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.





Flibe Inventory

Flibe inventory will be calculated based on volumes of the FHR sub-systems and components containing the primary salt. A list of these sub-systems and components is provided here. Some volumes can be directly computed from the current FHR CAD model. When geometry/volumes are unknown, the sub-system or component has been italicized and sources for future calculations are provided where possible.


  • Active core region
  • Defueling chute

Reactor internals

  • Coolant injection channels
  • Control rod channels
  • Inner reflector bypass paths (estimate from graphite blocks geometry, thermal expansion and resulting gap size)
  • Pebble injection lines
  • Free space below pebble bed (will depend on pebble injection method as well as divider plate geometry)
  • Outer reflector bypass paths (estimate from graphite blocks geometry, thermal expansion and resulting gap size)

Cold leg

  • 2 x CTAH to drain tank (need to locate drain tank)
  • 2 x stand pipe (need to estimate level swell based on flow velocity and properly size the stand pipes)
  • 2 x drain tank to reactor cavity wall (need to locate drain tank)
  • 2 x reactor cavity wall to reactor vessel wall
  • 2 x reactor vessel to downcomer coolant distribution system
  • downcomer

Hot leg

  • Hot salt collector ring
  • Hot salt extraction (collector ring to reactor vessel wall)
  • Hot salt extraction (reactor vessel wall to reactor cavity wall)
  • Hot salt extraction (reactor cavity wall to hot salt well) (need to locate hot salt well)
  • Hot salt well (need to estimate volume; based on level swell, what else?)
  • 2 x hot salt well to CTAH (need to know relative locations of hot salt well and CTAHs)

CTAH (need to estimate CTAH tube bundle total volume)

DHX (primary side)

  • Hot leg to DHX (need to figure out pathway)
  • DHX (shell side) (need to adapt numbers from MSBR heat exchangers; look at Cristhian’s RELAP model; implement heat transfer coefficient multiplicator based on twisted tube HX performance)
  • Check valve
  • Valve to downcomer (need to figure out pathway; this will be a bypass path under normal operation)

DRACS loop

  • DHX (tube side) (need to adapt numbers from MSBR heat exchangers; look at Cristhian’s RELAP model; implement heat transfer coefficient multiplicator based on twisted tube HX performance)
  • NDHX (tube side) (need to adapt numbers from European Breeder Reactor and other metal-cooled reactors designs; look at Cristhian’s RELAP model)
  • DRACS piping (total inventory will be based on required elevation difference between the DHX and NDHX centerline and pipe size to drive natural circulation; performance will be based on a 2/3 rule and 2% nominal power extraction [or should this be 6%?], hence 1% nominal power [or 3%?] per DHX; need to determine design average and delta T between hot and cold leg of the DRACS loop, based on DHX LMTD)

Fuel handling system

  • Defueling mechanism
  • Storage space (use wet canisters? room filled with flibe? should we use some multiplication factor of the core salt inventory? Mike is working on this)

2013-08-02 Design Meeting Notes

Inner reflector design

  • Update: instrumentation channels should go all the way through (top to bottom).
  • In addition to tie rods in lobe corners, some of the channels will be instrumented with neutron flux measurement.
  • Bottom of buoyant control rod channels must be a blind hole, so that pressure differential on the control rods will induce proper snubbing. Need to calculate required length of blind hole below perforated region of the control rod channel (will affect rod length, therefore stacking problem).
  • Must be careful with stiffness of the channel to cope with pressure differential.
  • Consider having some kind of porous structure at the bottom of the channels to ensure that the rods stop without damaging the structure itself if the channel walls are cracked and the pressure differential is not enough to stop the rod. Need a way to monitor if structures have been damaged.
  • Have another inlet flow chanel in the thick bottom part of the reflector that merges into the control rod channels (more flow area, lower flow velocity). Refined flow areas and pressure differential calculations must be performed. Need to take into account entry losses at the bottom of the inflow channels.
  • Add small channels (1 cm diameter) for flow outlet from the control rod channels. Adjust number and spacing of these channels based on desired flow distribution (based on pebble bed dynamics). Probably 2 columns of small channels per control rod channel.
  • Tip of control rods: load with neutron poison? Amount of graphite? Need to make sure not to insert positive reactivity feedback (based on FHR core neutronics analysis)
  • Plan for the Fall: work on COMSOL coupled stress and fluid analysis (Jae? Alex?)



  • Taper entrance to the combustor (curvature radius should be 1/4 – 1/3 of line radius).
  • Secondary hot duct to vent stack should be smaller (can take higher velocity).
  • Annulus around combustor should be larger (1/4 of total area). Wall should be thin (at pressure equilibrium, takes highest temperature).
  • Diamond structures (insulation) should be thinner to be flexible.


  • Thickness of outer insulation will depend on heat losses and need to have outer wall temperature remain below regulatory limits (wall cannot be >50°C for workers protection). Based on fiberglass insulation thermal conductivity and natural convection heat transfer coefficient.

Core design

  • Block with hot leg can be larger to accommodate size of hot leg (low dose rate).
  • Hot duct liner should extend a bit down into the graphite reflector.
  • Need to analyze thermal stress transient when flow reversal through DHX.
  • Need to have small gap (1 cm?) between fixed and movable metal rings in upper core structure.
  • Flow direction in defueling chute: probably want it to be downards so that upper core structures don’t get heated up, but will make defueling more complicated (pebbles cannot be entrained in flow outside of the core).
  • Shell structure (above cold leg, below hot leg): get elevation transition closer to hot leg.
  • Need to work on detailed design of reactor vessel/fire brick type of insulation/liner/concrete.

Central Reflector Design

Structural integrity of the lobes

  • constant cross-section  in the region that is not irradiated, so that it can bend without creating stress concentrations
  • shrinkage due to differential dose
  • talk to Madicken about central reflector irradiation dose
  • we’ll probably run the inner reflector until it have reached turn-around (why?). turn-around is when graphite goes from irradiation shrinkage to swelling.

Cooling considerations

  • additional coolant channels needed radially through the lobes?
  • COMSOL model of top-view?
  • consider what the dimensional changes are,  and how they affect the change in flow resistance
  • thicker, then narrowing slots, to limit pressure drop (maybe this is not needed)?

Flow in the Defueling Chute and S/D rod channel

  • design for downflow through the defueling chute? this will compete with buoyancy forces, and may lead to instabilities.
  • the alternative is upward flow, and collecting all bypass flow, then joining them to the hot leg
  • diode at the top of the S/D rod channel to allow for hot downflow when pump is off.
  • Show inserted and pulled up S/D rod in the channels.


  • What is the neutron shielding thickness needed at the top of the core, for stadoff distance of metallic components?
  • Is there a way to do concentric piping for hot metal ducts, and route cold flow through the outer shell, and hot flow through an inner, flexible tube. similar to the air ducts?