Why borated water for PWR and not necessary for BWR

[hazmatguy]

Hello all,

I am a firefighter hazmat tech in Illinois.  My primary hazmat interest resides in the rad/nuc field.  Most state hazmat tech courses cover rad/nuc for only about 20 minutes out of an 80 hour course.  I think its an injustice so I have tried to educate myself and read what I can get my hands on. 

I was wondering if someone could explain why PWRs use borated water for coolant and BWRs get by using demineralized water. 

Also, I have attended a FEMA course at the Nevada Test Site called rad/nuc emergencies for hazmat techs.  It was a good class but lacked the detail I was looking for.  If you can recommend any emergency response courses available to emergency service personnel I would appreciate it. 

I would love to go to Chernobyl and take the course put on by the Chernobyl Info Center but the costs are too much and I have not found any grant opportunities.

Many thanks.

[MacGyver]

Hello all,

Hi.

I am a firefighter hazmat tech in Illinois.  My primary hazmat interest resides in the rad/nuc field.  Most state hazmat tech courses cover rad/nuc for only about 20 minutes out of an 80 hour course.  I think its an injustice so I have tried to educate myself and read what I can get my hands on. 

You're correct.  It was an injustice.   

I was wondering if someone could explain why PWRs use borated water for coolant and BWRs get by using demineralized water. 

Alright let me use a systematic / building block approach to training here.

Pressurized Water Reactors use Boranted Water to matain "shutdown margin" {overly simplified}.  Boiling Water Reactors can maintain shutdown margin without Boranted Water.  Safe shutdown margin is important.  Because with the Control Rods inserted we desire to maintain a shutdown condition.  This makes everyone in the surrounding areas happy. 

Like an engine in your car we relate to Reactors as Beginning of Life and End of Life in regards to fuel cycles.  A pressurized water reactor will have more boranted water concentrations at the beginning of core life because it has more energy at this point in its life (e.g. just like your car, it runs stronger when new).  They (i.e. pressurized water reactor plants) will be diluting it as it proceeds to the end of core life (i.e due to less energy).  This cycle is dependent on re-fuel cycles and is typically 18 months.  Some plants are on a 24 month cycle.

Even Boiling Water Reactors can inject "chemical shim" (e.g. is a means to control neutron population with chemicals versus or in addition to the control rods) to help shutdown margin if necessary.

Also, I have attended a FEMA course at the Nevada Test Site called rad/nuc emergencies for hazmat techs.  It was a good class but lacked the detail I was looking for.  If you can recommend any emergency response courses available to emergency service personnel I would appreciate it.

Contact a Nuclear Plant and talk to there fire team leader about specifics.

I would love to go to Chernobyl and take the course put on by the Chernobyl Info Center but the costs are too much and I have not found any grant opportunities.

I can think of no good reason you need to go and not one important learning objective you could attain from said trip.

Many thanks.

Clear as mud isn't it?

[cheme09]

BWRs don't use boron during normal, at-power operation because you run into the problem of boron precipitation since you're boiling, versus a one phase system in a PWR;  I think boron is still used during refueling.  MacGyver did accurately give the reason why boron is used.

I am interested in the "chemical shim" MacGyver is talking about in BWRs.  What kind of chemical shims are used in boilers?

[Cycoticpenguin]

BWRs don't use boron during normal, at-power operation because you run into the problem of boron precipitation since you're boiling, versus a one phase system in a PWR;  I think boron is still used during refueling.  MacGyver did accurately give the reason why boron is used.

I am interested in the "chemical shim" MacGyver is talking about in BWRs.  What kind of chemical shims are used in boilers?

SLC. Standby Liquid Control system is used to inject boric acid during an anticipated transient without scram incident or other tech spec. Besides controlling flow and manipulation void coefficients, thats the only other way besides control rod manipulation that Im aware of, but Im just a stupid nub. Shame justin took off, hed be able to answer this better. 

[Cherenkov]

BWRs don't use boron during normal, at-power operation because you run into the problem of boron precipitation since you're boiling, versus a one phase system in a PWR;  I think boron is still used during refueling.  MacGyver did accurately give the reason why boron is used.

I am interested in the "chemical shim" MacGyver is talking about in BWRs.  What kind of chemical shims are used in boilers?

He must be referring to the SBLC, like Charlie said. BWRs use void control as normal daily reactivity control, and the control rods provide SDM. I guess it could be called a "chemical shim" since it is primarily used during ATWS, although I've never heard of it referred to as that... until now. Good reactor vessel level control and your SBLC tank injecting makes combatting an ATWS a "low pressure" event, as far as the operator stress goes... IMO.

