Why does a control rod work in a nuclear reactor?

[treddie]

Hello.

I know that control rods in reactors work by absorbing neutrons that would otherwise be used to collide with say, Uranium, to cause fission. But here is the part I do not understand:

As far as I understand, a Uranium atom fissions because it absorbs a neutron causing it to become inbalanced, thus causing the Uranium atom to split into two lighter elements and in the process a lot of radiation is released including...more neutrons.

But since a control rod absorbs neutrons too, why does IT not fission as well, since adding neutrons to stable atoms turns them into unstable isotopes that ALSO emit radiation. Does not the now radioactive control rod material also emit neutrons? Or is the control rod material much less likely to emit neutrons, even as it releases other radioactive particles and electromagnetic radiation? If so, what is different about a neutron rod that gives it this special quality?

Many thanks from an inquisitive mind!

 

[willem2]

Only very heavy atoms, such as uranium, thorium or plutonium can split when bombarded with neutrons.
For smaller atoms, an isotop with 1 more neutron is produced, which will be stable or decay by beta radiation (emmiting an electron, forming an element with one more proton)
In any case no neutrons will be produced.

 

[WatermelonPig]

Usually Boron-10 is used in control rods and it justs absorbs and goes to Boron-11.

 

[treddie]

Ahh, OK, so it is the mass that is the deciding factor. That makes complete sense, actually. So with the lighter elements, it sounds like the absorbed neutron decays into a proton and it's opposite charge electron plus other products? It reminds me of a neutron star in reverse, sort of, where the protons and electrons are squeezed back into neutrons.

I imagine that the only reason the electron does not stick around to balance out the proton is that it is too easily kicked out by all of the excess energy, thus creating an ion? If so, is the decay process completed at that point, and the heavier atom just looks for another atom to share an electron with?

 

[Astronuc]

Usually Boron-10 is used in control rods and it justs absorbs and goes to Boron-11.

B-10 undergoes an n,alpha reaction producing alpha + Li-7

http://web.mit.edu/nrl/www/bnct/info/description/description.html

 

[Astronuc]

Ahh, OK, so it is the mass that is the deciding factor. That makes complete sense, actually. So with the lighter elements, it sounds like the absorbed neutron decays into a proton and it's opposite charge electron plus other products? It reminds me of a neutron star in reverse, sort of, where the protons and electrons are squeezed back into neutrons.

I imagine that the only reason the electron does not stick around to balance out the proton is that it is too easily kicked out by all of the excess energy, thus creating an ion? If so, is the decay process completed at that point, and the heavier atom just looks for another atom to share an electron with?

It's not about mass as much as the particular nuclide. Certain isotopes of certain element shave a high absorption cross section for thermal (or epithermal) neutrons. Boron is one of the lightest elements with a large aborption cross-section, and it is less expensive and more abundant than others.

Other elements include Hf, Er, Dy and Gd.

In a BWR, the control rods (or blades, reflecting the cruciform geometry of the control element) are used in controlling the neutron chain reaction. The reactor is critical at constant power. Fine tuning is accomplished with boiling (void fraction), as well as burnable absorbers like gadolinia (Gd) that are intimately mixed with UO2 in selected fuel rods. At some point during the cycle, the control rods are readjusted to offset the depletion of the U-235, as well as to redistribute the power in the core, so that the same vintage of fuel (batch) achieves similar burnups (energy per unit mass of fuel) at the end of the cycle, and ultimately at end of production life.

The control blades contain tubes of boron carbide (which contain B-10 mixed with B-11). Some control blades/rods may contain Hf. If the reactor is critical, then inserting the control rods makes the reactor subcritical and power will decrease. If the control rods are withdrawn, the reactor becomes supercritical and power increases. We employ small movements to ensure that power changes at a controlled rate. If necessary, when the control rods are fully inserted or perhaps 80-90 percent inserted, the reactor shuts down.

In PWRs - a different LWR system - control rods are withdrawn from the core, and the control is acheived with soluble B-10 in the coolant (which is buffered with LiOH) to maintain a pH above 6.9 or perhaps closer to 7.1 - 7.4 for purposes of minimizing the accumulation of crud (metal corrosion products) on the fuel.

