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.


2013-09-20 Design Meeting Notes


  • DHX sits at bottom of metal lid
  • Arrangement on plate for modularity
  • Large radius elbows for DRACS loop to allow for flexible inspection instruments to be inserted
  • DHX to TCHX centerlines will be ~6m, which is a distortion compared to scaling from CIET (would be ~8.5m)
  • Each DHX should remove 2% of nominal power. 2/3 failure logic.

Hot well

  • Level difference between two hot leg penetrations in hot well creates a seal loop. 0.5m of head can be accommodated if isolation valve fails.
  • Look at calculation for hot well height required above penetrations for thermal expansion (600°C to maximum accident condition temperature) + level swell from pump operation.

Reactor cavity cover – refueling deck hatches

  • Need to have a center circular hatch to pull out center reflector
  • Hatch for reactor vessel
  • Hatches for 3 DRACS (see picture)
  • Seal for DRACS hatch could be integrated in frame design of DRACS
  • Because of high number of penetrations in cover, may need to have it be steel
  • Missile shield above has fewer penetrations and could be concrete

2013-09-20 Cavity Covers

Level swell

  • Design objective: 2.0 m of head from cold leg to hot leg (level swell in cold leg standpipe)
  • May need full 2.0 m available inside reactor vessel to accommodate full level swell (therefore need to increase vessel height to 13 m), although probably lower because the level swell in the control rod insertion channels doesn’t take head losses from cold leg and downcomer
  • If reactor vessel’s height increases by 1 m, control rod drive must be even longer (top must remain uncovered) (see picture)

2013-09-20 Vertical Stacking


2013-09-20 Vertical Stacking 2

Primary Coolant Routing for Defueling Chute and DHX

Meeting: Nicolas & Raluca

Top of DHX to Hot Leg

Connect top of hot leg to top (inlet) of DHX, above faulted level 1B. (because for FL2, heat removal is done through cavity wall cooling system, and salt level just needs to be sufficient to cover the fuel pebbles).

  • Faulted level 1: due to loss or removal of coolant from the hot and cold legs; at the bottom of the hot and cold leg reactor vessel penetrations
  • Faulted level 2: due to a reactor vessel break and primary coolant filling the free space in the reactor cavity, 0.6m below Faulted Level 1 (per PB-FHR-DESIGN-CALC-004-00A)
  • Faulted level “B”: due to drop in coolant temperature and volume shrinkage, Calculation: 7.2 m^3 [number needs to be updated] salt volume in the RV below Faulted Level 1. Assume salt temperature drops from 900oC to 600oC (same as the assumption in PB-FHR-DESIGN-CALC-004-00A, for dimensional change of reactor vessel in drain volume calculation). This would lead to 8% volume drop, so 0.6 m^3. The level drop of 0.6 m^3 would be 10 cm. (same cross-sectional area assumption as PB-FHR-DESIGN-CALC-004-00A)

Ensure symmetry among the three DHX to hot leg manifolds.

Note: It is important to have cooled downflow and heated upflow in the DHX.

DHX Inlet Manifold

To prevent sudden failure of the DHX when the level faults below the DHX inlet pipe, the top DHX coolant inlets can be designed such that as the level gradually drops the flow rate also gradually drops; (this might be at the expense of some loss in the effective elevation of the natural circulation loop, or might be mediated by internal detailed design of the DHX shell and tubes.) A manifold with most of the pipes at the top of the DHX and gradually fewer and fewer going down, would implement this concept.

Bottom of DHX to Downcomer

Done through penetrations in the core barrel, connecting to the bottom of the DHX shells. no need for a manifold here, just one hole sufices (although mechanical design considerations will need to be discussed here).

Top of the defueling chute to DHX

A penetration in the inner wall of the outer top lid will connect the top of the DHX to the annulus formed by the inner and outer top lids. This annulus will then be connected to the top of the two defueling chutes. The use of the annulus is important for flow redistribution because the DHXs are all located in one quadrant of the core, and the defueling chutes are located at 180oC from each other. From the annulus to the defueling chute, flow can be routed axially, or axially with a radially outwards component, depending on the axial distribution of the coolant ports connecting the defueling chutes to the annulus between the two top lids.

Flow through the defueling chute is downwards during forced circulation and upwards during natural circulation. Allowing upwards flow through the defueling chute in NC is important, to prevent formation of stagnant hot pockets at the very top of the defueling chute.

Fluidic diode functionality

The fluidic diode function needs to be implemented immediately after the dhx inlet manifold. If the check-valve option is chosen, then check valves can be put on the larger manifold lines, and some smaller manifold lines are kept without check valves, to ensure some amount of bypass flow.

We figured out that we can use the DHX bypass flow during forced circulation to provide cooling to the defueling chute. Calculations of the flow resistance in the different branches are needed to verify this, but it might be possible for all of the bypass flow to be routed through the defueling chute. This is achieved by using the two-branch check-valve concept to meet the diode functionality. The check-valve branch would connect the core to the top of the DHX. The small free bypass branch would connect the top of the defueling chute to the top of the DHX.

