Uranium & Diamonds: Heat Transfer in Nuclear Reactors

[John d Marano]

This article http://www.technologyreview.com/news/530981/new-nuclear-fuel-could-boost-reactors-but-also-safety-worries/ about new fuel rod design with better heat transfer than traditional designs reminded me of how diamonds are being used in electronics [/PLAIN] http://usapplieddiamond.com/thermal-management/[/URL] because they're such good heat sinks.

So I was wondering given that its been shown that a more thermally conductive material around uranium pellets improves the efficiency of a power plant and that diamond films are cheap [url]http://www.sciencedaily.com/releases/2013/06/130628102929.htm does coating uranium pellets with diamond make sense?Regards,
JDM

 

[Astronuc]

This article http://www.technologyreview.com/news/530981/new-nuclear-fuel-could-boost-reactors-but-also-safety-worries/ about new fuel rod design with better heat transfer than traditional designs reminded me of how diamonds are being used in electronics [/PLAIN] http://usapplieddiamond.com/thermal-management/[/URL] because they're such good heat sinks.

So I was wondering given that its been shown that a more thermally conductive material around uranium pellets improves the efficiency of a power plant and that diamond films are cheap [url]http://www.sciencedaily.com/releases/2013/06/130628102929.htm does coating uranium pellets with diamond make sense?Regards,
JDM

It's not clear that this would be economical since each pellet would have to be coated (on the circumferential surface), which is done for certain fuel pellet designs. The bulk of the thermal gradient is within the ceramic pellet. UO₂ simply has poor thermal conductivity compared to UN or UC, or some metal alloys. LWR fuel does require an enrichment of the U in U-235, and in the course of irradiation, some U-238 is converted to Pu-239/240/241, which also fission. The benefit of the oxide is that is traps a number of fission products.

Another consideration for a pellet coating is the pellet cracking due to differential thermal expansion within the ceramic pellet.

 

[John d Marano]

It's not clear that this would be economical since each pellet would have to be coated (on the circumferential surface), which is done for certain fuel pellet designs. The bulk of the thermal gradient is within the ceramic pellet. UO₂ simply has poor thermal conductivity compared to UN or UC, or some metal alloys. LWR fuel does require an enrichment of the U in U-235, and in the course of irradiation, some U-238 is converted to Pu-239/240/241, which also fission. The benefit of the oxide is that is traps a number of fission products.

Another consideration for a pellet coating is the pellet cracking due to differential thermal expansion within the ceramic pellet.

Wouldn't a durable diamond surface be another way to trap fission products if the differential thermal expansion can be solved? If I understand you correctly I think the process of coating might be able to overcome the problem as the coating only has to be a few microns thick so tiny uranium particles could be coated and then merged and coated again into ever larger particles until you have the desired pellet size . . .

JDM

 

[nikkkom]

The bulk of the thermal gradient is within the ceramic pellet. UO₂ simply has poor thermal conductivity compared to UN or UC, or some metal alloys.

I wonder how a UO2/Zr cermet would fare in this regard. (IIRC in cermets used for cutting tools metal content is usually only about 10%. Since in this case we aren't going for strength, we can use even lower percentage of Zr metal.)

 

[Astronuc]

Wouldn't a durable diamond surface be another way to trap fission products if the differential thermal expansion can be solved? If I understand you correctly I think the process of coating might be able to overcome the problem as the coating only has to be a few microns thick so tiny uranium particles could be coated and then merged and coated again into ever larger particles until you have the desired pellet size . . .

The differential thermal expansion is a consequence of the fission/energy distribution in the pellet, which at steady state establishes an approximately parabolic temperature distribution between the surface of the pellet and the center. The difference in temperature can be a few hundred °C to about 1000 °C depending on the pellet fission rate. The UO₂ ceramic has grain sizes on the order of 10 microns to 20 microns (with some grains smaller and some larger). Adding inert material such as diamond or metal like Zr as nikkkom indicated with UO2/Zr cermet means that U would be displaced, so the enrichment would have to be increased to get to the necessary enrichment.

UO₂ pellets are fabricated in a bulk/batch process where the power is pressed into pellet in a rotary press to a density of about 55-60% of theoretical density. The pressed (green) pellets are then collected in Mo boats or on sheets and are then sintered in a low humidity H₂ (from cracked ammonia) environment at 1600-1800 °C for 2 to several hours depending on the temperature.

