12-19-2013 Design Meeting

Flow from the central reflector, as currently designed, is potentially a source of lots of bypass flow.
You could change the geometry of the slots to increase the flow resistance.
The main problem with this type of bypass flow will mean that some of the flow coming out of the core region will be above 700 degrees.
if the delta T is 20%, that’s a problem, because the core outlet will have to e mixed.
thermal power is generally flow rate * delta T

There are two different atws events.
one is LOHS : the pumps though, are still running. so, we have the capability to tune the amount of bypass flow going through the DHX. The flow pattern in this case is that the flow going through the core is 98% of the flow. 2% of the flow may be going through the DHX heat sink. However, we may not be removing heat efficiently in the DHX because it’s designed to avoid being a parasitic heat sink. A good question is what kindof parasitic heat load we get out with the DHX? Another question is whether we want those pools to be constantly be sitting there at 100C? A lower temperature may be better to avoid the concrete degradation. Having a liner with active cooling for the water pools is good. That feature would also be a good example for best practice because you can add a leak checker.

the other LOFC is less of a problem because of natural circulation.

Mike : For completeness, there’s another ATWS scenario that includes just one pump shutting down. This is an imbalanced situation in which you may see in the pumps or other features as well.
We may need to think about the single component failure accidents as a design basis concern. For this reason, the situation in which one pump shuts down is no longer a BDBE, but, rather, a DBA.

Per: We have the question about when to interrupt the supply of energy to something. So, since there are many diverse ways to interrupt power to something, good failures are failures that occur during power interruption and fail to a safe state. Still, there is sometimes the situation in which, mechanically, the component won’t fail successfully to a safe state. Also, there may be something that keeps power interruption from happening successfully, so we want to try to make sure that the safety systems are effectively physically secured because all you have to do is to interrupt malevolent actors before thety can break into containment to access equipment that they could destroy to disable expected power interruption and other safety situations.

Nico : Using Tommy’s decay heat curve, the pools can be 30 cubic meters large. This is based on 24 hours of use, using two tanks and full evaporation. This is without any re-condensing.
Per: We also have to think about the way the freezing problem is handled. You may be able to do this by activating some sells and allowing air flow only into some cells under cold ambient conditions. In this case, then, you just cool a subset of the condenser surface.

Mike: could we use our very nice heat sink to help the condenser behavior ? Or, use our very nice heat source to help the condenser behavior in this freezing situation? or
Per: Maybe we could just actively cool the secondary liner system between the pools and the concrete. Additionally, the cooling water provided to the motors may be provided by some other heat management system in the building. (fan coolers, HVAC system… ) We also should have redundant ways to cool the reactor cavity, as an investment protection system.
Raluca: Do we have a plan for such a redundant manner of concrete cooling.
Per: We could have cooling tubes that are fed by more than one cooling system. You could have three tubes that alternate going up each of the flat surfaces that alternate between three trains of service water supplied by different manifold pipes. The more difficult thing is how do we provide cooling to the thermal shield.
Raluca: That’s a good project for the 170 class!

Raluca: So, for the design of the DRACS, in addition to a safe failed state, should we also design for no-power instrumentation.
Per: First, what is a safe failed state for the DRACS? It’s likely to be a fully operational state. That means that in the longer term, we need to have energy to support the dracs and possibly need heat to avoid freezing in the dracs and heat exchangers. It may be possible to design the dracs in such a way that they fail elegantly when the salt starts to freeze in them. The dracs will certainly stop heat removal from the core.

Raluca: If you froze the downcomer, then you could potentially have freezing in the core.
Per : Temperature measurements will be important. The question is, can we measure flow?
Raluca:  What about pressure drop?
Per: The delta Ps might not be an easy way to determine flow.
Raluca: It might be that the difference in elevation between the surfaces, plus the pressure drop would give the flow.
Per : That’s worth checking

Tommy: Whatever happened to us using dowtherm for freezing experiments?
Per: It may be difficult to argue that dowtherm would behave like salt.

Does sinap have a reports repository in english?