-C

[Cycoticpenguin]

He must be referring to the SBLC, like Charlie said. BWRs use void control as normal daily reactivity control, and the control rods provide SDM. I guess it could be called a "chemical shim" since it is primarily used during ATWS, although I've never heard of it referred to as that... until now. Good reactor vessel level control and your SBLC tank injecting makes combatting an ATWS a "low pressure" event, as far as the operator stress goes... IMO.

-C

I was under the impression we classified it as an over temperature event haha. BIIT sounds right....?? Just want to make sure my head is in the right place haha.

[Fermi2]

BWRs don't use boron during normal, at-power operation because you run into the problem of boron precipitation since you're boiling, versus a one phase system in a PWR;  I think boron is still used during refueling.  MacGyver did accurately give the reason why boron is used.

I am interested in the "chemical shim" MacGyver is talking about in BWRs.  What kind of chemical shims are used in boilers?

Part 1: BS

Part 2: BWRs have no such thing as a chemical shim.


Oh and no. Boron is not required for SD Margin.

[HydroDave63]

Oh and no. Boron is not required for SD Margin.

exactly....I'd hate to see EOL if it was....betting your reactivity control on some sleepy AO at the batch add tank....

[Fermi2]

exactly....I'd hate to see EOL if it was....betting your reactivity control on some sleepy AO at the batch add tank....

EOL is a somewhat limiting condition. On one hand Alpha T is hugely negative, on the other there's less fuel. In any event we're just like a BWR, provided all but one rod insert we have adequate shutdown margin. Now we might not be able to say so under all conditions, but the bottom line is post trip we meet the required SD Margin of 1.6% on Control Rods alone.

The boron is there to allow the Smaller PWR core to have more uranium without having to use fabricated burnable poison and to allow the Control and Safety Rods to operate fully withdrawn to ensure you can shutdown the reactor. Without the boron, post trip shutdown margin still meets TS requirements

[Cherenkov]

I was under the impression we classified it as an over temperature event haha. BIIT sounds right....?? Just want to make sure my head is in the right place haha.

I put "low pressure" in quotes meaning "low stress" on the operators. Not low pressure in the vessel.

-C

[Cycoticpenguin]

EOL is a somewhat limiting condition. On one hand Alpha T is hugely negative, on the other there's less fuel. In any event we're just like a BWR, provided all but one rod insert we have adequate shutdown margin. Now we might not be able to say so under all conditions, but the bottom line is post trip we meet the required SD Margin of 1.6% on Control Rods alone.

The boron is there to allow the Smaller PWR core to have more uranium without having to use fabricated burnable poison and to allow the Control and Safety Rods to operate fully withdrawn to ensure you can shutdown the reactor. Without the boron, post trip shutdown margin still meets TS requirements

Wow this post just cleared up like 90 questions I had haha. thanks mikey.

[MacGyver]

Dear Hazmatguy,

As you can guess by the other posts this topic is a little over your pay-grade currently.  Unless you wish to get a license and stand watch.

So, as I mentioned above, lets start with something you know and go to something you don't know.  Which is in essence the building block learning process (i.e. known to unknown; e.g. car engine to reactor example).

I was "oversimplifying" the issue.  As you can tell it is difficult to do that and maintain 100% accuracy.  Particularly in one (or more) posts here.

As mentioned before please contact a local nuclear plant and talk to their training department about your needs.  They may be able to help.

As for BZ's posts.  He is right.  Just at a PhD / NRC knowledge level.

Mac

[hazmatguy]

I think I understand the concept now.  And this is of no use in emergency response but just being curious.

So if I understand correctly.....

In BWR's void control is used under normal operating conditions to control criticality and regulate thermal output.  By void control I am guessing you are talking about steam bubbles which would be controlled by flow rates.  The faster the flow, less steam bubbles, less voids, power goes up.  Slow down the flow and the water takes longer to travel up through the fuel channels; it heats up and creates steam bubbles.  The voids reduce neutron moderation and power goes down.  So this is a negative void coefficient where the absence or reduction of the water moderator (and coolant) slows the chain reaction, totally opposite of graphite moderated reactors like RBMKs where graphite is the moderator and water is the coolant.  I'm guessing most BWRs have an emergency boric acid injection system.

In PWR's you cannot use void control since the primary loop is pressurized to prevent boiling.  So instead borated water is used under normal operating conditions.  So different concentrations can adjust power up and down.  Higher the boron content, more neutrons are absorbed and power goes down.