Modern PWR fuel cycles use enrichments up to 4.9 to 4.95% U-235 in U. The maximum enrichment is always just below 5% since 5% is the legal limit in commercial LWR fuel, and the 0.1 to 0.05% difference is there to allow for uncertainty in the manufacturing process.

 

[nikkkom]

People have been looking at Molybdenum matrix cermet fuel:

http://www.nrg.eu/docs/nrglib/2004/2004_nucl_techn_146_3_bakker_klaassen.pdf

Looks quite good, although they need to remove Mo-95.

 

[mheslep]

This article http://www.technologyreview.com/news/530981/new-nuclear-fuel-could-boost-reactors-but-also-safety-worries/ about new fuel rod design with better heat transfer than traditional designs reminded me of how diamonds are being used in electronics [/PLAIN] http://usapplieddiamond.com/thermal-management/[/URL] because they're such good heat sinks.

So I was wondering given that its been shown that a more thermally conductive material around uranium pellets improves the efficiency of a power plant and that diamond films are cheap [url]http://www.sciencedaily.com/releases/2013/06/130628102929.htm does coating uranium pellets with diamond make sense?Regards,
JDM

The metal fuel rod technology from your TR reference would be superior in heat conduction to any attempt to place a *coating* around some traditional oxide fuel, as the improvement of heat conduction from the center of the fuel to the edge of the fuel would greater than any aide offered by a coating alone.

 

[mheslep]

The differential thermal expansion is a consequence of the fission/energy distribution in the pellet, which at steady state establishes an approximately parabolic temperature distribution between the surface of the pellet and the center. The difference in temperature can be a few hundred °C to about 1000 °C depending on the pellet fission rate. The UO₂ ceramic has grain sizes on the order of 10 microns to 20 microns (with some grains smaller and some larger). Adding inert material such as diamond or metal like Zr as nikkkom indicated with UO2/Zr cermet means that U would be displaced, so the enrichment would have to be increased to get to the necessary enrichment.

UO₂ pellets are fabricated in a bulk/batch process where the power is pressed into pellet in a rotary press to a density of about 55-60% of theoretical density. The pressed (green) pellets are then collected in Mo boats or on sheets and are then sintered in a low humidity H₂ (from cracked ammonia) environment at 1600-1800 °C for 2 to several hours depending on the temperature.

Modern PWR fuel cycles use enrichments up to 4.9 to 4.95% U-235 in U. The maximum enrichment is always just below 5% since 5% is the legal limit in commercial LWR fuel, and the 0.1 to 0.05% difference is there to allow for uncertainty in the manufacturing process.

As I understand the metal rod concept from Lightbridge, the idea is to replace oxygen in UO2, not uranium.

 

[mheslep]

I wonder how a UO2/Zr cermet would fare in this regard. (IIRC in cermets used for cutting tools metal content is usually only about 10%. Since in this case we aren't going for strength, we can use even lower percentage of Zr metal.)

The reference from the OP refers to a U/Zr alloy metal.
http://www.technologyreview.com/news/530981/new-nuclear-fuel-could-boost-reactors-but-also-safety-worries/

The Lightbridge fuel is instead made of zirconium/uranium alloy, with a cross configuration and spiral shape, which makes it look like a piece of Twizzlers candy. The metal composition means heat transfers far faster, and the shape increases the contact area between fuel and water by more than 35 percent. To cope with the increased intensity, water must move through the reactor core more quickly, but existing water pumps can handle this because the fuel provides less resistance to the flow.
...
Inserted in a conventional reactor, the new fuel could boost power 10 percent. Replacing equipment including turbines with larger-size ones would increase that to 17 percent, Lightbridge says.

 

[Astronuc]

Lightbridge has an interesting concept that has yet to be proven in an LWR environment. Such fuel will have potential issues with fuel-cladding chemical interaction, depending on the fuel alloy. Metal fuel is beneficial from a fissile density and thermal conductivity perspective, but there are other disadvantages with respect to reduced melting temperature and fission product mobility.

The OP mentioned uranium pellets, and the bulk of commercial fuel is comprised of UO₂ ceramic pellets clad in zirconium alloy tubing. Even most fast reactor fuel is UO₂ or (U,Pu)O₂, although some carbide, nitride and carboxide or carbonitride fuel has been tested.

In addition to normal operation, a designer must also consider the off-normal and failed fuel situations, as well as accident (RIA and LOCA) situations, and the consequences of the fuel form and it's behavior in the different situations. UO₂ is more chemically stable in high temperature water coolant than UC/MC or UN/MN, where M = (U, Pu).

