Boron in a PWR

[Trinian23]

Was getting trained today on the David Besse stuff that happened not to long ago and I was wondering. Why did they decide to use both Boron and Control Rods for a PWR instead of just Control Rods? Is it because of the size of the core, low enrichment in the rods? Doesn't seem to make too much sense to me to be injecting an acid into the Coolant. Any input?

[hamsamich]

Here's what I'm thinking: Mostly Economics with a little more safety thrown in possibly?

1. Another redundant feature to help control reactivity
2. Less "flux tilting" due to more even fuel burnout due to homogenous mixture of netron absorber
3. Plant has to coast down when rods "fully out", wouldn't it be nice to just lower the concentration of something in the water to allow reactor to keep running?
4. Advantages: Higher power limits....Less control rods..less stuck rods...less frequent re-fuels....more even fuel burnout....more money....more safety.  Hopefully

[Already Gone]

Boric Acid is a very weak, non-corrosive (to stainless steel) acid.  If you are old enough, your mother used it to wash your eyes when you had something stuck in there.  Contact lenses usually come packed in Boric Acid.  It seems that Boric Acid has only two natural enemies - Carbon Steel and cockroaches. 
Normally, a PWR is kept at a neutral pH by using a caustic to balance the BA.  The ideal is LiOH.  Because Boron absorbs a neutron and becomes Lithium, burnout of Boron actually raises Lithium concentration, the balance between Boron and Lithium actually ends up resulting in higher pH.  To keep that neutral, the Lithium is removed with resin beds.
At beginning of a fuel cycle.  You might start with 1000 - 1500 ppm Boron and 2.0 - 2.5 ppm Lithium.  As Xenon builds up in the fuel, operators dilute the Boron to keep the reactivity stable.  This goes at a rate of about 3ppm per day.   When Boron gets below about 25 ppm, they switch from dilution to deborating with a demineralizer.  The whole time Boron is being balanced against xenon and Lithium is kept in line with Boron.
When Boron is reduced to zero, power is gradually reduced over a period of time because the neutron flux in that environment must increase in order to keep power steady in all that Xenon.

As a consequence of Boron reactivity control, there is a POSITIVE temperature coefficient of reactivity (alpha-T) early in the cycle.  As temperature rises, the less dense coolant not only makes the water moderator less efficient, but it causes the Boron molecules to be less dense.  Early in the cycle, the high Boron levels actually makes the less-dense Boron have a bigger effect on reactivity than the less-dense water moderator.

[cincinnatinuke]

So without Boron avail. could you still s/d via rods?  Also why no NaOH vice LiOH?  And no I havent seen a reactor past my Navy days.

BTW Any good literature I can read on this?  And my wife says I am not interesting.....it is just that I appreciate the finer things in life.

[Already Gone]

Yes, the rods will shut down the reactor.  Sodium is more corrosive than Lithim, it becomes devastatingly radioactive in a neutron flux, and it is not produced by neutron interaction with Boron.

[thenuttyneutron]

Cool welcome to DB.  I am a new guy as well there.  I am in Operations and hope to complete my education by learning the real deal about nuke power.  I learned alot in school but know almost nothing when it comes to nuke power plant operations.  The one thing I do feel comfortable disscussing is neutronics.

It is all about the neutronics.  Na is not as good as Li-7 because it has a larger cross-section for absorbtion of thermal neutrons.  Don't use Li-6 because that makes H-3(tritium).  Go to the following link and you can look at the cross sections of all the nuclides.  My best bud works at Wstinghouse and he told me about a reactor where they shove Li-6 in it to make tons of tritium.  Not sure what for but tritium can be used for not so nice purposes.  I have also seen them used in watches.

http://wwwndc.tokai.jaeri.go.jp/CN04/index.html

Na has a higher cross section than Li-7.  Also with a (n, alpha) reaction of boron with a neutron you will make Li-7 as the fuel burnout progresses.  I am sure the computer code is much nicer to model just one nuclide that is being made in the burn and being put in the reactor at startup.