I did read a small paper about using burnable poisons to replace borated water in some future reactor designs.  The Davis-Beese incident was due to borated water corrosion correct?

Ok.  If I am way off please let me know.  If not, another question.  I know zirconium cladding or zircalloy is used for fuel cladding because it is neutron transparent and has high temperature resistance characteristics.  But it also has the problem of hydrogen creation in a sustained LOCA like Fukushima.  Is there no better cladding available that would not create the hydrogen problem or is it a cost issue?

And while I am asking....  Do most commerical PWRs operate around 2000psi?  What pressure do most BWRs operate at?

I will be at the University of Missouri 10MW thermal research reactor this Saturday for a tour and rad/nuc emergency class.  From what I have heard they have the most powerful research reactor in the US.  I am looking forward to it!

[Cherenkov]

You have a pretty good grasp of the basics of each.

Like Broadzilla mentioned earlier, some PWRs have high boron concentrations early in life not just for shut down margin, but to offset the large amount of extra fuel (excess reactivity) that is added for the plant to make it through a fuel cycle. As I am sure you understand based on what you said thus far, boron concentration is reduced throughout the fuel cycle until it is finally almost all gone near the end. Now, to reduce the high amount of boron in the coolant at BOL, which could/does cause you to have a positive moderator temperature coefficient for a small period of time, they do load in burnable poisons. I am not sure if "they" plan on further reducing boric acid use or not, but I doubt it. Besides Davis Besse, which was a personnel failure, the system works very well the way it is. If I was in charge of such things, I would see no need to change even considering Davis Besse. Future designs? Sure, why not? It will all be weighed but in the end, whatever has the best cost/benefit will be used.

I can't speculate as to other metals available, but I am sure there is some cost/benefit reasons for not using something more exotic. To be frank, the Fukushima accident is not a failure of nuclear power, and the fuel assemblies we have now work very well, so I see no need to change the way we do things now in the USA. Although, I would bet there will be some changes, and at the very least, the regulators taking a very close look at some things.

BWRs operate at around 1000#, give or take.

-C

[MacGyver]

I think I understand the concept now.  And this is of no use in emergency response but just being curious.

So if I understand correctly.....

In BWR's void control is used under normal operating conditions to control criticality and regulate thermal output.  By void control I am guessing you are talking about steam bubbles which would be controlled by flow rates.  The faster the flow, less steam bubbles, less voids, power goes up.  Slow down the flow and the water takes longer to travel up through the fuel channels; it heats up and creates steam bubbles.  The voids reduce neutron moderation and power goes down.  So this is a negative void coefficient where the absence or reduction of the water moderator (and coolant) slows the chain reaction, totally opposite of graphite moderated reactors like RBMKs where graphite is the moderator and water is the coolant.  I'm guessing most BWRs have an emergency boric acid injection system.

In PWR's you cannot use void control since the primary loop is pressurized to prevent boiling.  So instead borated water is used under normal operating conditions.  So different concentrations can adjust power up and down.  Higher the boron content, more neutrons are absorbed and power goes down.

I did read a small paper about using burnable poisons to replace borated water in some future reactor designs.  The Davis-Beese incident was due to borated water corrosion correct?

Ok.  If I am way off please let me know.  If not, another question.  I know zirconium cladding or zircalloy is used for fuel cladding because it is neutron transparent and has high temperature resistance characteristics.  But it also has the problem of hydrogen creation in a sustained LOCA like Fukushima.  Is there no better cladding available that would not create the hydrogen problem or is it a cost issue?

And while I am asking....  Do most commerical PWRs operate around 2000psi?  What pressure do most BWRs operate at?

I will be at the University of Missouri 10MW thermal research reactor this Saturday for a tour and rad/nuc emergency class.  From what I have heard they have the most powerful research reactor in the US.  I am looking forward to it!

Maybe I shouldn't have dumbed it down as much as I did for you.  Ask all the question you can while you are there.  Have a good trip.

[MacGyver]

I did read a small paper about using burnable poisons to replace borated water in some future reactor designs.  The Davis-Beese incident was due to borated water corrosion correct?

If you haven't read this thread it would be worth your time.  The thread talks about the new reactor designs.  It mentions Boron, BWR, PWR and rlbinc saying "we are just one Davis-Beese away from exnukeworker.com" (toward the end of the thread).

It has the usual cast and characters and some that haven't posted in a while.

If you have already read it then so much the better.

