
If the reaction gets too heated, the moderator issues a warning or outright bans any further emissions.
Another thing: nuclear reactors must walk the fine line between delayed and prompt critical Prompt criticality - Wikipedia - each fission event causes at least one more.
Prompt means neutrons from the first fission alone produce more fission events.
After the first fission, the fission products continue to decay and produce more neutrons. Delayed means neutrons from the first and subsequent fissions are required for criticality.
Ah, yes; the Manhattan project.
Out-standing!
Reaction from the next cubical: “Are you reading that Dope thing again?”:)
heh, heh, heh
I believe that’s called the Hall effect. Which of course must be monitored very carefully.