Reactors are nothing more than a complex balancing act of neutrons.  You have to make the reactor prompt sub-critical and makeup with the difference with delayed neutrons.  Otherwise the reactor period would be fractions of a micro second long and how in the hell can you control that?  Neutrons are born in die in a matter of nanoseconds.  They either will leak from the reactor or will get absorbed somewhere in the reactor.  The key is to get the geometric and material buckling just right to support a chain reaction and have reactivity mechanisms in place that make the reactror stable.  Temperature coeffcients, 1/v cross sections for fission,   and the 6 groups of delayed neutrons allow us to control this reactor.  When a neutron is absorbed it is either in the fuel or not in the fuel.  When absorbed in the fuel it will either activate the U atom, both U-235 and U-238 can have this done(U-238---> U239---> Np-239----> Pu239 or U-235---> U-236+ gamma), or cause a fission in the U-235.  The fission will on average release 2.3 neutrons per fission event.  Look at the six-factor formula for more detailed info about this.

When you can get a steady state system going you will have to start fighting poison buildup.  Xenon and Sumarian are nasty poisons that can prevent some reactors from starting up after a shut down if you can't get enough positive reactivity in it.  Not only that, you will breed in Pu-239 and this makes the balancing act even more difficult.  Pu-239 is a bigger neutron maker in fission events than U-235.  I am constantly thinking in aww about the amount of engineering work  that just goes into designing a core to last 24 months.  

So in short boron is used to be a fine tuneing tool to control the reactor by modifying the material buckling of the reactor and it is used to make Li-7 to try and make the reactor coolant less acidic.



a new post just as I posted mine.  Here is a dig at the french.

They like to use Na as a reactor coolant.  Na becomes a very nasty gamma emmiter when activated by a neutron.  Imagine trying to fight a Na fire due to a LOCA and on top of that is is s**t hot with gammas.  That is one good thing about light water.  Even when you activate H-1, it will not become radioactive.  H-2, or deuterium is a stable nuclide.

Russians like to have graphite moderated and water cooled reactors.  Yuck...  That is why Chernobyl got in trouble. 

I will take a USA design anyday over all the others.

[Fermi2]

Actually it's not legal to design a core with a positive temp coefficient. Boron cores are designed with either no coefficent or a slightly negative one. If you find yourself there during physics testing you have to take certain precautions.  At EOL the Coefficient is VERY negative and in fact we have to run a test to determine it stays within the Steamline Break Analysis. BWRs on the other hand have a positive Alpha T EOL below 350 F.

The reason the Commercial industry chose boron cores is expense. They could have just as easily used more control rods, then you run into peaking factors and rod exchanges (see a BWR). It means a more expensive core.

Control Rods in a Commercial PWR can shut down the reactor, but only when it's hot. Before reaching certain temperatures during a cooldown you have to verify boron concentration above a certain value in order to ensure you're at least 1.6% Delta K shutdown at any temperature (1% when less than 200F).

Commercial PWRs actually have rod insertion limits, in other words at any given power you have to ensure the CR are above a certain limit in order to prove they can shut you down (Power defect). The reason for this is technically you can have all rods in at a commercial PWR and still generate a lot of neutron power if you're cold enough.


Mike

[thenuttyneutron]

At the BOL of a core for about 60 hours, the core has a slight positive coefficient of reactivity.  I just read the tech specs today.  It is very small but it does exist.  There are other mechanisms that will allow us to keep it stable.

[Fermi2]

Your tech specs wouldn't tell you that. What your techspecs WILL tell you is you can have a Positive Alpha T for up to 60 hours. Your Core DEsign Report (which is not a part of tech specs) will tell you if one is possible. You're still not allowed to design for a Positive Alpha T, but the NRC recognizes one may occur. Core design is not an exact science.

B and W Basis says your MTC must be less than zero at greater than 95% RTP.

Mike

[BuddyThePug]

Cool welcome to DB.  I am a new guy as well there.  I am in Operations and hope to complete my education by learning the real deal about nuke power.  I learned alot in school but know almost nothing when it comes to nuke power plant operations.  The one thing I do feel comfortable disscussing is neutronics.

 My best bud works at Wstinghouse and he told me about a reactor where they shove Li-6 in it to make tons of tritium.  Not sure what for but tritium can be used for not so nice purposes.  I have also seen them used in watches.

If one "knows" neutronics, then some of the uses of H-3 that should come to mind include: radioisotope tracers for medical research, D-T fuel for fusion reactors (my favorite disposable fusion reactor being the W88), boresight illuminators, exit signs and of course, wristwatches. Due to H-3s relatively short half-life, constant production is a necessity. Why fear the gentle glow?  

