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PostPosted: May 02, 2013 3:07 pm 
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I feel like a dummy for asking this but I can't recall if control rods in PWR or BWRs actually also help handle long term reactivity loss as fuel burns up (i.e. slowly withdrawn with time after a fueling)? Or is that all taken care of by burnable poisons? Are control rods really just for things like cold shutdown and over riding Xenon to some extent? I should know this but tend to follow CANDU more when it comes to water...

David LeBlanc


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PostPosted: May 02, 2013 3:51 pm 
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Yes. Burnable poisons only go so far.


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PostPosted: May 02, 2013 8:36 pm 
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Not "burnable poisons" - just straight poison, Boric acid: The running joke is that fresh LWRs reactors are "pumping boron" instead of water.

As the fuel reactivity decreases over time, the boric acid concentration is gradually reduced (ie. it doesn't burn out by itself, as burnable poisons would do).

Near the end of core life, boric acid concentration is reduced close to zero, so there is little or no reactivity margin to override a Xenon transient.

Control rods are NOT used for long term reactivity shimming.


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PostPosted: May 03, 2013 2:18 am 
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So what about BWRs? They don't use boric acid for reactivity control, they need completely neutral coolant chemistry. The recirculation pumps (for non-natural circulation designs) and the boiling in the core can help but even with optimized burnable poison in the fuel it's hard to get under 5000 percent milliRho with a fresh core. How is this offset? Control rods and grey rods are all I can think of with these large early excesses of reactivity.


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PostPosted: May 03, 2013 3:45 pm 
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I was a little lazy and didn't google the right things. Wiki has a pretty good entry talking about how most (but maybe not all?) pwrs use only boron for long term reactivity (perhaps a few designs use some control rod movement?). For BWRs , yes, no boron in the water but they by "flow control" which I would guess to mean they just allow the fraction of boiling in core to change. So allow more boiling and a lower density when fuel is fresh and then slowly increase the moderation by decreasing the amount allowed to boil (steam fraction) within the core.

David L.


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PostPosted: May 03, 2013 8:01 pm 
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I wonder if it could be used for complete control? A 3-D lattice, 5-10% enriched uranium could be placed vertically in a long vessel and water level is controlled by pressure. At increased pressure, the pressure pushes down the water and the fuel becomes sub-critical. As the bottom fuel burns up, water level increases to include more length in the burning core. The whole arrangement burns like a candle but only from bottom upwards for several years. 3-D metallic lattice can be made strong or reinforced to stay in shape. Thorium-Pu or Thorium-U233 could be used subject to availability of fissile feed.
UK would have no problem about fissile feed. Canada could get its uranium enriched.


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PostPosted: May 03, 2013 10:54 pm 
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I wonder if the Boron thing is still the main mechanism in any the new integral LWRs? The B&W mPower brochure says "no soluble boron". The old LWRs had the control rod drive mechanisms mounted outside of the pressure vessel. That means there is an awful lot of pressure trying to eject the rods which means that unprotected rod-bank run-out is an important accident scenario.

I noticed that in "Plentiful Energy", they describe the IFR's response to rod-bank run-out (not-so-energetic melting and dispersal of a few fuel rods).

In the 2012 report in the AHTR (ORNL/TM-2012/320), they say the low maximum speed of the control rod motors allows time for the thermally-triggered poison dispenser to work. (they use burnable poison, Eu2-03 plus control rods to compensate for fuel life).

[edited to show B&W info]


Last edited by Nathan2go on May 04, 2013 10:17 am, edited 2 times in total.

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PostPosted: May 04, 2013 3:49 am 
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David wrote:
I was a little lazy and didn't google the right things. Wiki has a pretty good entry talking about how most (but maybe not all?) pwrs use only boron for long term reactivity (perhaps a few designs use some control rod movement?). For BWRs , yes, no boron in the water but they by "flow control" which I would guess to mean they just allow the fraction of boiling in core to change. So allow more boiling and a lower density when fuel is fresh and then slowly increase the moderation by decreasing the amount allowed to boil (steam fraction) within the core.

David L.


A typical void worth for BWR is around 150 pcm/%void. If the excess reactivity at BOL is 7500 pcm for a properly Gd2O3 poisoned core, then that means a void fraction of well over 50% is used at BOL, and then very little void at EOL. For example it could operate on 70% void at BOL then at EOL at 20%.

This works in terms of reactivity, but does it provide sufficient cooling and stability at BOL?


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PostPosted: May 04, 2013 3:57 am 
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Nathan2go wrote:
I wonder if the Boron thing is still the main mechanism in the new integral LWRs? The old LWRs had the control rod drive mechanisms mounted outside of the pressure vessel. That means there is an awful lot of pressure trying to eject the rods. That means that unprotected rod-bank run-out is an important accident scenario.

I noticed that in "Plentiful Energy", they describe the IFR's response to rod-bank run-out (not-so-energetic melting and dispersal of a few fuel rods).

In the 2012 report in the AHTR (ORNL/TM-2012/320), they say the low maximum speed of the control rod motors allows time for the thermally-triggered poison dispenser to work. (they use burnable poison, Eu2-03 plus control rods to compensate for fuel life).


It seems very difficult to put the control rods inside the reactor, as you need electrical connections and seals. A canned drive (magnetic torque transfer) is theoretically possible, but it seems the NuScale reactor uses fairly conventional control rods.

The usual approach to preventing control rod ejection is to anchor the control rod mount/supports to a heavy steel plate. This prevents ejection even if the control rod pressure vessel extentions break off.

