h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I writing a piece for school on nuclear reactors, I have found most of the information I need on reactor design, how the fission processes work etc.

What I would like to know is what happens to the control rods?

I understand that they are good neutron absorbs and so can reduce the rate of a chain reaction by absorbing excess neurons; but by absorbing these neutrons this must affect the rods. I imagine that they become different isotopes of the control rod materials (boron, cadmium etc.) which may become unstable and so (again I imagine) decay, becoming other materials, requiring the rods to be replaced.

But this is all speculation, I just don't know and having searched the web cannot find any useful information.

SExperts, please can you help!

_LAWE_

I know this doesn't answer your question, but I was browsing BBC News this morning and came across this explanation of what went wrong at Chernobyl, which is informative to this thread: http://news.bbc.co.uk/2/shared/spl/hi/guides/456900/456957/html/nn2page1.stm

Cheers,
Ste

There's also this: http://en.wikipedia.org/wiki/Control_rod

According to the Wiki article, the very definition of a control rod requires that it be made of a substance which, while absorbing neutrons, does not decay. So the short answer to "what happens to the control rods?" is, "nothing."

Generally, control rods would seem likely to lose effectiveness over time, and so need eventual replacement, but it seems it is possible to design rods to be sufficiently durable (overspecified?) to outlive the design life of a reactor. In some cases, the control rods may even not be used during normal reactor operation if boron-containing water used as a coolant is performing the necessary fine-tuning of the reactor operation.

See:
http://www.eng-tips.com/viewthread.cfm?qid=99164&page=2
http://www.eng-tips.com/viewthread.cfm?qid=141202&page=1

Control rods are made of materials (i.e. isotopes of certain elements) that can absorb neutrons without decaying themselves (or, to be more precise, do not decay at an appreciable or significant rate). Boron is an excellent example. Cadmium is commonly used, as are cobalt, silver, indium, and some others.

- JD

Gah! Bad timing. I typed up a response, forgot to hit send, got distracted by something else well after lunch was over, and just now saw that I'd forgotten to hit "send" hours ago. In any event, I wound up sending something I meant to just delete and check what everyone else had said. Indeed, wikipedia confirms what I just said, which cheered me when I used it as a sanity check since I couldn't quite remember the other common neutron absorption materials besides boron and cadmium (where I work, there aren't any actual reactors so this is from memory of some literature and various course I'd taken over the years - silver and some of those other rare earth elements seem very exotic and expensive to me, but I guess if they work ...).

- JD

Traveller in Time not an expert
"But I did have some radiation courses. One thing I have not read is the rods do get warm !

The absorbtion of extra neutrons in the nucleus of the atoms makes them more heavy but less absobing over time. The core of these atoms is big to start with, big enough to contain more Protons then Neutrons. Durint the Absorbtion Neutrons are added to the atom core.

My question to an expert was:
This absorbtion will result in (more) saturated atoms on the surface of the rods. Saturated atoms 'bounce' and slow down the passing neutrons.
Will the surface atoms also exchange neutrons ?


< A2922103 > 'The Chernobyl Disaster' Twenty years ago . . . "

OK, if we've established that control rods absorb neutrons without decaying, why/how is that possible? When we talk about absorbing a neutron, it isn't literrally tacked onto the nuclei in the control rods - otherwise the material would be transformed into unstable isotopes. The energy/mass from the neutron must go somewhere. Do the rods re-emit gamma radiation?

>>"OK, if we've established that control rods absorb neutrons without decaying, why/how is that possible? "

*Have* we established that they absorb neutrons without being changed?

For a substance to absorb (capture) neturons, there's one one place they can end up - in the nucleus, which at least steps the atom up by one in atomic weight, and possibly leads to subsequent decay into some other kind of atom.

Thanks for what has been said, it's a very interesting subject.

Like Potholer, I would have thought that the rods would absorb neutrons into the nucleus and therefore decay into other atoms; but if there are enough control rods to continue absorbing neutrons for longer than the reactor is operational for, then there would be no need to replace the rods. Any decayed atoms within the rods would be trapped and I presume not affect the efficient working of the rods.

If anyone knows any better I would still love to know.

Thanks everyone

_LAWE_

I think I was a bit confused in my zeal to post (I don't often post any more) and in my elation at being able to recall something from about a decade in the past that I'd learned. Or thought I had learned - insert ish grin here. Jury's always out on that. I dusted off a few old references to refresh my memory and did a couple web searches, but I'm still probably at least fuzzy if not still confused.