 I am sure the computer code is much nicer to model just one nuclide that is being made in the burn and being put in the reactor at startup.

That is an assumption, and an incorrect one at that.

Reactors are nothing more than a complex balancing act of neutrons.  You have to make the reactor prompt sub-critical and makeup with the difference with delayed neutrons.  Otherwise the reactor period would be fractions of a micro second long and how in the hell can you control that?  Neutrons are born in die in a matter of nanoseconds.  They either will leak from the reactor or will get absorbed somewhere in the reactor.  The key is to get the geometric and material buckling just right to support a chain reaction and have reactivity mechanisms in place that make the reactror stable.  Temperature coeffcients, 1/v cross sections for fission,   and the 6 groups of delayed neutrons allow us to control this reactor.  

6 groups is a statistical model, and one hears the term and sees that table often, but  http://public.lanl.gov/jomc/PNE/spriggs2.pdf  on page 8 shows some analyses of an 8 group model and why it is useful. As far as neutrons dying in nanoseconds, I'll bet you a can of pop that isn't true , a little Google search shows you could eat a sandwich and still have the stable free neutron exist.

When you can get a steady state system going you will have to start fighting poison buildup.  Xenon and Sumarian are nasty poisons that can prevent some reactors from starting up after a shut down if you can't get enough positive reactivity in it.  Not only that, you will breed in Pu-239 and this makes the balancing act even more difficult.  Pu-239 is a bigger neutron maker in fission events than U-235.  I am constantly thinking in aww about the amount of engineering work  that just goes into designing a core to last 24 months.  

Sumarian, isn't that the people that discovered bronze, or like Conan the Sumarian ? oh, Samarium. Well, considering  that the fission yield of Sm is nearly zero  http://www.tpub.com/content/doe/h1019v2/css/h1019v2_69.htm  I wouldnt worry about a samarium-precluded startup. Xenon is usually mitigated by waiting until it has reached equilibrium to restart, changing the moderator temperature to gain positive reactivity, or deborate. Here we find ourselves back at Boron.

As far as 24 month fuel cycles......there are places in this world where one can operate 20 years on a core (no hints from the bubbleheads    ) without the use of boron. It all depends on the fuel loading and which burnable poisons are used.

So in short boron is used to be a fine tuneing tool to control the reactor by modifying the material buckling of the reactor and it is used to make Li-7 to try and make the reactor coolant less acidic.  

So, boron is there for neutron flux control?(yes) or chemistry control?(hmmm)
I'll bet there is something called "volatile chemistry control" that a chem tech can use for that, even in a plant without boron!

a new post just as I posted mine.  Here is a dig at the french.

They like to use Na as a reactor coolant.  Na becomes a very nasty gamma emmiter when activated by a neutron.  Imagine trying to fight a Na fire due to a LOCA and on top of that is is s**t hot with gammas.  That is one good thing about light water.  Even when you activate H-1, it will not become radioactive.  H-2, or deuterium is a stable nuclide.

 

well, even H-2 has a capture cross-section of 0.0253 eV = 550.0 micro barn , and then we once again have the beautiful soft beta glow of H-3.

Also, sodium coolant is used because of the excellent heat transfer qualities. Not exactly a French idea. Unless you want to tell GE and Argonne to dump the PRIZM design.

Russians like to have graphite moderated and water cooled reactors.  Yuck...  That is why Chernobyl got in trouble. 

No one has burned down or blown up a CANDU, which also fits your description. What DID get Chernobyl in trouble is doing a complex set of experiements with a large reactor, and violating their own testing safety parameters. 10CFR50.59 keeps us from being tempted to do the same here. An excellent description is found at http://www.chernobyl.co.uk

In summary, boron is used because it is the most-cost effective means of soluble neutron absorption (the cost of soluble hafnium compounds would be staggering!) first and foremost. It does have drawbacks, as the often seen pictures of DB's head testify. It doesnt respect mechanical seals, and it is a wonderful transport agent for CRUD. You can count on spending quality time with the RP techs if you get enough on ya...