We’ll want to focus in comsol on the biggest losses.
the ratio of DHX pressure drops to the core in the Annular AHTR was no more than two.
This points to having a strong basis for characterizing the DHX pressure drops, etc.
We’re likely to see the same stratification in natural circulation.
We should see reynolds drop for lower power levels as well.
Once we do this with better analysis, then there will be a reasonable basis with which to do some parametric analyses and sensitivity studies to see where we can make changes and gain efficiency.
Comparison between the DHX and TCHX will be important too.
Do we want to use twisted tubes in the TCHX?

Per: Discusses the plan for filling ciet.
Question: how do we pump this container if we want to empty it? (there is a little pump that can be used to prime it before siphoning.)
Also, how do we purge gas out of it?
Raluca: you can do it through ciet.

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ATWS Modeling

Inputs for TH from neutronics:

  1. power dristribution
  2. energy deposition in the graphite (neutron-gamma heating)
  3. energy deposition in the coolant (1% – generally neglected)
  4. averaging method for the coolant and kernel temperatures (right now: volumetric averaging weighed on power density)
  5. averaging method for graphite temperature (flux weighing)

does BU swelling/shringking change temp reactivity of the graphite?

  • BU-dependence of thermal expansion coefficient
  • Do we see less number of pebbles in the bed because of the swelling of the central reflector? – ask Mike

What’s the contribution of voiding vs. scattering in the graphite reactivity coefficient? – Madicken to check this

To Do: Add “ID key” for input decks – Madicken and Katy to do this

Temperature Reactivity Coefficients as of Right now:

  • All coolant (core + channels): -0.64 pcm /K
  • Coolant in the inner reflector channels: +0.36 pcm/K (likely on the order of +0.6 pcm/K, because the error bars are on this order)
  • Collant in IR + OR channels: +0.61 pcm/K
  • Coolant in the OR channels: ~0
  • Graphite IR: +1.45
  • Graphite OR: +0.67
  • Both graphite blocks: +1.89
  • Graphite pebbles: [Madicken to update]
  • Graphite in the fuel pebbles: [Madicken to update]
  • Graphite in reflector pebbles + fuel pebbles: [Madicken to update]
  • All graphite in the core (pebbles+blocks: [Madicken to update]

Uncertainty in reactivity coefficients:

  • Madicken to do a one-page design calculation on uncertainty calculation of thermal expansion coefficients; Katy/Jeff to review this; Raluca to approve it

DHX Outlet Flow Routing

The challenge with DHX flow routing to the cold leg is the we can’t use the inlet toruses at the top because they’re above the coolant fault level. I don’t know if it will be acceptable to break the azimuthal uniformity of the flow in the core during natural circulation decay heat removal, by feeding the DHX outlet directly into the downcomer.

  • Buoyancy forces are significant, and they will help work against the formation of hot spots in the core. (3-D modeling of core porous media flow is needed to check this.)
  • The pebble bed is quite good at mixing flow, so by the time coolant gets to the core outlets, azimuthal temperature uniformity will be reached anyway.
  • I don’t think that we worry about hot regions in the core, except for neutronics, which may be sensitive to funny temperature distributions in the core.
  • Do we worry about temperature gradients to which the graphite walls are exposed?

That being said, there are three options:

  1. We keep all 3 DHXs close to the hot leg, and feed the outlets to a DHX torus that then feeds into the downcomer. This torus needs to be below the elevation of the faulted level. The torus would ensure uniform cold flow distribution around the circumference of the core, thus uniform cooling of the down-comer, and azimuthally uniform temperature distribution at core inlet.
  2. We distribute the 3 DHXs equidistant around the circumference (and maybe change to 4 DHXs, so that we don’t break the 8-symmetry). The DHXs outlet routes directly to the down-comer. The problem is significantly alleviated, but maybe not entirely fixed (2-D modeling of the laminar flow in the down-comer is needed to check this).
  3. We keep all 3 DHXs close to the hot leg, and feed the outlets directly to the downcomer. (2-D modeling of the downcomer and 3-D modeling of the core are needed to check that this will work.)