An exception to the previous statement are power shaping grey (or gray) rods, which contain a Ni-bearing alloy, Inconel, which is not as strong an absorber as B or Hf, but absorbs enough neutrons to keep the local power down a bit. Grey rods can be inserted in the core for most of the cycle to control the radial power distribution in the core, or they may reside out of the core and only inserted and withdrawn to make quick small changes to the power level.

The Russians have used Dy in the form of Dysprosium titanate as a control rod material.

 

[treddie]

Great info. Thanks for all the responses.

So one last question. Uranium in its ore state is essentially still unstable as I can figure it, but that its radioactivity is very slight. But when the Uranium is extracted from the ore and therefore enriched (not enriched in the sense of separating U235 from U238), I gather that this purified Uranium is now quite radioactive and dangerous, AND generating a healthy amount of heat due to decay. So even if a reactor is scrammed, there is still the default level of heat being generated by each Uranium pellet, even though the entire system is subcritical at that point. Therefore, I assume that just like in external nuclear waste ponds, cooling fluid is still required to dissipate that heat inside the reactor. If so, then if the containment walls are too thick to effectively dissipate that default generation of heat adequately without cooling, then a meltdown can still occur as the internal vessel temperature continues to climb, albeit much more slowly. Or is the heat production rate slow enough to be easily handled by the heat conductivity of the thick steel/cement containment walls?

 

[QuantumPion]

Great info. Thanks for all the responses.

So one last question. Uranium in its ore state is essentially still unstable as I can figure it, but that its radioactivity is very slight. But when the Uranium is extracted from the ore and therefore enriched (not enriched in the sense of separating U235 from U238), I gather that this purified Uranium is now quite radioactive and dangerous, AND generating a healthy amount of heat due to decay. So even if a reactor is scrammed, there is still the default level of heat being generated by each Uranium pellet, even though the entire system is subcritical at that point. Therefore, I assume that just like in external nuclear waste ponds, cooling fluid is still required to dissipate that heat inside the reactor. If so, then if the containment walls are too thick to effectively dissipate that default generation of heat adequately without cooling, then a meltdown can still occur as the internal vessel temperature continues to climb, albeit much more slowly. Or is the heat production rate slow enough to be easily handled by the heat conductivity of the thick steel/cement containment walls?

U-235 has a half-life of 700 million years, it is only slightly more radioactive than U-238 (half life 4.5 billion years). Concentrating a radioactive substance does not make it more radioactive, the decay rate of any single atom is completely independent of its surroundings. The self-heat generated by uranium decay is practically zero (something like a microwatt per kilogram).

 

[treddie]

No, more radioactive in the sense that you have more concentration of Uranium in a given volume, not in the sense that the half-lives are changed. In other words, I could expect a lower likelihood of cancer if I was standing next to a 10 foot diameter ball of Uranium ore, than if I was standing next to 10 foot diameter ball of enriched Uranium. I would expect more neutron collisions in my body from the second case.

In regards to self-generated heat from enriched Uranium, it seems the heat generated should be much higher in even a single pellet since although the decay heat from a single U atom is very small, there are still a zillion more U atoms in the pellet, offering the chance for many neutron collisions to occur in that volume, even though a good percentage of these neutrons are fast neutrons and do not contribute to fission all that much in this ideal example.

 

[QuantumPion]

No, more radioactive in the sense that you have more concentration of Uranium in a given volume, not in the sense that the half-lives are changed. In other words, I could expect a lower likelihood of cancer if I was standing next to a 10 foot diameter ball of Uranium ore, than if I was standing next to 10 foot diameter ball of enriched Uranium. I would expect more neutron collisions in my body from the second case.

In regards to self-generated heat from enriched Uranium, it seems the heat generated should be much higher in even a single pellet since although the decay heat from a single U atom is very small, there are still a zillion more U atoms in the pellet, offering the chance for many neutron collisions to occur in that volume, even though a good percentage of these neutrons are fast neutrons and do not contribute to fission all that much in this ideal example.