For the molten salt reactor, I'm thinking of a gadolinium metal (heavy) control rod encased in silicon carbide, which is pushed into an all welded cavity (no penetrations or seals) by the normal fluid flow. So emergency control rod injection upon loss of flow is fully passive. Manual scram would be by tripping the primary pumps which then shuts down flow which inserts the control rod. This way we only need control on the pumps. The control rods, with their many seals, motors, would be difficult for the high temperature high radiation, molten salt environment of an MSR, and they cost a fair penny too. A couple of welded extentions would be cheap and much more reliable.

Control rod ejection with MSRs is not plausible (even purely physically) because there's no big pressure driver.


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PostPosted: May 04, 2013 1:48 pm 
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Here's a summary of reactivity control strategies to answer and confirm some of the previous posts:

Current PWRs - Control rods generally only used for startup and other maneuvers, as discussed in previous posts. Longer term reactivity control is through burnable absorbers (there are several different types) and soluble boron.

Current BWRs - Long term reactivity control is by burnable absorbers and control blade movement. Flow control is used for finer control and to help control axial power shape. Remember that a BWR must be producing steam in the core (there are no steam generators) and so you always will need an exit void fraction of 70-80% or more.

LWR SMRs - There are some concepts that do not use soluble boron and are controlled only by burnable absorbers and control rods. Others designs retain the standard PWR approach with burnable absorbers and soluble boron.

AP1000 - Operations with burnable absorbers, soluble boron and tungsten (gray) control rods.

AHTR - For the fixed fuel ORNL concept, burnable absorbers and control rod reactivity control. The "thermally-triggered poison dispenser" mentioned in previous post is only for a secondary passive shutdown system and would not be used during normal option.


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PostPosted: May 04, 2013 2:07 pm 
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Cyril R wrote:
Control rod ejection with MSRs is not plausible (even purely physically) because there's no big pressure driver.

Because of the Chernobyl thing (iirc, where the human operators disabled the automatic controls and withdrew all of the control rods), I believe it is traditional to analyze the unprotected control rod withdrawal scenario, however unlikely, and mitigate it as required to demonstrate that the containment is not breached (if it requires no mitigation, so much the better).


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PostPosted: May 04, 2013 2:28 pm 
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Cyril R wrote:
For the molten salt reactor, ... Manual scram would be by tripping the primary pumps which then shuts down flow which inserts the control rod.

Are you talking about a design that includes continuous reprocessing?

I thought all thermal designs that don't have on-line reprocessing (such as DMSR and IMSR) would need control rods to compensate for long term reactivity changes? (even with monthly topping up, surely there would be some undesirable power/temp swing?)


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PostPosted: May 04, 2013 2:38 pm 
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Nathan2go wrote:
Cyril R wrote:
Control rod ejection with MSRs is not plausible (even purely physically) because there's no big pressure driver.

Because of the Chernobyl thing (iirc, where the human operators disabled the automatic controls and withdrew all of the control rods), I believe it is traditional to analyze the unprotected control rod withdrawal scenario, however unlikely, and mitigate it as required to demonstrate that the containment is not breached (if it requires no mitigation, so much the better).


Just another reason to not allow control rods to be manipulated remotely, and go for the passive control rod arrangement. It can't be removed by operators. Removing is only possible by starting the pumps again.

Still, I agree we should analyse ATWS events, especially combined ATWS+SBO, as the limiting beyond design basis accident for MSRs.


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PostPosted: May 04, 2013 2:49 pm 
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Nathan2go wrote:
Cyril R wrote:
For the molten salt reactor, ... Manual scram would be by tripping the primary pumps which then shuts down flow which inserts the control rod.

Are you talking about a design that includes continuous reprocessing?

I thought all thermal designs that don't have on-line reprocessing (such as DMSR and IMSR) would need control rods to compensate for long term reactivity changes? (even with monthly topping up, surely there would be some undesirable power/temp swing?)


It is generic, but no-reprocessing designs are potentially nearer to market, so let's consider them.

Long term reactivity changes would be compensated by daily top-up. There would never be more than a week of excess reactivity. During shutdowns some salt could be removed temporarily to a holdup tank to compensate cold reactivity, but it's probably not necessary. A single high worth control rod made of gadolinium metal suffices for shutdown. Depending on the design, a few more control rods could be needed for long term cold shutdown, but it's not a big deal to weld on some more cavities to the vessel head.

Startup is easiest just as electric heated (first time) or decay heated startups (with enough FPs around later), at low flow rate to prevent the passive control rod from retracting. This prevents reactivity spikes when transitioning from cold to operating temperature.

This minimum reactivity approach allows <100 degree C heatup during a control rod stuck accident (though it seems impossible).


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PostPosted: May 04, 2013 2:52 pm 
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Nathan2go wrote:
Cyril R wrote:
For the molten salt reactor, ... Manual scram would be by tripping the primary pumps which then shuts down flow which inserts the control rod.

Are you talking about a design that includes continuous reprocessing?

I thought all thermal designs that don't have on-line reprocessing (such as DMSR and IMSR) would need control rods to compensate for long term reactivity changes? (even with monthly topping up, surely there would be some undesirable power/temp swing?)

If you allow neutron absorbing control rods during normal operation then you have to analyze the improbable condition that suddenly you are without them and probably have to prove you are safe without them. An MSR regulates fine on its own so there is no reason to include them for normal operation and create this beyond design basis accident scenario. Control rods as a hard shutdown mechanism would be OK but I think you still end up proving you can shut down even if they fail. Using the force of the flow to hold them out of the salt seems attractive to me as a passive means of control.


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