Meanwhile - the Lightbridge patents on a Th-inspired novel fuel design for LWR and CANDU fuel.
http://www.google.com/patents/US20110255651
http://www.google.com/patents/US20130322591
http://www.google.com/patents/US20110311016
http://apps.shareholder.com/sec/vie...=AMDA-16UEEM&docid=9884078#FORM10K_HTM_PAGE_5 (see page 7)
KEY FUEL DEVELOPMENTS IN 2013

I'm not aware of actual fuel fabricated by LB unless it's in Russia.

 

[nikkkom]

Carbide and nitride fuels have a problem of generating C-14, which bioaccumulates and has a very nasty half-life of 5700 years.

 

[mheslep]

Lightbridge has an interesting concept that has yet to be proven in an LWR environment.

The reference suggests metal alloys have been tried in subs, thus military LWRs.

but there are other disadvantages with respect to reduced melting temperature and fission product mobility.

I would think the melting point of UO2 is not the primary accident event issue, post Fukushima, but rather the hydrogen production from Zirc metal when oxidized by water, which is well under way at 700C. Fission product mobility, as you suggest, seems like an intractable problem for a system that won't well tolerate those hot isotopes in the loop.

 

[Astronuc]

The reference suggests metal alloys have been tried in subs, thus military LWRs.

or more generally naval reactors. I believe metal fuel has been used in reactors of Russian icebreakers, e.g., KLT-40 reactor.
http://scienceandglobalsecurity.org/archive/sgs14diakov.pdf
http://www.iaea.org/NuclearPower/Downloadable/aris/2013/25.KLT-40S.pdf

I would think the melting point of UO₂ is not the primary accident event issue, post Fukushima, but rather the hydrogen production from Zirc metal when oxidized by water, which is well under way at 700C. Fission product mobility, as you suggest, seems like an intractable problem for a system that won't well tolerate those hot isotopes in the loop.

Certainly the rapid oxidation and disintegration of the cladding (Zr-alloy or otherwise) is a concern. There are other design considerations on the cladding as well. Not too many systems handle high temperature steam conditions, especially as the temperature exceeds 500°C for long periods, or possibly as may have been the case with Fukushima, > 1000°C.

With regard to diamond coating of fuel or fuel particles, one has to look at the engineering feasibility and cost, and one must be familiar with the technology, much of which is proprietary or otherwise sensitive.

There is also the neutron damage to the diamond structure, which would also change the composition through transmutation, in addition to displaced atoms.

 

[jim hardy]

I can only speculate here..

Interesting thought. Mix diamond right in with the fuel pellet.

What would be effect of having so many carbon atoms in such proximity to the fuel ?
Seems to me moderator temperature feedback would be really quick.

But would such energetic neutrons break up the diamond crystal lattice?

 

[nikkkom]

I can only speculate here..

Interesting thought. Mix diamond right in with the fuel pellet.

In the limit, you end up with just having uranium carbide fuel. Which may be not that bad, although I don't like that neutron activation will produce lots of C-14.

 

[QuantumPion]

In the limit, you end up with just having uranium carbide fuel. Which may be not that bad, although I don't like that neutron activation will produce lots of C-14.

No it won't. Carbon does not absorb neutrons. That is why it is a good moderator. C-14 in the environment is produced from nitrogen.

 

[jim hardy]

The absorbtion cross section for C13 is small but finite, around a millibarn.
Nitrogen's is a bit shy of 2 whole barns.
http://www.isis.stfc.ac.uk/learning/neutron-training-course/downloads/general/sears---neutron-cross-section-table10655.pdf

useless and boring trivia -
That's why we purged our incore neutron detector tubes with CO2 not air. Flux in there was around 10^14nv.
We lubricated the steel drive cables with graphite. As best i recall the prominent activation product on the drive cables was cobalt. Of course the little fission chambers themselves (Oralloy) got the hottest.

old jim

 

[Hologram0110]

So I was wondering given that its been shown that a more thermally conductive material around uranium pellets improves the efficiency of a power plant and that diamond films are cheap http://www.sciencedaily.com/releases/2013/06/130628102929.htm does coating uranium pellets with diamond make sense?

I know a Czech researcher working with a group studying diamond coatings who is studying coating the outside of the cladding material to reduce oxidation during accidents. Another research group is looking at using diamond coating on the inside of the cladding to mitigate some chemical interactions between fission products and the Zr. Neither of these are trying to take advantage of the thermal conductivity but instead the chemical and mechanical stability. The coating in question is a mix of diamond and graphite (to give flexibility and toughness).

