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.





2013-08-02 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.

2013-07-22 Design Meeting Notes

Inner reflector

  • Stress analysis for inner reflector: use COMSOL as an easy first step (+ allows for coupling with TH analysis).
  • Trick the model into taking shrinkage and swelling due to neutron irradiation into account.


  • Find references on implementation of dose data.
  • Find ratio between peak stress for optimized design and simplified design.

Outer reflector

  • Outer reflector blocks slide in radially (cold core): use tie rods to lift up blocks and key into upper core structure.

Upper core structures

  • Dose limit on upper core structures should be ~ the same as that on core barrel.
  • Make sure metallic components in upper core structure don’t start to bulge in under force from buoyant fuel and graphite structures underneath.

Cold leg flowpath

  • Deliberate fraction of by-pass flow from the cold leg to small annular slot in the upper core region to keep it ~ at same temperature as downcomer.
  • This small by-pass will feed into the hot leg through a nozzle that shouldn’t see hot salt à use same design as combustor in turbine. The high temperature gradient along the hot leg pipe should be designed to be located a few 10s cm away from the core barrel, away from inside of the vessel and the joint with the cold leg by-pass.
  • Other by-pass to pebble injection lines (opposite of hot leg). Pebble injection system is part of the upper core internals.

Neutron shielding

  • Shielding of core barrel from recirculated fuel is not too big of an issue: gammas don’t affect integrity of materials, and it is a sub-critical multiplication region, so the only neutron irradiation comes from inside the core, meaning the fuel in the injection lines doesn’t change anything.
  • Both key rings for outer reflector blocks and core barrel need to be shielded from neutrons from the core though. Investigate where to optimally locate boron absorbers (currently between rings and core barrel; move inwards to protect rings as well?).

Core barrel design

  • Avoid stress concentration under creep regime.
  • Design to accommodate bends, changes in diameter, etc.
  • Keep access for inspection along core barrel.
  • Hot leg and cold leg should be at same elevation to have same influence on LOCA level. Have one above and one under the inner flange by having the flange make a 45° angle with core barrel.
  • Downcomer flow: want to remain ~ 2 m/s, 0.6 m3/s à need 0.3 m2 flow area. With a 3.5 m diameter vessel (12 m circumference), the gap should be ~ 2.5 cm. If using a 2 cm flange, this means only a 5 mm gap.
  • Assume possibility that all joints may self-weld, especially tapering; investigate gasket materials in the design.

2013-07-22 Core Barrel Design

Pebble defueling mechanism

  • Material flowpath schematic for defueling mechanism: separate graphite vs. fuel pebbles, detect broken pebbles.
  • Design of canister system if used vs. manipulate pebbles individually through pneumatic systems.


  • See IAEA paper on pebble fuel: rationale for letting the pebbles cool down for ~ 4 (?) days betwen several passes is to let some of the high gamma emitters decay before analysis for burnup (Cs accounting).

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.)