As I said, the activity of Uranium is very low. There may be a zillion atoms but only a few of them are decaying per second because Uranium is so stable. There is no risk of radiation poisoning or cancer from uranium just by standing next to it, since it only emits alphas which cannot penetrate your skin. There would only be a little risk if you inhaled/ingested it. The neutron flux from natural uranium is also practically nil, and would be due to other trace isotopes like spontaneous fission of U-234.

 

[Astronuc]

No, more radioactive in the sense that you have more concentration of Uranium in a given volume, not in the sense that the half-lives are changed. In other words, I could expect a lower likelihood of cancer if I was standing next to a 10 foot diameter ball of Uranium ore, than if I was standing next to 10 foot diameter ball of enriched Uranium. I would expect more neutron collisions in my body from the second case.

In regards to self-generated heat from enriched Uranium, it seems the heat generated should be much higher in even a single pellet since although the decay heat from a single U atom is very small, there are still a zillion more U atoms in the pellet, offering the chance for many neutron collisions to occur in that volume, even though a good percentage of these neutrons are fast neutrons and do not contribute to fission all that much in this ideal example.

Well - uranium ore is about 1-3% of natural uranium, which is in the form of some complex oxides, e.g., phosphates, carbonates, etc. A processor much separate the uranium compounds from the ore. Then one gets a uranium ore/oxide concentrate.

http://en.wikipedia.org/wiki/Yellowcake
http://en.wikipedia.org/wiki/Uranium

Here is an overview of the nuclear fuel cycle
http://www.world-nuclear.org/education/nfc.htm

The uranium oxide that one gets is a form like U₃O₈, with about an isotopic fraction of 0.0071 U-235 and 99.29 U-238 with a trace of U-234. The oxide is converted to UF₆, which is then processed in an enrichment facility to increase the concentration of U-235, typically up to a maximum isotopic fraction of 5% for commercial LWR fuel.

The UF₆ is then converted into UO2, which is then formed into ceramic pellets. The activity is very low. I have handled nuclear fuel powder and pellets, however I used gloves as is required to prevent the spread of fuel outside of controlled areas.

The uranium ore on the other hand generally has higher activity because it contains natural decay products from the early years when the uranium was formed in supernova somewhere nearby. The decay products have much shorter half-lives, and so they actual provide the bulk of the radioactivity in the ore.

http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radser.html
Look at the decay chains for U-235 and U-238

 

[treddie]

OK. After reading the above links and looking further, it appears I am confusing decay with spontaneous fission. The decay of U concerns only alpha and beta particles. But spontaneous fission and the release of a free neutron occurs too, but very rarely in any Uranium sample. Am I correct in assuming that it is these rare spontaneous events that initiate any chain reaction. After all, if there is enough enriched Uranium in a given volume, all it takes is one spontaneous fission...one neutron... to start the chain reaction.

 

[Astronuc]

OK. After reading the above links and looking further, it appears I am confusing decay with spontaneous fission. The decay of U concerns only alpha and beta particles. But spontaneous fission and the release of a free neutron occurs too, but very rarely in any Uranium sample. Am I correct in assuming that it is these rare spontaneous events that initiate any chain reaction. After all, if there is enough enriched Uranium in a given volume, all it takes is one spontaneous fission...one neutron... to start the chain reaction.

It's a bit more complicated than just one neutron.

The system must be critical which means for the range of production of neutrons must equal the rate of consumption of neutrons. If the system was critical, and only one neutron was produced, then in the next generation, one neutron would be produced. That's not a lot of energy.

Really what happens is that for the 2 or 3 fission neutrons produced (average of something like 2.3 or 2.4 per fission), at least one neutron must survive to cause the next fission. In addition to being absorbed by the fuel, neutrons are absorbed by the structural material, e.g., steel or Inconel, by the moderator (H+n -> D, O16+n -> O17), by various fission products (Xe-135 is a very strong neutron absorber), or they simply leak out of the core, where they are absorbed in the surrounding structure.