The biggest issue with UO2 fuel is that it performs best at low temperatures (thermal expansion, fission product retention, margin to melting ect) but it has a poor thermal conductivity. To improve this property people have suggested doping UO2 to produce heterogeneous materials with better macroscopic heat transfer properties. A number of materials are being looked at mostly BeO, SiC, diamond and nano-tubes. In these cases it isn't a coating, but instead bulk material added to the pellet. You could imagine a pellet with many strings of high thermal conductivity material carrying heat from the center of the fuel to the surface.

 

[nikkkom]

No it won't. Carbon does not absorb neutrons. That is why it is a good moderator. C-14 in the environment is produced from nitrogen.

At high neutron flux figures typical for power reactors at 100% power, some carbon in carbide _will_ absorb neutrons. Fuel is exposed to this flux for at least one year, typically more.

 

[nikkkom]

In these cases it isn't a coating, but instead bulk material added to the pellet. You could imagine a pellet with many strings of high thermal conductivity material carrying heat from the center of the fuel to the surface.

Yes, that's why cermet (a ceramic sintered with about 10% of metal powder) immediately pops into my mind. Its thermal conductivity is higher than of pure ceramic.

It is a well-known class of materials, used for cutting tools and other high tech applications. There are tons of research on it - for example, how different sizes of initial powders affect the resulting material, what happens if you add a powder (possibly of a different metal) which consists of tiny needle-like particles, not spheres (some tests show that it tends to reinforce material, like microscopic rebar)...

As a starting point, I'd try a cermet with uranium metal (added benefit of more uranium in the fuel, by weight). Also l'd try depositing a thin layer of Zircalloy on the cermet pellet's surface - there's hope this can eliminate chemical interaction with cladding tube.

 

[Hologram0110]

Yes, that's why cermet (a ceramic sintered with about 10% of metal powder) immediately pops into my mind. Its thermal conductivity is higher than of pure ceramic. As a starting point, I'd try a cermet with uranium metal (added benefit of more uranium in the fuel, by weight). Also l'd try depositing a thin layer of Zircalloy on the cermet pellet's surface - there's hope this can eliminate chemical interaction with cladding tube.

U metal in an LWR/PHWR is pretty though sell in terms of safety. From a reactor physics U metal is great (density, thermal conductivity) but it is far too chemically reactive during accidents and won't contain fission products. It would significantly improve reactor performance in a number of ways. It would be great if you could rely on no cladding failures to eliminate the need for fuel/coolant chemical compatibility, but historical evidence doesn't support that. There are too many failure modes for fuel cladding (core damage, debris fretting, corrosion, over pressure).

In reactors cooled with something other than water you may be on to something since you don't need to worry about fuel oxidation.

 

[nikkkom]

U metal in an LWR/PHWR is pretty though sell in terms of safety. From a reactor physics U metal is great (density, thermal conductivity) but it is far too chemically reactive during accidents and won't contain fission products.

I know about this concern. That's why I'd look into adding an additional barrier - a thin coating of Zr over the cermet pellet. Even if Zr tube fails and water touches pellets, it won't come into contact with uranium metal - it will touch zirconium.

Historical evidence is not very nice towards current breed of purely ceramic pellets either. Since specific powers have gone up, and burnup gone up too, even ceramic gets damaged: IIUC pellets crack and swell, and if Zr tube fails and water touches pellets, significant fraction of more volatile fission products, namely krypton and xenon, iodine, caesium, strontium get washed out.

Cermet pellets may fare better because they would be under far lesser thermal stress.

 

[Hologram0110]

I know about this concern. That's why I'd look into adding an additional barrier - a thin coating of Zr over the cermet pellet. Even if Zr tube fails and water touches pellets, it won't come into contact with uranium metal - it will touch zirconium.

Historical evidence is not very nice towards current breed of purely ceramic pellets either. Since specific powers have gone up, and burnup gone up too, even ceramic gets damaged: IIUC pellets crack and swell, and if Zr tube fails and water touches pellets, significant fraction of more volatile fission products, namely krypton and xenon, iodine, caesium, strontium get washed out.

Cermet pellets may fare better because they would be under far lesser thermal stress.

To prevent fuel coolant interaction your fuel would have to be resistant to cracking due to the temperature gradient. This is a significant issue for most fuels. For solid-metal fuels swelling is a big problem which would also destroy the integrity of any coating on the surface. The idea of diamond coating on the outer surface of fuel (or inner surface of the cladding) is certainly helpful, but doesn't fix the core of the problem steam interactions with fuel can still happen.

