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PostPosted: Jun 23, 2013 1:19 am 
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In the LFTR/MSR reactor core designs I've seen in various sources on the web, the core is typically a vertically oriented cylinder with various amounts of moderator and control rods.

Imagining a 2 salt system, why not do a horizontally arranged cylinder, with a fuel reservoir below and a control pump (as opposed to rod) carefully controlling the liquid level in the core and thereby the power output?

In case of pump failure, the reactor drains right away into the reservoir (i.e. no waiting 10-15 minutes for a freeze plug to thaw).

Would the control mechanism be too slow? Too sensitive? (i.e. small change in fuel level => large change in power) Too much helium gas would risk getting into the fuel flow leading to sudden dips in power?


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PostPosted: Jun 23, 2013 7:07 am 
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jacobn wrote:
In the LFTR/MSR reactor core designs I've seen in various sources on the web, the core is typically a vertically oriented cylinder with various amounts of moderator and control rods.

Imagining a 2 salt system, why not do a horizontally arranged cylinder, with a fuel reservoir below and a control pump (as opposed to rod) carefully controlling the liquid level in the core and thereby the power output?

In case of pump failure, the reactor drains right away into the reservoir (i.e. no waiting 10-15 minutes for a freeze plug to thaw).

Would the control mechanism be too slow? Too sensitive? (i.e. small change in fuel level => large change in power) Too much helium gas would risk getting into the fuel flow leading to sudden dips in power?


Just some thoughts, not conclusions.

This type of control has been considered in the solid fuel realm with fluidized beds of ball or particulate fuel. I would try to look those up for a quick picture. I also think someone mentioned in this forum graphite balls vs block to get around the swelling/shrinkage. You could fluidized the graphite instead of maintaining a fluid level. You might have to weight the graphite or have down flow (hurts natural circulation).

Having said that, those would probably mess up the self controlling negative temperature aspects, but maybe that is ok. Otherwise, pump power would need to be maintained clean to prevent power oscillations (doable), and also need to prevent accidental over power of the pump (doable).

How would you maintain flow to the Hx for various core levels. Optimizing flow for both criticality AND heat transfer with changing core pressure drop due to temperature and graphite swelling would be tricky


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PostPosted: Jun 23, 2013 2:08 pm 
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jacobn wrote:
In the LFTR/MSR reactor core designs I've seen in various sources on the web, the core is typically a vertically oriented cylinder with various amounts of moderator and control rods.

Imagining a 2 salt system, why not do a horizontally arranged cylinder, with a fuel reservoir below and a control pump (as opposed to rod) carefully controlling the liquid level in the core and thereby the power output?

The power output in an MSR is controlled by how much power you pull out. If you pull out the same power it won't matter what level you make the fuel. This will only impact what temperature the fuel in the core is at. This is tough to get your mind around but since the nuclear reaction is exponential you can draw essentially infinite amount of power from a small volume. In nuclear the key is not how much power you can generate - rather it is how much power can you extract. If you can pull the heat out the exponential increase in power available means that the reactor core will settle in at your new power level.

In an MSR the control rods aren't needed to control the power level. They are there simply to provide an emergency cold stop. Without them the reactor core will idle along almost generating power if you stop drawing power. For servicing or protective shutdown in case of an earthquake or ... one would like a cold shutdown where the reactor is very far from critical.


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PostPosted: Jun 25, 2013 1:19 am 
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Interesting!

What I interpreted that as is something along the lines of: for a given core configuration (fuel density, moderators, physical layout) and a negative temperature coefficient, you'll promptly reach some stability temperature T_s. If you send in fuel salt with a lower temperature T_in (because you've extracted energy from the system in the HX), then the fuel salt will rapidly get heated to T_s upon entering the reactor, since until it hits T_s there's criticality going on.

But.

Having a control pump should be somewhat equivalent to being able to take fuel rods in and out of a regular solid fuel reactor. And beyond some level, the reactor should no longer be critical at any temperature.

So I guess the question becomes: how many "rods" do you need to take out to reach 0.95 of criticality (or whatever the safety margin typically required is)?


