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)

2013-07-22 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).

Inner and Outer Reflector Design (July 18, 2013 meeting notes)

Center reflector design

  • Depth of key holes: currently 4 cm, probably more than we need. We want to make sure that the keys remain inserted into following blocks as the column buckles under DBEs. 1 cm for 25 cm thick blocks is probably enough. Also, instead of sharp breaks, use small fillets. Change the key hole shape accordingly.

Stress concentration:

  • Thickness of the inner reflector blocks is currently 25 cm –> lobes are nearly square. Having this roughly square ratio works best for irradiation induced stresses.
  • Control rod channels: move holes outward so that distances to edges of the lobes are ~ equal. Important for resulting shrinkage of the lobes from neutron irradiation.
  • Coolant injection channels: change shape from circular to key shape to increase distance between channels. This will help to cope with tensile stress from pressure differential between the coolant channels and out of the reflector.
  • For a more uniform neutron irradiation region geometry at surface of reflector, get the center of the curvature radius closer to the surface.
  • Slot size: need to perform pressure loss calculations to determine the right size.
  • Check if the changes in geometry have an impact on neutronics (bigger region of graphite absorber).
  • Small coolant channels between core and control rod/coolant injection channels : have some that go to the control rod insertion channel and some that go to the coolant injection channel. These channels should be ~ 1 cm diameter. Number of rows of channels will be determined based on pressure drop.
  • Injection channels: as we get higher than ~ 70 cm above bottom of the active core region, we don’t want to have too much flow coming through the center reflector à narrow down the size of the coolant injection channels (having a small circular region at the end of the slots will also help to reduce stress concentration at the end of the slots).
  • Add instrumentation channels (8?) at the center of the inner reflector, with keys for alignment.
  • Tie rods to carry tensile stress: possibly locate them at the outer corners of the lobes.

Lower region of the center reflector:

  • Also reduce thickness of the keys to ~ 1 cm. Reduce radial thickness of the keys to ~ 2 cm. All these dimensions will be determined through buckling and FEM analyses. Key structure should not be symmetric (use odd number of keys).

Outer reflector design

  • Block thickness can be double that of the inner reflector blocks (50 cm instead of 25 cm) because of lower neutron dose.
  • Successive layers of blocks are staggered azimuthally so that when gaps open between blocks under thermal expansion (1% of the 6 m circumference, divided by 24 blocks = 2.5 mm) by-pass flow through the cracks is blocked.
  • At the top, transition into metal for neutron shielding, where the DHX wells begin.

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.)

Outer Reflector and Upper core structures

Coolant flow path design

  • move dhx down, and integrate into lid
  • what is the stand-off, for neutron irradiation? – ask Tommy/Madicken
  • bring outlet plenum down, and run hot duct up through the outer reflector
  • downwards flow through the defueling chute?
  • dead space in the top metalk structure is to be filled with graphite
  • beyond design basis – if the lid looses structure, we want to be able to carry loads through the graphite
  • outlet can be channels, rather than slots, because outer reflector is shielded by graphite pebbles
  • talk to Madicken/Tommy to estimate doses on outer reflector surface, and consider issue of cracking due to differential shrinking

Rationale for the elevation of the skirt

  • at faulted salt level
  • additional salt faulting: 50-60 cm, in the event of vessel faulting, and leaking into the reactor cavity (need to know residual porosity of cavity insulation)
  • need room for the vessel to expand downwards when heating up
  • if the metal structure fails, everywhere, there needs to be structure from the ceramics
  • cavity wall, frozen interface – water boiling, or let the concrete heat up
  • use concrete with modest off-gassing
  • cavity wall: metal composite, tie rods etc, so good conduction path

100 DT:

  • pressure drop vs. materials
  • below 600: changes average temperature on power conversion system
  • above 700: limited by metallics
  • less than 100 DT, high pressure drop, but also higher temp to power conversion system

To Discuss Further:

  1. Stacking problem, elevation for (a) DHX, (b) start point for metallic upper structures
  2. Design of outlet plenum torrous, to ensure uniform azymuthal flow distribution.
  3. Outlet and inlet lines: channels vs. slots
  4. Consider again Option 2 for coolant routing, with hot leg extending down into the central reflector

(notes taken by Raluca, design meeting on 25 Jun 2013)

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?






(design meeting notes)

First Stage

first nozzle stage: should be an impulse stage. flow goes to sonic velocity, choke flow regime, pressure drop is significant. The blades are impulse blades, and pressure drop will be very low. The air will cool, across the nozzle.

gives you a way to control mass flow

the air that strikes the blades will be much cooler because of expansion to high velocity (20% temp drop)

How many rows of blades do we need, to get expansion ratio of about 4?

Second Stage

may be difficult to also get it in choked flow. so we have to choose between first or second stage being in chocked flow.

inlet manifold and nozzle must have about double the flow area, compared to those of the first stage

challenge: peak temperature is on the second stage, rather than the first stage. does this mean that impulse blades should be used here, rather than reactive blades?

Constant air mass flow

power varies by varying turbine inlet temperature. so when running at part load, these types of turbines will be inefficient.

Turbine casing width must stay below 3.5 m, to remain rail-transportable, excluding the external combustor, which connects via a flange

Combutor cooling flow:

420+1300 -> 670


torrous: you want to keep the velocity constant, so that you don’t have to accelerate and decelerate, and you get constant pressure

applies to inlet plenum upstream of each of the two turbine nozzles. also applies to the outlet plenum on the reactor vessel.

Cold Air Line

what is the design assumption on the diameter of the cold air pipe? also, need to transition to higher diameter, because we shift to insulated piping

need to rotate around so it comes off the bottom

Hot Air Line

add the extension

to mention:

  1. adding notes in wordpress
  2. design of torrous
  3. posting edrawing of latest design