I've read about a fuel design which was briefly tested which used liquid metal fuel in a stainless steel cladding. The idea is that you design the fuel to melt by design, thus you do not have complicated fuel-cladding interaction, virtually no fuel swelling and good thermal contact. Unfortunately the problem was chemical reactions with the cladding (due to fission products and radiation). Such a design might benefit from a diamond coating.

Personally I think the simpler answer to increase the surface area to volume ratio on fuel by moving away from cylindrical fuel pins (like the lightbridge design). This lowers the temperate improving all the other properties.

 

[nikkkom]

Here's a link where cermet properties with different metals, ceramics and grain sizes are looked at. They look at it from cutting tool POV, specifically woodcutting. It turns out some grades of wood are somewhat acidic, they literally dissolve the tool as it cuts:

http://www.carbideprocessors.com/pages/carbide-parts/making-cermet-material.html

Other attempts at reducing fuel temperature gradients are looked at by Russians, they experiment with fuel in a form of small ceramic grains, almost powder, vibro-packed into a Zr tube.

 

[QuantumPion]

At high neutron flux figures typical for power reactors at 100% power, some carbon in carbide _will_ absorb neutrons. Fuel is exposed to this flux for at least one year, typically more.

You said "lots". Specifically, you inferred that uranium carbide fuel would be undesirable merely due to C-13 activation. The amount of C-14 produced would be totally insignificant compared to activation products from any other impurities in the fuel, cladding, or coolant.

 

[nikkkom]

Unlike other activation products, C-14 can volatilize during accident, it bioaccumulates and has a bad half-life. For example, Chernobyl's main contaminant, Cs-137 will eventually be almost gone after ~300 years. If Chernobyl would release C-14, there would be no hope of waiting it out, even for our grand-grand-grand-grand-grand-grand-grandchildren.

You are right that it is not a show-stopper, it is merely a downside, but oxide or metal fuels don't have this downside.

 

[QuantumPion]

Unlike other activation products, C-14 can volatilize during accident, it bioaccumulates and has a bad half-life. For example, Chernobyl's main contaminant, Cs-137 will eventually be almost gone after ~300 years. If Chernobyl would release C-14, there would be no hope of waiting it out, even for our grand-grand-grand-grand-grand-grand-grandchildren.

You are right that it is not a show-stopper, it is merely a downside, but oxide or metal fuels don't have this downside.

Chernobyl was a graphite moderated reactor and released tons of burning activated graphite right into the atmosphere. While C-14 increases were detected, the biological consequences of the C-14 contamination were and remain completely negligible compared to everything else that was released. Your original point is unfounded.

 

[nikkkom]

A paper on Chernobyl C-14:

https://journals.uair.arizona.edu/index.php/radiocarbon/article/download/2026/2029

6 months after accident, C-14 activity in ejected graphite particles was 100-300 times less than Cs-137. Since Cs-137 half-life is 190 times shorter, it looks like there are about equal amount of atoms of both nuclides there.

Now, and 10 years from now, and 100 years from now, Cs-137 activity will still dominate around Chernobyl. But in 900 years, Cs-137 activity will fall by about 1 million times, while C-14 will barely change.

 

[QuantumPion]

A paper on Chernobyl C-14:

https://journals.uair.arizona.edu/index.php/radiocarbon/article/download/2026/2029

6 months after accident, C-14 activity in ejected graphite particles was 100-300 times less than Cs-137. Since Cs-137 half-life is 190 times shorter, it looks like there are about equal amount of atoms of both nuclides there.

Now, and 10 years from now, and 100 years from now, Cs-137 activity will still dominate around Chernobyl. But in 900 years, Cs-137 activity will fall by about 1 million times, while C-14 will barely change.

Good find. According to the paper, the C-14 released was between ~ 0.04 to 6 PBq as a result of 1.7 million kg of graphite moderator released. This is about 1-3 orders of magnitude less than the release of other major isotopes (compare to ~80 PBq of Cs-137). The carbon mass content of a carbide fueled core would be on the order of 10 tons or so - 5 orders of magnitude less mass than Chernobyl's graphite moderator. So we're talking about 6-8 orders of magnitude less activity compared to other products. Thank you for verifying my point.

 

[jim hardy]

There is a biological multiplier that validates Nikkom's concern

Asimov pointed out in his "At Closest Range" essay that C14 is the only naturally radioactive isotope that shows up in quantity in the middle of the DNA molecule.
DNA molecule is a small target for cosmic rays, but from inside the molecule it's a far larger one.
So when we increase the amount of C14 in our DNA we likely increase our mutation rate ,
Linus Pauling picked up on this and got a nobel prize...