[Already Gone]

No one has burned down or blown up a CANDU, which also fits your description. What DID get Chernobyl in trouble is doing a complex set of experiements with a large reactor, and violating their own testing safety parameters. 10CFR50.59 keeps us from being tempted to do the same here. An excellent description is found at http://www.chernobyl.co.uk

CANDU reactors are water-cooled and water-moderated.  The coolant (Heat Transport System) and Moderator are contained in separate systems, but both are heavy water with no graphite to be seen.

[thenuttyneutron]

6 groups is a statistical model, and one hears the term and sees that table often, but  http://public.lanl.gov/jomc/PNE/spriggs2.pdf  on page 8 shows some analyses of an 8 group model and why it is useful. As far as neutrons dying in nanoseconds, I'll bet you a can of pop that isn't true , a little Google search shows you could eat a sandwich and still have the stable free neutron exist.

hmm ok so where are you going to put a neutron in a reactor where it will last long enough to decay on it's own?  I have never heard of a "magic cup" that can do that.  Like I said before the neutron will leak or get absored, they will exist in the reactor only small fractions of a second.

I also stand by my sodium remark.  I would rather have the high temperature gas cooled reactor.  Na only makes sense if you want a harder neutron flux. 
H-3 is used for not so nice things either at the tip of a missle or in a needle.  I hate being stuck by big burly nurses named Gina.

I never heard about a CANDU using graphite.  Also the CANDU is not a light water reactor. 

24 months is a long time for a light water reactor.  Unless you somehow get a harder neutron spectrum, you will not beable to breed enough fuel in to overcome the poisons that a being made in the fuel.

One more thing.  I would hold several liters of deuterium/tritium in milk jugs over holding a few grams of activated sodium anyday.  Betas don't scare me but nasty gammas do.

[Fermi2]

24 months is a long time for a light water reactor.  Unless you somehow get a harder neutron spectrum, you will not beable to breed enough fuel in to overcome the poisons that a being made in the fuel.

Not true. Tell  that to the USN. 24 Months was chosen because it's the longest surveillance period the NRC will let a utility get away with. No one ever analyzed for anything past that. Give GE or Westinghouse enough money, convince the NRC to extend surveillance intervals and I guarantee you there will be longer cycles than 24 months. That interval was chosen to meet legal requirements, not core requirements.

Also, boron was NOT chosen because it it partly ends up as Lithium. That's something that just happened. For PH control the commercial industry could have just as easily went all volatile.

I hope you're not getting this stuff from Davis Besse instructors!

Mike

[thenuttyneutron]

no this is from school and 2+ years of running code for neutronics.  If you read my post I said Boron is just there to change the material bukling of the core.  The fact that it makes Li-7 is just a nice side effect.  Li-7 is added at the BOL and dilluted out during the burn.

I would never want to run a core so long that I dont have any boron left in the coolant.  It is so easy to add and remove boron.  I did the code runs, 5% max enrichment does not make running a core longer than 24 months practical.

[shayne]

How many plants can go breaker to breaker in a 24 month period?

[Fermi2]

Throw that crap you learned in school out.

A BWR can run longer than 24 months on a 5% core if they so chose.

Also, Samarium is taken into account at core design. There's no such thing as a Samarium precluded start up.

Shayne I believe a couple of plants have done it. If you can go breaker to breaker during an 18 month cycle it's not that tough to go in a 24 month cycle. You go breaker to breaker in 18 months you've obviously done something right. When Fermi gets into better material condition and does a better job at root cause they'll do a breaker to breaker.  Both units I'm at have gone breaker to breaker, and if we were on an  24 month cycle I guess we could do it. I don't think you can design a commercial PWR core to go 24 months, my guess is unless you want a real expensive core the MTC at BOL would be too positive and at EOL WAY too negative.

Mike

[thenuttyneutron]

How many plants can go breaker to breaker in a 24 month period?

I don't know but I bet not many.  Most are on 18 month cycles.  We could design a graphite pile that would stop burning long after the last stars in our universe stop shining, but how practical is that?

PWR cores don't have Xenon problems either.  With just boron tweaking we can add enough positive reactivity to start it up at the Xenon peak.

[RDTroja]

PWR cores don't have Xenon problems either.  With just boron tweaking we can add enough positive reactivity to start it up at the Xenon peak.

Tell that to an operator trying to recover from a trip at the end of a fuel cycle in the middle of a coast down.