Mac







If the link doesn't work then cut and paste the following:
http://www.nukeworker.com/forum/index.php?topic=8869.0;all#top

[hazmatguy]

I have not read it yet.  I will dive into this right now.  Thanks.

[Fermi2]

Dear Hazmatguy,

As you can guess by the other posts this topic is a little over your pay-grade currently.  Unless you wish to get a license and stand watch.

So, as I mentioned above, lets start with something you know and go to something you don't know.  Which is in essence the building block learning process (i.e. known to unknown; e.g. car engine to reactor example).

I was "oversimplifying" the issue.  As you can tell it is difficult to do that and maintain 100% accuracy.  Particularly in one (or more) posts here.

As mentioned before please contact a local nuclear plant and talk to their training department about your needs.  They may be able to help.

As for BZ's posts.  He is right.  Just at a PhD / NRC knowledge level.

Mac

Trust me when I crossed over and saw they had to do Shutdown Margin calcs post trip I was like WHAT?

Our EOPs are geared to stop cooldowns. If you cool down too much you simply trip the Steam Dump Valves and let the atmospheric reliefs or gland seal remove decay heat. If you have a steam leak you close the MSIVS and the Atmospheric Reliefs remove decay heat. (Not really an option in a BWR)

If the cooldown continues you have a steam leak upstream the MSIVs, you initiate an SI and ECCS will supply PLENTY of water borated between 2500 to 2700PPM to help you stay shutdown.

Most limiting time is Zero Power, EOL, Hot Standby.

[cheme09]

Part 1: BS

Part 2: BWRs have no such thing as a chemical shim.


Oh and no. Boron is not required for SD Margin.

Why is that BS?

I understand that reactivity control requirements are met by other means, but isn't that BECAUSE of the fact that you can't use boron since your coolant boils?

Or is your point that controlling void fraction and pressure is enough to compensate for excess reactivity throughout the fuel cycle, and boron simply isn't needed in boilers?

[Fermi2]

Voids are more than enough and in fact are stronger than any coeefficient in a PWR

Additionally, Boron simply won't work, precipitation isn't the issue in a covered core, Flux profile is and boron will totally screw up a BWR flux profile.

[Cycoticpenguin]

Can someone please explain how coolant purification works on a commercial PWR please? Adding an ionic compound (boric acid) to the water makes me think you'd burn out the cationic resin super quick. Or are there provisions for that?

OP -> Zircaloy-4 alloy mostly has the best of all worlds. Two issues that we can mostly control are the only real downsides. Flourine reacts with it pretty poorly, and obviously, at extreme temperatures you start getting into the zirc-water reaction that releases H2 gas.  So, we routinely monitor Fl ion concentrations,  and we have temperature limits on our reactor (not just for this reason, but its part of it). It takes a LOCA or extreme corrosion(more likely) to cause real issues.  the other reason we use ZrA-4 is that it has an extremely low macroscopic cross section for absorbing neutrons. When looking at this aspect, aluminum is the only other even remotely cost effective solution. However, aluminum does not work well with PWR temperatures or basic chemistry.  hope this helps a bit?

[Fermi2]

Westinghouse PWRs don't use Zirc 4. They use a zirc alloy called M5. IIRC it's tougher than Zirc 4.

Also I told you in a PM. We saturate the bed with boron. The resin picked is designed to allow it.

[Cycoticpenguin]

Westinghouse PWRs don't use Zirc 4. They use a zirc alloy called M5. IIRC it's tougher than Zirc 4.

Also I told you in a PM. We saturate the bed with boron. The resin picked is designed to allow it.

thanks. And I believe you may have forgot to press send on that specific PM  

[Cherenkov]

Like Broadzilla said, the mixed bed demins are boron saturated to prevent boron take up or release, but they are temperature sensitive. Cool down the coolant entering the demin and it can take up more boron, heat it up and it can release boron. Both potential reactivity excursions.

Along with the mixed beds, there is a cation demin for de-lithiating, that can be placed into service from time to time to remove the Li7 that builds up from the B10-neutron interaction. Also, there is a de-borating demin to assist in removing boron when around 90% of the fuel cycle is over since it takes a huge amount of water to dilute when boron concentration is low, near the end of the cycle.

Hope that helps give you a better picture of PWR demin control. This is usally known as the "letdown" side of the "chemistry and volume control system" or CVCS.

-C

[Smooth Operator]

As a former PWR Chem tech....I am not missing the days of Boron sampling, 15 minute Borons, 30 min Borons, De-lithiating, calcs, samples.....pain in the pooper.

BWR...so much simpler.