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PostPosted: Nov 26, 2013 7:49 am 
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It is simple to design a graphite moderated MSR with control rods...It could be placed similar to the PBMR in the graphite neutron reflectors on the sides. For a fast reactor it is more challenging.

The question is...Does a Fast MSR need control rods???

The first person I asked if a MSFR, MCFR needs control rods was my father who was responsible for the reactor safety in the German State of Baden Wuerttemberg* for many years until retirement. Later I asked a guy who is doing this job in Switzerland. Both told me that a MSR reactor needs to have control rods to get licenced. I assume the answers would not be much different in the US.

Question:

Are there any designs of control rods and control system published for Fast MSR?
What materials are preferred for neutron absorption for fast reactors?

Holger


*Germany consists of 16 States (Bundesländer). The states are doing the regulatory supervision, control and licencing of nuclear facilities in their states based on Federal Law and regulations. BaWue had 5 commercial power plants and a research center with 2 test reactors.


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PostPosted: Nov 26, 2013 8:32 am 
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Fast fluid fuelled reactors don't need control rods. They can get very negative power coefficients.

They do need either fast fuel draining or shutdown rods for total shutdown.

Fast neutrons are difficult to absorb due to poor cross sections of all materials.

So you need to slow down the neutrons and absorb them, without producing more reactivity in the process (from the thermal neutrons).

B4C with a high B-10 enrichment is probably the best choice for a shutdown rod. Or make sure you have ultra reliable fast salt dump.


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PostPosted: Nov 26, 2013 10:12 am 
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I've wondered about using Xenon as a fast stop mechanism. Seems like we have plenty of it around and if we have a gas insertion system (like the helium sparge) then we have a means to insert it fast.

Alternately, I've wondered if there is a neutron absorber that can be inserted to the liquid that is easy to remove later.


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PostPosted: Nov 26, 2013 11:02 am 
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Lars I had similar ideas...injecting helium or having a helium chamber that is filled from the top by overpressure. A valve that opens as soon as electricity fail....But if you take a pencil and a piece of paper you will find out these options are more complex than to have control- or fast shut down rods.

Holger


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PostPosted: Nov 26, 2013 1:18 pm 
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In most LFTR designs there is some form of offgas extraction and sparge gas insertion.
In ORNL designs the sparge gas insertion was a large volume of helium.
In recent conversations with Cyril we discussed the possibility of trapping iodine and doing gas extraction on the output of the iodine trap. We did not discuss whether that would include any helium but I could see that IF we are able to collect the iodine on silver then when it decays it is already collected to a surface so perhaps the large surface area of helium bubbles aren't needed and we could do the extraction with little or nor helium.

If so, then we have offgas containers with lots of 135Xe in them. We have the fuel salt being pumped already. So a bubbler to insert the gas could inject a lot of negative reactivity (perhaps more than 6000 pcm!) within 10 seconds. And this mechanism is one that can easily be reversed when we want to restart the reactor.

I'm not sure if this is better than a hard stop control rod but it is good to have an idea of possible alternatives - especially those that are new to the trade space because our fuel is liquid.


I don't think inserting helium is effective enough to serve though. The helium has virtually no absorption cross-section so the only effect inserting helium would have is to reduce the density of fuel salt and I expect that drains or thermal expansion would be more effective.

My favorite idea so far though is a few hard stop control rods that are held out of the core by fluid pressure of the upflowing salt. Stop the salt flow and automatically (w/o electronic control) the hard stop control rods drop down and won't be removed until you get the fuel salt flowing at full rate again.


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PostPosted: Nov 26, 2013 2:57 pm 
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Hi Lars....the main challenge for a control rod is the gasket that works at 700°C and a corrosive environment. Another option is to make hull pipe and guide the control rod in the dry environment. Both options require a lot of manufacturing.

The helium gas would work by replacing salt volume in the reactor core. The liquid salt would flow in the pressurizers.

Holger


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PostPosted: Nov 26, 2013 3:58 pm 
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No gaskets... the control rod would be "canned". Just an extension of the pressure vessel, fully welded. The flow of the pump normally pushes the control rod into the welded orifice. Loss of flow would mean insertion of the control rod. Doesn't matter if it takes 5 or 50 seconds due to the inherent negative reactivity.


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PostPosted: Nov 26, 2013 4:20 pm 
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Indeed one of the larger reactivity insertions in a liquid fueled reactor is the delayed neutrons that were lost out of core during normal flows contribute to the reactivity when the flow stops. The floating control rod idea naturally more than compensates for this since we will lose much more reactivity when the control rods are inserted than the delay neutrons contribute.

I also like this idea because it means that there is no way to screw up and try to get the reactor to go critical with the control rods in place. (Screwing up this way would be bad since then any accident that ejected those control rods would lead to super-criticality). Since the control rods are physically tied to the flow you can't bring the reactor up to criticality w/o removing the control rods unless you are trying to get it critical without any heat removal.