Spontaneous fission in U is very low. U-235 and U-238 decay by alpha emission. Their decay products decay by beta emission. And there is the sequence of other decays - some beta and some alpha. The half-life of U-238 is very long, longer than U-235, so it's activity is very low. U-235 with the shorter half-life would have a little stronger activity, but it normally represents less than 5% of the U atoms.

When a fresh reactor core starts from zero power, a special neutron source is placed in the core to get it started. The core become slighly supercritical to allow the power to increase slowly. At some point, after the plant heats up, and the turbine has started to roll, the reactor power is slowly increased to full rated power.

In a mature core, one that contains fuel from one, two or more cycles of irradiation, the core can be started from zero power using the spontaneous fissions of transuranic radionuclides that are produced by successive neutron capture in U-238 and the subsequent decay products. That is called conversion, when U-238 is used to produce Pu-239, and heavier isotopes. If more Pu-239 is produced than U-235 and Pu-239, that is considered breeding - the basis of breeder reactors.

See also this thread - https://www.physicsforums.com/showthread.php?t=481702

 

[treddie]

It's a bit more complicated than just one neutron.

The system must be critical which means for the range of production of neutrons must equal the rate of consumption of neutrons. If the system was critical, and only one neutron was produced, then in the next generation, one neutron would be produced. That's not a lot of energy.

Yes, that much I understood but did not explain myself very well. And your other thread did a good job of explaining to me the startup source for the neutrons. But if one fast neutron from a spont. fissioning Uranium nucleus hits another nucleus, I thought that in those rare moments when a fast neutron can be absorbed by that other nucleus, the nucleus that was hit was split and in the process produced MORE than one fast neutron...not very likely to get absorbed by another nucleus itself without moderation, but if the fuel rod is composed of pellets with a moderator fluid in the spaces between the pellets, not SUCH a long shot?

 

[Astronuc]

Yes, that much I understood but did not explain myself very well. And your other thread did a good job of explaining to me the startup source for the neutrons. But if one fast neutron from a spont. fissioning Uranium nucleus hits another nucleus, I thought that in those rare moments when a fast neutron can be absorbed by that other nucleus, the nucleus that was hit was split and in the process produced MORE than one fast neutron...not very likely to get absorbed by another nucleus itself without moderation, but if the fuel rod is composed of pellets with a moderator fluid in the spaces between the pellets, not SUCH a long shot?

In general, nuclei have very low cross-sections for fast (fission) neutrons. Fast neutrons would likely escape from a system before causing a fission.

Fast reactors use enrichments of 20% of fissile material, and usually a tighter lattice in order to achieve criticality.

The point of moderation is to slow the neutrons to thermal energies, that is the neutron kinetic energies are in thermal equilibrium with their environment - the reactor core. At thermal energies, the neutron cross-sections for capture and fission by U-235 are quite high - higher than the cross-sections of most structural materials, most of which is Zr-2 (a Zr alloy) in a BWR or Zr-4 in a PWR.

Another element in reactivity control is the use of burnable absorbers or burnable poisons, such as gadolinia or erbia, and boron. Burnable poisons are mixed with the fuel in which they absorb neutrons in competition with the fuel, but the burnable poisons (Gd and Er) transform to isotopes of lower cross-section, or in the case of boron are transformed into different elements. B-10 + n => alpha + Li-7.

BWR fuel generally has gadolinia distributed in a limited population of fuel rods, while PWR design use Gd or ZrB₂. In the case of ZrB₂, a thin coating is applied to the circumferential surface of the fuel pellets. It's slow depletes over the first cycle. Some PWR fuel designs have used erbia instead of gadolinia.

Also, boron can be used in Burnable Poison Assemblies, which look like Neutron Source Assemblies, but contain neutron absorber (B-10) instead of neutron source material.

 

[treddie]

But can't a fast neutron from a spontaneous fission event, when exiting a pellet and traversing the moderator fluid (I am assuming the fluid flows between all of the pellets in the rod) be slowed down enough to cause fission in another pellet, or am I getting it all wrong here?