Lots of plants have gone breaker to breaker on 24 month cycles and althogh I have not done the research, I am willing to bet just from recent experience that there are as many 24 month cycles out there as 18 month (or more). The major drawback as far as I have seen (and this is strictly anecdotal, again no research) is that 24 month cycles seem harder on the equipment and harder on the fuel as far as integrity is concerned. Back in the day, (there he goes again!) when the cycles were 12 months, I don't remember having nearly as many degraded fuel problems (manifested as hot particles) as we are having in a lot of plants now. Of course, there were also 3 month outages and maintenance was not rushed through or postponed for schedule reasons.

[Fermi2]

I don't know but I bet not many.  Most are on 18 month cycles.  We could design a graphite pile that would stop burning long after the last stars in our universe stop shining, but how practical is that?

PWR cores don't have Xenon problems either.  With just boron tweaking we can add enough positive reactivity to start it up at the Xenon peak.

So your 5 weeks of comercial experience qualifies you to say not many plants have gone 24 months breaker to breaker.

Commercial PWRs are very susceptible to Xenon Transients, particularly after run backs. Compared with a BWR they're a LOT more susceptible to Xenon Oscillation, in fact in my whole time as a Licensed Operator at Fermi we never once woried about Xenon Oscillation, at the PWR I'm at we have operating strategies to avoid it.

How can you say a boron plant can't have Xenon Problems.? I know at EOL PWRs are very concerned about having  to shutdown because they'll be Xenon precluded on start up. In fact we have a item in one of our procedures that says after shutdowns at certain times of life verify if it's even possible to startup. We never had that issue at a BWR (non boron plant)

Where exactly are you getting this info??? Have you taled to any Licensed Operators at the Bess?

Right now the biggest issue with 24 month cycles appears to be Rod worth EOL, thermal limits EOL and Fuel Channel Bow (which is becoming HUGE in the BWR world!)

Mike

[HydroDave63]

I did the code runs, 5% max enrichment does not make running a core longer than 24 months practical.

Then I hope you were not running code for any site's TRIGA reactor at 20% enrichment, see http://www-rcp.ijs.si/~ric/safety_parameters-a.html

[Trinian23]

Wow, been gone a couple of days, and off this thread goes! Thanks for the input! So to make sure I have this straight. It was basically a design and cost decision because boron is very plentiful and it would allow for a smaller and less complicated core. Makes sense. Can anyone point me in the right direction for where to find out more for myself?

[rlbinc]

PWRs operate with 0% Rod Density at Full Power. Yep, all rods out. That preserves shutdown margin (SDM).
PWRs dilute and borate to oppose transient Xenon reactivity, to keep rods out past Rod Insertion Limits - which keeps SDM happy.
End of Life, Boron concentration is LOW. That means mucho dilution is required to produce significant reactivity changes - and that makes a bunch of radwaste water to process. And that converts into serious money. So, uh, yeah, Xenon transients are a big deal EOL.
Some plants have Boron Thermal Regeneration Systems - which were designed to retain Boron on one end of a Xenon Transient and then release it again on the other side. They don't work all that well.
Boric Acid - in solution is mild. Solid Boric Acid crystals subjected to heat contribute to Molten Salt Corrosion like you read about at Davis Besse. Bad news.

BWRs have a Standby Liquid Control System which uses Sodium Pentaborate solution to shut down the reactor in the event of an ATWS. (Anticipated Transient Without Scram). That's a salt, not an acid. The SLC system is maintained separate from the reactor by leakproof, explosive squib valves.
BWR control rods have Boron Carbide in them, but that Boron should never see reactor coolant.
In other words, if a BWR has Boron in the Reactor Coolant, they either have entered Emergency Procedures or somethings broke.

[HydroDave63]

Long time no see RLB, you were sorely missed!

Does anyone know the PWR vessel head inspection criteria offhand?

[870xprs]

As I recall from STA training at Maine Yankee (ages ago), at startup they pulled all the rods and diluted to power from the VCT via the charging pumps.  And they diluted the boron out very slowly.  So I'm guessing the boron gives finer reactivity control than the rods are capable of, especially at low reactor power, i.e. approach to criticality.  Don't want any surprises there.

Just a guess.

[Fermi2]

Intial start up post RFO is via dilution. I think it's to support physics testing. Most start ups during a cycle are via control rods.

Mike