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PostPosted: Nov 26, 2013 5:52 pm 
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A point that is adressed by engineers working in power stations is that it is a requirement to run reactor hydraulics and reactor power independent from each other.

For this purpose dry control rods are peferable. You can drive them with a motor and in case of a loss of electricity they will fall down.

Holger


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PostPosted: Nov 26, 2013 6:08 pm 
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With shut down rods based on flow, what happens if we have
a big excursion without loss of low, eg some idiot/cretin
dumps a big glob of fuel into the reactor?
Looks to me like you will have to trip the pump
which trip could fail.


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PostPosted: Nov 26, 2013 6:32 pm 
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djw1 wrote:
With shut down rods based on flow, what happens if we have
a big excursion without loss of low, eg some idiot/cretin
dumps a big glob of fuel into the reactor?
Looks to me like you will have to trip the pump
which trip could fail.


No more unreliable than a scrammed control rod. That trip could fail just as well. Nothing inherently more reliable about that.

Very easy to design the fuel addition line to not allow big sudden fuel insertions.

This type of accident isn't limiting with proper design.

Reliable pump trip is needed anyway, for freeze protection. Passive control rod simply eliminates the non-necessary systems, such as control rod logics and voting. In stead you're working with dual function on vital systems such as pump trip.


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PostPosted: Nov 26, 2013 6:44 pm 
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HolgerNarrog wrote:
A point that is adressed by engineers working in power stations is that it is a requirement to run reactor hydraulics and reactor power independent from each other.


Uhm, no. A molten salt reactor must keep hydraulics and power matched at any time it is running. Any mismatch means either subcooling or overheating, both are very bad. Avoiding thermal cycling during normal operation is also a major requirement. Yes you can use the inherent negative reactivity to shut down the reactor, but this is NOT normal operating procedure in the sense of controlling large reactor power swings quickly. It is an emergency feature. Hydraulics is what we use to sloooowly change the power output of the reactor. Less power, means slowly dialling down the pumps.

Below a certain hydraulic flow, to be determined based on thermal limits, the reactor would be subcritical, protected by the passive control rod that is fully inserted. The reactor would not go critical till the minimum flow to remove the control rod is reached, and the secondary pumps should not be started till the reactor goes critical.

Hydraulic control and matching is more important in MSRs than in other reactors. Due to freezing issues, reactivity issues (subcooling reactivity), and thermal limits.

A passive control rod is a good way to do this.


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PostPosted: Nov 26, 2013 7:09 pm 
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HolgerNarrog wrote:
A point that is adressed by engineers working in power stations is that it is a requirement to run reactor hydraulics and reactor power independent from each other.

For this purpose dry control rods are peferable. You can drive them with a motor and in case of a loss of electricity they will fall down.

Holger

I don't understand a requirement to be able to run the reactor at power even if the coolant isn't flowing. I believe it is good that the reactor power be coupled to the reactor hydraulics in the sense that when the hydraulics aren't flowing the reactor can't create power.

The reactor power and reactor hydraulics are independent in the sense that even when the reactor hydraulics are running you don't have to produce power.

Motorized control rods are OK. But you no longer have a passive safety system. If the fluid stops flowing but the electricity fails to cut off then you will gain reactivity and it will take a considerable temperature rise before the reactor shuts down. I don't think there is a risk of release but I think there is a risk of over temperature stress so that you might lose your reactor. There are a variety of ways to address this. Motorized control rods are one. The passive control rods are another.


Also, I'm not sure I understand the terminology "dry control rods".


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PostPosted: Nov 27, 2013 2:55 am 
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Hi Lars...

I don't understand a requirement to be able to run the reactor at power even if the coolant isn't flowing. I believe it is good that the reactor power be coupled to the reactor hydraulics in the sense that when the hydraulics aren't flowing the reactor can't create power.

If there is a power generation without flow the reactor will overheat...

In a MSR there is a temperature where the criticality is 1 depending on the share of fissile nuclides. That means you pump in 620°C salt and it will become 740°C at the outlet. The power generation is controlled by the volume flow and the feed pumps.

The requirement is the other way around. It is to run the reactor hydraulic - pumping the fluid thru the reactor - without generating the according power by nuclear fission. It is for start-up, shut down, maintenance reasons. Example..after a scram shut-down by control rods the reactor produces some % of power - several 100 MW - from decay heat the first seconds. It is to pump the fluid thru the circuit including the heat exchangers to transfer the heat to the secondary coolant. ...Example..during the start-up it is to pump the fluid thru the reactor and increase the power gradually to equalize the temperature in the primary circuit.

I will prepare a sketch of the "dry rods" soon and attach it.

Holger


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PostPosted: Nov 27, 2013 3:27 am 
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That is why the passive control rod would not be ejected at low flow speeds. Probably in the range of 10-20% flow. So your reactor stays shut down while you cool for decay heat. The exact value will have to be determined by the operational requirement after shutdown (temp, temp drop, etc.) and thermal limits of the vessel, etc.


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