Pauling calculated that the output of C-14 from the then scheduled weapons tests would cause 55,000 children to be born with gross physical and mental defects, result in more than 500,000 miscarriages, still births, and newborn deaths, and cause as much leukemia and bone cancer as that caused by all the fission products from the explosions combined.
The public controversy, sustained by Pauling's robust contributions, eventually induced the superpowers to suspend the testing of atomic bombs in the atmosphere; they signed the treaty in 1963, and it went into effect on the very day of the bestowal of the Nobel Peace Prize for 1962 on Linus Pauling. Throughout his campaign against the weapon tests in the polarized American political climate of the 1950s, Pauling had to endure the impugning of his citizenship and even the official affront of the lifting of his passport for a time by the Department of State. As late as 1963, his Nobel Peace Prise was headlined in Life magazine as a "Weird Insult from Norway."

http://internetwks.com/pauling/alp.html

 

[QuantumPion]

I suspect that is entirely political in nature. Those figures were not part of peer-reviewed research, but calculations and estimations made by one man. Note he won a nobel peace prize, not physics. In fact, the guy that actually invented C-14 dating, whom did win a nobel prize in chemistry, countered Libby's claims. But this line is out of my area of expertise so I may be full of it, this is just my guess based on superficial knowledge.

 

[jim hardy]

But this line is out of my area of expertise so I may be full of it, this is just my guess based on superficial knowledge.

It's beyond mine, too... i'd have preferred to quote Asimov's line of reasoning but can't find that essay anywhere. I read it in his 1961 collection "Fact and Fancy".

After reading it I surmised that Mother Nature made C14 occur naturally so as to assure evolution...
but i digress.

 

[nikkkom]

After reading it I surmised that Mother Nature made C14 occur naturally so as to assure evolution...

I think DNA mutation rate is tailored by evolution to be somewhere near an optimal point where mutations are not happening too often to kill significant percentage of offspring, yet they happen often enough to constantly generate new DNA sequences.

We discovered many ways how DNA damage is getting repaired, and surprisingly, there are different mechanisms in play _and not all of them are employed by any single species! If somehow you'd create a new species which use all known mechanisms, it would be very resistant to mutations, cancer, and radiation - and not be able to adapt.

If we'd live on a planet with much lower natural background, I think we would just have fewer DNA repair mechanisms...

 

[Astronuc]

Carbon-12/-13 absorb few neutrons, since the cross-section is so low. I looked at the elemental cross-section, which is mostly C12, and it is <1e-2 b, so activation would be very little since other nuclides have much high cross sections. Even spallation reaction (n,p), (n,d), (n,α) cross-sections are low, and would also require neutron energies in the upper fission energy range or greater. The other part of the activation process is the resident time. In modern US LWRs, residence time may be about 1000 - 1100 days (two high capacity 18-mo cycles), 1320 - 1460 days (two moderate to high capacity 24-mo cycles), or possibly 3 18-mo, or 24-mo cycles, for a minority of the fuel. Many other LWRs are still on annual cycles, so some fuel can spend 5 or 6 annual cycles in a reactor, and some might go as long as 7 or 8 annual cycles.

When incorporating an additive into a fuel matrix, one has to be concerned about displacing the fuel (U, Pu, Th) atoms, particularly in an LWR.

Furthermore, in an LWR, the fuel designer has to be concerned with fuel-coolant (water/steam) chemical interactions and hydrogen pickup, more so if moves away from Zr-alloys for cladding.

 

[jim hardy]

That's an interesting train of of thought , Astronuc

We doubtless make a minute amount of C14 in the air surrounding a LWR via leakage neutrons and gamma.

aha somebody has studied that.
Measurements at Ginna report ~a half curie per year from containment air

http://nepis.epa.gov/Exe/ZyNET.exe/9100BW8L.txt?ZyActionD=ZyDocument&Client=EPA&Index=1976 Thru 1980&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&UseQField=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:\ZYFILES\INDEX DATA\76THRU80\TXT\00000012\9100BW8L.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=p|f&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=30

page 2-16The UN (Uranium monoNitride) LWR fuel being contemplated would use nitrogen enriched to >90% N15 so it won't make so much C14
http://www.inl.gov/technicalpublications/Documents/5869831.pdf section 4
...you nukes are a thoughtful lot.