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PostPosted: Jun 25, 2013 1:22 am 
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Ed P wrote:
How would you maintain flow to the Hx for various core levels. Optimizing flow for both criticality AND heat transfer with changing core pressure drop due to temperature and graphite swelling would be tricky

I was thinking two pumps: One Fuel Pump for circulating the Fuel Salt, and one Control Pump for controlling the level.

(It becomes very similar topologically to the MSRE setup, but instead of having a salt plug you have one pipe going down to the reservoir constantly draining the reactor, and another pipe with the Control Pump constantly filling it back up again to the desired level. They'd probably be physically closer, but details, details... ;)


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PostPosted: Jun 25, 2013 1:32 am 
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jacobn wrote:
Interesting!

What I interpreted that as is something along the lines of: for a given core configuration (fuel density, moderators, physical layout) and a negative temperature coefficient, you'll promptly reach some stability temperature T_s. If you send in fuel salt with a lower temperature T_in (because you've extracted energy from the system in the HX), then the fuel salt will rapidly get heated to T_s upon entering the reactor, since until it hits T_s there's criticality going on.

But.

Having a control pump should be somewhat equivalent to being able to take fuel rods in and out of a regular solid fuel reactor. And beyond some level, the reactor should no longer be critical at any temperature.

So I guess the question becomes: how many "rods" do you need to take out to reach 0.95 of criticality (or whatever the safety margin typically required is)?


It is surprising how much you have to remove. The information is in the French papers showing reactivity change with volume change but I recall being disappointed in finding out how long it took the freeze plug drain to get the fuel to a safe condition.


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PostPosted: Jun 25, 2013 12:16 pm 
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jacobn wrote:
(It becomes very similar topologically to the MSRE setup, but instead of having a salt plug you have one pipe going down to the reservoir constantly draining the reactor, and another pipe with the Control Pump constantly filling it back up again to the desired level. They'd probably be physically closer, but details, details... ;)

Fuel salt is expensive, and there needs to be as little outside the core as possible since it drags delayed neutrons with it. We need those delayed neutrons in the core for controlability. About 1/2 of the salt outside the core (in HX, pumps, pipes,..) is in general considered OK.


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PostPosted: Jun 25, 2013 7:25 pm 
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Why not make the primary HEX into the reactor vessel?

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PostPosted: Jun 25, 2013 7:39 pm 
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Inside the reactor vessel you have lots of neutrons flying around. You really would rather not get them embedded into the structure of your HX or into your secondary fluid. ORNL did look at putting the reactors inside but decided that it was just too expensive in terms of neutrons.


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PostPosted: Jun 26, 2013 3:37 pm 
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Even if you have vessel integrated HXs (like say SmAHTR), the neutrons outside of the actual core do not contribute to the total reactivity considerably, since the flux is very low outside of the actual core.


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PostPosted: Jun 29, 2013 7:09 am 
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Fast reactivity control is done by having enough negative and fast acting feedback coefficient. It appears a simple question of arranging the right amount of graphite and fuel salt.

Pumps draining a sump is an attractive option for long term reactivity control. Since some sort of drain is needed for maintenance, a holdup tank or overflow sump type tank is attractive. The biggest design problem is probably the terrible location for a pump: if the pump hydraulic portion needs maintenance, the salt must be drained, but since the sump is the lowest part in the system... a two pump arrangement, with one pump for main flow, and another for level control, could work, but the sump pump still needs to be drainable for maintenance or inspection. Perhaps the level control pump could be at somewhat above the lowest level, but one way or another it appears a difficult maintenance/inspection issue.

Decay heat still has to be removed from the core, whether the fuel salt is in the sump tank or in the reactor vessel. A sump tank has no neutronic requirement, so could have a large surface area to volume ratio, and just transmit the heat through the sump tank wall to some passive cooling system. Since it's easy to design a compact sump with a non-compact overflow space, this would be an interesting avenue to explore - if a maintainable and inspectable pump arrangement can be achieved, that is.


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PostPosted: Jul 14, 2013 9:13 pm 
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We may nowdays regulate the flow of fuel salt going through the core varying the speed of the fuel salt pump using a variable frequency driver (VFD). We may perform a closed control loop using a temperature transmitter at the fuel salt outlet of the reactor and varying the speed of the pump. I envisage some advantanges such as saving fuel and avoinding thermal stress in the reactor.


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