 

[Astronuc]

But can't a fast neutron from a spontaneous fission event, when exiting a pellet and traversing the moderator fluid (I am assuming the fluid flows between all of the pellets in the rod) be slowed down enough to cause fission in another pellet, or am I getting it all wrong here?

Yes, that it what happens in an LWR and even CANDU. It also why when the first core is loaded, or when the plant is refueled, neutron absorbers are in the core to absorb more neutrons that the fuel, thus keeping the core subcritical.

In a PWR, neutron absorbing boron in the form of boric acid is in the coolant, and some assemblies will have burnable poison assemblies (clusters of rods) inserted in them, and some assemblies have control rods inserted in them. In BWRs, boric acid is not used (except during dire emergencies) in the coolant because of the boiling in the core. Instead, the control rods are fully inserted in the core.

The coolant/moderator flows upward on the outside of the cladding and between the fuel rods which are in an regular square (Cartesian) array/lattice.

 

[treddie]

Then cannot a reactor self-start without an additional neutron source so long as one spontaneous neutron can be moderated and if it is heading toward the core giving it a higher chance of a collision, releasing multiple neutrons? Or would it take an unpredictable amount of time to do it, whereas an additional neutron source guarantees startup very quickly?

 

[QuantumPion]

Then cannot a reactor self-start without an additional neutron source so long as one spontaneous neutron can be moderated and if it is heading toward the core giving it a higher chance of a collision, releasing multiple neutrons? Or would it take an unpredictable amount of time to do it, whereas an additional neutron source guarantees startup very quickly?

The reason why you need a neutron source (either external or due to burned fuel) to start up a reactor is because the criticality of the reactor is independent of the power. There is no way to know if the reactor is supercritical or subcritical except by measuring the rate of change of the neutron population. So if you had no source neutrons, it would be possible to be highly supercritical without knowing it, and as soon as just a few stray neutrons start multiplying, power can rise faster than you can control it. When you do have source neutrons, it is easy to tell what the state of the reactor is by measuring the subcritical neutron multiplication and that way slowly approach criticality at a controlled rate.

 

[treddie]

I need a good book on all of this. Is there a great single book out there that covers all of these things in good detail? Each answer you give me leads me to more and more questions and I don't want to talk your ear off! :)

 

[Astronuc]

I need a good book on all of this. Is there a great single book out there that covers all of these things in good detail? Each answer you give me leads me to more and more questions and I don't want to talk your ear off! :)

Become a nuclear engineer!

 

[treddie]

Heheh! I'm a little old for that career decision now!

Without a good textbook, my only other option is to...talk your ear off!

 

[Astronuc]

Check out these areas of the NRC public library:

http://www.nrc.gov/reading-rm/basic-ref/teachers.html

Nuclear Power for Electrical Generation
http://www.nrc.gov/reading-rm/basic-ref/teachers/01.pdf

The Fission Process and Heat Production
http://www.nrc.gov/reading-rm/basic-ref/teachers/02.pdf

BWR
http://www.nrc.gov/reading-rm/basic-ref/teachers/03.pdf

PWR
http://www.nrc.gov/reading-rm/basic-ref/teachers/04.pdf

Radiation Terminology
http://www.nrc.gov/reading-rm/basic-ref/teachers/05.pdf

Natural and Man-made Radiation Sources
http://www.nrc.gov/reading-rm/basic-ref/teachers/06.pdf

Radiation Sources at Nuclear Plants
http://www.nrc.gov/reading-rm/basic-ref/teachers/07.pdf

Dose Standards and Methods for Protection Against Radiation and Contamination
http://www.nrc.gov/reading-rm/basic-ref/teachers/08.pdf


http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/

http://www.nrc.gov/reading-rm/doc-collections/

 

[treddie]

I will definitely read all of these. Thank you for taking the time to show these links. I wish I could take your mind and put it in book form!

 

[Astronuc]

I will definitely read all of these. Thank you for taking the time to show these links. I wish I could take your mind and put it in book form!

So do I, and so does the company where I work. I know too much and can't get it our fast enough.