Two types of "neutron poisons" exist,: "burnable" and, of course, the imaginatively named "non-burnable," according to many sources like another wiki article which came about via a DOE handbook (which, shame on me, is actually sitting in this very office as I type this!). The former refers to those neutron absorbers that react and become other isotopes that have much smaller cross sections, and do not absorb neutrons effectively. The latter refers to other elements like hafnium, which decay into other nuetron absorbers which decay into other neutron absorbers, so on down for a long enough period of time to make them essentially everlasting for the life of a nuclear reactor. I think this is what was meant by "without decay" in that wiki article that both Blathers and I had hit upon, but I don't know for certain.

Boron is an interesting example of a "burnable" poison used for reactor control. Boron-10 absorbs thermal neutrons to become an excited state of boron-11 which instantaneously undergoes a particle ejection type of neutron absorption reaction (I've seen this reaction also called a "fission," but what I've read, fission refers to a more specific particle ejection reaction) to become lithium-7 and an alpha particle. To quote DOE-HDBK-1019 "Nuclear Physics and Reactor Theory," p.46, "In a particle ejection reaction the incident particle enters the target nucleus forming a compound nucleus. The newly formed compound nucleus has been excited to a high enough energy level to cause it to eject a new particle while the incident neutron remains in the nucleus. After the new particle is ejected, the remaining nucleus may or may not exist in an excited state depending upon the mass-energy balance of the reaction."

So, while boron-11 is normally a stable nucleus, in the case of its excited state after neutron absorption it will decay via particle ejection, and then most often if the neutrons are moving slower (high cross section). This is critically important for a possible form of cancer treatment (boron neutron capture therapy, BNCT) using this very reaction and the the unusual results of the reaction. I only hammer on this because doing research on neutron absoprtion reactions as a means of control for nuclear reactors (when using boron as an example as I did) tends to come up with some confusing stuff based on some unique phenomena and uses of the material in reactors versus medicinal purposes versus simply trying to read the chart of the nuclides.

Sorry for carrying on like that, but I found the topic fascinating and happened upon another interesting topic within nuclear medicine that is related to this sort of thing. I'll retreat to lurk mode soon enough.

- JD

With regard to this question, "This absorbtion will result in (more) saturated atoms on the surface of the rods. Saturated atoms 'bounce' and slow down the passing neutrons. Will the surface atoms also exchange neutrons ?"

I think I'm only agreeing with the premise of your question rather than answering it (I think), but I read somewhere (some lecture/presentation slides from some source in Sweden) that careful control rod design allows for some decay of neutron absorbers to neutron moderators that then act to aid the absorption of further neutrons as the rods become "seasoned" from the outside in, if that makes sense. Indeed, they can become a little too effective in their absorption, thus requiring some adjustment over time so as not to reduce the output of the reactor to below effective operating parameters. As I understand it, in the case of neutron moderators, the nuclei of such atoms do not absorb neutrons but slow down fast neutrons simply by undergoing inelastic collisions with them and not absorbing them.

- JD

Traveller in Time also browsing the WEB
"One thing I remember is that all materials lose integrity after prolonged exposure to neutrons.
This will be another reason why they have to be replaced.


Twenty years ago I did some training work in a Reactor plant and almost thirty years ago I wrote my piece for school "

thanks JD! that was a good explanation

Thanks JD. that made alot of sense, I really think I understand what is going on now.

_LAWE_

Can I ask a completely different question, but about control rods?

I can't remember much about reactor design... Is there not one type where the fuel-rods and the control-rods are emersed in water, and the water is heated up (and this goes to make steam, to drive turbines, etc...)

Assuming that such a construction exists (!) I've never understood if the radioactivity (ie, neutrons) from the fuel rods heats the water directly, or if the control rods warm up from absorbing the neutrons, and the *control rods* heat the water... Or is it both, or neither

Traveller in Time on the lead dome
"All three heat up the water.


Listed according to contribution:

The core elements, containing the uranium. ( fuel in container )
The control rods. ( brakes )
The water, seperating the elements and the rods, as lubrication and coolant. "

Almost completely offtopic: one of the coolest things I've ever heard of was that certain nuclear reactors are cooled by circulating liquid sodium through them.

LIQUID SODIUM!!!

Even cooler, consider this: how do you pump liquid sodium? What kind of pump impeller would stand up for any length of time?

Answer: none. You don't need an impeller. Sodium is a conductor, and you can just impose a magnetic field on it from outside the pipe, and it flows. How cool is that??

I know, I'm a

SoRB

I've also read about experiments being carried out with Lead (Pb) being used as reactor coolant - god only knows how that works!