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.

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?