If it looks good, then we can ask David to sketch some of these (relatively minor) changes in SW, and from there the next steps are (1) pressure drop calculations (Alex can add these to his current set of DP calcs), and (2) COMSOL modeling of the forced and natural circulation cooling modes for the defueling chute.




2013-08-28 Design Meeting Notes

Design Report

  • Need to have a design report done in time to generate a summary for ICAPP (final papers due January 2014).
    • Need to have a draft design report for December.
    • Also need to have a draft white paper for the January workshop by December.
    • Regis Matzie (and Jim Rushton?) to review preliminary design. October 7-8 (preferred). Need to send draft design report in advance (mid-september) and have plant layout figured out.
    • Outcome of the meeting: 1/ take action for Mark-1 design, or 2/ implement changes for Mark-2 design.

Plant Layout

  • Low pressure air duct diameter might have to bump up to 2 m to stay at reasonable circulating power (high pressure air duct is ~ 1.5 m).
  • Most compact configuration could be ~11.5 m from center of RV to center of CTAHs. Resulting thermal expansion could be up to 12 cm.
  • Shutdown conditions:
    • All plant is stopped and put in hot standby
    • One CTAH is drained, the other is used for normal shutdown cooling
    • Elevation of crane above grade must accommodate pulling out the longest object from below grade (probably RV).

2013-08-29 - Plant layout top and side view

CTAH Design

  • CTAH head and bottom: ASME standard dished head (flanged and dished)
  • CAD model should start with external vessel of the CTAH

2013-08-29 - CTAH

2013-08-19 Design Meeting Notes

Isolation Valves

  • Free surface on hot well makes it physically impossible to pressurize the hot leg (hence the containment). Same with free surface on stand pipes on the cold leg.
  • Need isolation valves for containment and CTAHs. For hot well, use gate valve to seal entrance from hot leg (see picture).
  • In hot well, for each primary pump, have a plug for inlet to isolate the pump. This way, one loop can be operational while the other is offline.
  • Other option: 2 hot wells, with 2 hot leg valves, and dam between the 2 wells to prevent communication between the 2 if needed (with additional requirement that elevation change in the wells across all operating conditions shouldn’t be too high). In that case, have 2 parallel hot legs coming out of the vessel all the way to each well. This way, one can perform maintenance on one well while keeping the other full for decay heat removal.
  • Also option to have one hot leg coming to a hot well with 2 compartments, 2 seats, 2 gate valves (see picture).
  • On cold leg, stand pipe might look like “cold well” with cold trap filter. Filter must be at higher elevation than drain tank. Isolation valve can be another gate valve (like hot well isolation valve). Design cold well to overflow into hot well.
  • Surface area in hot well must be large to reduce level swell. Not that critical everywhere else in the system (including cold leg stand pipe). In cold leg stand pipe, add neutrally buoyant volume displacer (graphite?) to reduce salt inventory (see picture). This will have a thin diaphragm for water hammer (if explosion in CTAH).

Decay Heat Removal

  • MSBR had a DRACS for the drain tank, using flibe (so that if there was a leak, they wouldn’t contaminate primary salt)
  • For FHR, use DRACS loop with salt, transfer heat to water, then to air stack. No need for huge air stack since some decay heat will just heat up and boil water.
  • See picture: if following CIET scaling, NDHX would end up being in the space between top of reactor cavity and control rod drive missile shield.
  • Need to isolate water from NDHX to limit parasitic heat removal.
  • Water would be on tube side of NDHX.
  • The water will be at 100°C (heat removal through boiling). This way, not too low temperature (avoid freezing) and not too high temperature (worse heat transfer). Air cooled condenser will condense water back (at 100°C). Any steam that doesn’t condense will be exhausted not to generate extra pressure in the system.

2013-08-09 Design Meeting Notes


  • New Items
    • Cavity cap penetrations
    • Hot salt well sizing
    • Pebble HIS volume
  • Old Items
    • Full pull length
    • Level swell

Level swell

  • Reactor vessel should extend at least 2 m above hot leg level to accommodate level swelling.

Cavity cap

  • Top cap of reactor cavity should be at least 1m thick (biological shield). Should be lined with 1/2m thick insulation. Outside of the insulation must be actively cooled (see picture).
  • There should be ports in the top dome for shutdown and control rods, instrumentation and inspection lines, DRACS loops, defueling machines. Start with ports for the electrical heater as proof of concept for more ports and plugs with insulation (see picture).

Cavity Top w/ Plug and Cavity Wall Lining

Hot well

  • Assume total salt inventory is ~4 times core inventory: 25 m3
  • Expansion of the salt from 600 to 700°C is ~3%: 0.75 m3
  • Assuming a 3 m2 hot well surface (approximately 2 m diameter circular, or equivalent oval), need ~0.3 m high tank to accommodate level swell. Probably more for safety purposes if we go to higher temepratures under some transients.


  • Relocate them closer to reactor: lines from hot well to CTAHs would come back at an angle from hot well (helps to accommodate for thermal expansion, see picture).

Hot Well - CTAH Piping (must accommodate thermal expansion)