2013-12-04 Design Meeting Notes

Upper core internals

  • With the 3 DHXs in the space between the inner and outer lids, there is significant room for pebble handling containers.
  • Refueling deck could be thinner (1 m rather than 2 m; higher at the bottom, same at the top) and add steel or lead locally where additional shielding is needed.


  • Fill tank: needs to be big enough to hold whole DRACS salt inventory. Locate it inward compared to TCHX (needs to fit in footprint of DRACS hatch).
  • Hot and cold leg should not penetrate the RV wall if we want to mount the DRACS modules on frames. Extend upward (above cap).
  • Hot and cold leg elbows should be bends.
  • Space out the TCHXs 120° rather than 90° (Aligned with DHXs).
  • Frame should be key shaped structure going through cavity cap. Reactor will have to be defueled when pulling out the DRACS (breach in containment).
  • Condenser can be on the outside, at the base of the chimney structure, at grade level.
  • Water tanks for makeup water: look at ESBWR design. Bottom can be aligned with bottom of TCHX. Shape doesn’t matter. Locate inside key shape, inside containment, against wall.s
  • Partition the water tanks with independent valve systems so that if a line breaks, they don’t drain completely.

Reactor cavity

  • May want to change the shape of the reactor cavity wall to key shape (not flat walls everywhere but rather circular walls where not interfering with hot/cold leg of primary system.
  • Having flat outer walls of the cavity would help for manufacturing (cf AP1000). Could be done by having 12 flat sections (spaced by 30°).
  • Outer reactor building wall below grade could be thinner (~ 50 cm). Look at IAEA report for water proofing.

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

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)

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.)