I also have too much brain clutter, and I need a better filing system.

 

[treddie]

You need a 4G mind downloader!

 

[agadou]

Great info. Thanks for all the responses.

..So even if a reactor is scrammed, there is still the default level of heat being generated by each Uranium pellet, even though the entire system is subcritical at that point. Therefore, I assume that just like in external nuclear waste ponds, cooling fluid is still required to dissipate that heat inside the reactor. If so, then if the containment walls are too thick to effectively dissipate that default generation of heat adequately without cooling, then a meltdown can still occur as the internal vessel temperature continues to climb, albeit much more slowly. Or is the heat production rate slow enough to be easily handled by the heat conductivity of the thick steel/cement containment walls?

After scramming it is not the uranium that is producing the bulk of heat, but the radio active nuclei, daugter nuclei of fission and nuclei activated by neutron absorption during the continuous operation of the reactor. Just after scamming the heat produced by these nuclei is about 7% of the heat produced during operation. It is decaying exponentially. One hour after shut down it is about 2% of pre-shutdown power.

 

[treddie]

Ha! You read my mind! I was going to ask about the amount of heat being given off by the decay chain isotopes. But even at 7% of power or even 2%, I imagine the reactor containment is a very good insulator, so that even at very low power levels and with no liquid cooling available, that heat would still collect. Could not a meltdown still be possible for a relatively long time period (whatever that is), albeit at a much slower rate? If so, how long does it take for a typical BWR to cool down to the point that liquid cooling is no longer necessary?

 

[Astronuc]

Here is a reasonably good text and one can find other examples through Google books.

Nuclear engineering: theory and technology of commercial nuclear power
Ronald Allen Knief
http://books.google.com/books?id=EpuaUEQaeoUC

 

[treddie]

Thank you very much for the link. My next book to read.

 

[RickVS]

Can I get in on this thread? I'd like to know if there is a limit to how long a control rod can be effective. Do they at some point become unable to absorb more electrons? Is this a theoretical limitation (like maybe taking 10,000 years) or a practical one (like not lasting the life of the plant it is operating in requiring new rods to replaced the worn out ones)? Thanks!

 

[Astronuc]

Can I get in on this thread? I'd like to know if there is a limit to how long a control rod can be effective. Do they at some point become unable to absorb more electrons? Is this a theoretical limitation (like maybe taking 10,000 years) or a practical one (like not lasting the life of the plant it is operating in requiring new rods to replaced the worn out ones)? Thanks!

Yes - there is both a nuclear limit and a mechanical limit.

The nuclear limit is based on the depletion of boron 10 (B¹⁰). When the control blade looses so much worth (a measure of the ability of the rod to absorb neutrons) it is moved (shuffled or relocated) to a new less critical position, and a fresh rod is put in the place, which is usually the 'shutdown' bank.

Control blades are also subject to mechanical limits due to irradiation effects, e.g., irradation-assisted stress corrosion cracking, or mechanical deterioration of the handle and structural parts, and also the internal pressure due to He production from the neutron capture by B¹⁰, which produces an (n, alpha) reaction.

At some point the control rod is removed from the core and placed in the spent fuel pool. Ultimately it is cut up and sent to a disposal repository.

 

[M. Bachmeier]

Yes - there is both a nuclear limit and a mechanical limit.

The nuclear limit is based on the depletion of boron 10 (B¹⁰). When the control blade looses so much worth (a measure of the ability of the rod to absorb neutrons) it is moved (shuffled or relocated) to a new less critical position, and a fresh rod is put in the place, which is usually the 'shutdown' bank.

If I understand correctly, you are saying that control rods (shutdown) are the newest, but that there may be variability in their age (life-cycle as (shutdown) control rods)? Older rods would be used to control the arrangement or geometry of (power generation) normal reactivity?

 

[daumphys]

How about making small RTG(radioactive thermoelectric generator) assembled nuclear power plant? First make small RTGs in the battery plant, and power station only assemble them.
And used battery is retreated, and use it again. At the present high voltage technology is not developed, but we can make high voltage by using solar energy collection method.